Motors preferred for rolling mill drive is
Right Answer is:
Any of the above
SOLUTION
Depending upon the type of mill selection of motor has to satisfy the following conditions.
- Superior efficiency
- Robust frame and shaft construction to withstand the severest duty
- Less demand for maintenance
- A high degree of control and responsiveness
- Great capacity for acceleration and deceleration.
- Greater flexibility for extreme operating conditions.
- it should be capable of developing short-time torque to the extent of 2 or 3 times of rated torque.
- It should be capable of having wide speed variation.
- It should be in a position to maintain preselected speed within tolerances from no load to full load. Therefore it should have a quick response to change.
Various motors used for Rolling Mills
- Induction Motor
- DC Motor
Induction Motors:
AC slip ring motors are suitable for roughing and re-rolling mills where very precise speed control is not required. We can change the speed of the slip induction motor by introducing resistance in the rotor circuit.
Slip ring motors are suitable for roughing stands of the rolling mill. Wastage of energy in the rotor of the slip motor which is proportional to the reduction of speed can be avoided by employing a cascaded induction motor.
DC Motor
The DC motors, because, of their inherent characteristics (smooth and precise speed control over the wide range), are best suited for the rolling mills. Speed control is effected either through the Ward Leonard system or by grid-controlled mercury arc rectifiers. D.C. motor is the only type of motor which can satisfy the stringent requirements of modern rolling mills. By varying the applied voltage to the armature and keeping the field constant it is possible to change the speed of the motor at constant torque up to the rated speed. In this way requirement of higher constant torque at the low speed needed for effecting the heavy reduction in first passes is achieved.
In the case of reversible rolling mills, in order to ensure a high rate of acceleration, it becomes necessary to reduce the moment of inertia of the motor to the minimum. This is achieved by increasing the length and diameter of the armature.