Rotor is rotating part of any electrical machine like alternator, motor. In alternator rotor serves as rotating magnetic flux source and in motor it serves as source rotating mechanical force or torque.
Electrical cable connection to rotor:
As it is a part of an electrical machine, the electrical connection to it comes in to consideration. In fact there is no fixed rule of electrical connection to rotor. In case of alternator the rotor produce magnetic flux so it has to be fed with DC current, this current is supplied to rotor either by direct connection (brush & slip ring arrangement) or brush less induction.
On the other hand in case of induction motor ac current is supplied to rotor by means of induced current principle. But in synchronous motor power supply connection to rotor is done by brush & slip ring arrangement.
Construction of rotor:
There are two types of rotor construction. 1.Squirrel Cage rotor, 2. Phase wound rotor.
01. Squirrel cage rotor construction.
Squirrel cage rotor construction can be stated as follows “Lamination or motor core material are stacked in cylindrical shape, metal bars are placed in to this lamination slots perpendicularly.Bothy sides of metal bars are shorted by end rings.The metal bars are in skew or with some angle with the rotor shaft.”
Below image describes the Squirrel cage rotor construction.
02. Phase Wound rotor construction and functions
Three phase, double layer, distributed winding is the characteristics of phase wound rotor. Rotor pole number should be equal to stator pole. And always the rotor will be three phase even when the stator wound for two phase.
In phase wound rotor , three phase is internally connected in star. The other side of phase coil is taken outward and three phase current is supplied to them with the arrangement of slip ring and brush.
How to improve starting of phase wound rotor by adding external resistance.
It is obvious that the complexity of starting of motors can be solved if an external resistance can be added before starting (thus improving the low torque in starting)and remove it while the motor is running in full speed. The specialty of phase wound rotor is that an external resistance can be added and removed while the motor is running.
The arrangement is adding an external resistance in series with phase wound rotor circuit.The removal is done with the aid of slip ring and brush arrangement.Below two picture will clear the facts.