AC Motor Duty Cycle Types as Per IEC Standards

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The IEC Duty Cycle ratings are different from those of that of NEMA. While NEMA specifies continuous, intermittent or special duty (typically expressed in minutes) cycles for AC motors, the IEC uses nine different duty cycle designations  according to IEC 60034-1. The table below lists the nine IEC duty cycles and their individual characteristics:

Designation Duty Cycle Type Description
S1 Continuous running Operation at constant load of sufficient duration to reach the thermal equilibrium
S2 Short-time duty Operation at constant load during a given time, less than required to reach the thermal equilibrium, followed by a rest enabling the machine to reach a temperature similar to that of the coolant (2 Kelvin tolerance).
S3 Intermittent periodic duty A sequence of identical duty cycles, each including a period of operation at constant load and a rest (without connection to the mains). For this type of duty, the starting current does not significantly affect the temperature rise.
S4 Intermittent periodic duty
with starting
A sequence of identical duty cycles, each consisting of a significant period of starting, a period under constant load and a rest period.
S5 Intermittent periodic duty
with electric braking
A sequence of identical cycles, each consisting of a period of starting, a period of operation at constant load, followed by rapid electric braking and a rest period.
S6 Continuous operation
periodic duty
A sequence of identical duty cycles, each consisting of a period of operation at constant load and a period of operation at no-load. There is no rest period.
S7 Continuous operation
periodic duty with electric
braking
A sequence of identical duty cycles, each consisting of a period of starting, a period of operation at constant load, followed by an electric braking. There is no rest period.
S8 Continuous operation
periodic duty with related
load and speed changes
A sequence of identical duty cycles, each consisting of a period of operation at constant load corresponding to a predetermined speed of rotation, followed by one or more periods of operation at another constant load corresponding to the different speeds of rotation (e.g., duty ). There is no rest period. The period of duty is too short to reach the thermal equilibrium.
S9 Duty with non-periodic
load and speed variations
Duty in which, generally, the load and the speed vary non-periodically within the permissible range. This duty includes frequent overloads that may exceed the full loads.