How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Question 892

I would like to know a little more about the ac slip ring motor and its uses, particularly in elevators. - M
A normal induction motor uses a set of stationary electromagnets to produce a magnetic field that seems to rotate rapidly around the motor's rotating central component—its "rotor." The rotor consists of a cylindrical aluminum metal cage and the rotating magnetic field causes currents to flow in the cage so that it becomes magnetic. The nature of the magnetism in the rotor causes it to be dragged along with the rotating magnetic fields around it and it begins to turn with those fields. When you first turn on the induction motor, the stationary rotor leaps into rotation as it tries to follow the spinning magnetic fields. That sudden start is acceptable for many applications, but you wouldn't want it in an elevator—the sudden starting of the elevator car that would accompany the sudden starting of its motor would throw the occupants to the floor. Instead, the aluminum cage in the rotor is replaced by a group of wires that are connected by way of metal ring (the "slip rings") and some stationary conductive brushes to some components outside the rotor. During the starting process, the currents that are induced in the rotor's wires are limited by the components outside the rotor. The rotor starts spinning gradually and gracefully. When the rotor has reached full speed, the brushes are retracted from the slip rings and the slip rings are shorted together so that the rotor behaves like the aluminum cage of a normal induction motor.
         

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