- Email: [email protected]
Heating systems with PLC and frequency control
Energy Conversion and Management 49 (2008) 3356–3361
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Heating systems with PLC and frequency control Salah Abdallah, Riyad Abu-Mallouh * Department of Mechanical and Industrial Engineering, Applied Science University, Amman 11931, Jordan
a r t i c l e
i n f o
Article history: Available online 18 July 2008 Keywords: Heat treatment PLC control Frequency control Tempering
a b s t r a c t In this work, medium capacity controlled heating system is designed and constructed. The programming method of control of heating process is achieved by means of integrated programmable logic controller (PLC) and frequency inverter (FI). The PLC main function is to determine the required temperatures levels and the related time intervals of the heating hold time in the furnace. FI is used to control the dynamic change of temperature between various operating points. The designed system shows the capability for full control of temperature from zero to maximum for any required range of time in case of increasing or decreasing the temperature. All variables of the system will be changed gradually until reaching their needed working points. An experimental study was performed to investigate the effect of tempering temperature and tempering time on hardness and fatigue resistance of 0.4% carbon steel. It was found that increasing tempering temperature above 550 °C or tempering time decreases the hardness of the material. It was also found that there is a maximum number of cycles to which the specimen can survive what ever the applied load was. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Heat treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, and restore ductility after a cold working operation. Thus it is a very enabling process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics [1,2]. A new single-switch parallel resonant converter for induction heating was introduced in [3]. The circuit consists of an input LCfilter, a bridge rectifier and only one controlled power switch. The switch operates in a soft communication mode and serves as a high frequency generator. A voltage-fed resonant LCL inverter with phase shift control was presented in [4]. It was seen that the control strategy offered advantages in the megahertz operating region, where a constant switching frequency is required. The inverter steady state operation is analyzed using fundamental frequency analyses. A cost-effective high efficiency inverter with phase-shifted pulse modulation scheme was proposed for medium power (5–
* Corresponding author. Tel.: +962 5 3740026. E-mail addresses: [email protected], Salahabdalah_1964@hotmail. com (S. Abdallah), [email protected] (R. Abu-Mallouh). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.11.017
30) kW induction heating applications is discussed in [5]. The proposed inverter accomplishes soft switching operation over a wide power regulation range. The actual power conversion efficiency reached was 96.7%. A control method of reducing the size of the dc-link capacitors of a converter–inverter system was presented in [6]. The main idea is to utilize the inverter operation status in the current control of the converter. This control strategy is effective in regulating the dc-voltage level. Even the dc-link capacitor is arbitrarily small and the load varies abruptly. In [7] a method was proposed to accurately predict the minimum required temperature recovery considering repeatability and accuracy of the leak detector by investigating the relation between temperature recovery time and theoretical thermal time constant for various test volumes and applied pressures using PLC system. In [8] a methodology was demonstrated to design a PLC program that organized the relation between the physical inputs and outputs of the pumping tools in manufacturing systems. In [9] an experimental study was performed to investigate the effect of using two axes tracking with PLC control on the solar energy collected. The two axes tracking surface showed better performance with an increase in the collected energy up to 41% compared to the fixed surface. The PLC main function is to control the required temperature levels and the related time intervals of the heating hold time in the furnace [10]. Frequency inverter is used to control the dynamic change of temperature between various operating points [11,12].
S. Abdallah, R. Abu-Mallouh / Energy Conversion and Management 49 (2008) 3356–3361
This integration of the PLC and frequency control shows the capability for full control of temperature from zero to maximum in dynamic and static conditions, in case of increasing or decreasing the temperature. The properties and microstructure as a function of tempering time at intercritical temperatures in HY-80 steel castings were evaluated by [13]. They varied the time for which the steel was held in the intercritical temperature range. An important finding of this study is that, contrary to normal behavior during tempering HY-80 steel tempered in the intercritical range demonstrates a severe loss of toughness; which can be exaggerated for longer hold times and higher temperatures. A fractography survey on high cycle fatigue failure in Fe–C–Cr– Mo–X alloys was made by [14]. They found that various parameters are likely to influence high cycle fatigue failures, the most significant one dealing with the nature and location of embedded precipitates and the forging reduction ratio. The ageing effect on cyclic plasticity of a tempered martensitic steel was studied by [15]. They carried out specific isothermal cyclic deformation tests on a tempered martensitic steel 55NiCrMoV7 at four hardness levels in the temperature range 20–600 °C. They found that the cyclic stress response generally shows an initial exponential softening for the first few cycles, followed by a gradual softening without saturation, hardness dramatically decreases when the specimen is simultaneously subjected to ageing and fatigue at elevated temperature, cyclic softening intensity increases with testing temperature from 300 to 600 °C, but the maximal softening intensity occurs at room temperature. Back-propagation neural networks were used by [16] to optimize the heat treatment technique of high-vanadium high-speed steel including predictions of retained austenite content, hardness and wear resistance according to quenching and tempering temperatures. A novel concept for the heat treatment of martensite, different to customary quenching and tempering was proposed by [17]. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Reversible martensitic transformation, ageing and low-temperature tempering of iron–carbon martensite were studied by [18]. In this study, reversible martensitic transformation was observed in Fe-based high-carbon alloys at temperatures when none lattice defects (including divacancies) could diffuse. Formation and behavior of carbon-vacancy clusters was studied and discussed. Chemical composition of hexagonal e-carbide was determined as Fe3C. The effect of particle size and e-carbide/martensite orientation relationship on the e-carbide ? cementite transformation was discussed.
