A compressor that tripped the motor breaker on every startup. The compressor was a reciprocating unit, 22 kW, with a 30 kW motor. The motor nameplate said 30 kW, 400V, 1470 RPM. The breaker was sized per the motor FLA. On every startup, the motor drew locked-rotor current for 8 seconds and tripped. The maintenance team thought the breaker was undersized. The breaker was correct. The motor was undersized for the compressor’s startup torque.
The torque the motor delivers vs the torque the compressor needs
A motor’s torque curve is not flat. At start, it delivers locked-rotor torque (about 150-200% of full-load torque for a standard squirrel-cage motor). As it accelerates, the torque dips (pull-up torque, about 120-150%), then rises to breakdown torque (200-250%) near 80% speed, then falls to full-load torque at rated speed.
The compressor’s torque requirement is the opposite. A reciprocating compressor has a high breakaway torque (the gas in the cylinders resists rotation). As the motor accelerates and the compressor unloads, the torque requirement drops, then rises again as the compressor loads up. If the motor’s torque curve dips below the compressor’s required torque at any speed, the motor stalls — it can’t accelerate. It sits at that speed, drawing locked-rotor current, until the thermal protection trips.
This compressor was tripping at about 70% speed. The pull-up torque of the motor was below the compressor’s torque requirement at that speed. The motor was on the edge of stalling every startup. It eventually got past it, but only after 8 seconds of near-stall current. The breaker (set to the motor’s locked-rotor curve) should have allowed 10 seconds. The motor drew enough current at 70% speed to trip the breaker before the compressor unloaded.
The five startup checks
Before matching a motor to a compressor (or any high-inertia load), run these five checks:
- Breakaway torque. Can the motor deliver more torque than the load needs at zero speed? For a reciprocating compressor with gas in the cylinders, breakaway torque can be 150% of running torque. The motor’s locked-rotor torque must exceed it with margin.
- Acceleration torque at every speed. Plot the motor torque curve and the load torque curve. The difference (acceleration torque) must be positive at every speed. The minimum difference is usually at pull-up speed (60-80% of rated). If it’s negative anywhere, the motor stalls.
- Acceleration time. Calculate the time to accelerate the total inertia (motor rotor + coupling + compressor) to full speed. The formula: t = (J × Δω) / T_acc, where J is total inertia, Δω is the speed change, and T_acc is the average acceleration torque. Compare with the motor’s safe stall time (usually 10-15 seconds for a standard motor).
- Thermal capacity. The motor heats up during startup. A motor that starts once an hour can tolerate a longer acceleration than one that starts every 5 minutes. Check the motor’s duty cycle rating (S1 continuous, S4 intermittent with starts per hour).
- Breaker and starter coordination. The overload relay and the circuit breaker must be set to the motor’s actual startup current-time curve, not the nameplate FLA. A standard Class 10 overload relay trips in 10 seconds at locked rotor — that’s often too fast for a high-inertia start.
What the calculation showed
The motor was 30 kW, 4-pole, with 2.0 kg·m² rotor inertia. The compressor had 4.5 kg·m² of inertia (flywheel plus crankshaft plus gas effects). Total J = 6.5 kg·m². The average acceleration torque from 0 to 1470 RPM was about 140 Nm (the motor curve minus the load curve, averaged). The speed change is 1470 × 2π/60 = 154 rad/s.
t = (6.5 × 154) / 140 = 7.15 seconds
The motor’s safe stall time is 10 seconds. The startup took 7 seconds with the compressor loaded. With the compressor unloaded (suction valve open), it took 5.5 seconds. The breaker was tripping at 8 seconds — right at the edge. The compressor was starting loaded (the discharge check valve was leaking, so the cylinders pressurized on startup). Fix the check valve, the startup time dropped to 5.5 seconds, and the breaker stopped tripping.
The motor was actually fine. The compressor’s check valve was leaking, causing a loaded start. The fix was a $40 check valve, not a bigger motor or breaker.
When you do need a bigger motor
If the acceleration torque is negative at any speed, a bigger motor is the fix. The rule: pick the motor size so the acceleration torque at the worst point is at least 15% of full-load torque. For a compressor, that usually means a motor one frame size up, or a motor with a higher breakdown torque (a “high torque” or “D” design motor). A soft starter or VFD also fixes the startup — the VFD ramps the frequency slowly, keeping the current at 150% instead of 600%.
But before buying a bigger motor, check the five items. The leaking check valve, the unloaded start, the breaker setting, the starter class, and the actual inertia. The biggest motor in the world won’t help if the load torque exceeds it at some speed — and the smallest fix (a check valve) can solve what looks like an undersized motor.
Motor sizing for compressors is a torque curve comparison, not a power check. Plot both curves, calculate the acceleration time, and verify it against the motor’s stall time. The tripping breaker wasn’t undersized — the compressor was starting loaded because of a leaking check valve. Five checks, a $40 part, and the startup is clean. Check the cheap fixes before you buy a bigger motor.
