AIM: Comparative Study of Different Types of Starters Used For Three-Phase Induction Motor

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EXPERIMENT NO: 1 DATE: 11/08/ 2021
AIM: Comparative study of different types of starters used for three-phase

Induction motor.
THEORY:
A 3-phase induction motor is similar in action to a poly-phase transformer with a short

circuited rotating secondary. Therefore, if a normal supply voltage is applied to the stationary
motor; very large initial current is taken by the primary at least for a short while. Induction
motors when direct-switched take 5 to 7 times their full load current and develop only 1.5 to
2.5 times their full load torque. This initial excessive current is objectionable because it will
produce large line voltage drop which in turn affect the operation of the other electrical
equipment connected to the same lines. Starting torque can be increased by increasing the
resistance of the rotor circuit, which is only feasible in slip ring motors.
METHODS OF STARTING:
The following methods can be used to obtain a reduced voltage for starting a 3-phase induction
motor.
A. Squirrel cage motors,
(i) Primary resistance (or reactors) starter
(ii) Auto-transformer starter
(iii) Star-delta starter
B. Slip- ring motors,
(i) Rotor rheostat starter
A. Squirrel Cage Motors:
(i) Primary Resistance Starter:

Primary resistors are connected in the stator circuit. Some voltage drop takes place across the
primary resistors and reduced voltage is applied across the motor terminals. In this way the initial
current drawn by the motor is reduced. It should be noted that whereas current varies directly as
the voltage, the torque varies as square of the applied voltage. This method is useful for the smooth
starting of small machines only.
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(ii) Auto-Transformer Starter:

In auto-transformer starters some fixed tapping such as 80, 60 or 50% can be used. To obtain
a safe starting current, normally 60% tapping is used. This method can be used both for star
and delta connected motors. With starting connections (when starter is on START position) a
reduced voltage is applied across the motor terminals. When the motor has run up to 80% of
its normal speed connections are so changed that auto-transformers are cut out and full supply
voltage is applied across the motor (when starter is on RUN position) The switch making these
changes from `START' to `RUN' may be air break (for small motors) or oil immersed (for
large motors) to reduce sparking. There is also provision for no voltage and over load
protection along with a time delay device so that momentary interruption of voltage or
momentary over load do not disconnect the motor from supply line.
(iii) Star-Delta Starter:

This method is used in the case of motors which are built to run normally with a delta
connected stator winding. In this method, the phases of the stator winding are connected in
star at starting so that the voltage applied to each phase is 1//3 of the rated voltage and hence
the torque developed becomes 1/3 of that which would have been developed if motor were
directly connected in delta. The line current is also reduced to 1/3. Once the motor has started
and run up to 80% of its normal speed, the phases of stator windings are connected in delta
so that the rated voltage appears across each phase. For connecting the phases in star and then
in delta, each phase is brought out to two terminals of a two way switch. This method is cheap
and effective provided the starting torque required is not to be more than 1.5 times the full -
load torque. It is used for machine tools, pumps, and motor generators etc.
B. Slip Ring Motors:
(i) Rotor Rheostat Starter:

These motors are practically always started with line voltage applied across the stator
terminals. The value of starting current is adjusted by introducing a variable resistanc e in the
rotor circuit. The controlling resistance is in the form of a rheostat connected in star, the
resistance being gradually cut out of the rotor circuit as the motor attains speed. By increasing
the rotor resistance not only the rotor (and hence stator current) is reduced at starting but at
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the same time the starting torque is also increased due to improvement in power factor. The
controlling rheostat is either of stud or contactor type and may be hand operated or automatic.
The starter unit usually includes a line switching contactor for the stator along with no-voltage
(or low voltage) and over current protective device. There is some form of interlocking to
ensure proper sequential operation of the line contactor and the starter. This interlocking
prevents the closing of stator contactor unless the starter is `all in'. Introduction of additional
external resistance in the rotor circuit enables a slip-ring motor to develop a high starting
torque with reasonably moderate starting current. Hence such motors can be started under
load. This additional resistance is for starting purpose only. It is gradually cut out as the motor
comes up to speed. The slip rings are short circuited later on and brushes lifted from them
when motor runs under normal conditions.
Auto tansformer starting :

