28 April 2011

Defective Rotor of a SCI Motor


Most of the induction motors that we use are Squirrel Cage Induction (SCI) Motor. This name comes from the shape of the rotor. Some motors copper bars / round conductors are inserted in to the slots and they are short at both ends with end ring. Some other motor rotors are aluminium die-cast type. In both cases the shape of the rotor looks likes that of a Squirrel’s cage.

One of the common complaints that we receive on SCI Rotor is broken copper bars or open die-casting. The main indication of a defective SCI Rotor is that motor does not take load. Most often the no-load current drawn by the motor will be same as that by a good motor. But in Some cases fluctuating current is also seen. There will magnetic noise & excessive vibration while running the rotor. The above figure is of a defective SCI Rotor. This rotor is aluminium die-cast type.

The above picture shows a more clear view of the defective part of rotor. We can see open aluminium casting on slot. Melted aluminium has come out from this portion. We have to re-cast the rotor to make the motor working. It is must to ensure dynamic balance of rotor after re-casting. If the rotor is not dynamically balance then there will be vibration and abnormal noise from the motor. It will also reduce the life of bearings.

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