23 Electric Motors Apart from the normal motor operating range, there are two braking ranges. • n/n0 > 1: the motor is driven by the load above its synchronous speed (n0) operating as a generator. In this region, the motor produces a counter torque and simultaneously returns power to the supply grid. • n/n0 If two phases of a motor are suddenly interchanged, the rotating fi eld changes direction. Immediately afterwards, the speed ratio n/n0 is 1. The motor, previously loaded with torque T, now brakes with its braking torque. If the motor is not disconnected at n = 0, it will continue to run in the new rotational direction of the magnetic fi eld.
1.3.4 Typical Operating Conditions In principle, asynchronous motors have six coils: three coils in the stator and three coils in the squirrel-cage rotor (which behaves magnetically as if consisted of three coils). A subset of these coils can be used as the basis for generating an equivalent circuit that makes the operating principle of the motor easier to understand, especially when the frequency of the supply voltage changes or varies. R Fe Fig.
1.15 Equivalent circuit diagram (one phase) for a motor operating under load Applying a supply voltage (U1) results in a current in the stator (I1) and the rotor (I2) which is limited by the resistance in stator (R1) and rotor (R2) and the reactance in stator (X1 σ) and rotor (X2 σ). While the resistance is independent of the supply frequency the reactance has an infl uence. X L = 2 x π x f x L X L = reactance [Ω] f = frequency [Hz] L = inductance [H]