The Motor That Burned (No Overload): Duty Cycle and Thermal Protection
A 0.5 kW gearmotor running a conveyor. It burned out after 3 months. The motor overload (thermal relay) didn’t trip. The motor was rated for continuous duty (S1). But the conveyor started and stopped 6 times per minute — that’s intermittent duty (S4/S5). The motor heated up between starts. The thermal relay was sized for the running current, not the starting current. This is about motor duty cycle, not about the motor being defective.
The duty cycles
| Duty cycle | Description | Use |
|---|---|---|
| S1 (continuous) | Runs at rated load, long enough to reach thermal equilibrium | Fans, pumps, conveyors that run continuously |
| S3 (intermittent periodic) | Start, run, stop, cool — repeated cycles | Batch machines, short cycles |
| S4 (intermittent with starting) | Starting current contributes to heating | Pick-and-place, frequent starts |
| S5 (intermittent with braking) | Start, run, brake, stop | Fast-cycling machines |
What happened
Our conveyor started and stopped every 10 seconds. The motor ran for 8 seconds, stopped for 2 seconds. That’s S4 duty. Each start draws 5-7x the rated current (locked rotor current). That current heats the windings. Over 6 starts per minute, the motor heats up faster than it can cool during the 2-second stops. The winding insulation degraded. After 3 months, it shorted.
The thermal relay was sized for the running current (1.5 A). The starting current (8 A) lasted only 0.5 seconds each start. The relay didn’t see it as a sustained overload. It never tripped. The motor cooked itself.
What we changed
1. Upsized the motor. We replaced the 0.5 kW S1 motor with a 0.75 kW S4-rated motor. The S4 motor is designed for frequent starts. It has a heavier-duty winding and better thermal capacity. It doesn’t overheat.
2. Soft starter / VFD. We installed a VFD on the conveyor. The VFD ramps the motor up slowly (3 seconds instead of instant). The starting current drops from 8 A to 2 A. The heating per start is minimal. The motor stays cool.
3. Proper overload protection. We set the thermal relay to the motor’s rated current (not 1.5 A but 1.8 A for the new motor). The VFD has its own electronic overload protection that accounts for starts. The motor is protected.
The calculation
For an S4 duty cycle, the motor’s effective heating is:
I_eff = sqrt((I_run² · t_run + I_start² · t_start) / (t_run + t_stop))
For I_run = 1.5 A, t_run = 8 s, I_start = 8 A, t_start = 1 s, t_stop = 2 s: I_eff = sqrt((1.5² × 8 + 8² × 1) / 11) = sqrt((18 + 64) / 11) = sqrt(7.45) = 2.7 A. The effective current is 2.7 A — 80% higher than the running current. The motor sees 2.7 A continuously, even though the meter reads 1.5 A. That’s why it burned.
The duty cycle rule: A motor that starts and stops frequently needs an S4/S5-rated motor, not an S1 continuous-duty motor. Use a VFD to reduce starting current. Size the thermal relay for the effective current (including starts), not just the running current. The motor that burned wasn’t overloaded — it was the wrong duty cycle.
