When using a VFD to control speed you are adjusting?

When using a VFD to control speed you are adjusting?

• A. Frequency only
• B. Voltage
• C. Torque
• D. Current

Answer: (A)

Speed is set by changing the supply frequency. A VFD controls how fast an AC motor spins by adjusting the frequency of the voltage it feeds the motor, since the motor’s rpm is essentially proportional to that frequency. The drive also tweaks voltage to keep the voltage-to-frequency ratio steady, so flux remains good and torque is available across speeds. Torque and current respond to the load; increasing load makes the motor draw more current to develop more torque, which can slow the shaft if the frequency isn’t raised. But the direct parameter that determines speed is the frequency, not voltage or current alone.

Speed is set by changing the supply frequency. A VFD controls how fast an AC motor spins by adjusting the frequency of the voltage it feeds the motor, since the motor’s rpm is essentially proportional to that frequency. The drive also tweaks voltage to keep the voltage-to-frequency ratio steady, so flux remains good and torque is available across speeds. Torque and current respond to the load; increasing load makes the motor draw more current to develop more torque, which can slow the shaft if the frequency isn’t raised. But the direct parameter that determines speed is the frequency, not voltage or current alone.