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REACTANCE CONTROL:
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STAR – DELTA STARTER:
CONCLUSION:
In this experiment we learn the starting of the induction moto as inherently the motor is self excited,
but due to the resistance of rotor winding is low due to slip is 1 at standstill motor will extract 4 to
5 % of that rated I at starting , this high current may burn the winding so, we need to limit the
current and due to that we use different types of starter as DOL starter, autotransformer as starter,
reactance control on the rotor side of slip ring rotor, resistance control, star-delta starter .
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LEARNING OUTCOMES:
In this I learnt that to limit the current and torque by means of starter, and how we can avoid the
voltage dip on other appliance and how to minimize effect , cost of the human effects buy using
program timer and controller in star delta starter.
Relevant IS: IS 13947 (1993) Voltage switchgear and control gear part – 4
Contactors and motor starters, sec – I (Electromechanical control devices)
QUIZ:
1. Why the starting current drawn by the motor is so high?
2. How the starting current can be reduced to permissible value?
3. State the limitations of the primary resistance starters.
4. Name a type of starter, which reduces the starting current and increases the starting
torque?
5. For what size of motors, direct – on line starter is generally used?
**************
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ANS1.) BECAUSE DUE TO THE RESISTANCE OF THR ROTOR WINDING DURING

STAND STILL POSITION IS LOW AS SLIP IS 1 WHERE ROTOR RESISTANCE IS
R/S .SO STARTING CURRENT IS HIGH IN INDUCTION MOTOR.
ANS2.)BY APPLYING THE SOME MEANS WE CAN REDUCE THE VOLTAGE AT

INPUT SIDE THEN WE CAN LIMIT THE STARTING CURRENT SO THAT MEANS
IS VARIETY OF STARTER AS STAR-DELTA,DOL,REACTANCE CONTROL AT
ROTOR SIDE,RESISTANCE CONTROL AND AUTOTRANSFORMER.
ANS3.) THE LIMITATIONS OF THE PRIMARY RESISTANCE STARTER IS DUE

TO THE I^2R LOSSES THE HEAT ACROSS THE RESISTANCE INCREASES AND
LEAD TO RUPTURE OF RESISTANCE WINDING AND ALSO DUE TO
UNCONTROLALBLE OF THE RESISTANCE WE CAN’T SET THE VOLTAGE
DROP AND LEADING TO REGULATE VOLTAGE DROP WE HAVE TO REPLACE
THE RESISTANCE AS PER THE CONVIENCE OF DROP ACROSS IT.
ANS4.) THE REACTANCE CONTROL STARTER TYPE ON THE SLIP RING TYPE
ROTOR WE CAN CONTROL CURRENT AND STARTING TORQUE AS PER
CONVENIENCE BECAUSE REACTOR CAN CONTROL VOLTAGE AS WELL
CURRENT LEAD TO TORQUE.
ANS5.) THE SMALL TYPE OF INDUCTION MOTOR DUE RATING OF THE

CURRENT IS LOW AND THE SUPPLY SIDE VOLTAGE WILL BE HANDLED
SO ,THAT AT LOAD SIDE THE VOLTAGE DIP WON’T OCCUR.
THANK YOU,

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19BEE036

EXPERIMENT NO: 1 DATE: 11/08/ 2021

AIM: Comparative study of different types of starters used for three-phase

A 3-phase induction motor is similar in action to a poly-phase transformer with a short

A. Squirrel Cage Motors:

(i) Primary Resistance Starter:

(ii) Auto-Transformer Starter:

(iii) Star-Delta Starter:

B. Slip Ring Motors:

(i) Rotor Rheostat Starter:

Auto tansformer starting :

STAR – DELTA STARTER:

ANS1.) BECAUSE DUE TO THE RESISTANCE OF THR ROTOR WINDING DURING

ANS2.)BY APPLYING THE SOME MEANS WE CAN REDUCE THE VOLTAGE AT

ANS3.) THE LIMITATIONS OF THE PRIMARY RESISTANCE STARTER IS DUE

ANS5.) THE SMALL TYPE OF INDUCTION MOTOR DUE RATING OF THE

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