The International Electrotechnical Commission has classified motors into various classes based on duty cycles

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

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. |

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. |

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. |

S5 | Intermittent periodic duty with electric braking | Sequential, identical cycles of starting, running at constant load and running with no load. No rest periods. |

S6 | Continuous operation with intermittent load | Sequential, identical cycles of running with constant load and running with no load. No rest periods. |

S7 | Continuous operation with electric braking | Sequential identical cycles of starting, running at constant load and electric braking. No rest periods. |

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. |

source : The internet