A variable-frequency drive (VFD) or variable speed drive (VSD), AC drive, microdrive, or inverter drive is a type of adjustable-speed drive used in electro-mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage. VFDs are used in applications ranging from small appliances to large compressors. About 25% of the world’s electrical energy is consumed by electric motors in industrial applications, VFDs can make these motors more efficient. Over the last four decades, power electronics and technology has reduced VFD cost and size and has improved performance through advances in semiconductor switching devices, drive topologies, simulation and control techniques, and control hardware and software.
It is important to consult a qualified electrician when considering employing a VFD due to their wide variety of characteristics and the fine tuning needed to get the most out of their use. Compared to conventional motor drives, VFD has greater functionality and operation capabilities with the potential to lessen energy use and reduce maintenance in the long term. In addition to adjustable speed control, variable frequency drives offer protections like phase, under, and over-voltage protection; while, software and interfacing options of the VFD’s allow the user to control the motors at desired levels.
A VFD contains the following;
- Rectifier – It converts AC power feed from mains to DC power. This section can be unidirectional or bidirectional based on the application used like the four-quadrant operation of the motor. It utilizes diodes, SCR’s, transistors, and other electronic switching devices.
- DC bus – DC power from the rectifier section is fed to DC link. The main function of the DC link is to store DC power. This section contains capacitors and inductors.
- Inverter – This section comprises of electronic switches like transistors, thyristors, IGBT, etc. It receives DC power from DC link and converts into AC, which is delivered to the motor. It uses modulation techniques like pulse width modulation to vary output frequency for controlling the speed of induction motor.
- Control circuit – It consists of a microprocessor unit and performs various functions like controlling, configuring drive settings, fault conditions, and interfacing communication protocols. It receives a feedback signal from the motor as current speed reference and accordingly regulates the ratio of voltage to frequency to control motor speed.
The use of the VFD can benefit many types of machinery increasing energy savings and reducing wear and tear. Here are few common benefits of VFD’s:
- Controlling start up current
- Reduced power line disturbances
- Controlled accelerations
- Adjustable operating speeds
- Adjustable torque limit
- Controlled stopping