Lecture 6: Third Class
Electrical Engineering Department
College of Engineering
University of Kerbala
2017/2018
By
Dr. Ali Altahir
Speed Control Using External Rotor Resistance
From the expression for the torque of the induction machine, torque is
dependent on the rotor resistance. The maximum value is independent of the
rotor resistance, but the slip at maximum torque is dependent on the rotor
resistance. Therefore, we may expect that if the rotor resistance is changed,
the maximum torque point shifts to higher slip values, while retaining a
constant torque. Figure below shows a family of torque-speed
characteristic obtained by changing the rotor resistance.
Note, that while the maximum torque and synchronous speed remain constant,
the slip at which maximum torque occurs increases with increase in rotor
resistance, and so does the starting torque. whether the load is constant
torque type or fan-type, it is evident that the speed control range is more with
this method. Further, rotor resistance control could also be used as a means of
generating high starting torque.
For all its advantages, the scheme has two serious drawbacks.
Firstly, in order to vary the rotor resistance, it is necessary to connect
external variable resistors (winding resistance itself cannot be changed).
This necessitates a slip -ring machine, since only in that case rotor terminals
are available outside. For cage rotor machines, there are no rotor terminals.
Speed Control Using External Rotor Resistance
Secondly, the method is not very efficient since the
additional resistance and operation at high slips entails
power loss in the resistor
.
The set up speed control using external rotor resistance is
show in figure above.
The variable rotor resistance is connected through the slip ring and
brushes.
The values of critical slip Sm corresponding to the maximum torque
increases with the increasing in the value of rotor resistance and the
torque slip characteristics.
Assume that the load torque TL is constant (Fan torque), then the
operating point of the I.M shifts from point A to point B.
So, the motor speed changes from N₁ to N₂ where N₂ is less than N₁.
Thus, the speed decreases with increase in the rotor resistance
Ex: Determine the resistance to be inserted in the rotor circuit
of the three – phase slip ring induction motor to provide
maximum torque at starting. Take the rotor resistance and
standstill reactance per phase as 0.01 ohm and 0.08 ohm,
respectively.
Solution: At the time of starting, the slip of the motor is equal to
one and the condition of torque is: 𝑅2 = 𝑠
𝑥2
Therefore, at the time of starting: 𝒙𝟐 = 𝑹𝟐
Let the resistance to be inserted per phase in the rotor circuit be R,
then R + 0.01 = 0.08 this implies R = 0.07 ohm
H.W: Determine the rotor resistance per phase to be added externally to
have 80 % of the maximum torque at starting. Consider the following
date of the three – phase six pole induction motor.
A) Rotor resistance per phase = 0.02 ohm.
B) Rotor standstill reactance per phase = 0.1 ohm
C) Frequency of supply = 50 Hz.
Also, determine the slip at maximum torque.
Hint: Tstarting / Tmaximum = 0.8
Speed Control of Induction Motors using cascade connection
Three phase voltage applied to the stator of a slip ring induction motor (P1 – poles) –
slip ring voltage applied to the stator of squirrel cage induction motor (P2 – poles) – two
rotors are coupled.
This cascaded set of two motors will now run at new speed having number of poles (P1 +
P2). In the above method the torque produced by the main and auxiliary motor will act
in same direction, resulting in number of poles (P1 + P2). Such type of cascading is called
cumulative cascading.
There is one more type of cascading in which the torque produced by the main motor is
in opposite direction to that of auxiliary motor. Such type of cascading is called
differential cascading; resulting in speed corresponds to number of poles (P1 - P2).
In this method of speed control of three phase induction motor.
Four different speeds can be obtained:
1. When only main induction motor work, having speed corresponds to:
2. When only auxiliary induction motor work, having speed corresponds to.
3. When cumulative cascading is done, then the complete set runs at a speed of.
4. When differential cascading is done, then the complete set runs at a speed of
120 𝑓 120 𝑓 120 𝑓 120 𝑓
𝑁𝑠1 = , 𝑁𝑠2 = ,𝑁 = 𝑁=
𝑝1 𝑝2 𝑝1 + 𝑝 𝑝1 − 𝑝
2 2
Cascade Connection of Three –Phase Induction Motors
Slip Power Recovery Scheme
This scheme applied to slip ring induction motor only: This implies, rated
voltage applied to the stator - the rotor voltage is rectified using a diode bridge
rectifier – the resulting DC voltage is inverted using line commutated inverter.
The AC voltage is fed back to the supply through appropriate transformer –
slip power is thus recovered from the motor and the speed reduced.
𝑠𝑙𝑖𝑝 𝑝𝑜𝑤𝑒𝑟 = 𝑠𝑃𝑔 . It is a part of the air-gap power, which is not
converted into mechanical power is called slip power.
