True or False: A motor controlled by a VFD is sped up or slowed down by increasing or decreasing the voltage on each phase.

True or False: A motor controlled by a VFD is sped up or slowed down by increasing or decreasing the voltage on each phase.

• A. True
• B. False
• C. Depends on motor type
• D. Only by adjusting frequency

Answer: (A)

A VFD controls motor speed primarily by changing the output frequency, but it also adjusts the phase voltage to keep the motor’s flux and torque in proper range as speed changes. As you raise or lower the frequency to speed up or slow down the motor, the drive increases or decreases the voltage on each phase to maintain the desired volts-per-hertz ratio. That voltage adjustment is part of how the VFD makes smooth speed changes and keeps torque available, so the idea that speed is changed by altering the phase voltage is accurate in the context of how a VFD operates.

A VFD controls motor speed primarily by changing the output frequency, but it also adjusts the phase voltage to keep the motor’s flux and torque in proper range as speed changes. As you raise or lower the frequency to speed up or slow down the motor, the drive increases or decreases the voltage on each phase to maintain the desired volts-per-hertz ratio. That voltage adjustment is part of how the VFD makes smooth speed changes and keeps torque available, so the idea that speed is changed by altering the phase voltage is accurate in the context of how a VFD operates.