A gearmotor that tripped its thermal overload every 4 hours. It was a 0.75 kW helical gearmotor, ratio 20:1, driving a screw feeder. The feeder ran 5 minutes on, 5 minutes off — 50% duty cycle. The gearmotor was rated for S1 (continuous). The customer thought a continuous-duty motor should handle anything. But the gearbox was running hot (85°C surface) and the thermal relay kept tripping. The issue was the duty cycle classification, not the motor size.
What S1, S2, S3 actually mean
These are IEC duty cycle classifications. People see “S1 continuous” and assume it can run forever at rated load. That’s true — but only if the cooling is adequate. The rating assumes a standard ambient (40°C) and the motor running at rated speed for full cooling.
- S1 (continuous running): the motor runs at constant load long enough to reach thermal equilibrium. Rated for 100% duty.
- S2 (short-time): runs for a short period, then cools. Example: S2-15 min means 15 minutes on, then off long enough to cool. The motor can produce more than rated power during those 15 minutes.
- S3 (intermittent periodic): a cycle of on/off. S3-25% means 25% on, 75% off. The motor doesn’t reach thermal equilibrium in either state. The allowed power is higher than S1 because of the cooling-off time.
The feeder ran 5 on / 5 off = 50% duty cycle (S3-50%). The gearmotor was rated S1 at 0.75 kW. At S3-50%, it could handle 1.2 kW. So the motor was actually oversized. Why did it overheat?
The thermal time constant
The motor winding heats up with a time constant of about 30 minutes. At 50% duty cycle (5 min on, 5 min off), the winding never reaches equilibrium. It oscillates. But the gearbox oil has a much longer time constant (2-3 hours). The gearbox heats up cumulatively — each on-cycle adds heat, the off-cycle doesn’t cool it fully. After 4 hours, the gearbox oil reached 95°C. The thermal relay (set at 90°C on the winding) tripped because the heat conducted from the gearbox into the motor winding.
The root cause: the screw feeder’s starting torque was 2x the running torque (it starts from a plugged condition). Each start dumped thermal energy into the gearbox. At 6 starts per hour, the gearbox couldn’t dissipate the heat.
What I changed
1. Upsized the gearbox by one frame size. The next size up (0.75 kW but larger gearbox housing) had 2x the oil volume and 1.5x the surface area. The running temperature dropped from 95°C to 65°C. The thermal relay stopped tripping. The upsized gearbox cost 30% more but required no control changes.
2. Added a soft-start. I installed a VFD and ramped the feeder speed from 0 to full over 5 seconds. The starting current dropped from 6x to 1.5x rated. The starting heat dropped by 70%. The gearbox ran cooler. The feeder started smoothly — less mechanical stress on the screw and the gear teeth.
3. Reduced the start frequency. I programmed the feeder to run continuously at a lower speed (rather than start/stop). The product flow was controlled by a slide gate instead of starting/stopping the motor. At continuous running, the motor reaches equilibrium at 55°C. No more thermal cycling. This was the best solution — but it required modifying the process, not just the hardware.
The duty cycle cheat sheet
| Application | Typical duty | Motor sizing |
|---|---|---|
| Conveyor, continuous | S1 | 1.0x running power |
| Screw feeder, cycling | S3-50% | 1.0x running power, but upsized gearbox for heat |
| Hoist, frequent starts | S4-25% | 1.5x running power (for inrush heat) |
| Valve actuator, occasional | S2-15 min | 2x running power (short time) |
The gearbox I size: not just for power, but for heat. A feeder that starts 6 times an hour generates more heat than the same feeder running continuously. Use a VFD for soft-start, or upsized the gearbox housing. The thermal trip wasn’t overloaded — it was cumulative heat from frequent starts.
