Page 88 - Handbook of Electrical Engineering

68 HANDBOOK OF ELECTRICAL ENGINEERING

The field leakage reactance is dependent on the shape of the pole yoke,

circumference of the yoke
X f ∝
radial length of the yoke

Therefore a low value of X f is obtained by having a radially long yoke of small cross-sectional
area. Hence the overall diameter of the rotor tends to increase as the reactance decreases.

The damper bars or winding act in a manner very similar to an induction motor and provide
a breaking torque against the transient disturbances in shaft speed. To be effective the damper needs
to have a steep torque versus slip characteristic in the region near synchronous speed. The equivalent
impedance of the damper requires a low resistance and a high reactance. High conductivity copper
bars are embedded into the pole face to provide a low reluctance path for the leakage flux.
The variation in X kd with slot dimensions is similar to the armature leakage,

axial length of slots × depth of slots
X kd ∝
width of slots
Increasing X kd tends to slightly increase the overall diameter of the rotor.
Reference 10 gives a full description of the physical design of electrical machines.

3.5 ACTIVE AND REACTIVE POWER DELIVERED FROM A
GENERATOR

3.5.1 A General Case

If the steady state, transient and sub-transient phasors in Figure 3.1 are considered separately, then
there is seen to be a similar structure. The terminal voltage V is resolved into its two-axis components

V d and V q .The emfs E, E and E can also be resolved into their components; E d , E q , E , E ,

q
d

E and E . In practical machines E d does not exist (except for an interesting prototype built for the
d q
CEGB in approximately 1970, called the Divided Winding Rotor generator, see References 12 and
13). E d would require a second exciter to produce it.
The variables can be regarded as ‘sending-end’ and ‘receiving-end’ variables. The sending-end
variables are the emfs E, E d and E q , whilst the receiving-end ones are V , V d and V q . The current
I, resolved into I d and I q , is common to both ends. The emfs, voltages and volt drops along each
axis can be equated as,
For the d-axis
(3.9)
E d = V d + I d R d − I q X q
For the q-axis
(3.10)
E q = V q + I q R q + I d X d
Where R d and R q are the resistances present in their respective axis, usually both are equal
to R a the armature resistance.