Page 682 - Basic Electrical Engineering

When relative speed is (Ns − Nr), the requency, f , is
r

= Sf

therefore, f = Sf (8.3)
r

i.e., rotor frequency = Slip × Stator frequency

The rotor-induced EMF is a function of the frequency. If the induced EMF in
the rotor under standstill condition is E , the induced EMF, E in the rotor
20
2
when it is rotating is, E = S E .
20
2

Figure 8.9 Rotor circuit when the rotor rotates at a slip, S

The rotor reactance at standstill, X = 2πfL.
20
The rotor reactance X when the rotor is rotating will be corresponding to
2
the rotor frequency f . Thus,
r

X = 2πf L = 2πSfL = S2πfL = SX 20
r
2

That is, rotor reactance under running condition is equal to slip times the
rotor reactance at standstill.

The resistance of the rotor circuit is R . R is independent of rotor speed.
2
2
Resistance does not change with speed. Rotor current is I . The rotor circuit
2
when the rotor is rotating is represented as shown in Fig. 8.9 (a).

The EMF induced in the rotor when it is rotating is SE , the rotor-circuit
20
resistance is R , the rotor-circuit reactance is SX , and the rotor current is I .
2
20
2
The current in the rotor circuit is