COMPRESSOR SPEED

  Open type compressor operating from a truck engine by means of a power take-off or by a belt driver are subject to extreme speed ranges. A typical truck engine may idle at 500 RPM to 700 RPM, run at 1,800 RPM at 30 MPH, and run at 3,600 RPM over the highway at high speed. Whatever the power take-off or belt ratio, this means the compressor must operate through a speed ratio range of 6 to 1 or greater unless it is disconnected from the power source by some means.

  The compressor speed must be kept within safe limit to avoid loss of lubrication and physical damage. Operation within the physical limitations of the compressor may be possible for example from 400 RPM to 2,400 RPM. It may be possible to use a cut-out switch ta disconnect the compressor from the power source at a given speed. The compressor manufacturer should be contacted for minimum and maximum speeds of specific compressors.

  If the compressor is of  the accessible-hermetic type, there is no problem concerning speed so long as the electrical source is operating at the voltage and frequency for which the motor was designed. If the speed of the generator is varied in order to obtain variable speed operation, the voltage and frequency on the normal alternating current generator will vary proportionally. Since the compressor speed and motor load will vary directly with the frequency, it is often possible to operate over a wide speed range with satisfactory results.

  However, it should be born in mind that increasing the frequency and voltage of the generator above the level for which the compressor motor was designed will increase the load on the compressor, may overload the motor, and can result in bearing or other compressor damage. Operation at speeds too low to provide adequate compressor lubrication must also be avoided, although normally lubrication can be maintained on Copelametic compressors down to 600 RPM and possibly lower speeds.

  Each new application involving operation of the compressor at a voltage and frequency differing from its nameplate rating should be submitted to the Copeland Application Engineering Department for approval.

  One other problem that may arise with operation from a variable speed generator is the operation of electrical contactors, relays, etc. on voltages below or above their nameplate rating. Field tests have shown that the winding design and physical construction of electrical components can cause wide variation in voltage tolerance. The drop-out voltage of various types of commercially available 220 volt contactors may vary from 145 volts to 180 volts depending on construction. If it is planned to operate at variable voltage and frequencies, the electrical  components which are to be used should be extensively tested at the electrical extremes in cooperation with the manufacture to insure proper operation