Rotor construction

Two types of rotor are used in induction motors. In both the rotor 'iron' consists of a stack of steel laminations with evenly spaced slots punched around the circumference. As with the stator laminations, the surface is coated with an oxide layer, which acts as an insulator, preventing unwanted axial eddy currents from flowing in the iron.

The cage rotor is by far the most common: each rotor slot contains a solid conductor bar and all the conductors are physically and electrically joined together at each end of the rotor by conducting 'end-rings' (see Figure 5.10). The conductors may be of copper, in which case the

Figure 5.10 Cage rotor construction. The stack of pre-punched laminations is shown on the left, with the copper or aluminium rotor bars and end-rings on the right

end-rings are brazed-on. Or, in small and medium sizes, the rotor conductors and end-rings can be die cast in aluminium.

The term squirrel cage was widely used at one time and the origin should be clear from Figure 5.10. The rotor bars and end-rings are reminiscent of the rotating cages used in bygone days to exercise small rodents (or rather to amuse their human captors).

The absence of any means for making direct electrical connection to the rotor underlines the fact that in the induction motor the rotor currents are induced by the air-gap field. It is equally clear that because the rotor cage comprises permanently short-circuited conductor bars, no external control can be exercised over the resistance of the rotor circuit once the rotor has been made. This is a significant drawback that can be avoided in the second type of rotor, which is known as the 'wound-rotor' or 'slipring' type.

In the wound rotor, the slots accommodate a set of three phase-windings very much like those on the stator. The windings are connected in star, with the three ends brought out to three sliprings (see Figure 5.11). The rotor circuit is thus open, and connection can be made via brushes bearing on the sliprings. In particular, the resistance of each phase of the rotor circuit can be increased by adding external resistances as indicated in Figure 5.11. Adding resistance in appropriate circumstances can be beneficial, as we will see.

Cage rotors are usually cheaper to manufacture, and are very robust and reliable. Until the advent of variable-frequency inverter supplies, however, the superior control which was possible from the slipring type meant that the extra expense of the wound rotor and its associated control gear were frequently justified, especially for high-power machines. Nowadays comparatively few are made, and then only in large sizes. But many old motors remain in service, so they are included in Chapter 6.

3-phase rotor winding

3-phase rotor winding

Sliprings T T T and brushes I-1-1

Figure 5.11 Schematic diagram of wound rotor for induction motor, showing sliprings and brushes to provide connection to the external (stationary) 3-phase resistance

Rotor circuit resistance

Sliprings T T T and brushes I-1-1

Figure 5.11 Schematic diagram of wound rotor for induction motor, showing sliprings and brushes to provide connection to the external (stationary) 3-phase resistance

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