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How to Control the Speed of an AC Motor

By G.K. Bayne

The most efficient and inexpensive method for controlling the speed of an AC motor is a frequency drive inverter. Besides helping you conserve electric power, the inverter offers precise speed control. Whether your motor is single phase or three phase, you can find an economical means for speed control.

Install the frequency drive in the location specified by the manufacturer. Generally this will be an isolated area away from high heat, moisture and dust. Another concern is that the drive does not interfere with other electrical signaling devices such as two-way radios or wireless communications. Be sure to follow all grounding requirements as stated in the specifications. Improper grounding may cause the drive to malfunction.

Use the wire strippers to remove three-quarters of an inch of the outer insulation from the incoming power wires and the wires leading to the motor. The incoming power wires will be connected to the terminals marked L1, L2 and L3. Single-phase wiring will use only one or two of the terminals, depending on the unit. The outgoing wires, to the motor, will be connected to T1, T2 and T3; again, the specific connections will vary with the unit.

Attach any auxiliary devices, such as power resistors for stopping the motor, external relays and remote speed control devices. All frequency drives will have many of these devices built in to the unit.

Power up the drive unit and watch the LCD or LED screen. In almost all cases the frequency drive will be ready to operate the motor after the unit goes through an on board check and reads the motor windings for correct connections.

Program the frequency drive unit according to the instructions in the manual. The more extras on the drive unit, the more instructions you'll have to plow through.


Things You Will Need

  • Frequency drive appropriate for the size of your motor
  • Incoming power wires and motor wiring
  • Wire strippers
  • Philips or slotted screwdrivers
  • Manufacturer's instructions for setup


  • Frequency drives can save an immense amount of electricity by "soft starting" a motor. New private well installations use a frequency drive to extend the life of submersed pumps to almost a full-lifetime operation.