Slip Ring Induction Motors Basics
written by: sriram balu • edited by: KennethSleight • updated: 11/27/2012
Slip ring motors usually have a “phase-wound” rotor. This type of rotor has a 3-phase, double-layer, distributed
winding consisting of coils used in alternators. The rotor core is made up of steel laminations which have slots to
accommodate formed 3-single phase windings.
Introduction
In previous articles I have discussed the construction, operation, starting, speed control, and classes of squirrel cage
induction motors. This article is devoted to slip ring induction motors. As discussed earlier, a slip ring induction
motor is an asynchronous motor, as the rotor never runs in synchronous speed with the stator poles. Learn more
about the construction and operation of a slip ring induction motor.
Construction
Stator:
The construction of a stator is the same for both the squirrel cage and slip ring induction motor. The main difference
in a slip ring induction motor is on the rotor construction and usage. Some changes in the stator may be encountered
when a slip ring motor is used in a cascaded system, as the supply for the slave motor is controlled by the supply
from the rotor of the other slip ring motor with external resistance mounted on its rotor.
Rotor:
The slip ring induction motors usually have a “Phase-Wound” rotor. This type of rotor is provided with a 3-phase,
double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel
laminations which have slots to accommodate formed 3-single phase windings. These windings are placed 120
degrees electrically apart.
The rotor is wound for as many poles as the number in the stator and is always 3-phase, even though the stator is
wound for 2-phase. These three windings are “starred” internally and the other end of these three windings are
brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends
touch these three slip rings with the help of carbon brushes which are held against the rings with the help of a spring
assembly.
These three carbon brushes are further connected externally to a 3-phase start connected rheostat. The slip ring and
external rheostat makes it possible to add external resistance to the rotor circuit, enabling them to have a higher
resistance during starting and thus higher starting torque.
When running under normal conditions, the slip rings are automatically short-circuited by means of a metal collar,
which is pushed along the shaft, thus making the three rings touch each other. Also, the brushes are automatically
lifted from the slip-rings to avoid frictional losses, wear and tear. Under normal running conditions, the wound rotor
acts the same as the squirrel cage rotor.
What happens when external resistance is added?
In the case of a squirrel cage induction motor, the rotor resistance is very low so that the current in the rotor is high,
which makes its starting torque poor. But adding external resistance, as in the case of a slip ring induction motor,
makes the rotor resistance high when starting, thus the rotor current is low and the starting torque is maximum. Also
the slip necessary to generate maximum torque is directly proportional to the rotor resistance. In slip ring motors, the
rotor resistance is increased by adding external resistance, so the slip is increased. Since the rotor resistance is high,
the slip is more, thus it's possible to achieve “pull-out” torque even at low speeds. As the motor reaches its base
speed (full rated speed), after the removal of external resistance and under normal running conditions, it behaves in
the same way as a squirrel cage induction motor.
Thus these motors are best suited for very high inertia loads, which requires a pull-out torque at almost zero speed
and acceleration to full speed with minimum current drawn in a very short time period.
Advantages of slip ring induction motors

The main advantage of a slip ring induction motor is that its speed can be controlled easily.

"Pull-out torque" can be achieved even from zero RPM.

It has a high starting torque when compared to squirrel cage induction motor. Approximately 200 - 250% of its
full-load torque.

A squirrel cage induction motor takes 600% to 700% of the full load current, but a slip ring induction motor
takes a very low starting current approximately 250% to 350% of the full load current.
What happens if the motor is started as a normal induction motor?
If the slip motor is started with all the slip rings or the rotor terminals shorted, like a normal induction motor, then it
suffers extremely high locked rotor current, ranging up to 1400%, accompanied with very low locked rotor torque as
low as 60%. It is not advised to start a slip ring induction motor with its rotor terminals shorted.
In my next article, read about various starting methods & speed control of a slip ring induction motors.
Source:
http://www.brighthubengineering.com/diy-electronics-devices/43725-slip-ring-induction-motorsbasics/