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state the required percentage output and the related hold time intervals. The digital output of PLC is ranging from zero to 32,700 quantization levels. The analog unit function is to transfer the digital output value at the output of PLC into analog value, which ranging from zero to 10 VDC at the output of analog unit. In the program a different percentages of output voltage is supplied to the furnace by the frequency inverter which is originally stated by the analog unit output where 0 DVC equals 0% at the output of the frequency inverter and 10 VDC equals 100% at the output of frequency inverter. Frequency inverter is a single phase input, three phase output, 220 VAC rated output voltage, 50 Hz rated output frequency and 3 kW power. In this work a frequency inverter of type SINAMICS G110 is used [11]. Frequency inverter according to the different incoming instructions of PLC through analog unit operates the three phase heater with the required percentage of voltage and frequency. Parameter unit is a type of programmers used to program the ramp up and ramp down time between each two controlled levels. So, frequency inverter has two types of commands: (1) Type of commands which is supplied by the PLC to the analog unit then to the frequency inverter to state the hold time intervals. (2) Type of commands which is supplied by parameter unit td control the ramp up and ramp down time to make a soft transition conditions between various operating levels [20]. The furnace consists of a three phase heater in a room which is 3 kW rated power, with 220 VAC rated input voltage, 50 Hz rated frequency and star connection.
3. Mathematical description of frequency controlled heating system The main important parts of the PLC and frequency controlled three phase heating system are the frequency inverter and the three phase furnace. 3.1. Modeling of the frequency inverter From all types of frequency inverters for regulation of AC motors, the more important is thyristor frequency inverter with clear dc part as shown in Fig. 2. This type of frequency inverter has high technical and economical specifications. The equation of the frequency control channel can be written as
F ¼ kF uF
ð1Þ
where kF is the inverter amplification coefficient by frequency control channel and uF the input voltage of frequency generator. The equation of the voltage control channel will be
u ¼ kv uu
ð2Þ
2. The heating system design and control In this work, the design of PLC and frequency controlled heating system were performed using an open loop and programming method of control in which stored instructions in memory of PLC was used to control the actuation of heating process. The block diagram of the hardware components of the automatically controlled heating system is shown in Fig. 1. Personal computer is used to write the control program then download it to the PLC [19,10] through communication cable. The PLC is S7-200 type, which has 12 inputs, 8 outputs and 220 VAC supply voltage.PLC S7-200 uses ladder logic diagram programming language describes in Refs. [19,10]. The PLC main function is to instruct the analog unit to go on or off and to
where uu pand ffiffiffi u is the output and input voltages of the inverter. kv ¼ 2=ðp 3Þ. The dc circuit of thyristor frequency inverter contains LdC-filter, where Ld is the inductance of the filter and C is the capacitance of the filter condenser. The equation of the dc circuit will be
uu ¼ Ed Rd id Ld
did dt
ð3Þ
where
id ¼ iu ic ic ¼ Cðduu =dtÞ
ð4Þ
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S. Abdallah, R. Abu-Mallouh / Energy Conversion and Management 49 (2008) 3356–3361
Program to control the slope of temperature
Program to cntrol the temperature level
Personal Computer
Parameter Unit
Analog Unit
PLC
Frequency Inverter
Three Phase Heater
T
Fig. 1. Block diagram of PLC and frequency controlled heating system.
Ld id
iu
+
Ciruit 1
Ciruit 2
ic
To V1
V2
C
V3
V7
V8
T
V9
u q1
A B C
V4
V5
V6
V10
V11
q2
V12
iu Three phase heater
Furnace
-
program
Controller Fig. 2. Principle circuit of frequency controlled three phase heater.
where Ed is the rectified voltage at the output of converter, Rd is the active resistance of coil and id is the coil current, iu is the input current of the inverter and ic the current of the condenser.
where Ro is the resistivity of the walls. Substituting for q2 gives
3.2. Modeling of the furnace
Hence
The right hand side of Fig. 2 illustrates a furnace consisting of a three phase heater in a room. The three phase heater emits heat at a rate of q1 and the room loses heat at a rate of q2.
Ro co
q1 ¼ 3kh iu
ð5Þ
where Kh is the heater coefficient. Assuming that the air in the room is at a uniform temperature T and that there is no heat storage in the walls of the room, we can derive an equation describing the time rate of change in room temperature.
q1 q2 ¼ co
dT dt
ð6Þ
where Co is the thermal capacity of the air in the room. If the temperature inside the room is T and that outside the room is To, then
q2 ¼
T To Ro
ð7Þ
q1
T To dT ¼ co dt Ro
dT þ T ¼ Ro q 1 þ T o dt
ð8Þ
4. Experimentation and results 4.1. Testing the control system performance In order to test the ability of the designed control system to perform the desired temperature control properly, successive heat treatment experiments were performed and their results are described as follows. Fig. 3 shows the input frequency vs. time and Fig. 4 shows the temperature vs. time for the two specimens 1 and 2 where this process represent a heat-treatment process carried out with ramping function only (i.e. the furnace and frequency inverter only are used), the holding time in this case was counted off from the moment the furnace is actuated (form room temperature). That is why the graph starts at 20 °C and then temperature rise up using the ramp up function in the frequency inverter on a range of time for
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S. Abdallah, R. Abu-Mallouh / Energy Conversion and Management 49 (2008) 3356–3361 450 400 350
Temperature (°C)
1 h ramp up, then temperature reaches 400 °C and maintained for 2 h. Then a ramp down time for 1 h is carried out. Fig. 5 shows the input frequency vs. time and Fig. 6 shows the temperature vs. time for the second two specimens 3 and 4 where this process represent a controlled heat-treatment process (i.e. the furnace and frequency inverter integrated with the PLC and an analog unit are used), and as referred before the holding time in this case was counted off from the moment the furnace is actuated (form room temperature). That is why the graph starts at 20 °C and then temperature rise up using the ramp up function and in steps sequence depending on the PLC program, the first ramp up process takes approx. 2 h then temperature decrease to 200 °C in half hour and ramp ups again in steps for 1 h to reaches 400 °C again and finally decreases to room temperature in a process that takes 0.5 h.
