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What Is a Variable-Frequency Drive?

Paul Reed
Paul Reed

A variable-frequency drive is used on alternating current motors to control motor speed. Although more expensive in first cost than a fixed-speed motor, this type of system can provide cost savings by lowering utility expenses and increasing equipment life. Variable-frequency drive systems change the motor voltage and frequency to increase or decrease pump flow rates.

This system contains an alternating current (AC) to direct current (DC) converter, a DC processing circuit, and a DC to AC inverter. The result is an AC supply similar in character to the AC supplied by the utility, but with the ability to raise and lower the voltage and frequency. When connected to a pump, a variable-frequency drive motor can provide flow rate control without the use of control valve systems.


The advantage of variable-frequency drive systems comes from the reduced power required to run the pump at lower speeds. It is inefficient to run a pump at one speed and reduce flow rates with valves. Utility companies charge customers for lower power factors, or electrical efficiency, and it can be an advantage to use variable-frequency drive systems with higher power factors.

These systems are used in a wide variety of industries. Some chemical processes have narrow acceptable temperature limits for product quality. Varying product flow rates can provide a more rapid temperature adjustment than varying the heat or cooling source.

Water cooling towers have a high variability in water flow for process cooling. During cooler weather, less water flow is required and a variable speed system can maintain required temperatures at much lower costs. Connecting the motor control to the system temperature regulators allows for the increase in water flow necessary for cooling.

Another advantage of variable speed motors is soft-starting capability. Motors connected to a constant voltage start quickly, and stresses occur in the motor from sudden pressure changes. Starting a motor with variable-frequency control allows the motor to increase speed slowly, and lower stresses occur in the system. Less stress can result in longer equipment life and a reduction of costs.

Most electric motors are cooled with fans connected to the motor drive shaft. One disadvantage of variable-frequency drive systems is the loss of fan cooling when running at lower speeds. This type of system can also cause additional heating of motor windings. Installation of motor temperature sensors or auxiliary cooling fans may be needed in warm climates to prevent motor overheating.

Variable-frequency drive motors can have higher noise levels than a constant-speed motor. The frequency has a slightly different AC wave form than the AC supplied by the utility. This can cause vibration of the motor windings resulting in noise that can increase with lower motor frequencies and speeds.

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