Which device is used to control the speed of AC motors by adjusting the frequency of the supply?

Which device is used to control the speed of AC motors by adjusting the frequency of the supply?

• A. Inverter
• B. Transformer
• C. Resistor
• D. Variable Frequency Drive

Answer: (D)

Changing the speed of an AC motor is done by altering the supply frequency, since the motor’s speed is tied to the frequency of the power it receives. A Variable Frequency Drive is the device built for this purpose: it converts incoming power to DC and then back to AC at a controllable frequency, delivering the motor a precisely set frequency to set its speed. It also manages voltage to keep the current and torque appropriate as the frequency changes, preserving performance and preventing motor saturation. Transformers only change voltage levels and don’t affect frequency, so they can’t control motor speed. Resistors waste power and offer no practical or reliable speed control for motors. An inverter can produce variable-frequency AC, but the standard, complete motor-speed-control unit is the Variable Frequency Drive, which encompasses the necessary circuitry and control logic to adjust frequency (and voltage) for the motor.

Changing the speed of an AC motor is done by altering the supply frequency, since the motor’s speed is tied to the frequency of the power it receives. A Variable Frequency Drive is the device built for this purpose: it converts incoming power to DC and then back to AC at a controllable frequency, delivering the motor a precisely set frequency to set its speed. It also manages voltage to keep the current and torque appropriate as the frequency changes, preserving performance and preventing motor saturation.

Transformers only change voltage levels and don’t affect frequency, so they can’t control motor speed. Resistors waste power and offer no practical or reliable speed control for motors. An inverter can produce variable-frequency AC, but the standard, complete motor-speed-control unit is the Variable Frequency Drive, which encompasses the necessary circuitry and control logic to adjust frequency (and voltage) for the motor.