300 250 200 150 100 50 0 0
1
2
3
4
5
Time (hrs) Fig. 4. Temperature vs. time for specimens 1 and 2.
4.2. Heat treatment experiments
60 50
Frequency (Hz)
50 40
50
37.5
30 25
25 20 12.5
10 0
0
0 0
0
2
1
3
0 0 4
5
Time (hrs) Fig. 5. Input frequency vs. time for specimens 3 and 4.
Temperater (ºC)
Sixty specimens were machined according to the standard dimensions suitable for the fatigue testing machine. These dimensions are shown in Fig. 7. These specimens were made of 0.4%C steel. Four of these specimens were left without any further heat treatment. The rest were heat treated. The heat treatment process of typical steel involves heating the object (austenizing) and then causing a quick and sharp drop in its temperature (quenching). Together, these two processes produce an extremely hard microstructure in medium-carbon or high-carbon steels, which can then be ‘‘tempered” to prevent the material from shattering. As mentioned above, the first stage of the heat treatment is water quenching. This was done by heating the specimens to 860 °C for a period of 1 h. That results in transforming the microstructure to austenite (face centered cubic). This heating process was followed by water quenching the specimens to room temperature. This means that the microstructure was transformed to martensite (body centered tetragonal). These quenched specimens were dried and kept in plastic bags to prevent corrosion of the specimens. The second heat treatment stage was tempering. Chemically, the process of tempering is a transformation from metastable martensite to ferrite and cementite. This change is accomplished by annealing at a temperature below the austenizing temperature, but high enough that nucleation of cementite particles can occur. The formation of cementite draws carbon from the surrounding alloy, allowing it to transform to ferrite. Cooling the object ends the annealing process, stopping cementite formation by slowing down the diffusion of carbon. In this paper, the effect of tempering temperature and tempering time on hardness and fatigue resistance of 0.4% carbon steel was studied. Specimens were divided into four categories accord-
450 400 350 300 250 200 150 100 50 0
400 300
400
300
200
300
200
200
100 0 0
0 1
2
3
4
5
Time (hrs) Fig. 6. Temperature vs. time for specimens 3 and 4.
Frequency (Hz)
60 50 40 Fig. 7. Fatigue test specimen.
30 20 10 0 0
1
2
3
4
Time (hrs) Fig. 3. Input frequency vs. time for specimens 1 and 2.
5
ing to tempering temperature (400, 500, 550 and 600 °C), and to three subcategories according to tempering time (1, 2 and 3 h). The hardness of the specimens was then tested using standard brinell testing machine and the results are recorded and plotted in Fig. 8. The specimens were then tested using standard fatigue testing machine and the results are reported and plotted in Figs. 9–11.
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5. Discussion In this paper, the effect of tempering temperature and tempering time on hardness and fatigue resistance of 0.4% carbon steel was studied. Fig. 8, shows that, for temperatures less than 550 °C,
116 TIME=1hr TIME=2hrs TIME=3hrs
114
BHN (MPa)
112
110
the effect of tempering temperature on hardness of the tested specimens is small. But it becomes significant for temperatures greater than 550 °C (4%–8% decrease in hardness). Accordingly, for tempering to reduce the hardness effectively, the tempering temperature should be greater than 550 °C. For tempering temperatures greater than 550 °C, we note that hardness decreases linearly with increasing temperature. In regard to the tempering time, this figure shows that increasing the tempering time decreases the resulting hardness (4%–8% decrease in hardness). For now, if we have a look at fatigue test results, we can see that the number of cycles to failure increases with decreasing the applied cyclic load. We can also note that the number of cycles to which the specimen can survive have a maximum value over which the specimen will fail whatever the applied load was. This can be clearly seen from the sudden sharp increase of the slope of the curve as number of cycles increases.
108
Time=3hrs
1.6
106
102 400
450
500
550
600
Temperature (ºC)
Log (F)
104
Fig. 8. Hardness vs. tempering temperature.
1.5
Temp=400
1.4
Temp=500 Temp=550
1.3
Temp=600 1.2 1.1
1 0.9
Time=1hr
0.8
1.6
1
1.5
3
4
5
6
7
Log (N)
Temp=500
1.4
Fig. 11. Fatigue life curve for a 3 h tempering time.
Temp=550
1.3
Temp=600
1.2
6.5
1.1
6
1
5.5
Log (N)
Log (F)
2
Temp=400
0.9 0.8 1
2
3
4
6
5
F=8N 3hr 2hr 1hr
5 4.5
7 4
Log (N)
3.5 400
Fig. 9. Fatigue life curve for a 1 h tempering time.
450
500
550
600
Temp Time=2hrs
Fig. 12. Log(N) as a function of temperature for a force of 8N.
1.6 Temp=400
1.5
Temp=500
Temp=400 1.6
Temp=550
1.3
1.5
Temp=600
3hr
1.4
1.2
Log (F)
Log (F)
1.4
1.1 1
2hr
1.3
1hr
1.2 1.1 1
0.9
0.9
0.8
0.8 1
2
3
4
5
Log (N) Fig. 10. Fatigue life curve for a 2 h tempering time.
6
7
1
2
3
4
5
6
Log (N) Fig. 13. Fatigue life curve for a 400 °C tempering temperature.
