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Reliability assessment of the single motor drive of the belt conveyor on Drmno open-pit mine
Electrical Power and Energy Systems 113 (2019) 393–402
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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Reliability assessment of the single motor drive of the belt conveyor on Drmno open-pit mine Saša Štatkića, Ilija B. Jeftenićb, Milan Z. Bebićc, Žarko Milkića, Srđan Jovića,
T
⁎
a
Faculty of Technical Sciences, University of Pristina in Kosovska Mitrovica, Knjaza Milosa 7, 38220 Kosovska Mitrovica, Serbia Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, Belgrade, Serbia c Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, Belgrade, Serbia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Reliability Single-drive Frequency converters Belt conveyors
Reliable operation of controlled electrical drives is one of the standard requirements set as a part of tender conditions and they must be met when designing technological systems in mining industry. Elements of power electronics play a crucial role in controlled electrical drives. In this paper reliability calculations, of one controlled drive with frequency converter and high-power cage induction motor, are shown. This setup is part of the multi-motor conveyor belt station in open-pit coal mine. Available data on reliability of individual components, automation, power electronics and mechanics have been provided by manufacturer or defined by appropriate standards. Possibilities for optimizing the reliability of one single drive have been analyzed depending on frequency converter and drive load.
1. Introduction
driven by cage induction motors, themselves powered by frequency converters, are shown in [3]. During the acceleration period, there are no significant oscillations or amplitude changes of the belt tension force. After reaching the constant belt speed, the tension force gradually grows to a constant value, at which it stays during operation. Utilization of controlled drives with frequency converters in belt conveyors enables the acceleration and deceleration process to be controlled, with no abrupt changes in the tension force amplitude, which has a positive influence on the rubber belt and mechanical parts reliability. By comparing the previously described examples of the two different belt conveyor electric drives, it can be concluded that electric drive with the frequency converters has significant advantage for the conveyor belt technology. Variable speed drive is capable to adjust the speed when starting and stopping with a predefined acceleration and deceleration rate [4]. Analysis and research in the field of the conveyor belt drives can show that in many industrial applications there is a large number of drives with frequency converters, and that the installed powers of these drives are high [5,6]. Achieving complex functions for equipment maintenance on belt conveyor stations with the purpose to increase the reliability of a single drive on any belt conveyor station is possible only through a modern computer control and management system and through reliable communications [7]. [8] proposed a modified model for reliability analysis of frequency
Conveyor belts are components in the production process at openpit mines and in other industrial sectors where transport of material is carried out. For the purpose of work, automatization and productivity increase, up-to-date designs that include utilization of controlled electrical drives and Industrial Programmable Logic Controllers are envisioned as part of the design of the new conveyors and the reconstruction of the existing ones. Remote surveillance and control of electric drive, as well as collection and archiving of data in the control center, are all important factors for efficient maintenance, which influence the increase in productivity and reliability of the entire belt conveyors system at open-pit mines [1]. In [2], characteristics of the startup of a belt conveyor, driven by induction motors with wound-rotors and rotor resistance starters, are demonstrated using a dynamic model of the drive and experimental results. The obtained results show that largest changes in belt tension force occur at the period of drive startup, as well as that there are significant abrupt changes in belt tension force when the contactors of rotor resistor segments are switched on. All of this indicates that old conveyor belt drives, with induction motors having wound rotors and rotor resistance starters, have a negative influence on transient belt stresses during startup, and that they are more likely to cause breakup of rubber belts at conveyors they drive. Experimental results recorded during the startup of a belt conveyor ⁎
Corresponding author. E-mail address: [email protected] (S. Jović).
https://doi.org/10.1016/j.ijepes.2019.05.062 Received 12 February 2019; Received in revised form 13 April 2019; Accepted 26 May 2019 Available online 29 May 2019 0142-0615/ © 2019 Elsevier Ltd. All rights reserved.
Electrical Power and Energy Systems 113 (2019) 393–402
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converters based on failures and outages reports according to the load conditions. In paper [9], a comparative analysis of the main standards for failure rate prediction of low voltage electric drives with induction motors supplied by frequency converters was made taking into consideration the influence of environmental conditions on the failure rates of the system components. [10] paper proposes an alternative method to significantly improve the availability of the high-capacity belt conveyor systems with medium-voltage variable-speed induction motor drives by using faultresilient medium-voltage ac drives. A reliability assessment for this improvement was made in paper [11]. This paper provides the reliability assessment of a single motor drive on the belt conveyor station with the changeable structure of the frequency converter power modules which facilitates the increase of the belt conveyor reliability.
Fig. 2. The drive pulley is driven by two motors via two gearboxes.
shown in Fig. 3.e. and Fig. 3.f, respectively. Induction cage motor has rated power 1000 kW, powered by frequency converter with nominal voltage of 690 V, with nominal speed 992 rpm and power factor 0.96.
2. Case description 3. Methodology In this paper the influence of the reliability assessment of semiconductors, electrical and mechanical components has been conducted for one drive within the belt conveyor station in open-pit mine Drmno. Also, the paper discusses the topology influence of LV frequency converters for high power rated drive on drive reliability. Single-pole diagram of one frequency converter (FC) on belt conveyor station is shown in Fig. 1. High rated power of considered drive (1 MW, 690 V) implies parallel connection of multiple rectifier modules in AC/DC section (2) and parallel connection of inverter units (5) in DC/AC section of frequency converter [12,13]. Due to high power, all frequency converters are placed in separate cabinets. Fig. 1 shows that between cabinets with rectifier modules (2), inverter cabinets (5) and braking chopper cabinet (4) exists electrical connection through DC bus-bars. Each frequency converter consists of rectifier units (AC/DC) and inverter units (DC/AC). Rectifier consists of two 12-pulse parallel connected rectifier modules (Fig. 3a) which are located in one cabinet section, while the inverter part consists of three inverter IGBT modules connected in parallel (Fig. 3b), also located in separate cabinet section. LV frequency converter, in its cabinet version ACS800-07-1740-7, has 12-pulse rectifier units which are supplied by three-winding transformer, whose secondary windings have 30° phase shift [14]. Induction motor is through mechanical transmission connected with drive pulley, Fig. 2. In addition to this, each frequency converter has cabinet section for braking chopper (DC/DC converter Fig. 3c) which turns on the braking resistors for electrical braking in frequency converter DC link. Two three-winding transformers 2,500kVA/2x1250kVA, 6/2x0,69 kV, Dy5D0 are located on belt conveyor platform, Fig. 3d. Two drive sections motor and gearbox from the drive pulley’s left and right side, are
Reliability assessment of the single-motor belt conveyor drive has been conducted using analytical reliability calculations for the considered drive structures and control systems. The regulated drive is complex functional group which can be divided into the following subgroups based on certain component nature: control, electrical, electromechanical and mechanical subgroups respectively. Reliability of one single drive with frequency converter and cage induction motor which drives drum within the belt conveyor station through coupling devices and gearbox, is defined as a serial connection reliability of all individual elements of controlled electrical drive system. Fig. 4 shows two row of reliability block diagram for two different configurations of frequency converters and all the rest subgroups of the single-motor drive. The Reliability block diagram in Fig. 4 is also a graphic representation of the subsections organization in the third section of this paper. Number of reliability blocks is the same as number of subsections. The same subsection numbers are also assigned in the fourth chapter and refer to the same reliability blocks as in the third section.
4. Theory Basic requirement in a belt conveyor design is for its drive to provide the needed load torque. In addition to this, there is an additional requirement for achieving high availability of the drive. Therefore, belt conveyor drive operation in emergency operating conditions should also be investigated in advance, allowing continuous use of the belt conveyor system with reduced performances in case of a failure at the rectifying or inventory unit of the frequency converter [5]. A system that used to have a serial configuration in terms of reliability, but which can actually tolerate the occurrence of faults and does not switch off due to a failure of one of its components, no longer has the serial character in terms of reliability [15,16]. In a serial system configuration, failure of any of components, switches the entire system off. If the system tolerates a certain kind of a failure, it continues to be operational even if one of the components switches off. Thus, the system in question is no longer a serial system in terms of reliability, and becomes more reliable, as defined by all of the working conditions. For a system with n identical components, each with a constant failure rate (λ = const.), where k is the minimal number of components that needed to be operational, the equivalent reliability for k out of n components (Rk/n (t)) is calculated by the generic equation [15,16]:
Fig. 1. Single-pole diagram of controlled single-motor drive system with cage induction motor 1000 kW, 690 V and frequency converter, [14], (1) power supply from three-winding transformer, (2) two 12-pulse rectifier modules, (3)DC bus-bars, (4) Chopper with braking resistors, (5) three IGBT inverter units.
n
Rk / n (t ) =
∑ i=k
394
n! ∙ (e−λt )i∙ (1 − e−λt )n − i (n − i) !∙i!
(1)
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a) Two 12-pulse rectifier modules
b) Three inverter modules
c) Brake chopper
d) Two three-winding transformers
e) Motor –gearbox- right, drive pulley
f) Motor –gearbox- left, drive pulley
Fig. 3. Photos of basic elements taken from one drive on belt conveyor station 4 × 1000kW, belt width 2000 mm, open-pit mine Drmno.
