The advantage of 3-speed A.C cage Motor as compared to simple squirrel cage induction motor used in winches are
Right Answer is:
Both 1 and 2
- Present-day cargo winches must be able to meet high cargo handling rates involving frequent starting and stopping.
- To overcome the inherent disadvantages associated with direct switching cage motors a specially designed dual stator/rotor low inertia, high torque motor is necessary.
- This minimizes the starting and accelerating losses which are responsible for a large proportion of the overall losses and subsequent heat generated.
The three-speed motor has two stators and three contractors; a low-speed stator with a 24-pole winding controlled by contactor 1 and a medium/ high-speed stator with two independent windings, one wound for 8 poles and the other for 4 poles controlled by medium speed contactor 2 and high-speed contactor 3. All windings are star-connected. These are in turn fed by reversing contactors x and y.
To run the motor through its speed range, first the low-speed contactor 1 is closed and then the forward or reverse contactor is energized and the motor will run at low speed, the electromagnetic brake being simultaneously released by an additional contactor. As the controller is moved to steps 2 and 3 the motor accelerates first to medium speed and then to high speed. To make the system foolproof, much interlocking is provided and, in addition, electrical deceleration is provided to reduce wear on the brake shoes.
Auxiliary contacts on each succeeding contactor open to trip out the preceding contractor but each succeeding contractor comes under the control of a timing device to ensure the current peak which occurs while accelerating has died down before the next acceleration takes place.