An electric motor that tripped on overload every time a conveyor started. The motor was a 5.5 kW, 4-pole (1450 RPM), 400V three-phase. The conveyor running load was 4.5 kW. The motor had a service factor (SF) of 1.0 — rated for exactly 5.5 kW continuously. The conveyor started against a full load (the hopper was full). The starting current was 6x rated for 2 seconds. The overload relay tripped. The customer thought the motor was undersized. It was correctly sized for running load. The issue was the starting duty — the motor needed a higher service factor or a soft-start. This is about motor service factor and starting current.
What is service factor
The service factor (SF) is a multiplier on the rated power. A motor with SF 1.15 can produce 5.5 × 1.15 = 6.3 kW continuously without overheating. The SF provides margin for: voltage fluctuations, ambient temperature above 40°C, occasional overloads, and starting duty. A standard motor has SF 1.0-1.15. A severe-duty motor has SF 1.25-1.35.
The failed motor had SF 1.0. It could run at 5.5 kW continuously at 40°C ambient. But the starting current (6x rated = 87 A for a 5.5 kW motor) heats the windings. Each start dumps thermal energy equivalent to about 30 seconds of running at rated current. At 10 starts per hour, the motor sees 5 minutes of equivalent heating per hour. At SF 1.0, the motor has no thermal margin. The winding temperature climbs. The overload relay trips.
What was changed
1. Upsized to a 7.5 kW motor with SF 1.15. The 7.5 kW motor has more thermal mass. At 4.5 kW running load, it’s at 60% load. The starts don’t overheat it. The overload relay stops tripping. The upsized motor costs 30% more but eliminates the start problem. The motor also runs cooler, extending bearing life.
2. Installed a soft starter. A VFD or soft-start ramps the voltage from 0 to full over 5 seconds. The starting current drops from 6x to 2x rated. The starting heat drops by 90%. The motor starts smoothly. The conveyor belt doesn’t jerk. The VFD costs $300 but the motor doesn’t need upsizing. This was the preferred solution — the 5.5 kW motor stayed, but the VFD reduced the inrush.
3. Reduced the start frequency. The conveyor was started/stopped by a photo-eye (each box triggered a start). The system was changed to run continuously at a slow speed, with a brake stop. The start frequency dropped from 10 per hour to 1 per hour. The motor didn’t heat up. This was a process change, not a hardware fix.
The SF selection table
| Application | Typical starts/hour | Recommended SF |
|---|---|---|
| Fan, blower, continuous | 1 | 1.0 |
| Conveyor, steady load | 2 | 1.15 |
| Conveyor, loaded start | 10 | 1.25 + VFD |
| Crusher, mixer, shock load | 20+ | 1.35 + VFD |
| Hoist, frequent reversing | 30+ | 1.5 (severe duty) |
The thermal time constant
A motor winding has a thermal time constant of about 30 minutes. One start (2 seconds at 6x current) heats the winding by about 5°C. If starts are spaced 6 minutes apart, the winding cools between starts. If starts are every minute, the heat accumulates. At 10 starts per hour (every 6 minutes), the motor reaches equilibrium at about 10°C above normal. With SF 1.15, the motor has 15% margin — enough for the extra 10°C. With SF 1.0, it trips. The rule: count the starts per hour, then add 5°C of winding temperature per start. If the total exceeds the motor’s temperature class (Class B = 80°C rise), upsizing or VFD is needed.
The motor selection rule: SF 1.15 minimum for any conveyor, SF 1.25 for frequent starts. The tripping motor wasn’t undersized for running load — it was SF 1.0 with 10 loaded starts per hour. Use a VFD for soft-start, or upsizing to SF 1.15. Count starts per hour; each start adds thermal energy. A motor that runs at 50% load with SF 1.15 will outlast one running at 90% load with SF 1.0.
