Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 69

3

Pull-out torque

2

Motoring
region

1

0

-1

Generating
region

-2

-3

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Slip (pu)

Figure 3.14 Typical torque-speed characteristic of an induction machine

The value of the maximum torque is, from (3.14) and (3.15),

3 V s
4 pð n s = 60 Þð X s þ X r Þ

k
2 X ¼

T max ¼ ð 3 : 16 Þ

Note that the maximum torque is independent of the value of rotor resistance.
A typical relationship between torque and slip is presented in Figure 3.14.
At standstill the speed is zero and the slip, s , is equal to 1 per unit (pu). Between
zero and synchronous speed, the machine performs as a motor. Beyond synchro-
nous speed the machine performs as a generator.
Figure 3.15 illustrates the effect of varying the rotor resistance, R r , on the
torque of the induction machine. On the one hand a low rotor resistance is required
to achieve high efficiency at normal operating conditions. On the other hand, a high
rotor resistance is required to produce high slip.
One way of controlling the rotor resistance (and therefore the slip and speed of
the generator) is to use a wound rotor connected to external variable resistors. The
rotor resistance is then adjusted by means of electronic equipment. The wound
rotor may be connected to the external variable resistors through brushes and slip
rings. However, a more innovative solution is to locate both the resistors and the
electronic control equipment on the rotor itself. The required slip-controlling signal
is then passed on to the rotor via optical fibre communications.