19 Electric Motors The amplitude of the rotating fi eld (φ) is constant and 1.5 times the maximum value ( max ) of the alternating fi elds. It rotates at the synchronous speed resulting from the pole pair number and supply frequency (see also section 1.3.3 Slip, Torque and Speed). a) b) Fig. 1.10 Magnetic fi eld components The representation of the rotating fi eld as a vector with a corresponding angular velocity describes a circle, shown in Fig. 1.10 Magnetic fi eld components. The magnitude of the magnetic fi eld φ as result of the components (φ1, φ2, φ3) is constant also at diff erent moments (a and b).
As a function of time in a coordinate system, the rotating fi eld describes a sinusoidal curve. The rotating fi eld becomes elliptical if the amplitude changes during a rotation. With single phase motors the phase shift which determines the rotation direction of the motor is created by a capacitor or an inductance which also results in an elliptical fi eld. 1.3.2 Squirrel-cage Motor The squirrel-cage rotor is the most frequently-used rotor type, and is used in the squirrel-cage motor. Unlike the stator, where the coils have many windings, in the squirrel-cage motor, only one winding is placed in the slots of the rotor lamination. This is typically done with aluminium or copper rods.
The rods are short- circuited at each end of the rotor by a ring made out of the same material. Copper has the advantage that it has a better conductivity than aluminium which results in lower losses and a higher effi ciency. Drawbacks compared to aluminium are higher prices, lower starting torques and higher melting temperature which complicate the casting and leads to a higher tooling eff orts.