4.1. PLC reliability and its communication with FC – Control functional subgroup
Explanation of symbols in previous equation: λPLC – failure rate on the PLC λ CPU – failure rate on processing unit λPB – Profibus communication modules (two pieces) failure rate. λETH – failure rate on Ethernet module λBAT – battery failure rate λPS – power adapter failure rate λDP – failure rate on distributed Profibus interface modules (two pieces) λI/O – failure rate on I/O modules (16 pieces) For connection of frequency converter [14] with PLC into Profibus communication network, Profibus adapter was used within the frequency converter. We will assume that this Profibus adapter has the same MTTF value as the Profibus card within PLC. Mean time to failure (MTTF) for PLC system and communication between PLC and one frequency converter [12] is:
Control functional group includes a programmable logic computer (PLC) that supports distributed control system [17] and communication card within frequency converter (Profibus adapter). Basic modular PLC components are processor unit (CPU), two communication modules (PB), Ethernet module (ETH), battery (BTH), power supply adapter (PS), Profibus interface modems (DP) and I/O modules. According to paper [18], PLC failure rate is calculated as the sum of the failure rates of all components that must be faultless for the normal operation of the PLC.
λPLC = λ CPU + 2λPB + λETH + λBAT + λPS + 2λDP + 16λI / O
(2)
Also, according to [18], mean time to failure (MTTF) for PLC is:
MTTFPLC =
1 1 = λPLC λ CPU + 2λPB + λETH + λBAT + λPS + 2λDP + 16λI / O (3)
Frequency converter 3.1
3.2 PLC
3.1
Electrical equipment
3.2 PLC
3. Theory
3.3
Electrical equipment
Two Rectifieer Modulees in parallel
Three IGBT inverter modules in parallel
3.5 Sqquirrel cage induction motor
3.6 Mechanical transmission system
One of Two Rectifieer Modulees in parallel
Two of Three IGBT inverter modules in parallel
3.5 Sqquirrel cage induction motor
3.6 Mechanical transmission system
3.4
Frequency converter with redused ran capability
4. Calculations 5. Results Fig. 4. Reliability Block diagram of the single motor drive and graphic presentation of applicate Methodology for reliability assessment. 395
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MTTFPLC _FC = =
1 λPLC + λPB
module causes the breakdown of entire drive and equivalent failure rate is:
1 λ CPU + 3λPB + λETH + λBAT + λPS + 2λDP + 16λI / O
(11)
λINVΣ = 3λINV1
(4)
Mean time to failure of all three IGBT inverter units is:
1 1 = λINVΣ 3λINV1
4.2. Reliability of elements for the electrical equipment
MTTFINV Σ =
In electrical equipment functional subgroup, we can classify threewinding transformer for supplying frequency converters, LV switch, FC container cooling system, frequency converter and squirrel cage induction motor. Mean time to failure of functional subgroup which consists of threewinding transformer, LV switch and FC container external cooling system is calculated according to the following relation:
According to data of inverter modules with IGBT transistors [20], which are determined at 50% of nominal voltage, 50% of nominal current and ambient temperature of 40 °C, number of failures is 100 FIT units. Based on handbook [19] for diode, failure rate is 50 FIT units, while for the DC capacitor failure rate is 20 FIT units. Mean time to failure for the entire frequency converter with two 12 pulse rectifier modules connected in parallel and three IGBT inverter modules connected in parallel as well is shown in the following equation:
MTTFLV =
1 1 = λLV λTR + λ SW + λ CS
(5)
Explanation of symbols in previous equation: λTR – failure rate on a three-winding transformer λ SW – low voltage (LV) switch failure rate λ CS – failure rate on FC container external cooling system.
MTTFFC1 =
(6)
Also, according to [12], mean time to failure of one three-phase 12 pulse diode rectifier is:
1 λREC1
(7)
RREC Σ1/2 = e−2λt + 2(e−λt − e−2λt )
Explanation of symbols in two previous equations: λREC1 – failure rate on three-phase 12 pulse diode module rectifier λD – diode failure rate Mean time to failure of two three-phase 12 pulse diode module rectifiers connected in parallel is:
MTTFREC Σ =
1 λREC Σ
=
MTTFREC Σ1/2 = (8)
MTTFINV 1 =
λINV1
=
(16)
Mean time to failure during the operation of two out of three IGBT inverter modules, only for the inverter section of frequency converter, is shown by the following expression:
(9)
MTTFINV Σ2/3 =
5 1 6 λINVΣ1
(17)
Mean time to failure for the frequency converter operating with one out of two 12 pulse rectifier modules and with two out of three IGBT inverter units is given by the following expression:
1 λ C + 6λIGBT + 6λD
(15)
RINV Σ2/3 = 3∙e−2λt − 2∙e−3λt
As well, according to [21], mean time to failure of three-phase IGBT inverter unit is given with the following expression:
1
3 1 2 λREC Σ1
According to papers [15,16,22], if two out of three inverter modules are in function, inverter unit reliability of the frequency converter is given by the following function:
According to manual [19], for the energy diode, number of failures is 50 FIT. Unit for failure rate is 1 FIT (failure in time = 1⋅10−9⋅h−1). If the certain component has the unitary value of 1 FIT, that means that during the 109 operation hours of observed component one failure will occur on that component, [20]. Failure rate of three-phase inverter module with IGBT’s [21], is calculated with the following equation:
λINV1 = λ C + 6λIGBT + 6λD
(14)
Mean time to failure during the operation of one out of two 12 pulse diode rectifier modules, only for the rectifier section of frequency converter, is shown in expression:
1 2λREC1
(13)
Based on data [14], in case that one rectifier module and one inverter module have to be removed from the cabinet for the maintenance reason, it is possible for the frequency converter to continue operation with the rest of the modules (Reduced run capability). With reduced load values, rectifier unit (two modules) and inverter unit (three modules), modularity, within one frequency converter, provides possibility to run the convertor without one rectifier module and one inverter module. This possibility increases operational reliability at the level of one frequency converter. Further reliability analysis is possible with the presumption that electric drive is in operation with reduced load. According to papers [15,16,22], in the case that one out of two rectifier modules is in operation, reliability of the rectifier unit is given by the following function:
Analyzed frequency converter has two 12-pulse rectifiers modules which are connected in parallel. According to the topology of FC and according to [14], number of diodes in rectifier module is 12. With presumption that for normal operation of rectifier it is necessary that all elements are in function, failure rate of three phase 12 - pulse diode rectifier module is given with the following expression:
MTTFREC1 =
1 1 = λRECΣ + λINVΣ 2λREC1 + 3λINV1
4.4. Reliability of frequency converter with reduced run capability
4.3. Reliability of frequency converter at rated operational conditions
λREC1 = 12λD
(12)
(10)
Explanation of symbols in two previous equations: λIGBT – IGBT failure rate λD – diode failure rate λ C – capacitor failure rate Frequency convertor within the belt conveyor station which reliability is being analyzed has three inverter units connected in parallel. During the simultaneous operation of all three modules, failure of one
MTTFFC2 =
1 = λRECΣ1/2 + λ INVΣ2/3
1 2 λ 3 REC1
6
+ 5λ
INV1
(18)
4.5. Squirrel cage induction motor reliability Motor reliability can be described with mean time to failure (MTTF) 396
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expectancy is reduced by half to a 10.000 h. The third point is obtained at the temperature of 120 °C which is maximal allowed temperature of insulation class B. In case the actual thermal stress of the winding with insulation class F is 120 °C, the lifespan of insulation class F is increased to 200.000 h. According to paper [23], MTTF for the stator winding of insulation class F is 10 years or 87.600 h. According to paper [23], the cage induction motor reliability calculation considers bearing and winding stator failures as the most common faults. Three-phase cage induction motor failure rate is calculated with the following expression:
which is nearly equal to mean time between failures (MTBF), if the malfunction time is much shorter than the operating time, [15]. Based on various results obtained in several different studies and research concerning failures on squirrel cage induction motors, the conclusion can be derived that failures on bearings and stator windings presents three quarters out of the total failures, [23]. Motor bearings and stator windings are the most critical elements in terms of squirrel cage induction motor reliability. When supplying induction motors from the frequency convertors, due to the switching nature of PWM inverter, high-frequency voltage components are induced on the motor shaft [24]. As a result of closing of the current circuit which is made by the motor metal parts: motor shaft, motor bearings and the motor housing, there is a flow of a highfrequency current. During this transient process, frequent discharges of a stray capacitance occurs in the bearings with high current values. This leads to the metal transfer between the balls and ball bearing race. Bearing damage is reflected in the appearance of transverse notches (fluting or wash boarding raceway) in the bearing guides (bearing raceway), which increase over time and lead to the loss of friction bearing characteristics. Current density flowing through the bearing, depends on amplitude, the contact surface in the bearing and the type of a lubricant for baring lubrication. In literature [25], upper limit value of the current density flowing through the bearing is marked. This value is 0.56 A/mm2. If the current density flowing through the bearing is lower than this value, lifespan of 50.000 h without any damage caused by electrical influence can be achieved. In order to prevent any mechanical damage of the induction motor (powered by the frequency converter) bearings, due to the current flow through the bearings, motor manufacturers apply certain measures. Some of these methods are: insulated bearings (ideal case ceramic ball bearings), brushes or rings for shaft grounding and additional electrostatic shield installed in the slot openings [23]. In the motor reliability calculations according to [23], MTTF value is used for the bearings in 5-year period or 43.800 h, which is in accordance with literature recommendations [24,25]. These literatures are related to bearing life expectancy if all safety measures are applied. During the motor operation, stator winding is exposed to various types of stress, such as: thermal, electrical, mechanical and environmental influences, [23,26,27]. Allowed winding temperature is determined by the insulation class, and must be higher than the temperature of the hottest spot on the stator winding within ambient temperature of 40 °C. For the insulation class F, maximum allowed temperature is 155 °C. According to the recommendations of standard [28] for the F insulation class, life expectancy of the insulation is foreseen for 20.000 h. This applies in case of 155 °C maximum allowed temperature and if the insulation preserves its dielectric properties when applying 50% of the insulating breakdown voltage. Standard [28] defines the procedure for determining the thermal endurance graph for the insulation class F used in electrical machines. In paper [29], one of these graphs is shown, obtained in experiment with accelerated insulation thermal aging in laboratory conditions. This experiment has been performed according to the procedure defined in standard [28]. On the curve of thermal endurance graph for insulation class F, three characteristic points are distinguished. The first one refers to a maximal allowed temperature of 155 °C where the life expectancy of insulation class F is 20.000 h. At the second point at the winding temperature of 165 °C, which is an increase of 10 °C to a maximal allowed temperature of 155 °C, the insulation life
Motor M1
Brake
Gear box
Also, based on [23], mean time to failure of the three-phase cage induction motor is:
MTTFINM =
1 1 = λINM λB1 + λW + λB2
(20)
Explanation of symbols in two previous equations: λB1 – failure rate of the bearing no. 1, on the fan side λW – failure rate of the stator winding λB2 – failure rate of the bearing no. 2, on the drive side 4.6. Mechanical transmission system reliability Transport belt is moving over one or more driving drums. The number of drums depends on the transport belt length, terrain configuration and transport capacity [1]. Driving drum is connected to the motor through couplings and gearbox. For the purpose of providing the necessary torque, driving drum is driven with one motor and gearbox on both sides. Mechanical brake is located between motor and gearbox and it is meant for safe stop during the regular stop or in case of emergency stop. Mechanical part within the belt conveyor station of single drive consists of mechanical brake (thruster), mechanical gearbox, coupling and a drive drum as it is shown in Fig. 5 According to literature [30] for one single driving drum within the belt conveyor station which is driven by two motors through two gearboxes, MTTF is 3 years. In previous chapter, through valid sources MTTF is calculated for the squirrel induction motor and its value is 2 years. Because the motor has 2 bearings, while the observed drive mechanical part has two bearings in the gearbox and two more bearings on the drive drum, we have estimated MTTF for mechanical drive part. It is two times less than the value given in literature [30], 3 years/ 2 = 1,5year.
