and air conditioning
compressors comprise of an electric motor
and a compression
device inside a sealed housing. One way to vary the speed of an electric motor
is to make it operate from Direct Current
(DC) and then vary the operating voltage or use pulse modulation.
But that would be impractical for many reasons, the least of which would be the need to employ a commutator and brushes
, as a permanent magnet motor’s speed cannot be varied. This dictates that the motor
must be powered by Alternating Current
(AC), as is already common, but to somehow be able to vary the speed.
Standard AC powered compressors operate at a speed that is synchronous to the frequency, or Hertz (Hz), of the power supply. A typical motor running on 60Hz American power will spin at close to 3,600 RPM
, whereas the same motor operating on the same voltage but on 50 Hz European/rest-of-the-world power will run slower, at around 3,000 RPM
For a well-traveled vessel this presents a conundrum, because while this variation in speed from different power supplies may not be problematic for many types of application, i.e. fans and pumps, it can cause big problems in refrigeration and air conditioning
compressors, where lubrication, overheating
, and other issues can initiate compressor
failure. Single-phase air conditioning and refrigeration compressors are designed to operate safely only on either
50Hz or 60Hz power, but not both
. There is one way we can safely operate a standard compressor
within a limited range of speeds, and that is with a compressor built to operate from three-phase power. This is common practice with the large compressors used in chillers for super-yachts, where they can be safely operated between about 2,500 RPM and 4,000 RPM and ramp
up slowly from 0 RPM at start-up to eliminate starting surge.
It’s amazing to note that we have this same clever technology in the small variable-speed refrigeration compressors mentioned earlier.