In this type of
construction, rotor winding is exactly similar to the stator. The rotor
carries a three phase star or delta connected, distributed winding,
wound for same number of poles as that of stator. The rotor construction
is laminated and slotted. The slots contain the rotor winding. The
three ends of three phase winding, available after connecting the
winding in star or delta, are permanently connected to the slip rings.
The slip rings are mounted on the same shaft. We have seen that slip
slip rings are used to connect external stationary circuit to the
internal rotating circuit. So in this type of rotor, the external
resistances can be added with the help of brushes and slip ring
arrangement, in series with each phase of the rotor winding. This
arrangement is shown in the Fig. 1.
|Fig. 1 Slip rings or wound rotor|
This way the value of rotor resistance per phase ca be controlled. This
helps us to control some of the important characteristics of the motor
like starting torque, speed etc.
In the running condition, the slip rings are shorted. This is possible
by connecting a metal collar which gets pushed and connects all the slip
rings together, shorting them. At the same time brushes are also lifted
from the slip rings. This avoids wear and tear of the brushes due to
friction. The possibility of addition of an external resistance in
series with the rotor, with the help of slip sings is the main feature
of this type of rotor.