Variable Speed Drives Variable speed drives are used to vary the speed of AC and DC motors in order to control the output of driven equipment. DC variable speed drives are costly and offer very precise control. They are widely used in industry for precise speed control of conveyors and printing presses, but are not widely used in the HVAC industry. AC variable speed drives are less costly and offer good control for equipment, such as centrifugal compressors, fans and pumps. AC variable speed drives operate on the principle that the synchronous speed of an AC induction motor is directly proportional to the frequency of the AC power supplied to the motor.
Many drives are available with one or more drive status alarms.
Some are also available with digital communication interfaces that allow detailed status and fault monitoring by DDC control systems. Most drives use an AC to DC converter and a DC to AC inverter. The simple diode rectifier creates a constant DC voltage for input to the inverter. The addition of the DC chopper allows regulation of the voltage to the inverter. Silicon controlled rectifiers also allow regulation of the voltage to the inverter.