MTTFMeh. = 3year/2 = 1.5year
(21)
5. Calculations In this section, based on the theoretical settings from the previous section and data from the corresponding references, appropriate calculations have been made and presented in a tabular manner. 5.1. Calculation PLC reliability and its communication with FC MTTF data used for PLC components is provided by equipment manufacturer [17] and is shown in Table 1. Mean time to repair MTTR on PLC (cards, voltage supply, battery replacement) can be several hours during one shift which is negligible compared to mean time to failure MTTF. Practically mean time between failure (MTBF) of a
Brake
Driving drum
(19)
λINM = λB1 + λW + λB2
Fig. 5. Driving Pulley Driven on Both Sides, [1].
Couple Motor M2
Gear box
397
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Table 1 MTTF calculation for control system and Profibus communication between PLC and FC - MTTFFC-PLC. Control functional group
PLC
Component
Numb. of compon.
CPU module Base Unit 4 MB Fieldbus - Profibus module Fieldbus -Ethernet module Battery Backup module Power Supply 230 V AC Distributed I/O module Distributed Communication module Profibus module on FC
DP FC
1 2 1 1 1 16 2 1
MTTF
MTTF
λ
h
year
h−1
184,994 292,765 416,713 909,227 395,091 570,780 1,003,020 292,765 λPLC + λ PB – [h−1] MTTFPLC_FC – [h] MTTFPLC_FC – [year]
21.12 33.42 47.57 103.79 45.10 64.16 114.5 33.42
5.41E-06 3.42E-06 2.4E-06 1.1E-06 2.53E-06 1.75E-06 9.96E-07 3.42E-06 5.17E-05 19,339 2.21
Table 2 MTTF calculation for supply transformer, LV switch and FC container external cooling system.
1. 2. 3.
Component
Numb. of compon.
λ FIT
MTTF h
MTTF year
λ h−1
Transformer for frequency converters Low voltage switch External cooling system for FC container
1 1 1
2,400 180 1,000 λLV – [h−1] MTTFLV – [h] MTTF LV – [year]
416,667 5,555,556 1,000,000
47.56 634.20 114.16
2.40E-06 1.80E-07 1.00E-06 3.58E-06 279,330 31.89
Table 3 MTTF calculation of two parallel 12 pulse diode rectifiers.
1.
Component
Number of components
λ FIT
MTTF h
MTTF year
λ h−1
Diode
12
50 Rectifier module
20,000,000
2,283.11 λREC1 – [h−1] MTTFREC1 – [h] MTTF REC1 – [year] MTTFRECΣ – [h] MTTF RECΣ – [year]
5.00E-08 6.00E-07 1,666,667 190.26 833,333 95.12
Two parallel rectifier modules
Table 4 MTTF calculation – three IGBT inverter modules in parallel.
1. 2. 3.
Component
Number of components
λ FIT
MTTF h
MTTF year
λ h−1
IGBT Diode DC capacitor
7** 6 1
100* 50 20 Inverter module
10,000,000 20,000,000 20,000,000
1,141.55 2,283.11 5,707.07 λINV – [h−1] MTTFinv1 – [h] MTTF inv1 [year] MTTFinvΣ [h] MTTF invΣ [year]
1.00E-07 5.00E-08 2.00E-08 1.02E-6 980.392 111,92 326.797 37.30
Three parallel inverter modules
* – FIT- failure rate for IGBT of medium power from the 2000. year, [20]. ** – The seventh IGBT is for braking chopper unit.
system is expressed as MTBF = MTTF + MTTR ≈ MTTF, [22].
Table 5 MTTF calculation – frequency convertor with two rectifiers in parallel and three inverter units in parallel. Frequency converter
1. 2.
Rectifier part Inverter part
Number of components
1 1 MTTFFC1 [h] MTTF FC1 [year]
MTTF h
MTTF year
833.333 326.797
95,12 37,30 234.742 26,80
5.2. Calculation reliability of elements for the electrical equipment According to [13,31] data about MTTF for transformer which supplies frequency converters, LV switch and container external cooling system is shown in Table 2.Table 3. 5.3. Reliability of frequency converter at rated operational conditions According to [32], the experimental reliability assessment (ongoing 398
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Table 6 MTTF calculation – one out of two parallel connected 12 pulse rectifier modules.
1.
Component
Number of components
λ FIT
MTTF h
MTTF year
λ h−1
Diode
12
50 Rectifier module
20,000,000
2,283.11 λREC1 – [h−1] MTTFREC1 – [h] MTTF REC1 – [year] MTTFRECΣ – [h] MTTF RECΣ – [year] MTTF RECΣ 1/2 [h] MTTF RECΣ 1/2 [year]
5.00E-08 6.00E-07 1,666,667 190.26 833,333 95.12 2.500.000 285,30
Two parallel rectifier modules One out of two rectifier modules 1/2
Table 7 MTTF calculation – two out of three IGBT inverter units connected in parallel. Component
1. 2. 3.
Number of components
7** 6 1
IGBT Diode DC capacitor
λ
MTTF
MTTF
λ
FIT
h
year
h−1
100* 50 20 Inverter module
10,000,000 20,000,000 20,000,000
1,141.55 2,283.11 5,707.07 λINV – [h−1] MTTFinv1 – [h] MTTF inv1 [year] MTTFinvΣ [h] MTTF invΣ [year] MTTFinv 2/3 [h] MTTF inv 2/3 [year]
1.00E-07 5.00E-08 2.00E-08 1.02E-6 980.392 111,92 326.797 37.30 816,993 93
Three parallel rectifier modules Two out of three inverter modules
1,00
Table 8 MTTF calculation – frequency converter with reduced number of modules.
1. 2.
Rectifier section 1/2 Inverter section 2/3
Number of components
1 1 MTTFFC1_1/2+2/3 [h] MTTF FC1_1/2+2/3 [year]
0,99
MTTF
MTTF
0,98
h
year
0,97
2,500,000 816,993
285.30 93 615,764 70.29
0,96
R(t)
Frequency converter
0,95 0,94 0,93
Table 9 MTTF calculation – squirrel cage induction motor, insulation class F, energy efficiency class IE2. Squirrel cage induction motor
Number of components
MTTF
MTTF
h
year
43,800 87,600
5 10 17,520 2
RFC2
0,92 RFC1
0,91 0,90 0
1. 2.
Bearings Stator windings
2 1 MTTFINM1 – [h] MTTF INM1 – [year]
2000
4000
6000
8000 10000 12000 14000 16000 18000 20000
t(h) Fig. 6. Reliability function of the frequency converter, RFC1-all modules are working, RFC2 - One rectifier and one inverter module are not operational.
Table 10 Reliability results overview of one single drive within the belt conveyor station in open-pit mine Drmno.
