Motor Starting Analysis
Starting large motors can cause disturbances to the motor and other loads on other buses. In the worst cases the starting motor may stall and be unable to start the driven load. In general, a motor starting study should be made if the motor's horsepower exceeds approximately 30% of the supply transformers base kVA rating. If the generator supplies the motor, use 10 -15% of the generator kVA rating.
Motor starting studies can vary from basic voltage drop on the system to a detailed waveform presentation of motor bus voltage, motor speed and motor torque, acceleration torque, load torque, power factor, rotor and stator currents, motor slip, real, reactive and total power.
One of the most common side effects of starting large motors is a serious voltage dip on the buses throughout the facility. This voltage dip will cause other motors to slow down. In severe cases other motors may reach the stall point causing a domino effect to the voltage drop. Control relays may not hold and auxiliary equipment may be affected. In addition to these secondary effects the life of all motors on the system may be shortened. Ideally a transient motor starting study should be preformed which shows a time/voltage waveform for the motor bus. Motor starting studies should be performed prior to the ordering of large motors, such that the motor can be installed with confidence that the motor's life and applications performance will be satisfactory and the remainder of the power distribution system will not be adversely affected.
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