How2Day: Types of Variable Frequency Drives

Electric motors frequently use variable frequency drives. Pamela Moore/Photodisc/Getty Images
Variable frequency drives (VFD) are devices which help to allow motors to change their speed faster and more efficiently. This allows motors to quickly and easily adjust their speeds to whatever task is at hand. Usually, they achieve this function by using semi-conductor switching devices. Most often, they are used to help alternating current (AC) induction motors, which are standard single-phase devices, to achieve multiple speeds of operation. Several different major types of variable frequency drives are in wide use.

Alternating current (AC) variable frequency drives are by far the largest category of VFDs. They are designed to control vehicles that run on alternating current. They are quickly becoming the sole type of variable frequency drives.

Direct current (DC) variable frequency drives control shunt wound DC motors. This design of DC motor has separate armature and shunt circuits.

Variable voltage input (VVI) variable frequency drives are the simplest types of variable frequency drives. In this type, the output switching devices create a new sine wave for the motor's voltage by inputting a series of square wave at different voltages. These drives usually work with the help of a large capacitor.

Current source input (CSI) variable frequency drives are very similar to VVIs. The difference between the two designs is that current source input VFDs manage to force a square wave of current as opposed to voltage. Current source input VFDs require a large inverter in order to keep the current as constant as possible.

Pulse width modulated (PWM) is the most complex type of VFD. While it is more complicated than the other types, it also allows the engine to run more efficiently. PWMs accomplish this through the use of transistors. The transistors switch the direct current at different frequencies and thereby deliver a series of voltage pulses to the motor. Each of these pulses is portioned out to react with the motor's reactance and create the proper current in the motor.

Flux vector pulse width modulated VFDs are a newer type of VFD. They use a type of control system usually more closely associated with DC motors. These drives have a microprocessor which is connected to the motor through a closed loop control. This allows the processor to more closely regulate how the motor operates.

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