The formula for calculating the power necessary to start a synchronous motor with full load running torque is as follows:
Formula
Where
P - Power developed by the motor (in watts).
V - Line voltage (V)
Eb - Back EMF generated/phase (V)
- The load angle (in degrees) is the angle formed between the back EMF & the terminal voltage.
Xs - Synchronous reactance (in Ω).
Important Points
Starting Condition: At starting, the load angle (δ) is normally minimal, but increases with motor acceleration.
Full Load Torque: The torque at full load influences the load angle and, consequently, the power calculation.
Maximum Power: Maximum Power is achieved when δ = 90∘ resulting in:
This formula estimates the electrical power input required to start a synchronous motor at full load conditions, taking into consideration its electrical characteristics & operational factors.