MPCB for Compressor Motors
How do you size and set an MPCB for a compressor motor? A motor protection circuit-breaker (MPCB) protects a compressor's motor branch with a thermal dial set to the motor's nameplate full-load current (FLC) and a fixed magnetic trip near 12-13x In per IEC 60947-4-1, but compressor duty changes how you apply that rule: reciprocating units can start against crankcase pressure and carry flywheel inertia, screw compressors usually start unloaded, and most compressors cycle on a pressure switch far more often than a continuously-run pump or fan. Get the trip class or the dial setting wrong and the breaker either nuisance-trips on a routine start or fails to open fast enough during a stalled or overloaded run. This article covers starting torque and inertia by compressor type, trip-class selection, thermal and magnetic dial setting, duty-cycle effects on thermal memory, and pairing an MPCB with a contactor for a compressor starter.
Compressor Starting Characteristics by Type
Reciprocating (piston) compressors are the hardest case. A belt-driven or direct-coupled piston unit with an unloading valve closed at start draws current for 2-4 seconds or more on larger frames, and the flywheel adds mechanical inertia on top of the electrical inrush. Most industrial reciprocating compressors ship with an unloader that vents the cylinder heads on start; with the unloader working, the start looks close to a pump's — under 2 seconds, moderate inrush. Without it, or with a stuck unloading valve, the motor starts loaded against head pressure and the current stays high for longer.
Rotary screw compressors present lower peak torque relative to their displacement, but the meshing rotor pair plus a belt sheave or gearbox still adds inertia beyond a bare motor. Above roughly 15-22 kW, many screw packages use a star-delta or soft-starter to limit inrush, so the MPCB or magnetic-only device downstream sees a reduced, longer current profile rather than a single direct-on-line spike.
Scroll compressors, common in small hermetic HVAC and refrigeration units, start and settle in under a second and load closer to a fan. Their nameplate locked rotor amps (LRA) can still read 5-8x rated current for that brief interval, so the magnetic trip setting still needs checking even though the thermal side rarely gives trouble.
What we see in the field: a panel builder copies a pump's MPCB setting onto a reciprocating compressor's starter, then spends a week chasing nuisance trips before checking whether the unit actually starts loaded or unloaded.
Trip Class Selection for Compressor Starting
Class 10 (trips within 10 s at 7.2x the dial setting, from cold, per IEC 60947-4-1) covers most scroll and unloaded-start screw compressors — their start finishes well inside that window. A reciprocating compressor starting loaded, or a belt-driven screw unit with a slow soft-start ramp, can run past 10 seconds before reaching rated speed; forcing that start through a Class 10 MPCB produces a trip mid-start. Trip classes 20 and 30 extend the same 7.2x threshold to roughly 20 s and 30 s, which is why compressor and other high-inertia specs sheets frequently call for Class 20.
Formula: Trip-Class Verification Against Compressor Start Time — Source: IEC 60947-4-1, Annex D
ttrip (at 7.2 x In, cold) ≥ tstart
| Symbol | Description | Unit |
|---|---|---|
| ttrip | Time the MPCB's thermal element takes to trip at 7.2x the dial setting, from cold, for the declared trip class | s |
| In | Dial (overload) setting, matched to the compressor motor's nameplate FLC | A |
| tstart | Measured or specified time for the compressor to reach running speed and, if fitted, unload | s |
Manufacturers publish ttrip for each class in their coordination and technical data; a compressor OEM's motor data sheet usually states tstart. When tstart is not documented, a clamp-meter trace of the first start on a comparable unit is more reliable than assuming from horsepower.
Setting the Thermal Dial and Magnetic Trip
The dial goes to the motor's nameplate FLC, not to a catalog horsepower-to-amps table. Compressor motors frequently run near or slightly above nameplate on a hot day or at high discharge pressure, so a dial set from a generic table under-protects the exact unit in front of you. Set it wrong and it nuisance-trips, or worse, rides through a real overload.
The magnetic (short-circuit) trip is fixed, roughly 12-13x In on a standard thermal-magnetic MPCB, and is not field-adjustable on most models — it is sized at the factory to clear an inrush without responding to it. On larger screw compressor starters built with a soft starter or VSD ahead of the motor, a magnetic-only MPCB is common: it provides short-circuit protection and manual isolation only, while the soft starter or drive handles overload electronically and a separate thermal overload relay may still sit downstream depending on the drive's protection class.