The air - gap power which is getting wasted in rotor is fed back to the
supply mains this is called slip recovery.
H.W: Example: 4.23 in Your Text Book, Machines and Drive Systems
STARTING OF 3-PHASE INDUCTION MOTORS
There are many methods in use to start 3-phase induction
motors. Some of the common methods are;
For slip-ring motors:
Rotor resistance starting
For squirrel-cage motors
i. Direct On-Line Starter (DOL)
ii. Star-delta starting.
iii. Autotransformer starting.
ROTOR RESISTANCE STARTING FOR SLIP - RING I.M
By adding eternal resistance to the rotor circuit any starting
torque up to the maximum torque can be achieved; and by
gradually cutting out the resistance a high torque can be
maintained throughout the starting period. The added resistance
also reduces the starting current, so that a starting torque in the
range of 2 to 2.5 times the full load torque can be obtained at a
starting current of 1 to 1.5 times the full load current.
Direct On-Line Starter (DOL)
This is the most simple and in-expensive method of starting a
squirrel cage I.M .
The motor is switched on directly to full supply voltage. The
initial starting current is large, normally about 5 to 7 times the
rated current but the starting torque is likely to be 0.75 to 2
times the full load torque. To avoid excessive supply voltage
drops because of large starting currents, the method is
restricted to small scale motors only.
To decrease the starting current cage motors of medium and
larger sizes are started at a reduced supply voltage.
The reduced supply voltage starting is applied in the next two
methods.
Star-Delta Starting for Squirrel Cage I.M
This is applicable to motors designed for delta connection in
normal running conditions. Both ends of each phase of the
stator winding are brought out and connected to a 3-phase
change -over switch.
For starting, the stator windings are connected in star and when
the machine is running the switch is thrown quickly to the
running position, thus connecting the motor in delta for normal
operation. The phase voltages & the phase currents of the motor
in star connection are reduced to 1/3 of the direct -on -line
values in delta. The line current is 1/3 of the value in delta.
A disadvantage of this method is that the starting torque (which
is proportional to the square of the applied voltage) is also
reduced to 1/3 of its delta value.
Star-Delta Starting for Squirrel Cage I.M
AUTO - TRANSFORMER STARTING
This method also reduces the initial voltage applied to the motor
and therefore the starting current and torque. The motor, which
can be connected permanently in delta or in star, is switched first
on reduced voltage from a 3 - phase tapped auto -transformer and
when it has accelerated sufficiently, it is switched to the running (
i.e., full supplied voltage) position.
The principle is similar to star / delta starting and has similar
limitations.
The advantage of the method is that the current and torque can
be adjusted to the required value, (How?) by taking the correct
tapping on the autotransformer.
The disadvantage is more expensive because of the additional
autotransformer.
AUTO - TRANSFORMER STARTING
SHORT ANSWER QUESTIONS
1. Why are 3-phase induction motors very popular as drives for industrial
applications?
2. What are the various types of 3-phase induction motors as per the rotor
construction?
3. List the differences between squirrel cage and slip ring rotor.
4. Define slip of induction motor.
5. A 3-phase induction motor does not run at synchronous speed. Why?
6. Why is the no-load current drawn by 3-phase induction motor so high?
7. Compare the efficiency and operating power factor of single phase
induction motor with 3-phase induction motor.
8. Why single phase induction motors are not self-starting?
9. What are the various types of single phase induction motors?
10. How to change the direction of induction motor?
DETAILED ANSWER QUESTIONS
1- With the help of diagrams, explain how a rotating magnetic field is
produced in the air gap of a 3-phase induction motor.
2. Explain the principle of operation of 3-phase induction motor.
3. Derive the relationship between the rotor copper losses and the rotor input
in a 3- phase induction motor .
4. Explain the effect of slip on the following rotor parameters.
i) frequency. ii) induced emf iii) current. iv) power factor . v)reactance
5. Derive a general expression for the torque developed in a 3-phase
induction motor.
6. Sketch and explain the torque-speed characteristics of a 3-phase induction
motor.
7. List the various losses that take place in an induction motor.
8. Draw and explain the phasor diagram of a 3-phase induction motor.
9. Develop the equivalent of a 3-phase induction motor.
DETAILED ANSWER QUESTIONS
10. i) Why do we need a starter for starting a 3-phase induction motor?
ii) Draw a neat diagram showing the connections of 3-phase induction motor
with star - delta starter. Explain how the above starter reduces the starting
current.
11. Draw the diagram of an auto-transformer starter used for 3-phase induction
motor and explain its operation.
12. Describe the no-load test and blocked rotor test to determine the
parameters of equivalent circuit of 3-phase induction motor.
13. Explain the various techniques used for speed control of 3-phase induction
motor.
14. Explain rotor resistance speed control of 3-phase induction motor.
Thank you for your attention
Any Question
