In a motor control system, which parameters does a Variable Frequency Drive primarily vary to control speed?

• A. Voltage and Frequency
• B. Current only
• C. Power factor
• D. Resistance

Answer: (D)

VFDs control motor speed primarily by changing the electrical supply frequency. The motor speed is roughly tied to the frequency of the applied voltage because the synchronous speed of an induction motor depends on how fast the stator magnetic field rotates, which is set by that frequency. To keep the magnetic flux and torque appropriate as frequency changes, the drive also adjusts the output voltage so that the voltage-to-frequency ratio remains essentially constant (a constant V/Hz relationship). This combination lets the motor run at the desired speed without losing torque.

Current, while it relates to torque, isn’t the parameter used to set speed; it’s the result of the load and torque requirements. Power factor and winding resistance aren’t the primary parameters controlled to regulate speed either. The drive’s job is to modulate frequency (and the accompanying voltage) to set speed, using current to meet torque needs as needed.