Duty Cycle and Frequent Cycling
A receiver-tank compressor without a VSD cycles on a pressure switch — load, unload, stop, repeat — far more often per shift than a continuously-run pump or fan. The MPCB's bimetal element retains heat between starts. Two or three restarts within the trip class's specified cool-down window can trip the breaker on the second or third attempt even though each individual start, tested cold, was within limits. This is not a fault; it is the thermal element doing its job across a duty cycle the class curve was not built to smooth over.
This depends on the compressor's actual cycling frequency, which varies with tank size, air demand, and any minimum-run timer built into the control panel — a compressor with a 10-minute minimum-run timer rarely trips on thermal memory, one with none and a small tank can trip several times a shift.
MPCB Plus Contactor: Building the Compressor Starter
The common build pairs a thermal-magnetic MPCB with a contactor switched by the pressure switch or a PLC output — see MPCB plus contactor motor starter for the wiring pattern. Size the pair to a published Type 2 coordination table for the available fault current at the panel; Type 2 means the contactor survives a short circuit with, at most, light contact welding that separates easily, and the starter is back in service without parts replacement. For belt-driven screw compressors started star-delta, the same coordination logic applies across three contactors instead of one — see star-delta motor starting for the switching sequence.
Larger compressor packages with a soft starter or VSD typically place the magnetic-only MPCB ahead of the drive for isolation and short-circuit clearing only, since the drive itself limits starting current electronically and reports its own overload protection. Sizing that MPCB uses the drive's rated input current, not the motor's FLC directly. For guidance on matching the dial and class to the actual motor once you have confirmed loaded vs. unloaded starting, see how to select and set an MPCB for a motor.
Common Field Issues on Compressor MPCBs
Nuisance trip on every cold-weather start, no trip in summer: usually a loaded start, either because the unloading valve is stuck closed or the compressor has no unloader at all — check the valve before raising the trip class.
Trip only on the first start of the day: thermal memory is unlikely here since the element has had all night to cool; look instead at low ambient raising oil viscosity and start torque, or a check valve letting the receiver tank bleed down overnight so the compressor starts against near-zero pressure but higher friction.
MPCB holds during light load but trips as the plant load builds: the dial may be set below the motor's actual running current at full head pressure — verify with a clamp meter at peak demand, not at idle.
Breaker won't reset after a trip: on some models this is thermal memory, not a fault — wait for the rated cool-down time before forcing a second attempt, since forcing it early risks nuisance behavior on the next real start too.
Frequently Asked Questions
What trip class should I use for a reciprocating compressor?
Start with the compressor OEM's specified class if published. Absent that, check whether the unit starts loaded or unloaded: unloaded starts under 10 s usually work on Class 10, while loaded starts or units without an unloading valve often need Class 20.
Can I use a magnetic-only MPCB on a compressor motor?
Yes, when a soft starter, VSD, or separate thermal overload relay handles overload protection downstream. A magnetic-only MPCB provides short-circuit protection and manual isolation only — it will not protect the motor against overload on its own.
Why does my compressor MPCB trip on a hot start but not a cold one?
The thermal element is warmer to begin with after a recent run or several quick cycles, so it reaches its trip threshold sooner on the next start. This is thermal memory, a built-in feature of the bimetal element, not a wiring or calibration fault.
Should the MPCB dial match the compressor's rated horsepower or its nameplate FLC?
Always the nameplate full-load current in amps, read from the specific motor on the specific compressor. A generic horsepower-to-amps table can be off enough to under- or over-protect the actual unit, especially on motors that run above nameplate at high discharge pressure.
How does an unloading valve change MPCB sizing?
An unloading valve vents cylinder or rotor pressure at start, cutting starting torque and shortening start time close to an unloaded motor's — often enough to keep the MPCB on a standard Class 10 rather than needing Class 20 or 30 for a loaded start.
Conclusion
Compressor motors are not a generic MPCB application: starting torque and inertia vary sharply between reciprocating, screw, and scroll designs, and the same nameplate current can call for a different trip class depending on whether the compressor starts loaded. Set the dial to nameplate FLC, verify the trip class against measured or specified start time using the formula above, and account for duty-cycle thermal memory on tank-cycled units before assuming a nuisance trip is a wiring fault. For the broader selection and setting process across motor types, see the MPCB engineering guide, and browse manual motor starters for compressor-rated frame sizes.
