AC induction motors are used in many industrial applications as well as commercial and domestic products and appliances. Due to their design, the speed of these motors is determined by the frequency of the supply power, and the current determines the motor torque (turning force) in the motor, which is determined by the voltage of the supply. To effectively manage the speed of the motor, even when the load varies, it is necessary to control both the frequency and the voltage of the supply power. This opens the possibility of great improvements in efficiency and productivity and reductions in maintenance costs.

Managing the speed of an induction motor is achieved by using a solid-state electronics device called a Variable Frequency Drive (VFD), sometimes known as a Variable Speed Drive (VSD) or simply as an AC drive. This device provides the ability to modify supply power frequency and voltage, giving operators and manufacturers fine control over how a motor behaves in an application.

The motors used with a Variable Frequency Drive are usually three-phase induction motors due to their popularity and widespread usage. Still, VFDs can be used to achieve speed control with single-phase and synchronous motors as well.

How a VFD Works

The key goal of a Variable Frequency Drive is to modify the frequency and voltage of the supply power. Doing this converts the alternating current (AC) power of the supply to direct current (DC), adjusts the voltage, and then converts it back to AC power at the desired frequency.

The first stage of a VFD is the converter. This device is typically a six-pulse full-wave diode bridge designed to only allow a single polarity output regardless of the input's polarity. This means that the alternating current (AC) on the input is converted to direct current (DC) at the output.

In the second stage, voltage adjustment is carried out. Typically, this is achieved using a Pulse Width Modulator (PWM). This device takes the power at the input and pulses it at the output, which causes the voltage at the output to fluctuate. By controlling the pulse rate and using a capacitor on the output, the result is smoothed out at the desired voltage. This makes it possible to achieve a wide range of output voltages from an input source.

The final stage of a VFD involves converting the voltage-adjusted input from DC power back to AC power. The device used to do this is known as an inverter. The output of the inverter stage of a VFD is usually quasi-sinusoidal, which is suitable for a motor.

Benefits of VFDs

Regardless of the application, there are innumerable benefits to controlling the speed of a motor using a Variable Frequency Drive.

The main benefit is efficiency. When motors run at a constant speed that is not determined by the application, they are rarely operating near maximum efficiency. Depending on the application, this can result in substantial power costs and loss of productivity. With speed control, the optimum speed for each application can be determined and maintained. This doesn’t just mean savings in operating costs, but also the potential to claim benefits from energy efficiency tax incentives and utility rebates.

Another advantage of speed control using a Variable Frequency Drive is a reduction in maintenance costs. When motors run faster than they need to, the entire mechanical system, including the motor, bearings, gearboxes and peripherals such as pumps or conveyor belts, wear out faster. Additionally, the shock of starting at full speed can create stress on the entire system and result in more frequent mechanical failure. In the long term, this can add up to a high cost. By controlling the speed of the motor with a Variable Frequency Drive, the lifetime of the entire system can be extended, reducing maintenance costs and the requirement for new parts.

Using a VFD has the added benefit of protecting the motor and the entire system from fluctuations in the supply power that may be outside the control of the operator or manufacturer. Because VFDs maintain the supply's frequency and voltage, they provide over-voltage, under-voltage, and phase protection to the motor. This reduces stress, maintains optimal operating conditions and increases the lifetime of the system.


A Variable Frequency Drive is a handy device that provides you with fine control over your motor's speed and torque, allowing you to tailor the speed of the motor to the application, increasing efficiency and productivity, and reducing maintenance costs.