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  Shorttime 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
