How Does a VFD Work?

Introduction

A variable frequency drive (VFD) is an electrical variable speed drive that is added to motor-driven systems to help save energy. Motor-driven systems are designed to handle peak loads. When these operating systems work for extended periods of time at a reduced load, it wastes energy. Adding a VFD allows you to adjust the motor-speed capability and match it with motor-output load. This is how it saves energy. The motor-driven system is connected to the VFD and allows you to have speed control by changing the frequency of the motor-supply voltage.

Function

A motor-driven system is controlled by the frequency of the supply voltage and rotates on a fixed speed. An alternating current that is applied produces a magnetic field that rotates at synchronous speed. The only way to alternate synchronous speed is through a VFD, which converts the power in three stages. In the rectifier stage, the power is converted to a higher adjustable DC voltage. In the inverter stage, the power transistors in the rectified DC are switched off and on. This produces a voltage waveform at the frequency you want. In the control system, the voltage waveform receives information from the motor-driven system and will adjust the output voltage to the selected value.

Benefits

The benefit of a VFD is that you can control of the process temperature without using a separate controller. This will lower maintenance costs and lower the operating speed, allowing a longer life span for your motor. There is no need to use a soft starter anymore, and the controlled speed will eliminate any water-hammer problems. The ability of a VFD to control the torque in the motor-driven system protects equipment that can't tolerate excessive torque.