Compressors and Their Contactors

The compressor is the most expensive single component in a residential cooling system and the one most often blamed for problems it didn’t cause. A tech who condemns compressors based on symptoms alone — “it hums and doesn’t start, must be bad” — writes a lot of checks that didn’t need to be written. A tech who isolates electrical faults upstream of the compressor, proves the compressor itself bad through measurement, and only then recommends replacement builds a better reputation and fewer callbacks.

This chapter focuses on the electrical side of compressors — the windings, the starting components, the contactors that deliver power to them, and the diagnostic procedures that separate “compressor bad” from “something else making the compressor look bad.”

Compressor types

Scroll. Dominant in modern residential AC and heat pumps. Two interleaved scrolls, one fixed and one orbiting, compress refrigerant vapor between them. Electrical characteristics: typically a single-phase PSC (with run capacitor) or three-phase motor. Scroll compressors are relatively tolerant of occasional liquid slugging, start against suction pressure without much difficulty, and are very reliable.

Reciprocating. Traditional piston-cylinder compressor, still common in commercial and in older residential. Needs to equalize pressure between suction and discharge before restart — starting against high discharge pressure demands huge inrush current. Hard-start kits are often fitted to help.

Rotary. Common in mini-splits and some residential heat pumps. Rotating vane or roller geometry. Compact and efficient but less tolerant of liquid; rotary compressor burnouts from flooded starts are a recurring service category.

Starting components

A residential single-phase compressor typically has:

• Common (C) — shared terminal for both windings
• Run (R) — run-winding terminal
• Start (S) — start-winding terminal
• Run capacitor — connected between R and S, always in circuit
• Start capacitor (on some) — connected between R and S via a start relay; in circuit only during startup
• Start relay — potential relay, current relay, or PTC — disconnects the start capacitor once the compressor reaches running speed

On systems with just a run capacitor (most modern scrolls), startup torque is marginal but adequate for a healthy system. On older reciprocating compressors or systems with pressure-equalization issues, a start capacitor + start relay are added to boost starting torque.

Contactor problems masquerading as compressor problems

Before diagnosing the compressor, rule out the contactor. Common contactor failures:

Pitted/burned contacts. Resistance through the contacts creates voltage drop, compressor runs on reduced voltage, high amperage, overheating, eventual burnout. Symptoms: compressor hot, high amp draw, thermal overload cycling.

Chattering. Discussed in Chapter 37. Inadequate coil voltage causing rapid open-close. Contactor chatter can weld contacts within seconds on a compressor circuit because of the inrush currents involved.

Stuck closed (welded). Compressor runs continuously, even when the thermostat isn’t calling. Signals of stuck contactor: compressor running with no thermostat call, extremely high suction temperatures in off-cycle (compressor is working but nothing’s letting heat reject), short-cycling on high-pressure cutout.

Coil open. Contactor won’t pull in. 24V present at coil, no mechanical action. Measure coil resistance — OL = open.

Locked rotor

Locked rotor means the compressor is electrically energized but not rotating. The motor draws “Locked Rotor Amps” (LRA) — typically 4–6x running current — continuously, which quickly overheats windings and trips thermal overloads.

Causes of locked rotor:

• Mechanical seizure — compressor internals damaged. Typically terminal; time for replacement.
• Liquid slugging — refrigerant liquid has accumulated in the compressor shell, preventing compression. Can sometimes be recovered with proper startup procedure after equalizing pressures and heating the crankcase.
• Welded contactor — compressor has been running continuously, thermal cycling has caused issues.
• Pressure differential too high for start torque — common on reciprocating without a hard-start kit, especially after short-cycle.

Diagnostic: Measure amp draw during start attempt. Locked rotor shows sustained LRA (e.g., 60–80 amps on a residential compressor) without drop-off. Running current should drop to RLA (typically 10–20 amps) within seconds on a healthy start.

Hard-start kits

A hard-start kit adds a start capacitor + start relay to a system that originally had only a run capacitor, or upgrades the values to increase starting torque. Useful when:

• System has pressure equalization issues that leave high differential at start
• Compressor is aging and starting torque is marginal
• Low line voltage conditions reduce available start torque

A hard-start kit is not a compressor repair. It’s a helper that can extend life on a compressor with weak starting or correct for a system design issue. If a compressor won’t start without a hard-start kit, it’s probably reaching end-of-life, and the hard-start kit is buying time before replacement.

Compressor burnout

An “open burnout” means one or more windings have opened — usually from extreme heat causing the winding insulation to fail and the winding itself to part. Resistance measurement shows infinite where it should show a few ohms.

A “grounded burnout” means a winding has shorted to the compressor shell. Resistance between any terminal and ground reads low (often a few hundred ohms down to near zero in severe cases). On a grounded burnout, the refrigerant circuit is contaminated with burned winding insulation byproducts (acid and sludge) — the entire system requires acid cleanup, filter-drier installation (often suction-line filter also), and often an oil change on the new compressor. Skipping cleanup after a burnout means the new compressor will fail in months.

From the field

Callback on a 4-ton AC that had a compressor replaced three months earlier. New compressor was humming, not starting, overload tripping after 30 seconds. The previous tech had diagnosed and replaced the compressor but not investigated why the original had burned out.

Contactor contacts were severely pitted — arcing had burned deep pits that created high resistance through the contactor. Measured voltage drop across closed contacts: 18 volts. Meaning the compressor was receiving 222 VAC instead of 240 VAC, running high-amperage, overheating, thermal cycling, chronic stress. Replaced the contactor (a $25 part), startup cleaned up immediately, amp draw came into spec. The compressor replacement had been correct but incomplete — the root cause was in the contactor, and the original compressor died from chronic low-voltage stress induced by the failing contactor. Now the new one was on the same path until I caught it.

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Check your understanding

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01A compressor hums for a few seconds at startup, trips the thermal overload, and won't restart until it cools. It's been doing this for days. Before condemning the compressor, what should you verify?

AThe refrigerant chargeBThe contactor contacts (pitted contacts create voltage drop that reduces starting torque) and the run capacitor (failed cap prevents starting entirely)CThe air filterDThe thermostat setting

02Why is cleanup after a grounded compressor burnout not optional?

ACode requires it even when unnecessaryBAcid and sludge contaminating the refrigerant circuit will attack the new compressor's windings and internal components, causing another burnout within monthsCThe insurance company won't pay without itDIt's a convenience that shortens the job

03You install a hard-start kit on a compressor with 'weak starting.' It starts reliably for the next month. What's your honest interpretation?

AThe compressor is now fixedBThe hard-start kit is masking a compressor that's nearing end-of-life; plan for replacement within the next year or twoCThe hard-start kit is unnecessary — remove itDThe home needs larger electrical service

Compressors are expensive; diagnose them carefully. Rule out the supporting cast — contactors, capacitors, wiring — before you pronounce the compressor bad. And when a compressor really is gone, cleanup is not the place to economize.