1. 2. 3.1 3.2 4. 5. 6.1 6.2
Functional subgroup name
MTTF h
MTTF year
Control system (PLC) and Profibus communication – MTTFFC-PLC Supply transformer, LV switch and cooling system for the container with frequency converters Frequency converter FC, 2/2_AC/DC modules +3/3 _DC/AC modules Frequency converter FC, 1/2_AC/DC modules +2/3 _DC/AC modules Squirrel induction motor, two bearings +stator winding Mechanical part, gearbox +driving drum One regulated drive on belt conveyor station (1., 2., 3.1, 4. i 5.) One regulated drive on belt conveyor station (1., 2., 3.2, 4. i 5.)
19,339 279.330 234.742 615.764 17.520 13.140 5.189 5.261
2.18 31,89 26,80 70,29 2,00 1,5 0,592 0,601
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Fig. 7. Reliability function of the complete electric motor drive, taking into account the reliability of mechanical components, RSD1-all modules are working, RSD2 - One rectifier and one inverter module are not operational.
Fig. 10. Reliability of induction motor (RINM), mechanical transmission (RMEH) and overall reliability of all mechanical components (RSMEH). 1,00
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module in the analyzed case of the belt conveyor drive. The experimental result was 93 years. In this paper for the same module, using reliability assessment calculations result was 111.9 years, as shown in table 4. Higher value was obtained because the cooling process of the complete module in the housing was not taken into consideration. In the frequency convertor catalog [14], which is analyzed in this case, there are no any data concerning MTTF and operation reliability. Mean time to failure (MTTF) calculation of frequency convertors depends on several key factors, but the most important are: temperature of IGBT modules, DC rectifier voltage, elevation, ambient temperature, internal cooling system probity, external cooling system probity, frequency convertor configuration, power electronics components reliability etc (see Table 5). Based on data provided by other frequency converter manufacturer [33], mean time to failure (MTTF) of 12 pulse high power frequency converters (up to 1200 kW), supply voltage 690 V, where our considered frequency converter belongs as well, is around 200.000 h. Calculations of mean time to failure (MTTF) are done according to certain presumptions and collected information from the end users. Result obtained by calculations which are derived from the topology and specific modular configuration of an analyzed frequency converter [14] is MTTF = 234.742 h. This value is in accordance with declared MTTF value (200.000 h) for frequency converters with approximately similar nominal data of other manufacturer [33] (see. Table 6Table 7Table 8Table 9.).
1,00
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Fig. 11. Reliability of PLC controller and frequency converter communication (RPLC). Reliability of transformer, LV switch and cooling system (RLV).
Fig. 8. Reliability function of the frequency converter rectifier unit, RREC11 modules in operation, RREC12 - One of the two (1/2) rectifier modules is out of function.
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reliability test) was done in laboratory conditions for the IGBT inverter module, which has a size R8i and belongs to the ACS800 frequency converter generation. This inverter module corresponds to the inverter 400
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taking into account all mechanical drive components, it ends up that we get negligible reliability increase of an entire drive, regardless the reduced operation. For the full operation, MTTF is increased for 5,189 h to 5,261 h. Resulted difference of 72 h represents increase of 1,38%. Comparing the time diagrams of one single drive reliability on belt conveyor station, this difference is hardly noticeable in Fig. 7. Rectifier modules reliability in Fig. 8 is greater than the reliability of inverter modules in Fig. 9. This difference could be explained as a consequence of switching operation of IGBT inverter modules and greater dissipation, while diode rectifier bridge is basic semiconductor component where switching operation depends on supply network frequency, while the switching power losses are negligible. In Fig. 10 the reliability curve is shown for all mechanical components in one single drive on belt conveyor station. Mean time to failure has the lowest value in the mechanical part of the drive compared to the control system and LV voltage supply system (Fig. 11), as well as compared to the frequency converter (Fig. 6). Hereupon, the reliability curve of mechanical components is declining in time most rapidly and has the dominant influence on the reliability curve of entire drive.
5.4. Calculation reliability of frequency converter with reduced run capability In case of operation with one out of two rectifier modules and two out of three inverter units and under condition that load allows this kind of operation regime, calculated mean time to failure (70,29 years) is 2,62 times greater than mean time to failure of frequency converter (26,80 years) with all modules in function. 5.5. Calculation of squirrel cage induction motor reliability According to adopted data for MTTF for the bearings and stator windings, using the previous equation (19), the MTTF value for one squirrel cage induction motor with insulation class F, energy efficiency class IE2 is 17,250 h or 2 years. 6. Results of reliability assessment Overview of obtained reliability results for the certain components which are part of one single drive within the belt conveyor station located in open-pit mine Drmno, is shown in the following table. Mean time to failure, of one frequency converter when all modules (2/2_AC/DC+3/3_DC/AC) have to be in function, is given in expression (12). Mean time to failure of one frequency converter when it is possible to perform reduced operation with reduced number of modules (1/2_AC/DC+2/3_DC/AC), is determined with expression (17). Mean time to failure (MTTF) of one single drive within the belt conveyor station when all modules (2/2_AC/DC + 3/3_DC/AC) have to be in function, by applying conditions from Eqs. (4), (5), (8), (12), (13), (20) and (21), as well as the results from Table 10, can be defined with equation (22).
MTTFSD1 =
8. Conclusion In this paper, the assessment of the reliability has been made for one low voltage frequency converter with the high rated nominal power, which is used to drive rubber transport belt within the belt conveyor station in open-pit mine. Assessment has been made through calculation of mean time to failure (MTTF) in two cases. The first one, frequency converter can be in operation only when all of the rectifier and inverter modules connected in parallel are in function. In this case calculated mean time to failure is MTTF = 234,742 h. The second one, when the frequency converter can operate with reduced load without one rectifier module and one inverter module. In this case calculated mean time to failure is MTTF = 615.764 h. Redundant operation of the modules in frequency converter (RECT 1/2 + INV 2/3) significantly increases reliability of frequency converter itself, respectively it is 2,62 times greater than in the case when all modules have to be operational. This result of a reliability assessment of single-motor drive within the belt conveyor station can have a positive influence on increasing the operation time efficiency of the frequency converters, on improvement maintenance conditions and on reduction the cost for the spare parts during exploitation.
1 1 = λ SD1 λPLC_FC + λLV + 2λREC1 + 3λINV1 + λINM + λMEH (22)
= 5189h
Mean time to failure of one single drive within the belt conveyor station when the frequency converter can operate in reduced mode (1/ 2_AC/DC + 2/3_DC/AC), by applying conditions from Eqs. (4), (5), (8), (10), (15), (17), (18), (20) and (21), as well as the results from Table 10, can be defined with equation (23).
MTTFSD2 =
1 1 = 2 6 λ SD2 λ PLCFC + λLV + 3 ∙λREC1 + 5 ∙λINV1 + λINM + λMEH
Acknowledgement
(23)
= 5261h
Paper is a part of research within the project no. TR 33016 Research, development and implementation of programs and procedures Energy efficiency of electric drives, 2011-2019, financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia. Project period: 2011-2019.
Expression (24) represents time change of frequency converter reliability on the belt conveyor station. Thereby, mean time to failure (MTTF) values which are determined by expressions (13) and (18) represent time constants on these diagrams.
RFC1 (t ) = e
−
1 t MTTFFC1
1
= e− 234.742 t , RFC 2 (t ) = e
−
1 t MTTFFC 2
1
= e− 615.764 t ,
References
(24)
[1] Jeftenić B, Bebić M, Ristić L, Štatkić S. Design and Selection of Belt Conveying Equipment & Systems. In: Design and Selection of Bulk Material Handling Equipment and Systems: Mining, Mineral Processing, Port, Plant and Excavation Engineering, vol. I, J. Bhattacharya, editor, I ed Kolkata: Wide Publishing; 2012, p. 254, (chapter) p. 60. [2] Karolewski B, Ligocki P. Modelling of long belt conveyors. Eksploatacja i Niezawodnosc – Maintenance and Reliability 2014; 16 (2): 179–187. < http://ein. org.pl/sites/default/files/2014-02-02.pdf > . [accessed 6 April 2019]. [3] Bebic MZ, Ristic LB. Speed controlled belt conveyors: drives and mechanical considerations. Adv Electr Comput Eng 2018. https://doi.org/10.4316/AECE.2018. 01007. [4] Nuttall AJG, Lodewijks G. Dynamics of multiple drive belt conveyor systems. Part. Part. Syst. Charact. 2007. https://doi.org/10.1002/ppsc.200601118. [5] ABB Process Industries GmbH, Variable speed drive for belt-conveyors systems, [accessed 06. April. 2019] < https://library.e.abb.com/public/ b353080c210c00af832573fc005038b6/Project_Report_VSD_on_Conveyors.pdf > . [6] Daus, W., Körber, S., Becker, N., Raw Coal Loading and Belt Conveyor System at the Nochten Opencast Mine, translated from Braunkohle Surface Mining 50, No.2
7. Discussions Diagrams in Fig. 6 are obtained using relations (23) representing time change of frequency converter reliability on the belt conveyor station. Obtained values in these calculations show that in case of reduced operation (reduced number of rectifier and inverter modules) reliability of frequency converter is significantly increased, because the mean time to failure (MTTF = 615.764 h) is 2,62 times greater than in the case when all modules have to be in function (MTTF = 234,742 h). In Fig. 6, exponential reliability curve of the frequency converter with greater MTTF value is declining more slowly than the curve of the lower MTTF value. However, when the reliability of the entire drive is determined 401
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Reliability assessment of the single motor drive of the belt conveyor on Drmno open-pit mine Saša Štatkića, Ilija B. Jeftenićb, Milan Z. Bebićc, Žarko Milkića, Srđan Jovića,
T
⁎
a
Faculty of Technical Sciences, University of Pristina in Kosovska Mitrovica, Knjaza Milosa 7, 38220 Kosovska Mitrovica, Serbia Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, Belgrade, Serbia c Faculty of Electrical Engineering, University of Belgrade, Bulevar Kralja Aleksandra 73, Belgrade, Serbia b
A R T I C LE I N FO
A B S T R A C T
Keywords: Reliability Single-drive Frequency converters Belt conveyors
Reliable operation of controlled electrical drives is one of the standard requirements set as a part of tender conditions and they must be met when designing technological systems in mining industry. Elements of power electronics play a crucial role in controlled electrical drives. In this paper reliability calculations, of one controlled drive with frequency converter and high-power cage induction motor, are shown. This setup is part of the multi-motor conveyor belt station in open-pit coal mine. Available data on reliability of individual components, automation, power electronics and mechanics have been provided by manufacturer or defined by appropriate standards. Possibilities for optimizing the reliability of one single drive have been analyzed depending on frequency converter and drive load.
