What is a Motor’s IEC Duty Cycle?
The duty cycle ratings provided by the IEC are different from that provided by the NEMA. The National Electrical Manufacturers Association (NEMA) provides definitions for continuous, intermittent, and special duty cycles for alternating current (AC) motors. These duty cycles are commonly specified in minutes. On the other end, the International Electrotechnical Commission (IEC) apply nine distinct duty cycle designations.This is because the standard IEC 60034-1 states that the NEMA uses continuous duty cycles.
The frequency and length of normal operations are both included in the concept of the duty cycle. This includes starting, running with either no or full load, using the electric brakes, and stopping and resting.
The nine different IEC duty cycles, together with their particular characteristics, are outlined in the following table:
| Designation | Type of Duty Cycle | Description |
|---|---|---|
| S1 | Continuous Running Duty | Continuous operation with a constant load for a time period long enough to attain thermal equilibrium. |
| S2 | Short-Time Duty | The machine is run at a constant load for a period of time that is shorter than the time necessary to attain thermal equilibrium. This is followed by a period of rest that allows the machine to attain a temperature that is comparable to that of coolant, with a tolerance of two Kelvin. |
| S3 | Intermittent Periodic Duty | A series of identical duty cycles, each of which includes a time of operation at constant load & a rest (during which the device is disconnected from the mains), is called a duty cycle sequence. When it comes to this type of task, the starting current doesn’t have a major impact on the increase in temperature. |
| S4 | Intermittent Periodic Duty with Starting | A series of identical duty cycles, with each one including a large period of starting, followed by a period under steady load, and then a period of rest. |
| S5 | Intermittent Periodic Duty with Electric Braking | A series of cycles that are all exactly the same, with each one starting with a period of starting, continuing with a period of operation at a constant load, and then concluding with a period of quick electric braking and a period of rest. |
| S6 | Continuous-Operation Periodic Duty | A set of similar duty cycles, each of which consists of a time of operation while carrying a constant load & a period of operation while carrying no load. There is no time for pause or break. |
| S7 | Continuous-Operation Periodic Duty with Electric Braking | A series of similar duty cycles, each of which consists of a period of starting, followed by a operating period at constant load, and then an electric braking phase. There is no time for rest or break. |
| S8 | Continuous Operation Periodic Duty with related Load & Speed Changes | The duty cycle comprises a sequence of the identical duty cycles, wherein each cycle includes a consistent load time that corresponds with a predetermined rotational speed. |
| S9 | Duty with Non-Periodic Load & Speed Variations | A type of duty in which the weight and the speed are allowed to fluctuate at irregular intervals but remain within the allowable range. This duty involves regular overloads, some of which may even surpass the full load. |
