Power Electronics Circuits
When the speed of an induction motor is varied with an inverter, which of the following is the main reason the voltage-to-frequency ratio (V/f) is kept roughly constant?
- ATo keep the flux in the motor roughly constant and prevent magnetic saturation of the core or a lack of torque
- BBecause keeping the voltage constant while lowering the frequency increases the flux and the torque, which makes the motor run faster
- CBecause keeping the voltage-to-frequency ratio constant always makes the slip zero
Show answer and explanation
Answer: A. To keep the flux in the motor roughly constant and prevent magnetic saturation of the core or a lack of torque
The flux of an induction motor is roughly proportional to the applied voltage divided by the frequency. Lowering only the frequency while keeping the voltage increases the flux, saturates the core and draws a large magnetizing current. Lowering only the voltage, on the other hand, reduces the flux and makes torque hard to produce. For example, a 200 V, 50 Hz motor run at 25 Hz is supplied with about 100 V (V/f = 4 V/Hz). At low frequencies a torque boost, which raises the voltage slightly, compensates for the voltage drop across the winding resistance.
- A. Correct. Keeping V/f constant to maintain the flux is the idea behind V/f control.
- B. The speed is set by the frequency. Lowering the frequency also lowers the synchronous speed.
- C. Producing torque requires slip. Slip does not become zero under V/f control either.