__Motivation__

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.

__Objective__

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

__Theory__

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.

Also

Pg = Ir^2. (Rr + Rs)/S

Let

(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) = R_{ext}. / S

& X = Xr

Hence =n (A) becomes

T * Ws = __(E___{e})^2 . ( R_{r}
+ R_{ext} .) / S
. . …… (B)

{R_{e} + (R_{r}
+ R_{ext})/S}^2 + (X_{e} + X_{r}) ^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 T_{max}
occurs can be adjusted using the rotor resistance. The starting torque also
increases.