AC Motor Speed Control

AC motor speed control: An overview

Alternating current (AC) motors are among the most widely used electric motors in several industries today. As its name implies, an AC motor converts electrical energy (from alternating current source) into mechanical energy (in the form of shaft rotation).

AC motors offer several advantages over direct current motors. For example, they have no brushes or commutators (unlike the DC motors), making them cheaper, simpler, and more reliable in producing mechanical energy. But one challenge that has remained constant with AC motors is how to control the motor speed accurately.

This article will get back to the basics of AC motor speed controls. It will present the different methods by which AC motor speed can be controlled, the advantages and suitability of these methods for different application requirements. This article will serve as a guide to help engineers achieve different operational requirements with their AC motors.

A challenge with AC motors is the method by which speed can be controlled

Source: [disfoto1973/Adobe Stock]

How to control the speed of an AC motor

An AC motor typically features coils that produce a rotating magnetic field inside a rotor attached to an output shaft. This rotor produces a second magnetic field, which then causes the output shaft to rotate at a constant speed. The synchronous speed of this motor can be estimated using:


f = Incoming line frequency
P = Number of poles in the motor

For example, consider the United States, where the incoming line frequency is usually 60 Hz at 120 V. If an engineer has an AC motor with two poles (a pole pair motor), then the motor's synchronous speed would be 3600 revolutions per minute. In reality, the AC motor will run at a speed of less than 3600 rpm, and the difference in speed (usually expressed in percentages) is called the slip.

However, if there is a need to achieve lower output speeds with this same motor, engineers would have to control the incoming line frequency since the number of poles of a typical AC motor is constant. Engineers achieve this speed control requirement by using what is called the variable frequency drive (VFD).

What is a variable frequency drive, and how does it work?

A variable frequency drive controls the speed of an AC motor by varying the frequency and voltage supplied to the motor. They work by taking the existing AC supply and converting it into DC using a rectifier and then converting it back to a variable frequency using an inverter.

A typical variable frequency drive features a rectifier circuit, DC link capacitor, and an inverter circuit. The rectifier circuit comprises six diodes (a diode allows electric current to flow in one direction). These diodes provide six current pulses as each diode opens and closes, creating a DC voltage with an AC ripple.

The DC-link capacitor absorbs this AC ripple, converting it to a smooth (or fixed DC voltage). Next, an inverter circuit converts this DC back to AC, allowing engineers to make any phase on the motor positive or negative and generate any desirable frequency.

The circuit of a typical variable frequency drive

Source: C J Cowie/CC[SA][3.0]

What are the types of variable frequency drives?

Variable frequency drives come in three types:

  1. Current source inverter
  2. Voltage source inverter type VFD
  3. Pulse width modulation type VFD

Current source inverter VFD

The current source inverter type VFDs are designed to convert the incoming AC voltage into variable DC while controlling the frequency using silicon-controlled rectifiers (SCRs). Their DC-link typically features inductors, which help store DC energy and regulate the AC current ripple for the inverter.

Voltage source inverter VFD

Like the current source inverter type VFD, a voltage source inverter VFD also converts incoming AC voltage into variable DC while controlling the frequency. However, a major difference between these two systems is their energy storage method. A voltage source inverter VFD uses capacitors (or capacitive energy) to store DC energy and regulate AC current ripple between the rectifier and inverter. In contrast, current source inverters use inductors to achieve this energy storage.

Voltage source inverter VFDs are ideal for dynamic applications, where there is the need to achieve fast changes in motor torque and speed.

Pulse width modulation

Pulse width modulation (PWM) type VFD features a diode-bridge rectifier to convert incoming AC voltage into DC voltage. A typical PWM VFD provides a more sinusoidal current output to control the frequency and voltage supplied to the AC motor, giving engineers better control over the speed of the motor compared to other VFD types.

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AC motor speed control: Reach out to manufacturers

While this article presents helpful tips about AC motor speed controls, there are several other things an engineer must consider when specifying AC motors and variable frequency drives for a particular application. Engineers are advised to reach out to AC motor manufacturers to discuss their application requirements.