What are Variable speed(frequency) Drives

Definition
An electronic device that controls motor speed by varying the frequency of the electrical supply.

Description
Most heating, cooling and ventilation systems operate at peak capacity even though demands are not at peak levels all of the time. For example, the amount of heating or cooling energy required in commercial buildings varies throughout the year. The conventional response to changing demand for heating and cooling is to restrict flow to individual rooms, even while (peak) flow is maintained in the central system. This approach requires high motor energy and reduces the lifetime of the equipment. A better approach is to use a variable-speed drive on HVAC motors; these drives reduce air or water flow to meet changing loads.

Variable-speed drives vary the frequency of AC electricity in response to an electrical signal. When coupled to a fan or pump motor, the change in frequency will result in a corresponding change in motor speed. Since the power required to drive centrifugal fans or pumps is proportional to the cube of the fan or pump speed, large reductions in electricity are achieved when fans or pumps operate at reduced speeds.

Variable-speed drives are most commonly applied to supply and return fans for variable air volume systems, circulating pumps in hydronic systems and, domestic water booster pumps in highrise buildings. In most cases, the motors are controlled to maintain a constant pressure within air ducts or water pipes. A pressure sensor in the pipe or duct sends a signal to the building automation system which in turn sends an electronic signal to the drive. Thus, as valves and dampers close, the pressure rises which in turn causes the flow to be reduced.

Benefits reduces electricity consumption eliminates the need for a separate motor starter improves fan or pump control extends equipment life

Limitations not cost-effective for small (1 hp) motors can cause send unwanted signals back up the incoming utility line

Cost
The extra cost for the variable-speed drive and the control point in the building automation system must be justified by the savings in electricity costs. In addition, the use of a variable-speed drive saves the cost of a separate motor starter.