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Large VSD motors bring big solutions
24
Feature
WORLD PUMPS
March 2013
Energy efficiency
Large VSD motors bring big solutions The reliability of the VSD system and electric motor is vital considering the cost of unscheduled shutdowns can cause millions of dollars in losses. This article discusses the practical issues and latest developments for VSD electric motors for pumps.
T
he VSD electric-motor drivers for high-power applications often use the LCI (Load Commutation Inverter) system associated with the synchronous two-pole electric motor. The LCI technology suffers from some well-known drawbacks like high torque ripple, poor 'Power Factor,' elatively high losses, and harmonic pollutions. The LCI technology generates torque pulsations and a harmonic filter is usually required. These disadvantages can make LCI-based variable-speed drives inadequate to reach the increasingly demanding performance required in critical large pump applications. Because of the reactive power consumption of its thyristor bridge, an LCI converter cannot properly power an induction motor. However, the VSI (Voltage Source Inverter) technologies are more suitable for a large pump. A VSI can power both the induction motors and the synchronous motors. There is a very low harmonic content when using a VSI system (no harmonic filter required) with a better 'Power Factor' (PF). A VSI solution could also offer a cost advantage. For a large pump, a VSI solution is preferred.
VSD systems for pumps The VSI (voltage source inverter) converter technology is proposed for large pump drivers. The choice of VSI technology makes it possible to attain some important goals like: www.worldpumps.com
• A voltage output (converter output to an electric motor) that approaches the sinusoidal waveform. Offers the possibility to operate the electric motor at a near unity power factor.
Particular advantages over traditional LCI-based solutions are:
• A low harmonic injection.
2. Very low vibrations.
A cascaded multi-level converter topology could also be chosen for very large (high-power) applications. Each converter phase is obtained by series connecting several transistor cells. Two or four converters can be used in the PWM-VSI technology (PWM: Pulse Width Modulation). The decision to supply the electric motor with several (two, four or more) three-phase converter units leads to the splitting of the stator winding into independent three-phase sets, each to be fed by a converter.
3. High fault tolerance.
1. In terms of the torque ripple, typically lower than 1-2% peak to peak.
4. High electric motor efficiency (usually above 98%). The electrical transformers play an important role in any VSD system. In-rush current limitation requirements and protection philosophies of transformers are important. Various cooling water pumps are employed in a VSD electric motor system. The cooling pump normal operating
The stator design needed for this purpose is often referred to as 'splitphase' as it results from splitting the winding into multiple star-connected three-phase sets. The phase currents contain harmonics of orders 5, 7, 11, 13, 17, and 19. The resulting harmonic fields in the electric motor air-gap are globally low because of the mutual cancellation effects. A new electric drive solution based on a VSI-fed quadruple-star 100 Hz four-pole synchronous electric motor has been presented for large pumps.
0262 1762/13 © 2013 Elsevier Ltd. All rights reserved
Figure 1. An example of a large pump.
26
Feature
WORLD PUMPS
March 2013
the full-load full-speed performance tests of the entire drive system are mandatory. The following tests should be conducted: 1. The testing an electric motor alone. 2. The 'back-to-back test' to verify the electric motor and the VSD performance. 3. The 'string test' for a complete pump train.
Figure 2. An example of a large pump driven by an engine. The engine is a relatively unreliable driver which requires many days of shutdown per year (for the maintenance). An engine could also present unscheduled shutdowns. A large VSD electric motor driver is a far superior driver for a large pump.
point should be as close as practical to the 'Best Efficiency Point” (BEP) of the pump. Rated cooling flows should preferably be within 20% of the BEP flow. The cooling pump characteristic curve is very important for proper operation. A cooling pump curve should exhibit the characteristics of stable continuously-rising head from the rated capacity to the shutoff (preferably a 10% head rise from the rated to the shutoff ). Typically, a VSI system footprint is less than 75% of a comparable LCI system. A VSI system weight is usually less than 70% of a comparable LCI system.
rotational centre changes from the geometric mass centre to the local mass centre, which means the local unbalance in an elastic rotor varies with the speed. Therefore, the modal sets of unbalance weights should be used to balance each mode individually. As a minimum, n+2 balancing planes (n=the number of modes to balance) are necessary for the balancing.
