Transient Motor Starting Analysis
When motors start they require a high inrush of current, typically 5-7 times normal running current for a short period of time. This high current causes the voltage to drop excessively. This drop may cause the motor to stall or not start, and other motors to slow down, or in severe cases stall as well. This shortens motor life. Control relays and other equipment may be affected. The purpose of the analysis is to determine the amount of voltage drop that will occur under normal starting conditions and how it will affect other equipment. Typically, this initial low voltage condition is overcome by using variable frequency drives or other starting compensation. This study will also evaluate starting voltage swings under compensated conditions.
- In a transient motor starting analysis, the starting motors are represented by dynamic models to simulate the entire process of motor acceleration. The study output includes:
- Bus voltage
- Motor voltage
- Motor rotor current
- Load torque
- Motor speed
- Motor stator current
- Motor torque
- Accelerating torque
- Motor slip
- Reactive power (kVAR)
- Power factor
- Real power (kW)
- Total power (kVA)
This analysis should be performed before installing a large motor or anytime a change in the power supply is implemented. Typically a motor starting study should be made if the motor’s horsepower exceeds approximately 30% of the supply transformers base kVA rating. If a generator is supplying the motor, use 10 –15% of the generator kVA rating.