7
S. Abdallah, R. Abu-Mallouh / Energy Conversion and Management 49 (2008) 3356–3361
Log (F)
6. Conclusion
Temp=500
1.6 1.5
3hr
1.4
2hr
1.3
1hr
1.2 1.1 1 0.9 0.8 1
2
3
4
5
6
7
Log (N) Fig. 14. Fatigue life curve for a 500 °C tempering temperature.
Temp=550
Log (F)
1.6 1.5
3hr
1.4
2hr
1.3
1hr
1.1 1 0.9 0.8 2
3
4
5
7
6
Log (N) Fig. 15. Fatigue life curve for a 550 °C tempering temperature.
Temp=600
Log (F)
1.6 1.5
3hr
1.4
2hr
1.3
1hr
1.2 1.1 1 0.9 0.8 1
2
3
4
5
6
In this work, medium capacity controlled heating system is designed and constructed using PLC and frequency control. The designed system shows the capability for full control of temperature from zero to maximum and from maximum to zero for any required range of time. As results for the experimentation of the controlled heating system it was found for significant decrease in the hardness of 0.4% carbon steel to be achieved, tempering temperature should be more than 550 °C. Increasing the reheat time decreases the hardness of 0.4% carbon steel. There is a maximum number of cycles to which the specimen can survive whatever the applied load was. This maximum number of cycles increases with increasing tempering temperature. Increasing the tempering time increases fatigue life of 0.4% carbon steel. References
1.2
1
3361
7
Log (N) Fig. 16. Fatigue life curve for a 600 °C tempering temperature.
It can also be seen in the figure that this maximum possible life of the specimen increases with increasing temperature. To demonstrate this behavior clearly, a plot of Log(N) as a function of temperature is shown in Fig. 12 for a force of 8N. (5.5 103 cycles for T = 400 °C to 1 106 cycles for T = 600 °C). To investigate the effect of the tempering time on the fatigue behavior of 0.4% carbon steel, the same results shown in Figs. 9– 11 is redrawn in Figs. 13–16 shown. These figures show that for 8N load and T = 500 °C, the number of cycles is 66,958 for 3 h tempering period and 38,974 for 1 h tempering period. That means that the number of cycles increased by 72% due to increasing tempering time.
[1] Rajan TV, Sharma CP. Heat treatment principle and techniques. New Delhi: Prentice Hall of India; 1992. [2] Arafat A, Abu Hamid E, Abdel Fattah AK. Royal Scientific Society, introduction of heat treatment technology in designing and manufacturing of spare parts of local industries in Jordan. Higher Council Sci Technol Amman – Jordan, 1999. [3] Shenkman A, Axelrod B, Berkovich Y. Improved modification of the singleswitch AC–AC converter for induction heating applications. IEE Proc – Electr Power Appl 2004;151(1):1–4. [4] Mollov SV, Theodoridis M, Forsyth AJ. High frequency voltage-fed inverter with phase-shift control for induction heating. IEEE 2003:12–8. [5] Kifune H, Hatanaka Y, Nakaoka M. Cost effective phase shifted pulse modulation soft switching high frequency inverter for induction heating applications. IEE Proc – Electr Power Appl 2004;151(1):19–25. [6] Jung J, Lim S, Nam K. A feedback linearizing control scheme for a PWM converter–inverter having a very small DC-link capacitor. IEEE Trans Ind Appl 1999;35(5). [7] Harus LG, Cai M, Kawahima K, Kagawa T. Determination of temperature recovery time in differential-pressure-based air leak detector. Measur Sci Technol 2006;17:411–8. [8] Abdallah S, Al-Dahwi A. A method to design an automated pump plants with PLC control. Issued in the publications of the international engineering conference ‘‘Mutah 2004”, Jordan, in April 2004. [9] Abdalah S, Nijmeh S. Two axis sun tracking system with PLC control. Issued Energy Conversion Manag 2004;45:1931–9. [10] Siemens Sinamics G110 operating instruction manual. User documentation issued 04/02. [11] Siemens PLC S7-200 operating instruction manual. Issued 06/02. [12] Bolton W. Mechatronics electronic control systems in mechanical engineering. Addison Wesley Longman Limited; 1999. [13] Stephen D Funni, Michelle G Koul, Angela L Moran. Evaluation of prosperities and microstructure as a function of tempering time at intercritical temperatures in HY-80 ‘‘steel” castings. Mechanical Engineering Department, US Naval Academy; 2007. [14] Tchoufang J, Beckers J. Fractography survey on high cycle fatigue failure: crack origin characterization and correlations between mechanical tests and microstructure in Fe-C-Cr-Mo–X alloys. Universite’ de Lie’ge; September 2006. [15] Zhang Z, Delagnes D, Bernhart G. Ageing effect on cyclic plasticity of a tempered martensitic steel. China: Dalian Maritime University; 2006. [16] Xu L, Xing J, Wei S, Zhang Y. Ruilong. Optimization of heat treatment technique of high-vanadium high-speed steel based on back-propagation neural networks. Henan University of Science and Tech; 2005. [17] Edmonds D, He K, Rizzo F, Cooman B, Matlock D, Speer J. Quenching and partitioning martensite – a novel steel heat treatment. University of Leeds; 2006. [18] Kaputkin D. Reversible martensitic transformation ageing and lowtemperature tempering of iron-carbon martensite. Technology University; 2006. [19] Mandado E, Macros J, Perze S. Programmable logic devices and logic controllers. Prentice-Hall; 1996. [20] Abdallah S. The regulation of starting and breaking dynamic characteristics in water pumping systems with open-loop control. In: Proceedings of the 8th Cairo University conference on mechanical design and production. Cairo, Egypt; January 2004. p. 4–6.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Heating systems with PLC and frequency control Salah Abdallah, Riyad Abu-Mallouh * Department of Mechanical and Industrial Engineering, Applied Science University, Amman 11931, Jordan
a r t i c l e
i n f o
Article history: Available online 18 July 2008 Keywords: Heat treatment PLC control Frequency control Tempering