1. Introduction
driven by cage induction motors, themselves powered by frequency converters, are shown in [3]. During the acceleration period, there are no significant oscillations or amplitude changes of the belt tension force. After reaching the constant belt speed, the tension force gradually grows to a constant value, at which it stays during operation. Utilization of controlled drives with frequency converters in belt conveyors enables the acceleration and deceleration process to be controlled, with no abrupt changes in the tension force amplitude, which has a positive influence on the rubber belt and mechanical parts reliability. By comparing the previously described examples of the two different belt conveyor electric drives, it can be concluded that electric drive with the frequency converters has significant advantage for the conveyor belt technology. Variable speed drive is capable to adjust the speed when starting and stopping with a predefined acceleration and deceleration rate [4]. Analysis and research in the field of the conveyor belt drives can show that in many industrial applications there is a large number of drives with frequency converters, and that the installed powers of these drives are high [5,6]. Achieving complex functions for equipment maintenance on belt conveyor stations with the purpose to increase the reliability of a single drive on any belt conveyor station is possible only through a modern computer control and management system and through reliable communications [7]. [8] proposed a modified model for reliability analysis of frequency
Conveyor belts are components in the production process at openpit mines and in other industrial sectors where transport of material is carried out. For the purpose of work, automatization and productivity increase, up-to-date designs that include utilization of controlled electrical drives and Industrial Programmable Logic Controllers are envisioned as part of the design of the new conveyors and the reconstruction of the existing ones. Remote surveillance and control of electric drive, as well as collection and archiving of data in the control center, are all important factors for efficient maintenance, which influence the increase in productivity and reliability of the entire belt conveyors system at open-pit mines [1]. In [2], characteristics of the startup of a belt conveyor, driven by induction motors with wound-rotors and rotor resistance starters, are demonstrated using a dynamic model of the drive and experimental results. The obtained results show that largest changes in belt tension force occur at the period of drive startup, as well as that there are significant abrupt changes in belt tension force when the contactors of rotor resistor segments are switched on. All of this indicates that old conveyor belt drives, with induction motors having wound rotors and rotor resistance starters, have a negative influence on transient belt stresses during startup, and that they are more likely to cause breakup of rubber belts at conveyors they drive. Experimental results recorded during the startup of a belt conveyor ⁎
Corresponding author. E-mail address: [email protected] (S. Jović).
https://doi.org/10.1016/j.ijepes.2019.05.062 Received 12 February 2019; Received in revised form 13 April 2019; Accepted 26 May 2019 Available online 29 May 2019 0142-0615/ © 2019 Elsevier Ltd. All rights reserved.
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converters based on failures and outages reports according to the load conditions. In paper [9], a comparative analysis of the main standards for failure rate prediction of low voltage electric drives with induction motors supplied by frequency converters was made taking into consideration the influence of environmental conditions on the failure rates of the system components. [10] paper proposes an alternative method to significantly improve the availability of the high-capacity belt conveyor systems with medium-voltage variable-speed induction motor drives by using faultresilient medium-voltage ac drives. A reliability assessment for this improvement was made in paper [11]. This paper provides the reliability assessment of a single motor drive on the belt conveyor station with the changeable structure of the frequency converter power modules which facilitates the increase of the belt conveyor reliability.
Fig. 2. The drive pulley is driven by two motors via two gearboxes.
shown in Fig. 3.e. and Fig. 3.f, respectively. Induction cage motor has rated power 1000 kW, powered by frequency converter with nominal voltage of 690 V, with nominal speed 992 rpm and power factor 0.96.
2. Case description 3. Methodology In this paper the influence of the reliability assessment of semiconductors, electrical and mechanical components has been conducted for one drive within the belt conveyor station in open-pit mine Drmno. Also, the paper discusses the topology influence of LV frequency converters for high power rated drive on drive reliability. Single-pole diagram of one frequency converter (FC) on belt conveyor station is shown in Fig. 1. High rated power of considered drive (1 MW, 690 V) implies parallel connection of multiple rectifier modules in AC/DC section (2) and parallel connection of inverter units (5) in DC/AC section of frequency converter [12,13]. Due to high power, all frequency converters are placed in separate cabinets. Fig. 1 shows that between cabinets with rectifier modules (2), inverter cabinets (5) and braking chopper cabinet (4) exists electrical connection through DC bus-bars. Each frequency converter consists of rectifier units (AC/DC) and inverter units (DC/AC). Rectifier consists of two 12-pulse parallel connected rectifier modules (Fig. 3a) which are located in one cabinet section, while the inverter part consists of three inverter IGBT modules connected in parallel (Fig. 3b), also located in separate cabinet section. LV frequency converter, in its cabinet version ACS800-07-1740-7, has 12-pulse rectifier units which are supplied by three-winding transformer, whose secondary windings have 30° phase shift [14]. Induction motor is through mechanical transmission connected with drive pulley, Fig. 2. In addition to this, each frequency converter has cabinet section for braking chopper (DC/DC converter Fig. 3c) which turns on the braking resistors for electrical braking in frequency converter DC link. Two three-winding transformers 2,500kVA/2x1250kVA, 6/2x0,69 kV, Dy5D0 are located on belt conveyor platform, Fig. 3d. Two drive sections motor and gearbox from the drive pulley’s left and right side, are
Reliability assessment of the single-motor belt conveyor drive has been conducted using analytical reliability calculations for the considered drive structures and control systems. The regulated drive is complex functional group which can be divided into the following subgroups based on certain component nature: control, electrical, electromechanical and mechanical subgroups respectively. Reliability of one single drive with frequency converter and cage induction motor which drives drum within the belt conveyor station through coupling devices and gearbox, is defined as a serial connection reliability of all individual elements of controlled electrical drive system. Fig. 4 shows two row of reliability block diagram for two different configurations of frequency converters and all the rest subgroups of the single-motor drive. The Reliability block diagram in Fig. 4 is also a graphic representation of the subsections organization in the third section of this paper. Number of reliability blocks is the same as number of subsections. The same subsection numbers are also assigned in the fourth chapter and refer to the same reliability blocks as in the third section.
4. Theory Basic requirement in a belt conveyor design is for its drive to provide the needed load torque. In addition to this, there is an additional requirement for achieving high availability of the drive. Therefore, belt conveyor drive operation in emergency operating conditions should also be investigated in advance, allowing continuous use of the belt conveyor system with reduced performances in case of a failure at the rectifying or inventory unit of the frequency converter [5]. A system that used to have a serial configuration in terms of reliability, but which can actually tolerate the occurrence of faults and does not switch off due to a failure of one of its components, no longer has the serial character in terms of reliability [15,16]. In a serial system configuration, failure of any of components, switches the entire system off. If the system tolerates a certain kind of a failure, it continues to be operational even if one of the components switches off. Thus, the system in question is no longer a serial system in terms of reliability, and becomes more reliable, as defined by all of the working conditions. For a system with n identical components, each with a constant failure rate (λ = const.), where k is the minimal number of components that needed to be operational, the equivalent reliability for k out of n components (Rk/n (t)) is calculated by the generic equation [15,16]:
Fig. 1. Single-pole diagram of controlled single-motor drive system with cage induction motor 1000 kW, 690 V and frequency converter, [14], (1) power supply from three-winding transformer, (2) two 12-pulse rectifier modules, (3)DC bus-bars, (4) Chopper with braking resistors, (5) three IGBT inverter units.
n
Rk / n (t ) =
∑ i=k
394
n! ∙ (e−λt )i∙ (1 − e−λt )n − i (n − i) !∙i!
(1)
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a) Two 12-pulse rectifier modules
b) Three inverter modules
c) Brake chopper
d) Two three-winding transformers
e) Motor –gearbox- right, drive pulley
f) Motor –gearbox- left, drive pulley
Fig. 3. Photos of basic elements taken from one drive on belt conveyor station 4 × 1000kW, belt width 2000 mm, open-pit mine Drmno.
