Service of Elettric Motors
The norm CEI-EN 60034-1 and the international standard IEC 34-1 define the following ten kinds of service so that, according to the operation, suitable electric motors can be sized.

S1 Continuous operation
Operation under constant load, lasting long enough to allow the machine to reach thermal equilibrium.

S2 Short-time duty
Operation under repeated, constant load in specified cycles, for a time too short to reach thermal equilibrium. Machine idle time is long enough to allow the machine to cool down until the temperature of the machine and that of the coolant are within a tollerance of 2°C.

S3 Intermittent periodic duty
Operation under repeated, constant load in specified cycles, each including a resting period.
In this type of service, the starting losses are small and do not significantly affect the temperature rise. (1)

S4 Intermittent periodic duty with start-up
Operation under repeated, constant load in specified cycles, each with a significant start-up period that influences the temperature rise and a resting period. (1)

S5 Intermittent periodic duty with electric braking
Operation under repeated, constant load in specified cycles, each with a significant start-up, a period of constant operation, a rapid electric braking phase and a resting period. (1)

S6 Intermittent periodic duty
Operation under repeated, constant load in specified cycles, each including a non-load period.
There is no resting period. (1)

S7 Intermittent periodic duty with electric braking
Operation under repeated, constant load in specified cycles, each with a significant start-up period, a period of constant operation and an electric braking phase.
There is no resting period. (1)

S8 Intermittent periodic duty with pole changing
Operation under repeated, constant load in specified cycles, each including a period of operation at a constant load at a preset rpm, followed by one or more periods of operation at another constant load corresponding to a different rotation rate (carried out, for example, by means of a change in the number of poles in the case of asynchronous motors).
There is no resting period. (1)

S9 Operation with non-periodic load and speed variation
Operation where load and speed normally undergo non-periodic variation within the permissible operating range.
This service includes frequently applied overloads that may greatly exceed the full loads.

S10 Operation with discrete constant load
Operation consisting of an exact number of discrete load values (or equivalent loads), each value being maintained for sufficient time to allow the machine to reach thermal equilibrium.
The minimum load within an operating cycle may have a value of zero (no-load or rest).

(1) Note: periodic service impilies that thermal equilibrium is not reached during the time on load.