For medium-duty cranes the short time rating motor used is

Question

Examoogle · Accepted Answer

30 minutes

Explanation: Standard specifications for intermittent duty electrical machines categorize crane duty motors based on their thermal rating. For medium-duty cranes, the Indian Standard (IS 4722) specifies a 30-minute short-time rating, meaning the motor is designed to operate at rated load for 30 minutes without exceeding the permissible temperature limits.

Working Principle: The thermal design of an electric motor is governed by the duty cycle. A short-time rated motor is not intended for continuous operation; it is rated for a specific duration $t$ to reach thermal equilibrium at the maximum permissible temperature. After the specified time, the motor must be disconnected to cool down to the ambient temperature, preventing insulation breakdown.

Key Formulas: $\Delta 	heta = \Delta 	heta_m (1 - e^{\frac{-t}{	au}})$ — temperature rise calculation where $	au$ is the thermal time constant; $P_{rated} = \sqrt{\frac{\sum P^2 \cdot t}{\sum t}}$ — equivalent power calculation for duty cycles

Key Points: Short-time duty (S2) involves a defined period of load followed by a rest period long enough to cool the machine to ambient temperature.; Standard ratings for S2 duty motors include 10, 30, 60, and 90 minutes.; Medium-duty crane applications typically align with the 30-minute industry standard.; Thermal capacity is the limiting factor in intermittent motor selection.

Advantages: Reduced size and weight compared to continuously rated motors.; Optimized cost for applications with frequent cooling periods.

Disadvantages: Cannot be used for continuous process applications.; Higher risk of burnout if the duty cycle is exceeded.

Applications: Hoists and cranes; Valve actuators; Lock gates

Additional Info: Other common S2 ratings are 10, 60, and 90 minutes.. Option A (10 minutes) is typically used for very light or short-duration duty cycles.. The 30-minute rating is the most ubiquitous standard for general industrial lifting equipment.

Answer: C is correct — Medium-duty cranes utilize a 30-minute short-time rating as per standard industrial electrical specifications.

Exam Tip: Always relate the duty cycle (S1 to S10) to the specific machine application; S2 is exclusively for 'short-time' operations where cooling periods are guaranteed.

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