Variable frequency drive technology (VFD) has been developed to allow speed and torque control of an AC motor. This is achieved by connecting a variable frequency controller to an AC motor. Three phase induction motors are the most common though it is possible to connect it a single phase unit. In simple terms the controller converts AC to DC and then back to 'quasi-sinusoidal' AC power. This allows both the speed and torque of the motor to be controlled by varying the voltage and frequency.
Initially, the variable frequency drive was developed to give control of speed in various industrial processes, and to avoid the power surge associated with starting a single speed AC motor. However the variable frequency drive has found another purpose in compressor systems used throughout industry to provide a high pressure air supply. It has been calculated that 10% to 12% of power produced in the UK goes on creating compressed air. An air compressor that utilises variable frequency drive technology is able to switch speeds automatically, dependent on demand. This amounts to a real saving in compressor life as well as energy costs. With air compressors demanding such a significant amount of energy, it is easy to see why any potential reduction is of value to industry. However, this is not always the case.
The main advantage of utilising a variable frequency drive is in the name It ability to vary frequency and voltage, therefore speed. However, the drive unit is an expensive addition to a system and where, for example, the compressor is working at a high percentage of its capacity on a more or less permanent basis, there is little gained by the expense of having a variable speed capability. The cost of purchase and installation needs to be weighed against potential energy savings.
The concept and application of the variable frequency drive has found uses through industry in machine processes' allowing smooth start up sequences, variable speed control for different processes on the same equipment, control over production rate and generally, a more accurate degree of control over a myriad of industrial processes. With the high levels of energy required to power an air compressor, it could be argued that this singular process has most benefitted from the development of modern variable frequency drive systems; bringing both control and efficiency to industry.