a b s t r a c t In this work, medium capacity controlled heating system is designed and constructed. The programming method of control of heating process is achieved by means of integrated programmable logic controller (PLC) and frequency inverter (FI). The PLC main function is to determine the required temperatures levels and the related time intervals of the heating hold time in the furnace. FI is used to control the dynamic change of temperature between various operating points. The designed system shows the capability for full control of temperature from zero to maximum for any required range of time in case of increasing or decreasing the temperature. All variables of the system will be changed gradually until reaching their needed working points. An experimental study was performed to investigate the effect of tempering temperature and tempering time on hardness and fatigue resistance of 0.4% carbon steel. It was found that increasing tempering temperature above 550 °C or tempering time decreases the hardness of the material. It was also found that there is a maximum number of cycles to which the specimen can survive what ever the applied load was. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Heat treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. Heat treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, and restore ductility after a cold working operation. Thus it is a very enabling process that can not only help other manufacturing process, but can also improve product performance by increasing strength or other desirable characteristics [1,2]. A new single-switch parallel resonant converter for induction heating was introduced in [3]. The circuit consists of an input LCfilter, a bridge rectifier and only one controlled power switch. The switch operates in a soft communication mode and serves as a high frequency generator. A voltage-fed resonant LCL inverter with phase shift control was presented in [4]. It was seen that the control strategy offered advantages in the megahertz operating region, where a constant switching frequency is required. The inverter steady state operation is analyzed using fundamental frequency analyses. A cost-effective high efficiency inverter with phase-shifted pulse modulation scheme was proposed for medium power (5–
* Corresponding author. Tel.: +962 5 3740026. E-mail addresses: [email protected], Salahabdalah_1964@hotmail. com (S. Abdallah), [email protected] (R. Abu-Mallouh). 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2007.11.017
30) kW induction heating applications is discussed in [5]. The proposed inverter accomplishes soft switching operation over a wide power regulation range. The actual power conversion efficiency reached was 96.7%. A control method of reducing the size of the dc-link capacitors of a converter–inverter system was presented in [6]. The main idea is to utilize the inverter operation status in the current control of the converter. This control strategy is effective in regulating the dc-voltage level. Even the dc-link capacitor is arbitrarily small and the load varies abruptly. In [7] a method was proposed to accurately predict the minimum required temperature recovery considering repeatability and accuracy of the leak detector by investigating the relation between temperature recovery time and theoretical thermal time constant for various test volumes and applied pressures using PLC system. In [8] a methodology was demonstrated to design a PLC program that organized the relation between the physical inputs and outputs of the pumping tools in manufacturing systems. In [9] an experimental study was performed to investigate the effect of using two axes tracking with PLC control on the solar energy collected. The two axes tracking surface showed better performance with an increase in the collected energy up to 41% compared to the fixed surface. The PLC main function is to control the required temperature levels and the related time intervals of the heating hold time in the furnace [10]. Frequency inverter is used to control the dynamic change of temperature between various operating points [11,12].
S. Abdallah, R. Abu-Mallouh / Energy Conversion and Management 49 (2008) 3356–3361
This integration of the PLC and frequency control shows the capability for full control of temperature from zero to maximum in dynamic and static conditions, in case of increasing or decreasing the temperature. The properties and microstructure as a function of tempering time at intercritical temperatures in HY-80 steel castings were evaluated by [13]. They varied the time for which the steel was held in the intercritical temperature range. An important finding of this study is that, contrary to normal behavior during tempering HY-80 steel tempered in the intercritical range demonstrates a severe loss of toughness; which can be exaggerated for longer hold times and higher temperatures. A fractography survey on high cycle fatigue failure in Fe–C–Cr– Mo–X alloys was made by [14]. They found that various parameters are likely to influence high cycle fatigue failures, the most significant one dealing with the nature and location of embedded precipitates and the forging reduction ratio. The ageing effect on cyclic plasticity of a tempered martensitic steel was studied by [15]. They carried out specific isothermal cyclic deformation tests on a tempered martensitic steel 55NiCrMoV7 at four hardness levels in the temperature range 20–600 °C. They found that the cyclic stress response generally shows an initial exponential softening for the first few cycles, followed by a gradual softening without saturation, hardness dramatically decreases when the specimen is simultaneously subjected to ageing and fatigue at elevated temperature, cyclic softening intensity increases with testing temperature from 300 to 600 °C, but the maximal softening intensity occurs at room temperature. Back-propagation neural networks were used by [16] to optimize the heat treatment technique of high-vanadium high-speed steel including predictions of retained austenite content, hardness and wear resistance according to quenching and tempering temperatures. A novel concept for the heat treatment of martensite, different to customary quenching and tempering was proposed by [17]. This novel treatment has been termed ‘quenching and partitioning’ (Q&P), to distinguish it from quenching and tempering and can be used to generate microstructures with martensite/austenite combinations giving attractive properties. Reversible martensitic transformation, ageing and low-temperature tempering of iron–carbon martensite were studied by [18]. In this study, reversible martensitic transformation was observed in Fe-based high-carbon alloys at temperatures when none lattice defects (including divacancies) could diffuse. Formation and behavior of carbon-vacancy clusters was studied and discussed. Chemical composition of hexagonal e-carbide was determined as Fe3C. The effect of particle size and e-carbide/martensite orientation relationship on the e-carbide ? cementite transformation was discussed.
