Industrial Drives and Control Module 04: Slip Ring Induction Motor Drives

4.1 Introduction

The induction motors are widely used in industrial applications. Induction

motor can be classified as squirrel cage induction motor, and slip ring or
wound rotor induction motor and also linear induction motor.
Squirrel cage induction motor costs nearly one third of a DC motor of the
same rating. It is extremely rugged and required practically no maintenance.
Can be built for higher speeds, torques and power ratings.

Wound or Slip Ring Induction Motor:

A Slip Ring Induction Motor (SRIM), also known as a wound rotor induction
motor, is a type of three-phase induction motor where the rotor is wound with
insulated wire and connected to external resistance through slip rings and
brushes. Unlike the squirrel cage rotor, which is permanently short-circuited,
the SRIM allows external access to the rotor circuit. The stator of an SRIM is
similar to any three-phase induction motor, consisting of laminated steel core
and three-phase windings. The rotor is also wound with three-phase
windings, which are connected to slip rings mounted on the rotor shaft.
Through these slip rings, external resistances can be inserted into the rotor
circuit during operation. Wound rotor motors are more expensive than
squirrel cage motors. Their maintenance needs are more than Squirrel cage
but much less as compared to DC motors. They are also available in high
power ratings.
The slip ring induction motor has a number of disadvantages compared to
squirrel cage motor such as
1. Wound - rotor machine is heavier
2. Higher cost
3. Higher rotor inertia
4. Higher speed limitation
5. Maintenance and reliability problems due to brushes and slip rings

Speed Control
The stator side control methods such as stator voltage control, V/f control
and stator current control are applicable to both squirrel cage and slip ring
induction motors.
Additionally, speed control of slip ring induction motor can be achieved using

Lecture Notes by Mrs. Seema P Diwan

Industrial Drives and Control Module 04: Slip Ring Induction Motor Drives

rotor side control. This speed control method is very simplest and oldest
method. The speed can be controlled by mechanically varying rotor circuit
rheostat.
The main feature of this machine is that slip power becomes easily available
from the slip rings, which can be electronically controlled to control speed of
the motor. For limited range speed control applications, where the slip power
is only a fraction of the total power rating of the machine, the converter cost
should be reduced. The main applications of slip power recovery drives are,
1. Variable speed wind energy systems.
2. Large - capacity wind energy systems.
3. Shipboard VSCF (Variable - Speed/Constant - Frequency) systems.
4. Utility system flywheel storage system.
5. Variable- speed hydropumps / generators.

Types of speed control methods in SRIM

1. Conventional rotor resistance control
2. Static rotor resistance control
3. Slip power recovery scheme (Energy efficient drives)

4.1.1 Conventional Rotor Resistance Control

This method is only applicable for slip ring or wound-rotor induction motors.
3- phase ac supply is fed to the stator and a variable resistance R2 is
connected in the rotor side. Rr is rotor resistance.

Methods of rotor resistance variation:

- A no of methods are used for obtaining variable resistance.
1. In drum controllers, resistance is varied by using rotary switches and a
resistance divided in few steps.
2. Variable resistance can also be obtained by using contactors and resistors
in series.

Speed torque curves for rotor resistance control are shown. While the maximum
torque is independent of rotor resistance.

Lecture Notes by Mrs. Seema P Diwan

Industrial Drives and Control Module 04: Slip Ring Induction Motor Drives

Fig (4.2.1) conventional Rotor Resistance Control

Speed Torque Characteristics:
The torque equation is given as:

3 𝑉2 (𝑅𝑟 ′ + 𝑅2)
𝑇=
𝜔𝑚𝑠 (𝑅𝑟 ′ + 𝑅2) 2 𝑠
( ) + (𝑋𝑠 + 𝑋𝑟′)2
𝑠

Starting Torque

s = 1 at starting

3 𝑉2
𝑇𝑠𝑡 = 2
(𝑅𝑟 ′ + 𝑅2)
𝜔𝑚𝑠 ((𝑅𝑟 ′ + 𝑅2)) + (𝑋𝑠 + 𝑋𝑟′)2

As starting s =1 and in running condition s is of the order of 0.05. During

running condition Rr’/s is very large. In denominator it is square of Rr’/s which
is even larger.
As s = 1, during starting denominator of equation is not large and torque is
higher.

