Induction motor – winding rotor and squirrel cage

There are two types of three-phase (asynchronous) induction motors:

• Three-phase induction motor, wound-type rotor (ring motor):

In the wound-type rotor, the rotor is surrounded by an insulated winding similar to the stator winding. The phase windings of the rotor are led out through three collector rings mounted on the rotor shaft. The rotor winding is not connected to any power supply.

The collector rings and brushes have the function of connecting the rotor with the external variable resistors, which are intended to increase the rotor’s resistance during start-up. In addition, the variable resistors allow the control of the current in the rotor and the speed of the motor. When it reaches its normal speed, the windings are short-circuited, and then the operation becomes similar to that of a cage rotor. For these reasons, the motor has a high starting torque and low starting current.

• Three-phase induction motor, cage rotor (short-circuited rotor):

In this type of motor, the cage is composed of bars usually made of aluminum, brass or copper, and short-circuited rings. The bars and the rings form a single piece, connected with the magnetic package of the rotor. Because of these characteristics, the union between the bars and the rings end up forming a kind of cage, justifying the reference of its name. There is no need for insulation between the bars and the core because the induced voltages in the rotor bars are very low.

Applications and Maintenance Plans:

• Equipment used on a large scale in industries;
• Maintenance programs developed from cost-benefit evaluation cost-benefit evaluation and the impacts of a failure in service;
• Development of maintenance strategy:
• Make a list with all applications using the engine with reference equipment;
• Analyze the list and assign categories for each engine application;

Critical:

High cost in lost production, high safety concern and environmental risk if the engine fails;

• Important , high repair cost, loss of redundancy or environmental impact if the engine fails;

Non-critical:

No impact on production, safety and environmental if the engine fails.

The maintenance program for each engine category should be based on allocating the best resources to the critical engines. For each category, if necessary, they should include online monitoring, offline testing, and visual inspection with frequency of activities.