4.1. PLC reliability and its communication with FC – Control functional subgroup
Explanation of symbols in previous equation: λPLC – failure rate on the PLC λ CPU – failure rate on processing unit λPB – Profibus communication modules (two pieces) failure rate. λETH – failure rate on Ethernet module λBAT – battery failure rate λPS – power adapter failure rate λDP – failure rate on distributed Profibus interface modules (two pieces) λI/O – failure rate on I/O modules (16 pieces) For connection of frequency converter [14] with PLC into Profibus communication network, Profibus adapter was used within the frequency converter. We will assume that this Profibus adapter has the same MTTF value as the Profibus card within PLC. Mean time to failure (MTTF) for PLC system and communication between PLC and one frequency converter [12] is:
Control functional group includes a programmable logic computer (PLC) that supports distributed control system [17] and communication card within frequency converter (Profibus adapter). Basic modular PLC components are processor unit (CPU), two communication modules (PB), Ethernet module (ETH), battery (BTH), power supply adapter (PS), Profibus interface modems (DP) and I/O modules. According to paper [18], PLC failure rate is calculated as the sum of the failure rates of all components that must be faultless for the normal operation of the PLC.
λPLC = λ CPU + 2λPB + λETH + λBAT + λPS + 2λDP + 16λI / O
(2)
Also, according to [18], mean time to failure (MTTF) for PLC is:
MTTFPLC =
1 1 = λPLC λ CPU + 2λPB + λETH + λBAT + λPS + 2λDP + 16λI / O (3)
Frequency converter 3.1
3.2 PLC
3.1
Electrical equipment
3.2 PLC
3. Theory
3.3
Electrical equipment
Two Rectifieer Modulees in parallel
Three IGBT inverter modules in parallel
3.5 Sqquirrel cage induction motor
3.6 Mechanical transmission system
One of Two Rectifieer Modulees in parallel
Two of Three IGBT inverter modules in parallel
3.5 Sqquirrel cage induction motor
3.6 Mechanical transmission system
3.4
Frequency converter with redused ran capability
4. Calculations 5. Results Fig. 4. Reliability Block diagram of the single motor drive and graphic presentation of applicate Methodology for reliability assessment. 395
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MTTFPLC _FC = =
1 λPLC + λPB
module causes the breakdown of entire drive and equivalent failure rate is:
1 λ CPU + 3λPB + λETH + λBAT + λPS + 2λDP + 16λI / O
(11)
λINVΣ = 3λINV1
(4)
Mean time to failure of all three IGBT inverter units is:
1 1 = λINVΣ 3λINV1
4.2. Reliability of elements for the electrical equipment
MTTFINV Σ =
In electrical equipment functional subgroup, we can classify threewinding transformer for supplying frequency converters, LV switch, FC container cooling system, frequency converter and squirrel cage induction motor. Mean time to failure of functional subgroup which consists of threewinding transformer, LV switch and FC container external cooling system is calculated according to the following relation:
According to data of inverter modules with IGBT transistors [20], which are determined at 50% of nominal voltage, 50% of nominal current and ambient temperature of 40 °C, number of failures is 100 FIT units. Based on handbook [19] for diode, failure rate is 50 FIT units, while for the DC capacitor failure rate is 20 FIT units. Mean time to failure for the entire frequency converter with two 12 pulse rectifier modules connected in parallel and three IGBT inverter modules connected in parallel as well is shown in the following equation:
MTTFLV =
1 1 = λLV λTR + λ SW + λ CS
(5)
Explanation of symbols in previous equation: λTR – failure rate on a three-winding transformer λ SW – low voltage (LV) switch failure rate λ CS – failure rate on FC container external cooling system.
MTTFFC1 =
(6)
Also, according to [12], mean time to failure of one three-phase 12 pulse diode rectifier is:
1 λREC1
(7)
RREC Σ1/2 = e−2λt + 2(e−λt − e−2λt )
Explanation of symbols in two previous equations: λREC1 – failure rate on three-phase 12 pulse diode module rectifier λD – diode failure rate Mean time to failure of two three-phase 12 pulse diode module rectifiers connected in parallel is:
MTTFREC Σ =
1 λREC Σ
=
MTTFREC Σ1/2 = (8)
MTTFINV 1 =
λINV1
=
(16)
Mean time to failure during the operation of two out of three IGBT inverter modules, only for the inverter section of frequency converter, is shown by the following expression:
(9)
MTTFINV Σ2/3 =
5 1 6 λINVΣ1
(17)
Mean time to failure for the frequency converter operating with one out of two 12 pulse rectifier modules and with two out of three IGBT inverter units is given by the following expression:
1 λ C + 6λIGBT + 6λD
(15)
RINV Σ2/3 = 3∙e−2λt − 2∙e−3λt
As well, according to [21], mean time to failure of three-phase IGBT inverter unit is given with the following expression:
1
3 1 2 λREC Σ1
According to papers [15,16,22], if two out of three inverter modules are in function, inverter unit reliability of the frequency converter is given by the following function:
According to manual [19], for the energy diode, number of failures is 50 FIT. Unit for failure rate is 1 FIT (failure in time = 1⋅10−9⋅h−1). If the certain component has the unitary value of 1 FIT, that means that during the 109 operation hours of observed component one failure will occur on that component, [20]. Failure rate of three-phase inverter module with IGBT’s [21], is calculated with the following equation:
λINV1 = λ C + 6λIGBT + 6λD
(14)
Mean time to failure during the operation of one out of two 12 pulse diode rectifier modules, only for the rectifier section of frequency converter, is shown in expression:
1 2λREC1
(13)
Based on data [14], in case that one rectifier module and one inverter module have to be removed from the cabinet for the maintenance reason, it is possible for the frequency converter to continue operation with the rest of the modules (Reduced run capability). With reduced load values, rectifier unit (two modules) and inverter unit (three modules), modularity, within one frequency converter, provides possibility to run the convertor without one rectifier module and one inverter module. This possibility increases operational reliability at the level of one frequency converter. Further reliability analysis is possible with the presumption that electric drive is in operation with reduced load. According to papers [15,16,22], in the case that one out of two rectifier modules is in operation, reliability of the rectifier unit is given by the following function:
Analyzed frequency converter has two 12-pulse rectifiers modules which are connected in parallel. According to the topology of FC and according to [14], number of diodes in rectifier module is 12. With presumption that for normal operation of rectifier it is necessary that all elements are in function, failure rate of three phase 12 - pulse diode rectifier module is given with the following expression:
MTTFREC1 =
1 1 = λRECΣ + λINVΣ 2λREC1 + 3λINV1
4.4. Reliability of frequency converter with reduced run capability
4.3. Reliability of frequency converter at rated operational conditions
λREC1 = 12λD
(12)
(10)
Explanation of symbols in two previous equations: λIGBT – IGBT failure rate λD – diode failure rate λ C – capacitor failure rate Frequency convertor within the belt conveyor station which reliability is being analyzed has three inverter units connected in parallel. During the simultaneous operation of all three modules, failure of one
MTTFFC2 =
1 = λRECΣ1/2 + λ INVΣ2/3
1 2 λ 3 REC1
6
+ 5λ
INV1
(18)
4.5. Squirrel cage induction motor reliability Motor reliability can be described with mean time to failure (MTTF) 396
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expectancy is reduced by half to a 10.000 h. The third point is obtained at the temperature of 120 °C which is maximal allowed temperature of insulation class B. In case the actual thermal stress of the winding with insulation class F is 120 °C, the lifespan of insulation class F is increased to 200.000 h. According to paper [23], MTTF for the stator winding of insulation class F is 10 years or 87.600 h. According to paper [23], the cage induction motor reliability calculation considers bearing and winding stator failures as the most common faults. Three-phase cage induction motor failure rate is calculated with the following expression:
which is nearly equal to mean time between failures (MTBF), if the malfunction time is much shorter than the operating time, [15]. Based on various results obtained in several different studies and research concerning failures on squirrel cage induction motors, the conclusion can be derived that failures on bearings and stator windings presents three quarters out of the total failures, [23]. Motor bearings and stator windings are the most critical elements in terms of squirrel cage induction motor reliability. When supplying induction motors from the frequency convertors, due to the switching nature of PWM inverter, high-frequency voltage components are induced on the motor shaft [24]. As a result of closing of the current circuit which is made by the motor metal parts: motor shaft, motor bearings and the motor housing, there is a flow of a highfrequency current. During this transient process, frequent discharges of a stray capacitance occurs in the bearings with high current values. This leads to the metal transfer between the balls and ball bearing race. Bearing damage is reflected in the appearance of transverse notches (fluting or wash boarding raceway) in the bearing guides (bearing raceway), which increase over time and lead to the loss of friction bearing characteristics. Current density flowing through the bearing, depends on amplitude, the contact surface in the bearing and the type of a lubricant for baring lubrication. In literature [25], upper limit value of the current density flowing through the bearing is marked. This value is 0.56 A/mm2. If the current density flowing through the bearing is lower than this value, lifespan of 50.000 h without any damage caused by electrical influence can be achieved. In order to prevent any mechanical damage of the induction motor (powered by the frequency converter) bearings, due to the current flow through the bearings, motor manufacturers apply certain measures. Some of these methods are: insulated bearings (ideal case ceramic ball bearings), brushes or rings for shaft grounding and additional electrostatic shield installed in the slot openings [23]. In the motor reliability calculations according to [23], MTTF value is used for the bearings in 5-year period or 43.800 h, which is in accordance with literature recommendations [24,25]. These literatures are related to bearing life expectancy if all safety measures are applied. During the motor operation, stator winding is exposed to various types of stress, such as: thermal, electrical, mechanical and environmental influences, [23,26,27]. Allowed winding temperature is determined by the insulation class, and must be higher than the temperature of the hottest spot on the stator winding within ambient temperature of 40 °C. For the insulation class F, maximum allowed temperature is 155 °C. According to the recommendations of standard [28] for the F insulation class, life expectancy of the insulation is foreseen for 20.000 h. This applies in case of 155 °C maximum allowed temperature and if the insulation preserves its dielectric properties when applying 50% of the insulating breakdown voltage. Standard [28] defines the procedure for determining the thermal endurance graph for the insulation class F used in electrical machines. In paper [29], one of these graphs is shown, obtained in experiment with accelerated insulation thermal aging in laboratory conditions. This experiment has been performed according to the procedure defined in standard [28]. On the curve of thermal endurance graph for insulation class F, three characteristic points are distinguished. The first one refers to a maximal allowed temperature of 155 °C where the life expectancy of insulation class F is 20.000 h. At the second point at the winding temperature of 165 °C, which is an increase of 10 °C to a maximal allowed temperature of 155 °C, the insulation life
Motor M1
Brake
Gear box
Also, based on [23], mean time to failure of the three-phase cage induction motor is:
MTTFINM =
1 1 = λINM λB1 + λW + λB2
(20)
Explanation of symbols in two previous equations: λB1 – failure rate of the bearing no. 1, on the fan side λW – failure rate of the stator winding λB2 – failure rate of the bearing no. 2, on the drive side 4.6. Mechanical transmission system reliability Transport belt is moving over one or more driving drums. The number of drums depends on the transport belt length, terrain configuration and transport capacity [1]. Driving drum is connected to the motor through couplings and gearbox. For the purpose of providing the necessary torque, driving drum is driven with one motor and gearbox on both sides. Mechanical brake is located between motor and gearbox and it is meant for safe stop during the regular stop or in case of emergency stop. Mechanical part within the belt conveyor station of single drive consists of mechanical brake (thruster), mechanical gearbox, coupling and a drive drum as it is shown in Fig. 5 According to literature [30] for one single driving drum within the belt conveyor station which is driven by two motors through two gearboxes, MTTF is 3 years. In previous chapter, through valid sources MTTF is calculated for the squirrel induction motor and its value is 2 years. Because the motor has 2 bearings, while the observed drive mechanical part has two bearings in the gearbox and two more bearings on the drive drum, we have estimated MTTF for mechanical drive part. It is two times less than the value given in literature [30], 3 years/ 2 = 1,5year.
