What a load profile describes
A load profile determines how a motor is thermally stressed over time—does it run continuously under a constant load, or do periods of operation alternate with periods of rest? The required motor power and size depend directly on this: An undersized motor will overheat and fail, while an oversized one wastes energy and increases acquisition costs. The IEC 60034-1 standard describes ten standardized duty types, S1 through S10, which summarize typical load profiles.
S1 and S3: the two most important duty types
S1 (continuous operation) means: The motor runs continuously under a constant load until it reaches its thermal steady-state temperature—typical for pumps and fans in district heating systems. S3 (intermittent operation), on the other hand, describes repeated short-term operation with pauses during which the motor does not cool down completely to ambient temperature—typical for lifting platforms or crane systems. The other duty types (S2 short-term duty type, S4/S5 periodic intermittent duty type influenced by the starting process or, additionally, by electrical braking, S6 continuous periodic duty type with intermittent load, S7 continuous periodic duty type with starting and electrical braking, S8 continuous periodic operation with load and speed variations, S9 operation with non-periodic load and speed variations, S10 operation with individual constant loads) refine this basic pattern for more specialized load profiles.
A key factor in motor sizing: The more cooling pauses a duty type allows, the greater the load can be for the same motor size. An S3 motor with a duty cycle of ED = 40% can theoretically deliver about 15–30% more power than an S1 motor of the same size (see table), since it cools down between load phases.
Description and values of the figure
The top bar shows S1 (continuous operation): the load remains at the same height throughout the entire period; duty cycle ED = 100%. The bottom bar shows S3 (intermittent operation): a load cycle is followed by a pause without load, during which the motor cools down. A load cycle consists of the load duration plus the pause; the duty cycle (ED) is the ratio of load duration to load cycle, expressed as a percentage. Common ED values are 15, 25, 40, or 60 percent.
| Duty type | Specifications | Standard ED |
|---|---|---|
| S1 – Continuous operation | continuous load, no cooling break | 100 % |
| S3 – Intermittent Operation | Load cycles with a break; the motor cools down in between | 15 / 25 / 40 / 60 % |
Duty cycle and overload factor for intermittent duty
In intermittent operation (S3), the duty cycle (ED %) indicates what proportion of a cycle is spent under load. The shorter the duty cycle, the more cooling time remains—and the higher the overload factor for intermittent duty (f_Ü) may be, by which the required torque is divided to determine the necessary rated torque.
Do not confuse this with the application factor K_A from Module 3, which increases the torque (the factor is multiplied rather than divided).
- Marked point 1: Worked example: 10 Nm/1.2 ≈ 8.3 Nm
- Marked point 1: Worked example: 10 Nm/1.2 ≈ 8.3 Nm
Description and values of the figure
The range shows the typical values used in practice for each duty cycle; the highlighted line indicates the average value. For a short duty cycle (ED=15%), the permissible overload factor is highest (approx. 1.55); for a high duty cycle (ED=60%), it is lowest (approx. 1.10). The worked example shows that for ED = 40% and a required torque of 10 Nm, the minimum rated torque is approximately 8.3 Nm.
| Duty cycle ED (%) | Mittelwert f_Ü | Bandbreite f_Ü |
|---|---|---|
| 15 | 1.550 | 1.500 – 1.600 |
| 25 | 1.300 | 1.250 – 1.350 |
| 40 | 1.225 | 1.150 – 1.300 |
| 60 | 1.100 | 1.050 – 1.150 |
| ED % | 15 % | 25 % | 40 % | 60 % |
|---|---|---|---|---|
| Overload factor f_Ü | 1.50–1.60 | 1.25–1.35 | 1.15–1.30 | 1.05–1.15 |
Example: With ED = 40% and a required torque of 10 Nm, a motor with a rated torque of 10/1.2 ≈ 8.3 Nm is sufficient based on calculations—the overload factor for intermittent duty thus allows for a smaller, more cost-effective size. In addition, when selecting a motor, a safety factor of 1.1 to 1.25 is usually applied to the calculated torque—regardless of the above—to account for wear, contamination, and temperature fluctuations.
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