24 Electric Motors The coils mutually infl uence each other by means of magnetic induction. The rotor coil induces a current in the stator coil and vice versa. This mutual eff ect means that the two electric circuits can be interconnected via a common element consisting of R Fe and X h , which are called the transverse resistance and reactance. The current the motor draws for magnetising the stator and the rotor fl ows through this common element. The voltage drop across the ”transverse link” is the induction voltage (U q ). As R Fe is very small and is neglected in the following explanations.
Standard operation When the motor operates in its normal operating range, the rotor frequency is, due to the slip, lower than the rotating fi eld frequency. In the equivalent circuit diagram, the eff ect is described by a change in the rotor resistance R 2 by the factor 1/s. R2/s can be expressed as R2 + R2 (1 – s)/s where R2 (1 – s)/s represent the mechanical motor load. No-load situation The slip s is small at no-load (idle) operation. This means that R2 x (1 – s)/s is high. Consequently, almost no current can fl ow through the rotor. Ideally, this is comparable to removing the resistor that represents the mechanical load from the equivalent circuit.
The induced voltage (U q ) is often confused with the motor terminal voltage. This is due to the simplifi cation of the equivalent circuit diagram to make it easier to understand various motor conditions. However, the induced voltage only approximately corresponds to the terminal voltage in no-load operation. Locked rotor situation The slip increases when the motor is operating under load. Therefore R2 x (1 – s)/s will decrease. When the rotor is locked the slip is 1 and hence the current which increases with the load reaches its maximum. The equivalent circuit diagram thus corresponds to the conditions applicable to the asynchronous motor in normal practice.
It can be used in numerous cases for describing conditions in the motor. 1.3.5 Changing Speed The motor speed n is dependent upon the rotational speed of the magnetic fi eld and can be expressed as: n = n 0 – n s = (1 – s) x f p