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9 : Rotor Resistance Control of Wound Rotor Induction Motor


      The main advantage of slip ring induction motor over the squirrel cage motor is that it offers one more degree of freedom to the Engineers for starting & speed control. Slip ring Induction Motor is used where the load requirements are high starting toque & variable speed, or where the motor is to be started under heavy load. Typical applications of these motors are crane & hoist control. Resister controllers in the rotor circuit are used to achieve smooth start & speed control. Resent investigations have shown that certain desired torque speed characteristics can be achieved by insertion of relatively simple passive frequency sensitive networks. Careful selection of network parameters leads to highly reduced starting current & & improved torque/ current ratio. To understand the industrial systems incorporating the Slip ring Induction Motors it is therefore necessary to study the effect of rotor impedance on the performance of Induction Motors.


     The objective of this experiment is to determine the performance (speed, torque, current, efficiency, and power-factor) of Slip ring Induction Motor for Various values of rotor circuit resistance        


            The per phase equivalent circuit of a polyphase Induction Motor is shown in Fig.1. Passive two terminal network is connected externally to each rotor phase. The external network can be represented as impedance


                           Z(s) = R(s) + JX(s)

  At slip s.. The rotor impedance when referred to stator side becomes Z(s)/S as shown in Fig.1. To simplify the circuit Thevenins equivalent of the circuit is taken across the air gap. The thevenins equivalent circuit is shown in Fig.2 where

                           Re =  C^2 Rs, Xe = C . Xs, Ee = C . E

We know that the internal torque of Induction Motor is


T = Pg/Ws watts/phase

Pg = Airgap power

Ws= synchronous speed in rad/sec.


 Pg = Ir^2. (Rr + Rs)/S


        (Rr + Rs)/S = R    &   Xr + X(s)/S = X


Combine these =ns we get

T. Ws = R. E^2/[(R+Re)^2 + (X + Xe)^2] ...............(A)


  The above =n gives the torque slip relation for an Induction Motor with external rotor impedance.

Now the effect of different network parameters will be examined.


(1)    Resistance Control

             In this case

       R(s) = Rext. / S

       &    X = Xr

                   Hence =n (A) becomes

                   T * Ws =        (Ee)^2  .  ( Rr + Rext .) / S            .     .   …… (B)

                                                {Re + (Rr + Rext)/S}^2 + (Xe + Xr) ^2


       In =n (B) Ee, Re, Xe.Xr are constants therefore the torque developed (internal) is a function of rotor resistance & rotor .speed. It shows that the value of the torque can be varied for a particular speed of the rotor by varying the external resistance rotor resistance. The value of the maximum torque is independent of the rotor resistance & the speed at which the Tmax occurs can be adjusted using the rotor resistance. The starting torque also increases.


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