This invention relates to an improved method of modifying a slip ring induction motors
(SRIMs) to operate under variable frequency drive (VFD) applications.
BACKGROUND OF THE INVENTION
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.
A slip ring induction motor is an asynchronous motor, as the rotor never runs in
synchronous speed with the stator poles.
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.
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 phase 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.
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 inertial 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.

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.
Variable frequency drive (VFD) is now commonly applied to air handlers, pumps,
chillers and tower fans.
Variable Frequency Drive (VFD)
This device uses power electronics to vary the frequency of input power to the motor,
thereby controlling motor speed.
Variable Speed Drive (VSD)
This is more generic term applies to devices that control the speed of either the motor or
the equipment driven by the motor (fans, pump, compressor, etc.). This device can be
either electronic or mechanical.
Adjustable Speed Drive (ASD)
Again, a more generic term applying to both mechanical and electrical means of
controlling speed.
The three basic section of the VFD the rectifier, dc bus, and inverter.
The voltage on an alternating current (ac) power supply rises and falls in the pattern of a
sine wave (see Figure 1). When the voltage is positive, current flows in one direction;
when the voltage is negative, the current flows in the opposite direction. This type of
power system enables large amounts of energy to be efficiency transmitted over great
distances.

The rectifier in a VFD is used to convert incoming ac power into direct current (dc)
power. One rectifier will allow power to pass through only when the voltage is positive. A
second rectifier will allow power to pass through only when the voltage is negative. The
rectifiers are required for each phase of power. Since most large power supplies are
three phase, there will be a minimum of 6 rectifiers.
After the power flows through the rectifiers it is stored on a dc bus. The dc bus contains
capacitors to accept power from the rectifier, store it, and later deliver that power
through the inverter section.
The final section of the VFD is referred to as an “Inverter”. The inverter contains
transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor”
(IGBT) is a common choice in modern VFDs. The IGBT can switch on and off several
thousand times per second and precisely control the power delivered to the motor. The
IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine
wave at the desired frequency to the motor.
Many times production schedule requires operating the existing slip ring induction motor
with VFD converter for better control of Torque Vs Speed characteristics and save
electrical energy. In normal SRIM, Speed and Torque controlled by adding external
resistances to rotor winding through Liquid Resistance Starter/ Grid Rotor Resistance
(LRS or GRR) which leads to additional energy loss. This leads to additional energy
consumption due to resistive heat loss.
For making SRIM suitable for VFD application, it is required to short the rotor winding,
so that the SRIM acts like a squirrel cage induction motor (SCIM). If the shorting link is
placed in rotor terminal box, the slip ring and carbon brushes remain in continuous
operation and are to be replaced frequently due to wear tear of carbon brushes and slip
rings.

OBJECT OF THE INVENTION
The object of the invention is to propose an improved method of modifying a slip ring
induction motors (SRIMs) to operate under variable frequency drive (VFD) applications.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved method of modifying a slip ring induction
motors (SRIMs) to operate under variable frequency drive (VFD) applications, the
improvement is eliminating the slip rings and brush gear assembly from the SRIM; and
mounting a copper shorting ring on the rotating motor shaft to short-circuit the rotor
winding.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a schematic diagram of assembly of rotor shorting ring.
Figure 2 shows a schematic diagram of rotor shorting ring.
Figure 3 and 4 shows typical photo of rotor shorting ring arrangement.
DETAIL DESCRIPTION OF THE INVENTION
In rotor, brush gear assembly is connected to the slip rings through which the brushes
are mounted on the rotating shaft and used to connect external resistance of the rotor
starter (LRS/GRR) with the motor rotor winding. Slip ring arrangement consists of 3
numbers of slip rings, brush gear, brush holder and brushes.
For starting a SRIM with VFD drive, it is required to short the terminals of rotor winding
by shorting link, which according to the normal practice, is done by shorting rotor
winding leads inside rotor terminal box. In this arrangement, the slip ring and carbon

brushes remain in continuous operation and are required to be replaced frequently due
to wear/tear of carbon brushes and slip rings.
In the new invented method, slip rings and brush gear assembly are removed
completely and to short circuit the rotor winding a copper shorting ring , mounted on the
rotating shaft is introduced. This modification allows the SRIMs to operate under VFD
control, like the squirrel cage induction motors.

WE CLAIM
1. An improved method of modifying a slip ring induction motors (SRIMs) to operate
under variable frequency drive (VFD) applications, the improvement is
characterize in that:
- eliminating the slip rings and brush gear assembly from the SRIM; and
- mounting a copper shorting ring on the rotating motor shaft to short-circuit the
rotor winding.