20 Electric Motors A variant of the squirrel-cage rotor is the slip-ring rotor which has wound coils for each phase. The coils are connected to slip-rings. Brushes sliding on the slip-ring allow the connection of external resistors which modifi es the motor behaviour (see also section 1.3.5 Changing Speed). If the slip-rings are short-circuited, the rotor acts as a squirrel- cage rotor. I w (A) Fig. 1.11 Operational fi eld and squirrel-cage rotor The rotor movement of the squirrel-cage motor is created as follows: A rotor rod placed in the rotating fi eld is passed by a series of magnetic poles, as shown in Fig. 1.11.
The magnetic fi eld of each pole induces a current (I W ) in the rotor rod, which is infl uenced by a force (F). This force is determined by the fl ux density (B), the induced current (I W ), the length (L) of the rotor within the stator, and the angle (θ) between the force and the fl ux density. Assuming that θ = 90°, the force is: F = B x I W L The next pole passing the rod has an opposite polarity. It induces a current in the opposite direction to the previous one. Since the direction of the magnetic fi eld has also changed, the force acts in the same direction as before as shown in Fig. 1.12b Induction in the rotor rods. Fig. 1.12 Induction in the rotor rods