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state the required percentage output and the related hold time intervals. The digital output of PLC is ranging from zero to 32,700 quantization levels. The analog unit function is to transfer the digital output value at the output of PLC into analog value, which ranging from zero to 10 VDC at the output of analog unit. In the program a different percentages of output voltage is supplied to the furnace by the frequency inverter which is originally stated by the analog unit output where 0 DVC equals 0% at the output of the frequency inverter and 10 VDC equals 100% at the output of frequency inverter. Frequency inverter is a single phase input, three phase output, 220 VAC rated output voltage, 50 Hz rated output frequency and 3 kW power. In this work a frequency inverter of type SINAMICS G110 is used [11]. Frequency inverter according to the different incoming instructions of PLC through analog unit operates the three phase heater with the required percentage of voltage and frequency. Parameter unit is a type of programmers used to program the ramp up and ramp down time between each two controlled levels. So, frequency inverter has two types of commands: (1) Type of commands which is supplied by the PLC to the analog unit then to the frequency inverter to state the hold time intervals. (2) Type of commands which is supplied by parameter unit td control the ramp up and ramp down time to make a soft transition conditions between various operating levels [20]. The furnace consists of a three phase heater in a room which is 3 kW rated power, with 220 VAC rated input voltage, 50 Hz rated frequency and star connection.
3. Mathematical description of frequency controlled heating system The main important parts of the PLC and frequency controlled three phase heating system are the frequency inverter and the three phase furnace. 3.1. Modeling of the frequency inverter From all types of frequency inverters for regulation of AC motors, the more important is thyristor frequency inverter with clear dc part as shown in Fig. 2. This type of frequency inverter has high technical and economical specifications. The equation of the frequency control channel can be written as
F ¼ kF uF
ð1Þ
where kF is the inverter amplification coefficient by frequency control channel and uF the input voltage of frequency generator. The equation of the voltage control channel will be
u ¼ kv uu
ð2Þ
2. The heating system design and control In this work, the design of PLC and frequency controlled heating system were performed using an open loop and programming method of control in which stored instructions in memory of PLC was used to control the actuation of heating process. The block diagram of the hardware components of the automatically controlled heating system is shown in Fig. 1. Personal computer is used to write the control program then download it to the PLC [19,10] through communication cable. The PLC is S7-200 type, which has 12 inputs, 8 outputs and 220 VAC supply voltage.PLC S7-200 uses ladder logic diagram programming language describes in Refs. [19,10]. The PLC main function is to instruct the analog unit to go on or off and to
where uu pand ffiffiffi u is the output and input voltages of the inverter. kv ¼ 2=ðp 3Þ. The dc circuit of thyristor frequency inverter contains LdC-filter, where Ld is the inductance of the filter and C is the capacitance of the filter condenser. The equation of the dc circuit will be
uu ¼ Ed Rd id Ld
did dt
ð3Þ
where
id ¼ iu ic ic ¼ Cðduu =dtÞ
ð4Þ
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Program to control the slope of temperature
Program to cntrol the temperature level
Personal Computer
Parameter Unit
Analog Unit
PLC
Frequency Inverter
Three Phase Heater
T
Fig. 1. Block diagram of PLC and frequency controlled heating system.
Ld id
iu
+
Ciruit 1
Ciruit 2
ic
To V1
V2
C
V3
V7
V8
T
V9
u q1
A B C
V4
V5
V6
V10
V11
q2
V12
iu Three phase heater
Furnace
-
program
Controller Fig. 2. Principle circuit of frequency controlled three phase heater.
where Ed is the rectified voltage at the output of converter, Rd is the active resistance of coil and id is the coil current, iu is the input current of the inverter and ic the current of the condenser.
where Ro is the resistivity of the walls. Substituting for q2 gives
3.2. Modeling of the furnace
Hence
The right hand side of Fig. 2 illustrates a furnace consisting of a three phase heater in a room. The three phase heater emits heat at a rate of q1 and the room loses heat at a rate of q2.
Ro co
q1 ¼ 3kh iu
ð5Þ
where Kh is the heater coefficient. Assuming that the air in the room is at a uniform temperature T and that there is no heat storage in the walls of the room, we can derive an equation describing the time rate of change in room temperature.
q1 q2 ¼ co
dT dt
ð6Þ
where Co is the thermal capacity of the air in the room. If the temperature inside the room is T and that outside the room is To, then
q2 ¼
T To Ro
ð7Þ
q1
T To dT ¼ co dt Ro
dT þ T ¼ Ro q 1 þ T o dt
ð8Þ
4. Experimentation and results 4.1. Testing the control system performance In order to test the ability of the designed control system to perform the desired temperature control properly, successive heat treatment experiments were performed and their results are described as follows. Fig. 3 shows the input frequency vs. time and Fig. 4 shows the temperature vs. time for the two specimens 1 and 2 where this process represent a heat-treatment process carried out with ramping function only (i.e. the furnace and frequency inverter only are used), the holding time in this case was counted off from the moment the furnace is actuated (form room temperature). That is why the graph starts at 20 °C and then temperature rise up using the ramp up function in the frequency inverter on a range of time for
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Temperature (°C)
1 h ramp up, then temperature reaches 400 °C and maintained for 2 h. Then a ramp down time for 1 h is carried out. Fig. 5 shows the input frequency vs. time and Fig. 6 shows the temperature vs. time for the second two specimens 3 and 4 where this process represent a controlled heat-treatment process (i.e. the furnace and frequency inverter integrated with the PLC and an analog unit are used), and as referred before the holding time in this case was counted off from the moment the furnace is actuated (form room temperature). That is why the graph starts at 20 °C and then temperature rise up using the ramp up function and in steps sequence depending on the PLC program, the first ramp up process takes approx. 2 h then temperature decrease to 200 °C in half hour and ramp ups again in steps for 1 h to reaches 400 °C again and finally decreases to room temperature in a process that takes 0.5 h.
