**IEC 60034-1 Duty Cycles of Operating Electrical Motors**

**IEC (the International Electrotechnical Commission)** uses ten duty cycle designations to describe electrical motor operating conditions:

Duty Cycles |
Descriptions | |
---|---|---|

S1 |
Continuous duty | The motor works at a constant load for enough time to reach temperature equilibrium. The abbreviation S1 |

S2 |
Short-time duty | The motor works at a constant load, but not long enough to reach temperature equilibrium. The rest periods are long enough for the motor to reach ambient temperature. The abbreviation S2 is followed by an indication of the duration of the duty.E.g. S2 60min |

S3 |
Intermittent periodic duty | Sequential, identical run and rest cycles with constant load. Temperature equilibrium is never reached. Starting current has little effect on temperature rise. The abbreviation S3 is followed by a cyclic duration factor.E.g. S3 25% |

S4 |
Intermittent periodic duty with starting | Sequential, identical start, run and rest cycles with constant load. Temperature equilibrium is not reached, but starting current affects temperature rise. The abbreviation S4 is followed by a cyclic duration factor and the moment of inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ) both referred to the motor shaft.E.g. S4 25% Iм=0.15Kgm² Iɩᴑᴀᴅ=0.7Kgm² |

S5 |
Intermittent periodic duty with electric braking | Sequential, identical cycles of starting, running at constant load and running with no load. No rest periods. The abbreviation S5 is followed by a cyclic duration factor and the moment of inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ) both referred to the motor shaft.E.g. S5 30% Iм=0.2Kgm² Iɩᴑᴀᴅ=0.8Kgm² |

S6 |
Continuous operation with intermittent load | Sequential, identical cycles of running with constant load and running with no load. No rest periods. The abbreviation S6 is followed by a cyclic duration factor.E.g. S6 40% |

S7 |
Continuous operation with electric braking | Sequential identical cycles of starting, running at constant load and electric braking. No rest periods. The abbreviation S7 is followed by the moment of inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ) both referred to the motor shaft.E.g. S7 Iм=0.4Kgm² Iɩᴑᴀᴅ=7.5Kgm² |

S8 |
Continuous operation with periodic changes in load and speed | Sequential, identical duty cycles run at constant load and given speed, then run at other constant loads and speeds. No rest periods. The abbreviation S8 is followed by the moment of inertia of the motor (Iм) and the inertia of the load (Iɩᴑᴀᴅ) both referred to the motor shaft, together with the load, speed and cyclic duration factor for each speed condition.E.g. S8 Iм=0.5Kgm² Iɩᴑᴀᴅ=6Kgm²16kW 740r/min 30% 40kW 1460r/min 30% 25kW 980r/min 40% |

S9 |
Duty with non-periodic load and speed variations | Load and speed vary periodically within the permissible operating range. Frequent overloading may occur. The abbreviation S9 |

S10 |
Duty with discrete constant loads and speeds | Duty with discrete number of load/speed combinations, with these maintained long enough to reach thermal equilibrium. The abbreviation S10 is followed by the per unit quantities p/∆t for the respective load and its duration and the per unit quantity TL for the relative thermal life expectancy of the insulation system. The reference value for the thermal life expectancy is the thermal life expectancy at rating for continuous running duty and permissible limits of temperature rise based on duty type S1. For a time de-energized and at rest, the load shall be indicated by the letter r. The value of TL should be rounded off to the nearest multiple of 0.05.E.g. S10 p/∆t = 1.1/0.4; 1/0.3; 0.9/0.2; r/0.1TL = 0.6 For this duty type a constant load appropriately selected and based on duty type S1 shall be taken as the reference value ‘Pref’ for the discrete loads. |

**Illustration of S1 to S10 Duty Cycles**