What is Variable Frequency Drive?

The basic function of a variable frequency drive is to control the flow of energy from the power network to the process. Energy is supplied to the process through the motor shaft. Here, we review variable frequency drive operation and consider the available alternatives.

To control the flow of energy, the torque on the shaft, and its speed, need to be controlled. Normally only one of these parameters is controlled to a particular set point while the mechanical load determines the other. If the speed of the motor shaft is controlled by the variable frequency drive, the VFD is said to be in speed control while the load determines the torque on the shaft. Similarly if the torque on the motor shaft is controlled, the VFD is said to be in torque control with the load determining the speed.

In the past, variable frequency drives were largely either mechanical, such as hydraulic couplings or variable ratio gearboxes based on chains or belts, or were electrically based such as a dc drive system consisting of a dc converter and associated dc motor. The dc drive systems were the VFD of choice in areas classified as non-hazardous as they could achieve the required control accuracy of speed or torque without the use of complex electronics.

The development of the ac VFD has been stimulated to achieve the same or better quality of control as delivered by the dc drive system but using the rugged, inexpensive and virtually maintenance free squirrel cage motor. The dc drive system has always been the performance benchmark for the ac VFDs system by defining torque response and speed control accuracy.

The constituents of a VFD system
The industrial environment that the variable frequency drive system operates within includes:

• The load that is coupled to the motor.
• The power network that provides the energy required for the process.
• The control system that provides the overall process control.
• The physical environment in which the variable speed is located.

The VFD system comprises all the components that are found between the source of energy – the electrical network – and the load or working machine coupled to the motor.

These components are the energy controller, the motor and the mechanical transmission. With each of these components variable speed control is possible. If the energy control is a frequency converter,then the speed of the load can be varied by changing the frequency.If the motor is a two speed unit, achieved by using a two stator winding configuration, then two different load speeds are possible.If the mechanical transmission includes a multi ratio gearbox or hydraulic coupling, variable speed control of the load is possible.

Types of load successfully controlled by variable frequency drives
The various loads found in industry that the motor could be coupled to – directly or indirectly – through a gearbox include:

• Centrifugal fans, blowers, compressors.
• Centrifugal pumps, positive displacement pumps.
• Hoists, cranes, roller conveyors, belt conveyors, elevators,excavators.
• Mixers, stirrers, centrifuges, extruders, autoclaves, crushers,grinding mills.
• Cutting machine tools, forming machine tools, rolling mills,printing, textile, wood, paper machines.
• Industrial robots.
• Wind turbines and gas turbines.

The bulk of applications, both from a volume and a value basis, comprises the pump and fan variant.