Download Practical Variable Speed Drives and Power Electronics Malcolm

3-Phase AC induction motors 55
• Operation at constant mechanical load for a period of sufficient duration for
thermal equilibrium to be reached.
• In the absence of any indication of the rated duty type of a motor, S1
continuous running duty should be assumed.
• Designation example: S1
S2: Short-time duty
• Operation at constant load, for a period of time which is less than that
required to reach thermal equilibrium, followed by a rest and motor deenergised period of sufficient duration for the machine to re-establish
temperatures to within 2°C of the ambient or the coolant temperature.
• The values 10 min, 30 min, 60 min and 90 min are recommended periods for
the rated duration of the duty cycle.
• Designation example: S2 – 60 min
S3: Intermittent periodic duty not affected by the starting process
• A sequence of identical duty cycles, each comprising a period of operation at
constant load and a period of rest when the motor is de-energized.