300 250 200 150 100 50 0 0
1
2
3
4
5
Time (hrs) Fig. 4. Temperature vs. time for specimens 1 and 2.
4.2. Heat treatment experiments
60 50
Frequency (Hz)
50 40
50
37.5
30 25
25 20 12.5
10 0
0
0 0
0
2
1
3
0 0 4
5
Time (hrs) Fig. 5. Input frequency vs. time for specimens 3 and 4.
Temperater (ºC)
Sixty specimens were machined according to the standard dimensions suitable for the fatigue testing machine. These dimensions are shown in Fig. 7. These specimens were made of 0.4%C steel. Four of these specimens were left without any further heat treatment. The rest were heat treated. The heat treatment process of typical steel involves heating the object (austenizing) and then causing a quick and sharp drop in its temperature (quenching). Together, these two processes produce an extremely hard microstructure in medium-carbon or high-carbon steels, which can then be ‘‘tempered” to prevent the material from shattering. As mentioned above, the first stage of the heat treatment is water quenching. This was done by heating the specimens to 860 °C for a period of 1 h. That results in transforming the microstructure to austenite (face centered cubic). This heating process was followed by water quenching the specimens to room temperature. This means that the microstructure was transformed to martensite (body centered tetragonal). These quenched specimens were dried and kept in plastic bags to prevent corrosion of the specimens. The second heat treatment stage was tempering. Chemically, the process of tempering is a transformation from metastable martensite to ferrite and cementite. This change is accomplished by annealing at a temperature below the austenizing temperature, but high enough that nucleation of cementite particles can occur. The formation of cementite draws carbon from the surrounding alloy, allowing it to transform to ferrite. Cooling the object ends the annealing process, stopping cementite formation by slowing down the diffusion of carbon. In this paper, the effect of tempering temperature and tempering time on hardness and fatigue resistance of 0.4% carbon steel was studied. Specimens were divided into four categories accord-
450 400 350 300 250 200 150 100 50 0
400 300
400
300
200
300
200
200
100 0 0
0 1
2
3
4
5
Time (hrs) Fig. 6. Temperature vs. time for specimens 3 and 4.
Frequency (Hz)
60 50 40 Fig. 7. Fatigue test specimen.
30 20 10 0 0
1
2
3
4
Time (hrs) Fig. 3. Input frequency vs. time for specimens 1 and 2.
5
ing to tempering temperature (400, 500, 550 and 600 °C), and to three subcategories according to tempering time (1, 2 and 3 h). The hardness of the specimens was then tested using standard brinell testing machine and the results are recorded and plotted in Fig. 8. The specimens were then tested using standard fatigue testing machine and the results are reported and plotted in Figs. 9–11.
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5. Discussion In this paper, the effect of tempering temperature and tempering time on hardness and fatigue resistance of 0.4% carbon steel was studied. Fig. 8, shows that, for temperatures less than 550 °C,
116 TIME=1hr TIME=2hrs TIME=3hrs
114
BHN (MPa)
112
110
the effect of tempering temperature on hardness of the tested specimens is small. But it becomes significant for temperatures greater than 550 °C (4%–8% decrease in hardness). Accordingly, for tempering to reduce the hardness effectively, the tempering temperature should be greater than 550 °C. For tempering temperatures greater than 550 °C, we note that hardness decreases linearly with increasing temperature. In regard to the tempering time, this figure shows that increasing the tempering time decreases the resulting hardness (4%–8% decrease in hardness). For now, if we have a look at fatigue test results, we can see that the number of cycles to failure increases with decreasing the applied cyclic load. We can also note that the number of cycles to which the specimen can survive have a maximum value over which the specimen will fail whatever the applied load was. This can be clearly seen from the sudden sharp increase of the slope of the curve as number of cycles increases.
108
Time=3hrs
1.6
106
102 400
450
500
550
600
Temperature (ºC)
Log (F)
104
Fig. 8. Hardness vs. tempering temperature.
1.5
Temp=400
1.4
Temp=500 Temp=550
1.3
Temp=600 1.2 1.1
1 0.9
Time=1hr
0.8
1.6
1
1.5
3
4
5
6
7
Log (N)
Temp=500
1.4
Fig. 11. Fatigue life curve for a 3 h tempering time.
Temp=550
1.3
Temp=600
1.2
6.5
1.1
6
1
5.5
Log (N)
Log (F)
2
Temp=400
0.9 0.8 1
2
3
4
6
5
F=8N 3hr 2hr 1hr
5 4.5
7 4
Log (N)
3.5 400
Fig. 9. Fatigue life curve for a 1 h tempering time.
450
500
550
600
Temp Time=2hrs
Fig. 12. Log(N) as a function of temperature for a force of 8N.
1.6 Temp=400
1.5
Temp=500
Temp=400 1.6
Temp=550
1.3
1.5
Temp=600
3hr
1.4
1.2
Log (F)
Log (F)
1.4
1.1 1
2hr
1.3
1hr
1.2 1.1 1
0.9
0.9
0.8
0.8 1
2
3
4
5
Log (N) Fig. 10. Fatigue life curve for a 2 h tempering time.
6
7
1
2
3
4
5
6
Log (N) Fig. 13. Fatigue life curve for a 400 °C tempering temperature.