Lecture Notes by Mrs. Seema P Diwan

Industrial Drives and Control Module 04: Slip Ring Induction Motor Drives

Maximum Torque

3 (𝑉)2
𝑇𝑚𝑎𝑥 =
2 𝜔𝑚𝑠
𝑅𝑠 ± √𝑅𝑠 2 + (𝑋𝑠 + 𝑋𝑟′)2
[ ]

By varying the external rotor circuit resistance R2 the starting torque and
starting current can be controlled. Figure shows the speed - torque
characteristics and speed - stator current characteristics.
In this curve,
1. by increasing the rotor circuit resistance, the maximum torque remains
constant. The maximum torque occurs at slip
sm = Rr/Xr
2. As the rotor resistance increases, speed decreases, starting torque
increases.

Fig(4.2.2.) -Speed –torque characteristics of rotor resistance

control

Lecture Notes by Mrs. Seema P Diwan

Industrial Drives and Control Module 04: Slip Ring Induction Motor Drives

Efficiency of the drive

Fig(4.2.3.) -Speed –efficiency and Speed Torque characteristics of rotor

resistance control

Slip efficiency calculation:

Slip of Induction Motor Efficiency

Lecture Notes by Mrs. Seema P Diwan

Industrial Drives and Control Module 04: Slip Ring Induction Motor Drives

0.2 80%
0.6 40%
(Slip = 1 is standstill condition)
Efficiency of Drive is lower at lower speeds.

Advantages of this method

1. Absence of in-rush starting current
2. Availability of full-rated torque at starting
3. High live power factor
4. Absence of line current harmonics
5. Smooth and wide range of speed control

Main drawbacks of this speed control

1. Reduced efficiency because the slip energy is wasted in the
rotor circuit resistance.
2. Speed changes very widely with load variation
3. Need to control three resistances simultaneously.
4. Unbalance in voltage and current if rotor circuit resistances are not
equal.

Example
A 3-phase,400V, 50Hz,10KW,960rpm,6pole, Y-connected slip-ring induction
motor has following constants referred to stator.
Rs=0.4 Ω , R’r=0.6 Ω, Xs = X’r =1.4 Ω.
Calculate full load torque, starting torque and maximum torque if resistance
inserted is R = 0.1,0.2,0.3,0.4,0.5 ohms.

Ns = 120*f/p = 1000 rpm

wms = 4*pi*f/p = 104.7198 rad /sec

Lecture Notes by Mrs. Seema P Diwan

Industrial Drives and Control Module 04: Slip Ring Induction Motor Drives

s_fl = (Ns - N_fl)/Ns

N=(3/wms)*V*V*(Rr+R)/s_fl

D= ((Rs+(Rr+R)/s_fl)^2+(Xs +Xr)^2)

S=1

N=(Rr+R)*(3/wms)*V*V
D=((Rs+Rr+R)^2+(Xs+Xr)^2)

3 𝑉2
𝑇𝑠𝑡 = (𝑅𝑟 ′ + 𝑅2)
𝜔𝑚𝑠 ((𝑅𝑟 ′ + 𝑅2))2 + (𝑋𝑠 + 𝑋𝑟′)2

𝑅𝑟′
𝑠𝑚 = ±
√𝑅𝑠 2 + (𝑋𝑠 + 𝑋𝑟′)2
[ ]

3 (𝑉)2
𝑇𝑚𝑎𝑥 =
2 𝜔𝑚𝑠
𝑅𝑠 ± √𝑅𝑠 2 + (𝑋𝑠 + 𝑋𝑟′)2
[ ]

3 𝑉2 (𝑅𝑟 ′ + 𝑅2)
𝑇𝐹𝐿 =
𝜔𝑚𝑠 (𝑅𝑟 ′ + 𝑅2) 2 𝑠𝑓𝑙
( ) + (𝑋𝑠 + 𝑋𝑟′)2
𝑠𝑓𝑙

R2 N(sFL) D(sFL) N(s=1) D(s=1) Tst SFL TFL sm Tmax

0.1 8.0214e+04 328.25 3.2086e+03 9.0500 354.5374 0.04 244.3689 0.2109 709.8909
0.2 9.1673e+04 424 3.6669e+03 9.2800 395.1433 0.04 216.2105 0.2095 709.8909
0.6 1.3751e+05 932 5.5004e+03 10.4000 528.8841 0.04 147.5428 0.2018 709.8909

Lecture Notes by Mrs. Seema P Diwan