Variable Speed Drives in Air-conditioning Systems (VSD)

What is Variable Speed Drives (VSD)?

Induction motors that typically used for compressors and fans in air conditioning systems are sized to handle maximum load under worst case conditions and then leaving them to run at full power. They are basically fixed-speed motors. Their speed is determined by the constant frequency of the power supply (typically 50 or 60 Hz).

Variable speed drives (VSD) or frequency inverters are solid-state devices and save energy whenever electric motors run at less than full power. VSD is actually a frequency converter in which 50Hz or 60Hz ac input voltage is first rectified into dc which is then converted back into variable-frequency ac voltage. It must be noted that the power demand of motor varies with the cube of the motor speed, i.e. power is proportional to (speed)3.

This means that a reduction of speed by 20% will result in reduction of power consumption by nearly a half, i.e. 50% saving. Since most HVAC equipment seldom runs at full power, significant energy savings can be made with these variable speed drives.

Motor Energy Saving with VSD

Power is deduced by 50% at 80% speed

Variable speed drives in air-conditioning systems

The motors on chillers, pumps, cooling towers and fans account for a significant portion of the energy consumption in HVAC system. The use of retrofit variable speed drives (VSD) is one of the most effective technologies applied in recent years. The VSD can be added on to conventional equipment or can be part of the factory-supplied equipment.

VSD can be added on chiller water pump, chiller motor, condenser water pump, cooling tower fan, air handler motor

Energy Efficient HVAC Equipment & Systems

Application example – VSD installations at the EMSD Headquarters