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.
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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. |
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