Compressor Support

Compressor Testing Guide

Please use this guide if you suspect a faulty compressor

To verify basic compressor function in a field environment the following will demonstrate whether further investigation will be necessary or not.


1 Earth Test Using a 1000v Megger check each field winding to earth. 1st check the earth connection is sound by an earth to earth test & repeat this following the motor test.
  If reading < than 2 MΩ Stator probably burnt out, but do not rule out a contaminated / damp terminal plate.
  Reading 2 MΩ - 20 MΩ Probably not burnt out, terminal plate contaminated or damp.
    Separate terminal plate from main casting & repeat check to casting. If fault clears then terminal plate is faulty.
  Reading > 20 MΩ No earth Fault
  Precautions Do not effect Megger checks under deep vacuum.
    Do not Megger overloads.
2 Field Balance  
  3ø motors only Using an Ohmmeter set on Ohms verify balance of motor field coils.
    If balance is out by more than 10 % suspect motor is defective.
    Separate terminal plate & repeat measurements, if readings do not improve rewind is probably required.
3 Pump Test This is the best check of compressor function reasonable or not.
  Whilst effecting these tests keep head pressure low 
    Close in the compressor suction valve tight, these tests keep & link out LP control switch.
    Attach a compound gauge with a coarse Vacuum scale and operate compressor. Allow machine to pump down to below 24” Hg.
    If at least 18” Hg not achieved suspect valves are faulty. Do not allow compressor to run in Vacuum for more than 1 minute.
  Motor Overload If 24” Hg achieved stop compressor & monitor vacuum rise rate. Normal criterion for a new / re-manufactured compressor is not more than 5” Hg rise / minute. A reasonable field machine should not rise more than 10”/ minute.
4 Voltage Balance Under Load The motor should receive a balanced voltage across all terminals.
    Maximum deviation must not exceed 1% from the mean voltage.
  If voltage does vary compare with levels at supply point. Any variation indicates a break down along the supply to the compressor, most likely points being blown fuses, or defective contactors.
5 Dry Fuse A fairly common fault easily overlooked is a dry fuse.
  When a fuse blows the wire within the fuse casing explodes throwing molten wire around the interior casing. If these particles form a conductive path, the fuse may seem OK, however when the motor attempts to run the affected phase will become effectively open circuit. The obvious consequence of this is a phase loss to the motor with the distinct possibility of motor burnout. 
  Always renew all three fuses whenever one fuse of a set is found blown.
6 Current Balance (Amps) This should be measured in conjunction with Test 4 - Voltage Balance Under Load.
  Running current is proportional to compressor load, this in turn is a function of the suction pressure (evaporating temperature) and discharge pressure (condensing temperature). System design, refrigerant type & compressor selection should take account of the maximum loading to be expected upon the compressor. The running current should not exceed the Nameplate Full Load Amps (FLA).
  Providing the current is within the FLA figure (ignore start up conditions) the most important measurement is the current balance between phases. This should not exceed 10% at any load condition. Any deviation greater than this amount suggests either a partial phase loss or an imbalance in the supply voltage (> 1%).
  Supply Rotation Supply imbalance can sometimes be reduced by “Rotating” the supply. That is moving the supply phases around one notch i.e. Red / Yellow / Blue becomes Blue / Red / Yellow or Yellow / Blue / Red.
  Important Note If rotating the supply upon a Part Wind Start motor care must be taken to move both 1⁄2 motor field groups the same.
7 Oil Pressure Because the oil in the compressor crankcase (sump) is subject to low side system pressure the actual oil feed pressure to the bearings is the difference in pump output pressure and the low side pressure, referred to as Net Pressure.
  Low net oil pressure is a symptom of many other defects, and rarely is the oil pump the culprit. 
  Is it Low Pressure or No Pressure? Desired pressure depends upon model, but most like at least 2 Bar / 30 psi net pressure. Carrier range function quite happily on less than 1 Bar, but this is really an exception. In general reckon on 2 - 3 Bar as good.
  Low Pressure Liquid return / diluted oil; Partially blocked pick up filter; Low oil level; worn bearings; Faulty Relief valve; Internal oil Leak; Oil too thin.
  No Pressure Fully blocked filter; Broken pump drive; Very low oil level; Dry pump; Extremely worn bearings.

if you require further advice on field testing of Compressors, please call ThermaCom for more assistance.