Electric motor tests To reduce the risk of having non-performing drive systems shipped,
The open-circuit and short-circuit tests could be accomplished in order to determine the conventional motor losses. The no-load test, conducted at the rated speed, can give the open-circuit curve (which indicates various loss contributions). The majority of the losses come from the friction and windage. Strong cooling air flows produced by the cooling fans (an internal cooling) are associated with some losses. Typically, efficiencies in range of 97-99% could be expected. When at least two similar VSD and electric motor systems are being supplied, the VSD-motor back-to-back test can be done (one in the motoring mode and another in the generating mode). It is theoretically possible to supply the losses and reactive power demands. The bearing problems, excessive vibrations and oil systems issues are responsible for a considerable portion of failed performance tests of large electric motors. During a back-to-back test, observations are:
Large electric motors A large electric motor should handle the thermal unbalances properly. Because of the inevitable use of various materials with very different thermal expansion coefficients, combined with non-uniform temperature distribution and large sizes, a symmetrical mechanical and thermally insensitive design should be achieved. A small asymmetry can cause an unacceptable dynamic load. For large, high-speed electric motors, the flexible rotor concept could be used (the first critical speed usually lies below the operating speed range). The rotor should be dynamically balanced. The field balancing would not be required. When passing the first critical speed, the local www.worldpumps.com
Figure 3. An example of a large pump with a large electric motor driver.
WORLD PUMPS
Feature March 2013
1. Assessment of the Motor Thermal Performance: Heat-run tests should be performed to assess the electric motor full-load thermal behaviour at different operating and emergency modes. 2. Assessment of Electric Motor Vibration Performance. 3. Torque Ripple Measurement: At the rated conditions and the emergency cases, the torque ripple could be monitored, at least two ways: A. Electrical measurements at motor terminals. B. Mechanical direct measurement (i.e., strain gauges mounted on the motor shaft, torque transducers, shaft encoder methods, laser measurement methods, and others). The torque ripple estimated from electrical measurements is usually higher than that resulting from mechanical direct measurements because an electrical measurement contains some high-order frequency torque components (mechanically dampened while being transferred). 4. Assessment of Motor Torque Overload Capability: The torque required for the start-up of a pump train, which could be 120-150% of normal torque for around 30-100 seconds. 5. Motor Voltage and Current Waveforms.
Figure 4. An example of a large electrical transformer (during the delivery).
Case Study In a case study, during a large electric motor test (an electric motor for a pump drive), suspicious noises and smokes were identified on the electric motor. The first observation (after the trip) given below: • The motor shaft drop by 1.6 mm. • The maximum temperature on the bearing reached above 150°C.
• The sleeve bearings were damaged. The root cause was the lack of lubrication oil, because of the main oil pump failure (and also the failure of standby oil pump to start).
Contact Amin Almasi WorleyParsons Services Pty Ltd. Email: [email protected]
www.worldpumps.com
27
Feature
WORLD PUMPS
March 2013
Energy efficiency
Large VSD motors bring big solutions The reliability of the VSD system and electric motor is vital considering the cost of unscheduled shutdowns can cause millions of dollars in losses. This article discusses the practical issues and latest developments for VSD electric motors for pumps.
T
he VSD electric-motor drivers for high-power applications often use the LCI (Load Commutation Inverter) system associated with the synchronous two-pole electric motor. The LCI technology suffers from some well-known drawbacks like high torque ripple, poor 'Power Factor,' elatively high losses, and harmonic pollutions. The LCI technology generates torque pulsations and a harmonic filter is usually required. These disadvantages can make LCI-based variable-speed drives inadequate to reach the increasingly demanding performance required in critical large pump applications. Because of the reactive power consumption of its thyristor bridge, an LCI converter cannot properly power an induction motor. However, the VSI (Voltage Source Inverter) technologies are more suitable for a large pump. A VSI can power both the induction motors and the synchronous motors. There is a very low harmonic content when using a VSI system (no harmonic filter required) with a better 'Power Factor' (PF). A VSI solution could also offer a cost advantage. For a large pump, a VSI solution is preferred.
VSD systems for pumps The VSI (voltage source inverter) converter technology is proposed for large pump drivers. The choice of VSI technology makes it possible to attain some important goals like: www.worldpumps.com
• A voltage output (converter output to an electric motor) that approaches the sinusoidal waveform. Offers the possibility to operate the electric motor at a near unity power factor.
Particular advantages over traditional LCI-based solutions are:
• A low harmonic injection.
2. Very low vibrations.
A cascaded multi-level converter topology could also be chosen for very large (high-power) applications. Each converter phase is obtained by series connecting several transistor cells. Two or four converters can be used in the PWM-VSI technology (PWM: Pulse Width Modulation). The decision to supply the electric motor with several (two, four or more) three-phase converter units leads to the splitting of the stator winding into independent three-phase sets, each to be fed by a converter.
