The rotors are of two types:
2. Wound rotor.
1. Squirrel-cage. The squirrel-cage rotor is made up of stampings (Fig. 2) which are keyed directly to the shaft. The slots are partially closed and the winding consists of embedded copper bars to which the short-circuited rings are brazed. The squirrel cage rotor is so robust that it is almost indestructible.
The great majority of present day induction motors are manufactured with squirrel-cage rotors, a common practice being to employ winding of cast aluminum. In this construction the assembled rotor laminations are placed in a mould after which molted aluminum is forced in, under pressure, to form bars, end rings and cooling fans as extension of end rings. This is known as die cast rotor and has become very popular as there as no joints and thus there is no possibility of high contact resistance.
In this type of rotor, it may be noted that slots are not made parallel to the shaft but they are ‘skewed’ to serve the following purposes:
(i) To make the motor run quietly by reducing the magnetic hum.
(ii) To reduce the locking tendency of the rotor.
2. Wound rotor. The wound rotor has also slotted stampings and the windings are former wound. The wound rotor construction is employed for induction motors requiring speed control or extremely high values of starting torque. The would rotor has completely insulated copper windings very much like the stator windings. The windings can be connected in star or delta and the three ends are brought out at the three slip rings. The current is collected from these slip rings with carbon ends are brought out at the three slip rings. The current is collected from these slip rings with carbon brushes from which it is led to the resistance for starting purposes. When the motor is running, the slip rings are short-circuited by means of a collar, which is pushed along the shaft and connects all the slip rings together on the inside. Usually the brushes are provided with a device for lifting them from the slip rings when the motor has started up, thus reducing the wear and the frictional losses.
The number of slots in the rotor should never be equal to the number of slots in the stator. If they are, there would be a variation of reluctance of the magnetic path from maximum, when teeth are opposite slots, to minimum when teeth are opposite teeth. The resulting flux pulsations would have have a high frequency, since the periodic time would be the interval period for a tooth to occupy similar positions opposite two successive teeth. This will not only cause extra iron loss but the rotor will tend to lock with the stator it at the time of starting teeth are opposite teeth. The best plan is to make the number of the stator and the rotor teeth prime to each other.
Advantages of a squirrel-cage motor over a phase-wound induction motor.
As compared with a wound rotor a squirrel-cage induction motor entails the following advantages:
- Slightly higher efficiency.
- Cheaper and rugged in construction
- No slip rings, brush gear, short-circuiting devices, rotor terminals for starting rheostats are required. The star-delta starter is sufficient for starting.
- It has better space factor for rotor slots, a shorter overhang and consequently a small copper loss.
- It has a smaller rotor overhand leakage which gives a better power factor and a greater pull out torque and overload capacity.
- It has bare end rings, a large space for fans and thus the cooling conditions are better.
The major ‘disadvantage’ of squirrel-cage motor is that it is not possible to insert resistance in the rotor circuit for the purpose of increasing the starting torque. The cage rotor has a smaller starting torque and large starting current as compared with wound rotor.