A VFD is a type of motor controller that varies the ___ and ___ supplied to an electric motor in order to control its speed.

A VFD is a type of motor controller that varies the ___ and ___ supplied to an electric motor in order to control its speed.

• A. Frequency, Power
• B. Frequency, Voltage
• C. Power, Voltage
• D. Current, Power

Answer: (B)

The essential idea is that VFDs control motor speed by adjusting two things the motor actually sees: the output frequency and the output voltage. The speed of an AC induction motor is set mainly by how fast the AC waveform is changing, i.e., the frequency. So changing frequency directly changes speed. But to keep the magnetic flux in the motor stable and to provide the right torque, the voltage must be adjusted as well. The VFD keeps the voltage proportional to the frequency (a roughly constant V/Hz ratio) up to the motor’s base frequency. This lets the motor speed vary smoothly without overheating or losing torque. That’s why the correct pairing is frequency and voltage. Current and power aren’t the direct control inputs; they vary based on load and torque requirements, and while power can change as speed changes, the drive’s primary speed-control actions are on frequency and voltage.

The essential idea is that VFDs control motor speed by adjusting two things the motor actually sees: the output frequency and the output voltage. The speed of an AC induction motor is set mainly by how fast the AC waveform is changing, i.e., the frequency. So changing frequency directly changes speed. But to keep the magnetic flux in the motor stable and to provide the right torque, the voltage must be adjusted as well. The VFD keeps the voltage proportional to the frequency (a roughly constant V/Hz ratio) up to the motor’s base frequency. This lets the motor speed vary smoothly without overheating or losing torque.

That’s why the correct pairing is frequency and voltage. Current and power aren’t the direct control inputs; they vary based on load and torque requirements, and while power can change as speed changes, the drive’s primary speed-control actions are on frequency and voltage.