Q3Electrical Machine Design
Question
Q.3. Discuss the types of duties of a machine as per IS:4722-1968.
Answer
IS:4722-1968 classifies machine duty into continuous (S1), short-time (S2), and intermittent periodic duty (S3-S8 with variations including starting, braking, and varying load), each requiring different thermal design margins.
Indian Standard IS:4722-1968 (Specification for Rotating Electrical Machines) classifies the duty type of a machine — the pattern of loading and rest periods it experiences in service — into several standard categories, since this pattern critically affects the permissible continuous rating and hence the thermal design of the machine.
Continuous duty (S1): the machine operates at a constant load for a duration sufficient to reach thermal equilibrium (steady-state temperature). This is the most thermally demanding standard duty type, since the machine must be designed to continuously dissipate its full-load losses indefinitely.
Short-time duty (S2): the machine operates at constant load for a specified, limited time (insufficient to reach thermal equilibrium), followed by a rest period long enough for the machine to cool back to ambient temperature before the next operating period. Since the machine never reaches full steady-state temperature, it can be designed for a higher instantaneous output than its continuous rating would allow, for that same physical frame size.
Intermittent periodic duty (S3): the machine undergoes a sequence of identical duty cycles, each consisting of a period of constant load followed by a rest period, but unlike S2 the machine does not fully cool to ambient during the rest period — thermal equilibrium is instead reached considering the repeated cyclic pattern, characterized by a cyclic duration factor (ratio of loaded time to total cycle time, typically expressed as a percentage such as 25%, 40%, or 60%).
Further duty types (S4-S8): the standard also defines additional duty types incorporating starting and/or electric braking effects into the intermittent cycle (S4: intermittent periodic duty with starting; S5: intermittent periodic duty with starting and braking), continuous duty with intermittent periodic loading (S6), continuous duty with starting and braking (S7), and continuous duty with periodic speed changes (S8), each reflecting more complex real-world operating patterns encountered in specific applications such as crane motors, elevator motors, or machine-tool drives, where accurately specifying the duty type allows a substantially smaller, more economical machine to be selected for a given peak load than would be required under continuous duty.