MTTFMeh. = 3year/2 = 1.5year
(21)
5. Calculations In this section, based on the theoretical settings from the previous section and data from the corresponding references, appropriate calculations have been made and presented in a tabular manner. 5.1. Calculation PLC reliability and its communication with FC MTTF data used for PLC components is provided by equipment manufacturer [17] and is shown in Table 1. Mean time to repair MTTR on PLC (cards, voltage supply, battery replacement) can be several hours during one shift which is negligible compared to mean time to failure MTTF. Practically mean time between failure (MTBF) of a
Brake
Driving drum
(19)
λINM = λB1 + λW + λB2
Fig. 5. Driving Pulley Driven on Both Sides, [1].
Couple Motor M2
Gear box
397
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Table 1 MTTF calculation for control system and Profibus communication between PLC and FC - MTTFFC-PLC. Control functional group
PLC
Component
Numb. of compon.
CPU module Base Unit 4 MB Fieldbus - Profibus module Fieldbus -Ethernet module Battery Backup module Power Supply 230 V AC Distributed I/O module Distributed Communication module Profibus module on FC
DP FC
1 2 1 1 1 16 2 1
MTTF
MTTF
λ
h
year
h−1
184,994 292,765 416,713 909,227 395,091 570,780 1,003,020 292,765 λPLC + λ PB – [h−1] MTTFPLC_FC – [h] MTTFPLC_FC – [year]
21.12 33.42 47.57 103.79 45.10 64.16 114.5 33.42
5.41E-06 3.42E-06 2.4E-06 1.1E-06 2.53E-06 1.75E-06 9.96E-07 3.42E-06 5.17E-05 19,339 2.21
Table 2 MTTF calculation for supply transformer, LV switch and FC container external cooling system.
1. 2. 3.
Component
Numb. of compon.
λ FIT
MTTF h
MTTF year
λ h−1
Transformer for frequency converters Low voltage switch External cooling system for FC container
1 1 1
2,400 180 1,000 λLV – [h−1] MTTFLV – [h] MTTF LV – [year]
416,667 5,555,556 1,000,000
47.56 634.20 114.16
2.40E-06 1.80E-07 1.00E-06 3.58E-06 279,330 31.89
Table 3 MTTF calculation of two parallel 12 pulse diode rectifiers.
1.
Component
Number of components
λ FIT
MTTF h
MTTF year
λ h−1
Diode
12
50 Rectifier module
20,000,000
2,283.11 λREC1 – [h−1] MTTFREC1 – [h] MTTF REC1 – [year] MTTFRECΣ – [h] MTTF RECΣ – [year]
5.00E-08 6.00E-07 1,666,667 190.26 833,333 95.12
Two parallel rectifier modules
Table 4 MTTF calculation – three IGBT inverter modules in parallel.
1. 2. 3.
Component
Number of components
λ FIT
MTTF h
MTTF year
λ h−1
IGBT Diode DC capacitor
7** 6 1
100* 50 20 Inverter module
10,000,000 20,000,000 20,000,000
1,141.55 2,283.11 5,707.07 λINV – [h−1] MTTFinv1 – [h] MTTF inv1 [year] MTTFinvΣ [h] MTTF invΣ [year]
1.00E-07 5.00E-08 2.00E-08 1.02E-6 980.392 111,92 326.797 37.30
Three parallel inverter modules
* – FIT- failure rate for IGBT of medium power from the 2000. year, [20]. ** – The seventh IGBT is for braking chopper unit.
system is expressed as MTBF = MTTF + MTTR ≈ MTTF, [22].
Table 5 MTTF calculation – frequency convertor with two rectifiers in parallel and three inverter units in parallel. Frequency converter
1. 2.
Rectifier part Inverter part
Number of components
1 1 MTTFFC1 [h] MTTF FC1 [year]
MTTF h
MTTF year
833.333 326.797
95,12 37,30 234.742 26,80
5.2. Calculation reliability of elements for the electrical equipment According to [13,31] data about MTTF for transformer which supplies frequency converters, LV switch and container external cooling system is shown in Table 2.Table 3. 5.3. Reliability of frequency converter at rated operational conditions According to [32], the experimental reliability assessment (ongoing 398
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Table 6 MTTF calculation – one out of two parallel connected 12 pulse rectifier modules.
1.
Component
Number of components
λ FIT
MTTF h
MTTF year
λ h−1
Diode
12
50 Rectifier module
20,000,000
2,283.11 λREC1 – [h−1] MTTFREC1 – [h] MTTF REC1 – [year] MTTFRECΣ – [h] MTTF RECΣ – [year] MTTF RECΣ 1/2 [h] MTTF RECΣ 1/2 [year]
5.00E-08 6.00E-07 1,666,667 190.26 833,333 95.12 2.500.000 285,30
Two parallel rectifier modules One out of two rectifier modules 1/2
Table 7 MTTF calculation – two out of three IGBT inverter units connected in parallel. Component
1. 2. 3.
Number of components
7** 6 1
IGBT Diode DC capacitor
λ
MTTF
MTTF
λ
FIT
h
year
h−1
100* 50 20 Inverter module
10,000,000 20,000,000 20,000,000
1,141.55 2,283.11 5,707.07 λINV – [h−1] MTTFinv1 – [h] MTTF inv1 [year] MTTFinvΣ [h] MTTF invΣ [year] MTTFinv 2/3 [h] MTTF inv 2/3 [year]
1.00E-07 5.00E-08 2.00E-08 1.02E-6 980.392 111,92 326.797 37.30 816,993 93
Three parallel rectifier modules Two out of three inverter modules
1,00
Table 8 MTTF calculation – frequency converter with reduced number of modules.
1. 2.
Rectifier section 1/2 Inverter section 2/3
Number of components
1 1 MTTFFC1_1/2+2/3 [h] MTTF FC1_1/2+2/3 [year]
0,99
MTTF
MTTF
0,98
h
year
0,97
2,500,000 816,993
285.30 93 615,764 70.29
0,96
R(t)
Frequency converter
0,95 0,94 0,93
Table 9 MTTF calculation – squirrel cage induction motor, insulation class F, energy efficiency class IE2. Squirrel cage induction motor
Number of components
MTTF
MTTF
h
year
43,800 87,600
5 10 17,520 2
RFC2
0,92 RFC1
0,91 0,90 0
1. 2.
Bearings Stator windings
2 1 MTTFINM1 – [h] MTTF INM1 – [year]
2000
4000
6000
8000 10000 12000 14000 16000 18000 20000
t(h) Fig. 6. Reliability function of the frequency converter, RFC1-all modules are working, RFC2 - One rectifier and one inverter module are not operational.
Table 10 Reliability results overview of one single drive within the belt conveyor station in open-pit mine Drmno.
