15 minute rated motors are suitable for

Question

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Light duty cranes

Explanation: 15-minute rated motors are classified as short-time rated machines, designed to operate at full load for a duration insufficient to reach thermal equilibrium with the ambient temperature. These are specifically suitable for light-duty applications like small cranes or hoists where operation is intermittent and infrequent.

Working Principle: The thermal rating of an electric motor depends on its capacity to dissipate heat generated during load operation. Short-time rated motors utilize their high thermal capacity to handle rated power briefly, after which they must be switched off to cool down to ambient temperature. This prevents the insulation from exceeding its maximum permissible temperature limit ($T_{max}$), as the motor is not designed for continuous heat dissipation.

Key Formulas: $P_{rated} = P_{load} \cdot \sqrt{\frac{t_{on}}{t_{on} + t_{rest}}}$ — Equivalent continuous power rating for intermittent duty

Key Points: Motor duty classes are defined in IS: 325 and IEC 60034.; Short-time duty (S2) is characterized by a constant load for a fixed duration, followed by a rest period.; 15, 30, 60, and 90 minutes are common standard durations for S2 duty motors.; These motors are smaller in size compared to continuous rated motors of the same power rating because they do not require extensive cooling systems.

Advantages: Reduced size and weight; Cost-effective for intermittent operations; Higher power-to-weight ratio for short bursts

Disadvantages: Cannot operate continuously; High risk of thermal insulation failure if overloaded beyond rated time; Requires strictly monitored duty cycles

Applications: Small cranes and hoists; Gate valves; Household appliances like blenders or juicers

Additional Info: Duty cycle classification (S1 to S10) dictates the operating envelope of rotating electrical machines.. Option B (Medium) and C (Heavy) typically require S3 (Intermittent Periodic) or S4 (Intermittent Periodic with starting) duty motors with higher cooling capabilities.

Answer: A is correct — 15-minute rated motors belong to the S2 duty cycle category, which is designed for applications requiring operation only for short, defined periods followed by complete thermal cooling.

Exam Tip: Always relate motor duty cycles to heat dissipation; S2 is for strictly short bursts, while S1 is for continuous steady-state operation.

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