In overhead traveling cranes
Right Answer is:
Intermittent periodic Motor is preferred
SOLUTION
Electric cranes require huge torques for short time. It is desired to specify a motor with pull-out torque of 275 to 30090 of rated full load torque based on a 40% duty rating. The motor of a steelworks duty crane must be designed with the lowest moment of inertia and high pull-out torque to meet the need for repetitive operations. Motors with lower synchronous speeds are best suited.
Duty cycle
In overhead traveling cranes Intermittent periodic Motor is preferred.
For crane and hoist motors, duty types S3, S4, and S5 as specified in IS:325-1970 are generally applicable.
- Intermittent periodic duty (duty type S3): A sequence of identical duty cycles, each consisting of a period of operation at constant load and a de-energized rest period, these periods being too short to attain thermal equilibrium during one duty cycle. Duration of the duty cycle is generally 10 minutes, cyclic duration factor 15%, 25%, 40%, and 60%.
- Intermediate periodic duty with starting (duty type S4): A sequence of identical duty cycles each consisting of a period of starting, a period of operation at constant load, and a de-energized rest period.
- Intermittent periodic duty with starting and braking (duty type S5): A sequence of identical duty cycles each consisting of a period of starting, a period of operation at constant load, a period of braking, and a de-energized period.
The motor should be designed for any of the duty factors, e.g. 15, 25, 40, and 60%. The duty factors for different types of cranes have been standardized depending upon operation, e.g. in steel industries, the duty factors are: Hoisting 60%; Traversing 60%; Travelling 60%; Sewing 40%.
Indian Electrical Manufacturers’ Association (IEMA) has brought out standards on crane duty motors in which an attempt is made to list the outputs against the IEC frames for S3 (6 starts/hour), S4, and S5 (150 and 300 starts/hour).
Slip ring induction motors are widely used for crane duties where squirrel cage motors are not suitable, where speed control is necessary, etc. With the development of thyristors, the use of DC motors is also increasing.