1. 2. 3.1 3.2 4. 5. 6.1 6.2
Functional subgroup name
MTTF h
MTTF year
Control system (PLC) and Profibus communication – MTTFFC-PLC Supply transformer, LV switch and cooling system for the container with frequency converters Frequency converter FC, 2/2_AC/DC modules +3/3 _DC/AC modules Frequency converter FC, 1/2_AC/DC modules +2/3 _DC/AC modules Squirrel induction motor, two bearings +stator winding Mechanical part, gearbox +driving drum One regulated drive on belt conveyor station (1., 2., 3.1, 4. i 5.) One regulated drive on belt conveyor station (1., 2., 3.2, 4. i 5.)
19,339 279.330 234.742 615.764 17.520 13.140 5.189 5.261
2.18 31,89 26,80 70,29 2,00 1,5 0,592 0,601
399
Electrical Power and Energy Systems 113 (2019) 393–402
1,00
1,00
0,90
0,90
0,80
0,80
0,70
0,70
0,60
0,60
R(t)
R(t)
S. Štatkić, et al.
0,50
0,50
0,40
0,40
0,30
0,30
0,20
RINM
0,20
RSD2
0,10
RMEH
0,10
RSMEH
RSD1 0,00
0,00 0
2000
4000
6000
8000 10000 12000 14000 16000 18000 20000
0
2000
4000
6000
t(h)
8000 10000 12000 14000 16000 18000 20000
t(h)
Fig. 7. Reliability function of the complete electric motor drive, taking into account the reliability of mechanical components, RSD1-all modules are working, RSD2 - One rectifier and one inverter module are not operational.
Fig. 10. Reliability of induction motor (RINM), mechanical transmission (RMEH) and overall reliability of all mechanical components (RSMEH). 1,00
1,00
0,90
0,99
0,80
0,98
0,70
0,97
0,60
R(t)
R(t)
0,96 0,95
0,40
0,94
0,30
0,93
0,00
RREC11 0
2000
4000
6000
0
8000 10000 12000 14000 16000 18000 20000
0,99 0,98 0,97
R(t)
0,96 0,95 0,94 0,93 RIGBT23
0,91 RIGBT11 4000
6000
6000
8000 10000 12000 14000 16000 18000 20000
module in the analyzed case of the belt conveyor drive. The experimental result was 93 years. In this paper for the same module, using reliability assessment calculations result was 111.9 years, as shown in table 4. Higher value was obtained because the cooling process of the complete module in the housing was not taken into consideration. In the frequency convertor catalog [14], which is analyzed in this case, there are no any data concerning MTTF and operation reliability. Mean time to failure (MTTF) calculation of frequency convertors depends on several key factors, but the most important are: temperature of IGBT modules, DC rectifier voltage, elevation, ambient temperature, internal cooling system probity, external cooling system probity, frequency convertor configuration, power electronics components reliability etc (see Table 5). Based on data provided by other frequency converter manufacturer [33], mean time to failure (MTTF) of 12 pulse high power frequency converters (up to 1200 kW), supply voltage 690 V, where our considered frequency converter belongs as well, is around 200.000 h. Calculations of mean time to failure (MTTF) are done according to certain presumptions and collected information from the end users. Result obtained by calculations which are derived from the topology and specific modular configuration of an analyzed frequency converter [14] is MTTF = 234.742 h. This value is in accordance with declared MTTF value (200.000 h) for frequency converters with approximately similar nominal data of other manufacturer [33] (see. Table 6Table 7Table 8Table 9.).
1,00
2000
4000
Fig. 11. Reliability of PLC controller and frequency converter communication (RPLC). Reliability of transformer, LV switch and cooling system (RLV).
Fig. 8. Reliability function of the frequency converter rectifier unit, RREC11 modules in operation, RREC12 - One of the two (1/2) rectifier modules is out of function.
0
2000
t(h)
t(h)
0,92
RTLV
0,10
RREC12
0,91
0,90
RPLC
0,20
0,92
0,90
0,50
8000 10000 12000 14000 16000 18000 20000
t(h) Fig. 9. Reliability of Inverter Units, RIGBT11 IGBT Modules in operation, RIGBT23 - Two out of three (2/3) invertor IGBT modules are operational.
reliability test) was done in laboratory conditions for the IGBT inverter module, which has a size R8i and belongs to the ACS800 frequency converter generation. This inverter module corresponds to the inverter 400
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taking into account all mechanical drive components, it ends up that we get negligible reliability increase of an entire drive, regardless the reduced operation. For the full operation, MTTF is increased for 5,189 h to 5,261 h. Resulted difference of 72 h represents increase of 1,38%. Comparing the time diagrams of one single drive reliability on belt conveyor station, this difference is hardly noticeable in Fig. 7. Rectifier modules reliability in Fig. 8 is greater than the reliability of inverter modules in Fig. 9. This difference could be explained as a consequence of switching operation of IGBT inverter modules and greater dissipation, while diode rectifier bridge is basic semiconductor component where switching operation depends on supply network frequency, while the switching power losses are negligible. In Fig. 10 the reliability curve is shown for all mechanical components in one single drive on belt conveyor station. Mean time to failure has the lowest value in the mechanical part of the drive compared to the control system and LV voltage supply system (Fig. 11), as well as compared to the frequency converter (Fig. 6). Hereupon, the reliability curve of mechanical components is declining in time most rapidly and has the dominant influence on the reliability curve of entire drive.
5.4. Calculation reliability of frequency converter with reduced run capability In case of operation with one out of two rectifier modules and two out of three inverter units and under condition that load allows this kind of operation regime, calculated mean time to failure (70,29 years) is 2,62 times greater than mean time to failure of frequency converter (26,80 years) with all modules in function. 5.5. Calculation of squirrel cage induction motor reliability According to adopted data for MTTF for the bearings and stator windings, using the previous equation (19), the MTTF value for one squirrel cage induction motor with insulation class F, energy efficiency class IE2 is 17,250 h or 2 years. 6. Results of reliability assessment Overview of obtained reliability results for the certain components which are part of one single drive within the belt conveyor station located in open-pit mine Drmno, is shown in the following table. Mean time to failure, of one frequency converter when all modules (2/2_AC/DC+3/3_DC/AC) have to be in function, is given in expression (12). Mean time to failure of one frequency converter when it is possible to perform reduced operation with reduced number of modules (1/2_AC/DC+2/3_DC/AC), is determined with expression (17). Mean time to failure (MTTF) of one single drive within the belt conveyor station when all modules (2/2_AC/DC + 3/3_DC/AC) have to be in function, by applying conditions from Eqs. (4), (5), (8), (12), (13), (20) and (21), as well as the results from Table 10, can be defined with equation (22).
MTTFSD1 =
8. Conclusion In this paper, the assessment of the reliability has been made for one low voltage frequency converter with the high rated nominal power, which is used to drive rubber transport belt within the belt conveyor station in open-pit mine. Assessment has been made through calculation of mean time to failure (MTTF) in two cases. The first one, frequency converter can be in operation only when all of the rectifier and inverter modules connected in parallel are in function. In this case calculated mean time to failure is MTTF = 234,742 h. The second one, when the frequency converter can operate with reduced load without one rectifier module and one inverter module. In this case calculated mean time to failure is MTTF = 615.764 h. Redundant operation of the modules in frequency converter (RECT 1/2 + INV 2/3) significantly increases reliability of frequency converter itself, respectively it is 2,62 times greater than in the case when all modules have to be operational. This result of a reliability assessment of single-motor drive within the belt conveyor station can have a positive influence on increasing the operation time efficiency of the frequency converters, on improvement maintenance conditions and on reduction the cost for the spare parts during exploitation.
1 1 = λ SD1 λPLC_FC + λLV + 2λREC1 + 3λINV1 + λINM + λMEH (22)
= 5189h
Mean time to failure of one single drive within the belt conveyor station when the frequency converter can operate in reduced mode (1/ 2_AC/DC + 2/3_DC/AC), by applying conditions from Eqs. (4), (5), (8), (10), (15), (17), (18), (20) and (21), as well as the results from Table 10, can be defined with equation (23).
MTTFSD2 =
1 1 = 2 6 λ SD2 λ PLCFC + λLV + 3 ∙λREC1 + 5 ∙λINV1 + λINM + λMEH
Acknowledgement
(23)
= 5261h
Paper is a part of research within the project no. TR 33016 Research, development and implementation of programs and procedures Energy efficiency of electric drives, 2011-2019, financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia. Project period: 2011-2019.
Expression (24) represents time change of frequency converter reliability on the belt conveyor station. Thereby, mean time to failure (MTTF) values which are determined by expressions (13) and (18) represent time constants on these diagrams.
RFC1 (t ) = e
−
1 t MTTFFC1
1
= e− 234.742 t , RFC 2 (t ) = e
−
1 t MTTFFC 2
1
= e− 615.764 t ,
References
(24)
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7. Discussions Diagrams in Fig. 6 are obtained using relations (23) representing time change of frequency converter reliability on the belt conveyor station. Obtained values in these calculations show that in case of reduced operation (reduced number of rectifier and inverter modules) reliability of frequency converter is significantly increased, because the mean time to failure (MTTF = 615.764 h) is 2,62 times greater than in the case when all modules have to be in function (MTTF = 234,742 h). In Fig. 6, exponential reliability curve of the frequency converter with greater MTTF value is declining more slowly than the curve of the lower MTTF value. However, when the reliability of the entire drive is determined 401
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[7] [8]
[9]
[10]
[11]
[12] [13]
[14]
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