Diagnosing Commercial Compressor Failures

A compressor rarely dies of old age. Something in the system kills it, and if you do not find that something, the new compressor dies too. Your job on a failed compressor has two parts. First, prove it really failed, because good compressors get condemned every week. Then read the evidence and name the cause. This is lead-level work on high-value equipment, and it involves live electrical testing that only qualified technicians should do.

What You Will Be Able to Do

• Rule out power, control and safety faults before you condemn a compressor.
• Test windings for opens, shorts and grounds, and use a megohm meter correctly.
• Tell weak valves from an electrical fault.
• Match the evidence inside a failed compressor to its cause.
• Name the system repair needed before a replacement starts.

Prove It Failed First

A compressor that will not run is not always a bad compressor. Work outward from the power source.

1. Check voltage at the disconnect and at the contactor, under load if the unit will start.
2. Check the contactor points, and look for burned or pitted contacts.
3. Check for a tripped control, such as a high-pressure, low-pressure or oil safety.
4. On a single-phase unit, test the start and run capacitors and any start relay.
5. Feel the shell, carefully. A hot shell with an open winding reading usually means the internal overload is open. It may take hours to cool and reset.
6. On three-phase power, measure voltage imbalance.

Worked example, three-phase imbalance: the legs read 460, 455 and 445 volts. The average is 453.3. The largest difference from the average is 8.3 volts. Imbalance = 8.3 divided by 453.3, x 100, or about 1.8%. Many compressor makers set the limit near 2%, and a small voltage imbalance causes a much larger current imbalance and extra motor heat.

Warning: Lock out power and verify it is dead before you touch the terminals. Keep your face and hands away from the terminal box when anyone energizes a compressor that may have a short or ground. A failed terminal can blow out and release hot oil and refrigerant.

Electrical Tests

With power locked out and the leads removed, measure the windings.

On a single-phase compressor, the reading from run to start should equal the two other readings added together. On a three-phase motor, all three should match closely.

A regular ohmmeter misses early insulation damage. A megohm meter applies a high test voltage to find it. Use the voltage the compressor maker gives, on a system at normal pressure with the leads removed. Do not test a compressor that is under a vacuum. Thin gas around the windings insulates poorly, so the reading is false and the test voltage can arc.

Pass and fail values vary widely by maker, from about 1 megohm to 100 megohms or more. Use the maker's number. The most useful thing is the trend: a reading that falls every year is telling you something, even while it still passes. Liquid refrigerant in the shell also pulls a reading down, so a low reading on a cold, idle compressor may rise after warm-up.

Mechanical Tests

If the motor is sound, ask whether the compressor still pumps.

Running signature. Worn valves let gas leak back. Suction runs high, head runs low, and the two pressures sit abnormally close together. Amps are low, and the box does not pull down. A compressor that behaves this way with a good charge has a pumping fault.

Fast equalization. When the unit shuts off, a sudden rush of high side into low side points to leaking discharge valves.

Pump-down test on a semi-hermetic. With gauges on, front-seat the suction service valve, and let the compressor pull down its own crankcase. A healthy reciprocating compressor pulls down quickly and holds after it stops. A slow pull-down, or a pressure that climbs right back, means leaking valves or gaskets. Follow the maker's procedure and limits.

Warning: Do not run a scroll compressor into a vacuum, and never use a compressor to evacuate a system. A deep vacuum can damage the scroll and can cause a motor or terminal failure. Use the scroll maker's test instead.

Reverse rotation. A three-phase scroll wired backward runs loud, draws low amps and builds no pressure difference. Reversing two power leads fixes it, if you catch it fast. Check rotation at every start-up, and after any utility or panel work.

The Five Mechanical Killers

Mechanical failures mostly come from five causes. The damage pattern shows which one happened.

Loss of oil and floodback look alike at a glance. Loss of oil damages all bearing surfaces evenly. Liquid damage tends to be uneven and may leave plenty of thin, diluted oil behind.

Oil Pressure and Oil Return

A compressor with an oil pump needs net oil pressure, which is pump outlet pressure minus crankcase pressure.

Worked example: the pump reads 78 psig and the crankcase reads 45 psig. Net oil pressure is 33 psi. A gauge reading of 78 psig alone tells you nothing, because crankcase pressure moves with the load. Compare the net figure with the maker's minimum.

When the oil safety trips, do not just reset it. Common causes are:

• Low oil level, because oil is stuck in the system.
• Oil diluted with liquid refrigerant, which foams in the sight glass.
• A worn pump or plugged oil screen. Gray oil with bearing metal in the screen points to wear.
• A failed oil control or sensor.

Oil leaves the compressor all the time, so the system must carry it back. Traps in the wrong place, level suction lines, oversized risers and long low-load periods can leave oil lying in the piping or the coil. A system that keeps needing oil usually has a piping or velocity problem, and adding oil only hides it.

From Evidence to Root Cause

Replacing the compressor is the easy part. Each failure points to system work that must happen first.

Save the evidence. Keep the failed compressor sealed if warranty or a teardown is possible, take a sample of the oil, and record every reading you took before removal. Note the model and serial numbers, and the refrigerant, oil type and run hours if you know them.

Before starting a replacement, energize the crankcase heater for the time the maker requires, check the oil level, and set safeties. After start-up, record total superheat, discharge temperature, amps and voltage so the next person has a baseline.

Worked Diagnosis

A low-temperature walk-in freezer has lost its third semi-hermetic compressor in two years. The compressor runs but trips on oil pressure, and now it is seized. You find:

• All bearings evenly scored, and very little oil in the crankcase.
• Discharge temperature logged at normal levels before the failure.
• A 25-foot suction riser with no trap at the bottom, and a horizontal run that slopes back toward the coil.
• A defrost heater that failed months ago on one coil, so ice has been cutting load.

Even bearing wear and a dry crankcase mean loss of oil, not floodback. The piping cannot carry oil back at low load, and the iced coil reduced flow even further, so gas velocity dropped. Replacing the compressor alone guarantees a fourth failure. The real work is the suction piping, the defrost heater and a check of the oil level after the system stabilizes.

Common Mistakes

Check Your Understanding

A compressor reads open from common to run and start, and the shell is too hot to touch. What now? Let it cool, then retest. The internal overload is probably open. Find out why it overheated.

The oil pump reads 60 psig and the crankcase reads 40 psig. What is the net oil pressure? 20 psi. Compare that with the maker's minimum.

Suction and head pressures sit close together, amps are low, and the charge is correct. What does that point to? Failed compressor valves, with gas leaking back inside.

Teardown shows broken reeds and a broken rod. What should you look at in the system? Slugging, so check the TXV, oil traps and anything that could send liquid or oil to the compressor.

A new three-phase scroll is loud, amps are low, and there is no pressure difference. What is the cause? Reverse rotation. Lock out power and swap two leads.

Key Takeaways

• Prove the failure, because power, controls and a hot overload can all mimic a dead compressor.
• Never megohm test a compressor in a vacuum, and never pull a scroll into a vacuum.
• Close suction and head pressures with low amps point to failed valves.
• Floodback, flooded starts, slugging, overheating and loss of oil each leave their own evidence.
• Net oil pressure is pump pressure minus crankcase pressure.
• Fix the cause before the new compressor starts.