Phase Converters & Power Factor
Phase Converter Efficiency
Installing a Phase Converter
Rotary Phase Converters
Static Phase Converters
VFDs as Phase Converters
     • Harmonic Distortion
Three-Phase Motors
Phase Converters & Voltage Balance
Phase Converter Applications
     • Submersible Pumps
     • Woodworking Equipment
     • Dual Lift Stations
     • Phase Converters & Welders
     • Phase Converters & CNC Machines
     • Phase Converters & Air Compressors
     • Phase Converters & Elevators
     • Phase Converters & Wire EDM
     Phase Converters & HVAC
Phase Converters & Transformers
     • Step-up Transformers
     • Buck-Boost Transformers
     • Isolation Transformers
Phase Converter Experts
Digital Phase Converters
Regenerative Power
Three-Phase Power
     • Delta vs. Wye Configured Power
Motor Starting Currents

Phase Converters and Air Compressors


An air compressor is found in nearly every shop.  If large volumes of air are required, a three-phase compressor is a must because of the availability of large three-phase motors along with their efficiency and reliability.

There are two main types of air compressor, the reciprocating type, or the more modern rotary screw compressors.

Reciprocating compressors are one of the hardest starting three-phase loads to be found.

The motor must overcome the inertia of the large pulley on the compressor and the piston usually loads the pump before the motor has come to full speed. All this demands large starting current for a relatively long period of time before the motor is running at full speed. After starting, the motor may also be operating at full load, requiring good voltage balance to prevent damage to the motor.

Rotary screw compressors are usually used in more modern three-phase compressors. These compressors usually have electronic controls that start the compressor under very lightly loaded conditions, avoiding the large starting currents. The controls monitor power quality and control the load on the pump. A converter that cannot deliver balanced, sine wave voltage will not operate a rotary screw compressor, therefore static phase converters and VFDs should never be used on these compressors.

A static converter is not a good solution for reciprocating compressors because of its poor voltage balance.

A phase converting VFD can operate a reciprocating compressor because it ramps up the speed of the motor, reducing the starting currents. It must however, deliver enough torque at low speed to start the motor under load, which will require significant oversizing. Most compressors are automatically controlled by the pressure in the tank. Since a VFD cannot act as a power supply, the VFD must be wired directly to the motor leads, bypassing the factory controls of the compressor. The control must be rewired and be capable of starting and stopping the motor through the VFD.

A rotary converter for reciprocating compressors will have to be much larger than the compressor motor in order to generate enough starting current. This will compound the inherent inefficiency of rotary converters. This inefficiency is a real problem because the converter will probably have to be running

continuously because the compressor is automatic on/off. It may or may not have good enough voltage balance to operate the motor safely under load. For operating rotary screw compressors, starting currents are not an issue, but voltage balance is. Most of these compressors monitor power quality and will not run under unbalanced conditions. The rotary screw compressor is also likely to have a wide load range, creating more voltage balance problems for rotary phase converters.

Digital phase converters can operate reciprocating and rotary screw compressors safely and efficiently. They deliver large momentary current for hard starting loads and always supply perfectly balanced voltage under all load conditions. The sine wave output voltage allows digital phase converters to safely power the control electronics of rotary screw compressors. When used in the power supply mode, the converter can be continuously powered with very little power consumption when the compressor is not running and operate at 95-98% efficiency when powering the compressor at full load.