Generators - Wind Energy Systems for Electric Power Generation - page 37

3.2.2.6 Operation at Given Stator Current

Consider the case when the machine is fed by impressed currents or from a current-
source inverter, terminal voltage and torque are calculated, using the same equiva-
lent circuit model, as follows:

1 + j ω 2 στ 02

1 + j

U 1 = j ω 1 L 1 · I 1

ω 2 τ 02

2 ω τ

2 02

3 I 1 2

T = T k where T k = z p · · L 1 (1 − σ ) (3.12)

1 + ω 2 2 τ 02 2

Normally operation with impressed current is from an inverter, preferably under
current control maintaining constant rotor or stator flux. Care must be taken for lim-
iting the flux in this type of operation. This requires to take the effect of main field
saturation into account, for which the value of magnetizing current I m is indicative:

I m
I 1

L m
L 2

ω 2 τ 02
1 + j

j

= 1 − (3.13)

ω τ

2 02

Due to main field saturation the flux is a non-linear function of the magnetizing
current, Ψ m = L m I m , the saturation can be alternatively mod-

elled by the non-linear inductance L m ( I m ). Hence in the equations (3.12, 3.13) the
parameters are saturation dependant.
Figure 3.9 shows typical curves of the main field flux and inductance vs. magne-
tizing current in normalized representation.

Ψ m ( I m ); according to

Fig. 3.9 Flux and inductance under main field saturation