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For medium-duty cranes the short time rating motor used is
10 minutes
15 minutes
30 minutes
any of the above
30 minutes
Quick Summary: 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.
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.
╬Ф╬╕=╬Ф╬╕mтАЛ(1тИТe╧ДтИТtтАЛ) тАФ temperature rise calculation where ╧Д is the thermal time constant
PratedтАЛ=тИСtтИСP2тЛЕtтАЛтАЛ тАФ equivalent power calculation for duty cycles
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.
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.
Reduced size and weight compared to continuously rated motors.
Optimized cost for applications with frequent cooling periods.
Cannot be used for continuous process applications.
Higher risk of burnout if the duty cycle is exceeded.
Hoists and cranes
Valve actuators
Lock gates
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.
C is correct тАФ Medium-duty cranes utilize a 30-minute short-time rating as per standard industrial electrical specifications.
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.