A 7.5 kW motor on a scissor lift kept tripping its overload. The lift ran for 30 seconds, then sat for 3 minutes. The motor was sized for continuous duty (S1) at 7.5 kW, and the lift demanded about 6 kW during the raise. The motor should have been fine. It wasn’t. An S1-rated motor running an S3 duty cycle was being asked to carry 80% load during the on period, and the thermal rating for S1 assumes the load runs forever. The nameplate says S1. The application is S3. They’re different machines.

The duty classes that actually matter

IEC 60034-1 defines the duty types, and they change the usable torque.

S1, continuous duty. Constant load, long enough to reach thermal equilibrium. The nameplate rating applies.

S2, short-time duty. Load for a fixed period (say, 30 minutes), then a long off period so the motor cools fully. A motor can carry more than nameplate during the on period because it starts cold.

S3, intermittent periodic duty. A repeating cycle of load and rest, with the load period too short to reach equilibrium and the rest too short to cool fully. Defined by the cyclic duration factor.

S4-S10. The variants with starting, braking, and variable load. Most industrial machines land in S1 or S3.

The scissor lift is S3: 30 seconds on, 150 seconds off, a cycle factor of 16.7%.

The cyclic duration factor and what it buys

The CDF is the load time as a percentage of the cycle. For the lift: 30 / (30 + 150) = 16.7%. An S3 motor with a 16.7% CDF can carry roughly 1.5 to 2 times its continuous torque during the on period, because the off period lets it cool.

The lift’s 6 kW demand at 80% of a 7.5 kW S1 rating should be fine thermally. The motor heats during 30 seconds and cools during 150. If it ran at 80% load continuously, it would run at 80% of its thermal limit. Cycling, the average is 80% x 16.7% = 13%. It should run cool.

So why did it trip? The lift used a motor with a built-in brake and a high starting current. Each start draws 5-6x full-load current for about half a second. With 10 starts per hour, the starting current adds heat. The overload relay was set to nameplate FLA, but the starts, not the running load, were accumulating the heat. The relay tripped on its thermal model, not on the actual load.

The fix: set the overload for the duty, not the nameplate

The overload relay’s thermal model needs the duty type. A modern electronic overload has start time and rest time settings. For S3 duty, set the relay to allow the starting current but trip on sustained overload. The lift’s relay was set for S1. It treated every start as an overload.

The other fix: the motor itself. An S3-rated motor with a matching CDF has a smaller frame and a higher permissible torque during the on period. The lift’s 7.5 kW S1 motor could be replaced with a 5.5 kW S3 motor rated for the cycle. Smaller, cheaper, thermally correct. The S3 rating was on the original spec, but the buyer substituted an S1 motor because it’s bigger, so it’s better. Bigger isn’t better when the thermal model is wrong.

Service factor: the other nameplate number

The service factor (1.0, 1.15, 1.25) is the continuous overload the motor can carry at rated voltage and frequency. A 1.15 SF motor can run at 115% load continuously. But the SF doesn’t apply to a motor already running S3 duty with starts. The starts consume the thermal margin. Don’t stack SF on top of duty-cycle overrating.

The selection path

Define the duty cycle honestly: load time, rest time, starts per hour, and the load torque during each phase. Pick the duty class. If it’s S3, use the CDF to pick the motor size. Set the overload relay for the cycle. Check the starting current against the relay’s tolerance. The lift’s tripping was solved by setting the relay for S3, allowing the starts, and confirming the motor’s thermal rise stayed below the class B limit. No motor swap was needed. The original 7.5 kW had enough margin once the relay stopped treating every start as a fault.

S1 and S3 motors are different animals. The nameplate says continuous, the application says intermittent. Set the overload for the cycle, allow the starts, and match the motor’s duty rating to the CDF. The tripping lift wasn’t undersized. The thermal model was set for the wrong duty.