3. High fault tolerance.
1. In terms of the torque ripple, typically lower than 1-2% peak to peak.
4. High electric motor efficiency (usually above 98%). The electrical transformers play an important role in any VSD system. In-rush current limitation requirements and protection philosophies of transformers are important. Various cooling water pumps are employed in a VSD electric motor system. The cooling pump normal operating
The stator design needed for this purpose is often referred to as 'splitphase' as it results from splitting the winding into multiple star-connected three-phase sets. The phase currents contain harmonics of orders 5, 7, 11, 13, 17, and 19. The resulting harmonic fields in the electric motor air-gap are globally low because of the mutual cancellation effects. A new electric drive solution based on a VSI-fed quadruple-star 100 Hz four-pole synchronous electric motor has been presented for large pumps.
0262 1762/13 © 2013 Elsevier Ltd. All rights reserved
Figure 1. An example of a large pump.
26
Feature
WORLD PUMPS
March 2013
the full-load full-speed performance tests of the entire drive system are mandatory. The following tests should be conducted: 1. The testing an electric motor alone. 2. The 'back-to-back test' to verify the electric motor and the VSD performance. 3. The 'string test' for a complete pump train.
Figure 2. An example of a large pump driven by an engine. The engine is a relatively unreliable driver which requires many days of shutdown per year (for the maintenance). An engine could also present unscheduled shutdowns. A large VSD electric motor driver is a far superior driver for a large pump.
point should be as close as practical to the 'Best Efficiency Point” (BEP) of the pump. Rated cooling flows should preferably be within 20% of the BEP flow. The cooling pump characteristic curve is very important for proper operation. A cooling pump curve should exhibit the characteristics of stable continuously-rising head from the rated capacity to the shutoff (preferably a 10% head rise from the rated to the shutoff ). Typically, a VSI system footprint is less than 75% of a comparable LCI system. A VSI system weight is usually less than 70% of a comparable LCI system.
rotational centre changes from the geometric mass centre to the local mass centre, which means the local unbalance in an elastic rotor varies with the speed. Therefore, the modal sets of unbalance weights should be used to balance each mode individually. As a minimum, n+2 balancing planes (n=the number of modes to balance) are necessary for the balancing.
Electric motor tests To reduce the risk of having non-performing drive systems shipped,
The open-circuit and short-circuit tests could be accomplished in order to determine the conventional motor losses. The no-load test, conducted at the rated speed, can give the open-circuit curve (which indicates various loss contributions). The majority of the losses come from the friction and windage. Strong cooling air flows produced by the cooling fans (an internal cooling) are associated with some losses. Typically, efficiencies in range of 97-99% could be expected. When at least two similar VSD and electric motor systems are being supplied, the VSD-motor back-to-back test can be done (one in the motoring mode and another in the generating mode). It is theoretically possible to supply the losses and reactive power demands. The bearing problems, excessive vibrations and oil systems issues are responsible for a considerable portion of failed performance tests of large electric motors. During a back-to-back test, observations are:
Large electric motors A large electric motor should handle the thermal unbalances properly. Because of the inevitable use of various materials with very different thermal expansion coefficients, combined with non-uniform temperature distribution and large sizes, a symmetrical mechanical and thermally insensitive design should be achieved. A small asymmetry can cause an unacceptable dynamic load. For large, high-speed electric motors, the flexible rotor concept could be used (the first critical speed usually lies below the operating speed range). The rotor should be dynamically balanced. The field balancing would not be required. When passing the first critical speed, the local www.worldpumps.com
Figure 3. An example of a large pump with a large electric motor driver.
WORLD PUMPS
Feature March 2013
1. Assessment of the Motor Thermal Performance: Heat-run tests should be performed to assess the electric motor full-load thermal behaviour at different operating and emergency modes. 2. Assessment of Electric Motor Vibration Performance. 3. Torque Ripple Measurement: At the rated conditions and the emergency cases, the torque ripple could be monitored, at least two ways: A. Electrical measurements at motor terminals. B. Mechanical direct measurement (i.e., strain gauges mounted on the motor shaft, torque transducers, shaft encoder methods, laser measurement methods, and others). The torque ripple estimated from electrical measurements is usually higher than that resulting from mechanical direct measurements because an electrical measurement contains some high-order frequency torque components (mechanically dampened while being transferred). 4. Assessment of Motor Torque Overload Capability: The torque required for the start-up of a pump train, which could be 120-150% of normal torque for around 30-100 seconds. 5. Motor Voltage and Current Waveforms.
Figure 4. An example of a large electrical transformer (during the delivery).
Case Study In a case study, during a large electric motor test (an electric motor for a pump drive), suspicious noises and smokes were identified on the electric motor. The first observation (after the trip) given below: • The motor shaft drop by 1.6 mm. • The maximum temperature on the bearing reached above 150°C.
• The sleeve bearings were damaged. The root cause was the lack of lubrication oil, because of the main oil pump failure (and also the failure of standby oil pump to start).
Contact Amin Almasi WorleyParsons Services Pty Ltd. Email: [email protected]
www.worldpumps.com
27