Motor control surge

If there is a large number of motors to restart (or also called reaccelerate) then there will be a large surge of main feeder current if they all start at the same time. This will cause a severe volt-drop at the common busbars, which will cause the run-up times to be extended and the possibility of the overcurrent protection relay at the main feeder circuit breaker to trip. Similarly if there is a group of motor control centres all in the same state of restarting their motors, then their common busbar e.g. main generator switchboard, will experience a significant volt-drop which may not be acceptable. 

Prelorius, R.E. and Eriksson. A.J., Field studies of switching surge generation in high voltage vacuum contactor controlled motors , IEEE Conference Publication 210. 54-64 ( 1982). 

Compressors driven by steam mrbines, gas turbines, hot gas expanders, or variable speed electric motors usually employ variable rotation speed for capacity control. Generally, the surge limit slope varies witli rotation... 

Figure 3.23. Control of centrifugal compressors with turbine or motor drives, (a) Pressure control with turbine or motor drives, (b) Flow control with turbine or motor drives. SC is a servomechanism that adjusts the guide vanes in the suction of the compressor, (c) Surge and pressure control with either turbine or motor drive. The bypass valve opens only when the flow reaches the minimum calculated for surge protection.

Since 2007, Enova/IC posttransmission parallel PHEBs have operated in California and Florida school districts. In 2009, DOE funded Enova and IC/Navistar with Federal stimulus dollars to deploy 16 PHEB school buses nationwide. It couples a diesel engine with an 80-kW AC induction electric motor mounted behind the transmission (post-transmission) and a hybrid controller with a battery care unit (BCU) to monitor the battery voltage, SOC, amp-hours, kWh, and temperature. The BCU controls a safety disconnect unit to ensure the safety of the battery pack during charging and provides surge protection and automatic disconnect in case a ground fault occurs. 

If the compressor is of the variable speed type then we can use this to control the flow, as shown in Figure 11.6. By changing the speed we move from one compressor curve to another. This is energy efficient and, because the surge point moves, can operate over a wide range. However variable frequency drives (VFD) for large electric motors are costly. Variable speed steam turbine drivers have a mixed reputation. Most success has been had with gas turbine drivers. 

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