Overload Relays for Compressor Motors

What overload relay does a compressor motor need? A compressor motor overload relay is a thermal or electronic overload relay (IEC 60947-4-1) sized to the compressor's nameplate FLC and selected for the run-up profile the compressor imposes when it starts against residual head pressure rather than a Class 10 fan or pump load. Undersize the trip class or skip phase-loss sensitivity and a reciprocating or hermetic compressor motor burns a winding on the very fault the relay exists to catch — single-phasing on a sealed unit rarely announces itself until the motor is scrap. This article covers reciprocating vs screw/scroll starting current, run-up time and trip-class selection, phase-loss risk on hermetic units, thermal memory for start-stop cycling duty, and coordination with the upstream contactor and SCPD.

What Loads Does a Compressor Motor Present to an Overload Relay?

A reciprocating compressor's crankshaft produces a torque pulse on every piston stroke, so the motor current carries a ripple on top of the average draw. The overload relay measures average current through its sensing element and does not resolve stroke-by-stroke ripple, so this pulsation rarely changes the dial setting — it is still the motor nameplate FLC. What it does change is the starting profile, covered below.

Screw and scroll compressors present a smoother torque curve close to a standard centrifugal load, because the working fluid is compressed continuously rather than stroke by stroke. A scroll compressor's setting and trip class selection tracks a typical fan or pump motor more closely than a reciprocating unit does, and both draw from the same range of thermal overload relays sized to nameplate FLC.

Starting Current and Run-Up Time on Compressor Motors

A compressor without an unloading valve starts against full suction and discharge pressure. The motor accelerates a load already near full torque from zero speed, which stretches run-up time and starting current duration compared with a fan or pump motor that ramps up against a load also near zero at standstill. A relay set for a Class 10 fan can read this run-up as an overload and trip on every start — pick contactors and overload relays rated for the compressor's actual starting duty, not just its horsepower.

An unloading valve — a bypass that vents discharge pressure back to suction, or a solenoid that holds the suction valve open for the first seconds of a run — cuts the compression torque the motor must overcome at zero speed. Where the compressor package includes one, the motor accelerates closer to a Class 10 load and standard trip class selection applies.

What we see in the field: packaged reciprocating units without unloading, especially older or lower-cost compressor skids, nuisance-trip on a hot restart because the discharge line has not fully depressurized from the prior cycle. The relay is doing its job; the fix is a longer anti-short-cycle delay or a proper unloader, not a bigger dial setting.

Selecting the Trip Class for a Compressor Motor

Trip class picks how long the relay tolerates elevated current during a normal start before it calls the condition a fault. IEC 60947-4-1 defines the class by trip time at 7.2x the current setting from cold: Class 10A trips in 2-10 s, Class 10 in 4-10 s, Class 20 in 6-20 s, Class 30 in 9-30 s. A scroll or unloaded reciprocating compressor with a short, low-torque start typically clears on Class 10 or 10A, the same class most pump and fan applications use — see overload relay trip classes 10A, 10, 20 and 30 for the full definitions. A reciprocating compressor starting against full head, or a large screw compressor with a long soft-start ramp, needs Class 20 or 30 so the run-up current stays under the trip curve instead of tripping the relay before the motor reaches speed.

Set the dial to nameplate FLC regardless of trip class — class changes how long an elevated current is tolerated before tripping, not what current the relay treats as normal running. For the general procedure, see how to select and set an overload relay for a motor, then confirm the run-up time against the manufacturer's trip curve for the selected class at the expected starting current multiple.

Phase-Loss and Single-Phasing Risk on Compressor Motors

A lost phase forces roughly 1.7x current onto the two remaining windings. On an open, belt-driven compressor motor this is a serviceable fault; on a hermetic or semi-hermetic compressor, where the windings sit sealed inside the refrigerant circuit, single-phasing burns the motor before anyone notices a problem, because there is no external smell or visible smoke to prompt a shutdown. Refrigeration and air-conditioning compressors are disproportionately represented in overload relay warranty claims tied to phase loss for exactly this reason.

Phase-loss sensitive overload relays carry a differential trip mechanism that reacts to the current difference between phases, not only the absolute level, and trip faster than the plain thermal element would on a symmetric overload — the mechanism is covered in phase-loss and single-phasing protection. Specify this feature on any compressor overload relay, hermetic or open; the relay's datasheet will state phase-loss sensitivity as a selectable or standard feature.

Thermal Memory and Short-Cycling Duty

A refrigeration or air-conditioning compressor cycles on and off with the thermostat or pressure control, sometimes many times per hour under light load. A bimetal strip cools passively between starts and largely forgets the heat from the previous run; an electronic overload relay with thermal memory keeps a running model of winding temperature and carries heat forward from one start to the next, closer to how the motor windings actually behave.

This matters most where an anti-short-cycle timer is undersized or missing: a compressor that restarts before the motor has cooled sees a hotter starting point than the nameplate duty cycle assumed. Electronic relays account for that residual heat in the next trip calculation; a bimetal relay, reset and cooled, does not know the motor was hot ten seconds earlier. This depends on how tight the cycling interval is — a compressor with a generous anti-short-cycle delay may never stress a bimetal relay's blind spot in practice.

Coordination with the Contactor and SCPD

A compressor starter is the same three-device stack as any motor circuit: an SCPD clears short circuits, the contactor switches the motor, and the overload relay carries the running current and trips the contactor coil through its 95-96 NC contact on a sustained overload. On a compressor circuit the SCPD is typically a fuse or a motor protection circuit breaker. The manufacturer's coordination table, tested to IEC 60947-4-1, states whether that combination is Type 1 (starter may need repair after a fault but does not endanger persons) or Type 2 (no damage beyond light, separable contact welding).

Compressor packages with high inrush and long run-up push harder on this coordination than a typical pump or fan starter, because the SCPD and contactor must ride through the longer starting transient without nuisance-tripping on the short-circuit side while the overload relay still protects the winding on the thermal side. Follow the published coordination table for the specific SCPD, contactor and overload relay combination rather than assuming same-brand components coordinate by default — see Type 1 vs Type 2 coordination for how to read that table.

Conclusion

A compressor motor overload relay starts from the same nameplate FLC and IEC 60947-4-1 trip-class table as any other motor overload relay — what changes is which class fits, driven by whether the compressor starts loaded or unloaded, how often it cycles, and whether the motor is hermetic. Get the trip class and phase-loss sensitivity right and the relay rides through the compressor's starting transient without nuisance tripping, while still catching the single-phasing fault that would otherwise scrap a sealed motor. For the general sizing and setting procedure this article builds on, see the thermal overload relay engineering guide.