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S. Abdallah, R. Abu-Mallouh / Energy Conversion and Management 49 (2008) 3356–3361
Log (F)
6. Conclusion
Temp=500
1.6 1.5
3hr
1.4
2hr
1.3
1hr
1.2 1.1 1 0.9 0.8 1
2
3
4
5
6
7
Log (N) Fig. 14. Fatigue life curve for a 500 °C tempering temperature.
Temp=550
Log (F)
1.6 1.5
3hr
1.4
2hr
1.3
1hr
1.1 1 0.9 0.8 2
3
4
5
7
6
Log (N) Fig. 15. Fatigue life curve for a 550 °C tempering temperature.
Temp=600
Log (F)
1.6 1.5
3hr
1.4
2hr
1.3
1hr
1.2 1.1 1 0.9 0.8 1
2
3
4
5
6
In this work, medium capacity controlled heating system is designed and constructed using PLC and frequency control. The designed system shows the capability for full control of temperature from zero to maximum and from maximum to zero for any required range of time. As results for the experimentation of the controlled heating system it was found for significant decrease in the hardness of 0.4% carbon steel to be achieved, tempering temperature should be more than 550 °C. Increasing the reheat time decreases the hardness of 0.4% carbon steel. There is a maximum number of cycles to which the specimen can survive whatever the applied load was. This maximum number of cycles increases with increasing tempering temperature. Increasing the tempering time increases fatigue life of 0.4% carbon steel. References
1.2
1
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7
Log (N) Fig. 16. Fatigue life curve for a 600 °C tempering temperature.
It can also be seen in the figure that this maximum possible life of the specimen increases with increasing temperature. To demonstrate this behavior clearly, a plot of Log(N) as a function of temperature is shown in Fig. 12 for a force of 8N. (5.5 103 cycles for T = 400 °C to 1 106 cycles for T = 600 °C). To investigate the effect of the tempering time on the fatigue behavior of 0.4% carbon steel, the same results shown in Figs. 9– 11 is redrawn in Figs. 13–16 shown. These figures show that for 8N load and T = 500 °C, the number of cycles is 66,958 for 3 h tempering period and 38,974 for 1 h tempering period. That means that the number of cycles increased by 72% due to increasing tempering time.
[1] Rajan TV, Sharma CP. Heat treatment principle and techniques. New Delhi: Prentice Hall of India; 1992. [2] Arafat A, Abu Hamid E, Abdel Fattah AK. Royal Scientific Society, introduction of heat treatment technology in designing and manufacturing of spare parts of local industries in Jordan. Higher Council Sci Technol Amman – Jordan, 1999. [3] Shenkman A, Axelrod B, Berkovich Y. Improved modification of the singleswitch AC–AC converter for induction heating applications. IEE Proc – Electr Power Appl 2004;151(1):1–4. [4] Mollov SV, Theodoridis M, Forsyth AJ. High frequency voltage-fed inverter with phase-shift control for induction heating. IEEE 2003:12–8. [5] Kifune H, Hatanaka Y, Nakaoka M. Cost effective phase shifted pulse modulation soft switching high frequency inverter for induction heating applications. IEE Proc – Electr Power Appl 2004;151(1):19–25. [6] Jung J, Lim S, Nam K. A feedback linearizing control scheme for a PWM converter–inverter having a very small DC-link capacitor. IEEE Trans Ind Appl 1999;35(5). [7] Harus LG, Cai M, Kawahima K, Kagawa T. Determination of temperature recovery time in differential-pressure-based air leak detector. Measur Sci Technol 2006;17:411–8. [8] Abdallah S, Al-Dahwi A. A method to design an automated pump plants with PLC control. Issued in the publications of the international engineering conference ‘‘Mutah 2004”, Jordan, in April 2004. [9] Abdalah S, Nijmeh S. Two axis sun tracking system with PLC control. Issued Energy Conversion Manag 2004;45:1931–9. [10] Siemens Sinamics G110 operating instruction manual. User documentation issued 04/02. [11] Siemens PLC S7-200 operating instruction manual. Issued 06/02. [12] Bolton W. Mechatronics electronic control systems in mechanical engineering. Addison Wesley Longman Limited; 1999. [13] Stephen D Funni, Michelle G Koul, Angela L Moran. Evaluation of prosperities and microstructure as a function of tempering time at intercritical temperatures in HY-80 ‘‘steel” castings. Mechanical Engineering Department, US Naval Academy; 2007. [14] Tchoufang J, Beckers J. Fractography survey on high cycle fatigue failure: crack origin characterization and correlations between mechanical tests and microstructure in Fe-C-Cr-Mo–X alloys. Universite’ de Lie’ge; September 2006. [15] Zhang Z, Delagnes D, Bernhart G. Ageing effect on cyclic plasticity of a tempered martensitic steel. China: Dalian Maritime University; 2006. [16] Xu L, Xing J, Wei S, Zhang Y. Ruilong. Optimization of heat treatment technique of high-vanadium high-speed steel based on back-propagation neural networks. Henan University of Science and Tech; 2005. [17] Edmonds D, He K, Rizzo F, Cooman B, Matlock D, Speer J. Quenching and partitioning martensite – a novel steel heat treatment. University of Leeds; 2006. [18] Kaputkin D. Reversible martensitic transformation ageing and lowtemperature tempering of iron-carbon martensite. Technology University; 2006. [19] Mandado E, Macros J, Perze S. Programmable logic devices and logic controllers. Prentice-Hall; 1996. [20] Abdallah S. The regulation of starting and breaking dynamic characteristics in water pumping systems with open-loop control. In: Proceedings of the 8th Cairo University conference on mechanical design and production. Cairo, Egypt; January 2004. p. 4–6.