The Best Use of Variable Frequency Drives

A variable frequency drive (VFD) is a system for controlling the speed of an electrical motor by controlling the frequency of the power supplied to the motor. Also known as adjustable-frequency drives (AFD), AC drives, microdrives or inverter drivers; voltage can vary in these systems. Variable Frequency Drives are fairly common in their use. These drives can often save energy by allowing the volume of air moved to match system demand, making the system much more energy efficient as opposed to a motor that does not use a variable frequency drive.

Variable Frequency Drive Motors

The motor used in a variable frequency drive are often either single-phase or three-phase motors, however, three-phase motors are much more common. Induction motors are usually used when price is a major factor; however a complete audit should be made prior to implementation.

Variable Frequency Drive Controller

Variable frequency drive controllers are solid state electronic power conversion devices. A variable frequency drive controller first converts AC power to DC intermediate power. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit, arguably the most important section of a variable frequency drive. Next, the inverter switching circuit changes DC energy into three channels of AC energy so it can be used by an AC motor.

The standard method used to create variable motor voltage is pulse-width modulation, which takes place when inverter switches are used to create that quasi-sinusoidal output through what is called narrow-voltage pulses.

The Operation of a Variable Frequency Drive

When a variable frequency drive starts a motor it starts out at a low frequency and voltage, avoiding too high a current transfer that normally takes place when a standard motor is turned on. After the variable frequency drive has started and the motor’s voltage increases, it avoids pulling too much current. As a result, when a variable frequency drive is used, less power is required, ultimately reducing the chances of the motor overheating.

While the use of variable frequency drives are seemingly complicated, they will ultimately save power. Through this power savings, power cost savings can be discovered as well.