This document presents a project report on DC injection braking of an induction motor. The project aims to brake a 3-phase induction motor by disconnecting the 3-phase supply and connecting a DC supply to one of the motor windings. Various components are used including two contactors, three transformers in series to obtain 110V, a timer switch, two NO NC switches, a rectifier circuit, and the induction motor. When the motor is running, one contactor is energized to supply power, and when de-energized, the other contactor is energized to inject DC into the winding, causing braking. The DC supply is obtained by stepping down the voltage with transformers and rectifying it. The timer

1. MANMOHAN MEMORIAL POLYTECHNIC
DEPARTMENT OF ELECTRICAL ENGINEERING
PROJECT REPORT ON
“DC INJECTION BRAKING OF INDUCTION MOTOR”
Prepared by:
Arun Thapa (7120010)
Arjun Dhimal (7120009)
Meghraj Basnet (7120019)
Lokmani Bhandari (7120017)
Utshab Rana (7120047)
Samartha Dhungana (7120032)
Aman Kumar Shah (7120004)
(2071-74 Batch)
Under the supervision of :
Er. Khagendra Sapkota
Submitted to :
Department of Electrical Engineering(MMP)
Date 2074-04-11 BS

2. MANMOHAN MEMORIAL POLYTECHNIC
DEPARTMENT OF ELECTRICAL ENGINEERING
Hattimuda-7, Morang
CERTIFICATE
This is to certify that this thesis titled “DC INJECTION BRAKING OF INDUCTION
MOTOTR” is the bonafide work, which carried out final year project work under the well
supervision of Er. Khagendra Sapkota (Asst. lecturer) of Electrical Department. It is certified
further that to the best of knowledge the work reported here in doesn’t form part of any other
thesis or dissertation on the basis of which a degree or reward was conferred on an earlier
occasion on this any other candidate.
Roll no: Name of student Student's id number
10 Arun Thapa 7120010
09 Arjun Dhimal 7120009
19 Meghraj Basnet 7120019
17 Lokmani Bhandari 7120017
47 Utshab Rana 7120047
32 Samartha Dhungana 7120032
04 Aman Kumar Shah 7120004
……………… ………………….
Signature Signature
Er. Khagendra Sapkota Er. Ram Kumar Yadav
Supervisor coordinator
………………………
Signature
Dr. Subash Shree Pokhrel
Principal

3. Abstract
The project DC injection braking of Induction Motor aims at braking of 3-Phase Induction
Motor. This is done by disconnecting 3-phase supply from motor and connecting DC supply to
any one winding of motor. Due to constant flux created by DC supply motor gets brake. Higher
the DC voltage the brake will be strong. Two contactor,3 transformer in series to obtain 110 V, 1
timer switch, 2 NO NC switch, indicating bulb, rectifier circuit and Motor are the components
used in this project. One main contactor is used for main 2-phase supply and another one for DC
supply. Under motor running condition main contactor in energized when we de-energized main
contactor another contactor is energized and inject DC to the winding which cause brake on the
motor. DC supply is obtained by stepped down the voltage and further rectify with bridge
rectifier. Timer switch is use on the line supply of contactor use for DC for disconnecting this
contactor after brake is done. This is to be done to prevent winding from damage because DC
supply on winding for more than necessary cause damage to the winding.

4. Aknowledgement
We would like to express our gratitude to all those who gave us the possibility to complete this
project. We want to thank the Department of Electrical Engineering for giving us opportunity to
commence this project in the first instance and to do the necessary research work. We have
furthermore to thank our classmates and seniors who helped us and encouraged me to go ahead
with our project.
We are deeply indebted to our supervisor Asst. Lecturer Er. Khagendra Sapkota whose help,
stimulating suggestions and encouragement helped us in all the time of completeion of this
project.

5. Table of contents
CHAPTER 1 .................................................................................................................................................7
INTRODUCTION ........................................................................................................................................7
1.1 Background and objectives...........................................................................................................7
1.1.1 Background...........................................................................................................................7
1.1.2 Objectives .............................................................................................................................7
1.2 Application....................................................................................................................................8
1.3 System Overview..........................................................................................................................9
1.3.1 Block Diagram.............................................................................................................................9
1.3.2 Block Diagram Description .......................................................................................................10
1.4 Methodology.....................................................................................................................................11
CHAPTER 2 ...............................................................................................................................................12
2.1 Contactor...........................................................................................................................................12
2.3 Timer Switch.....................................................................................................................................14
2.4 NO NC Switch..................................................................................................................................15
2.5 Bridge Rectifier.................................................................................................................................16
2.6 Induction Motor ................................................................................................................................17
2.7 Indicating Bulb..................................................................................................................................18
CHAPTER 3 ...............................................................................................................................................19
3.1 DOL stator circuit .............................................................................................................................19
3.2 Rectifier Circuit ................................................................................................................................20
3.3 Contactor with Timer Switch............................................................................................................21
CHAPTER 4 ...............................................................................................................................................22
PRODUCT DESCRIPTION AND PERFORMANCE...............................................................................22
CHAPTER 5 ...............................................................................................................................................25
DISCUSSION AND CONCLUSION.........................................................................................................25
5.1 Work Accomplished .........................................................................................................................25
5.2 Problem Faced ..................................................................................................................................25
CONCLUSION...........................................................................................................................................26

6. Table of Figures
Figure 1: Block Diagram ..............................................................................................................................9
Figure 2: Methodology ...............................................................................................................................11
Figure 3: Contactor .....................................................................................................................................12
Figure 4: Transformer.................................................................................................................................13
Figure 5: Timer Switch...............................................................................................................................14
Figure 6: NO NC Switch.............................................................................................................................15
Figure 7: Bridge Rectifier...........................................................................................................................16
Figure 8: 3-Phase IM ..................................................................................................................................17
Figure 9: Indicating Bulb............................................................................................................................18
Figure 10: DOL stator circuit......................................................................................................................19
Figure 11: Rectifier circuit..........................................................................................................................20
Figure 12: Contactor with timer switch ......................................................................................................21
Figure 13: Circuit Diagram.........................................................................................................................23
Figure 14: DC injection braking of IM .......................................................................................................24

7. CHAPTER 1
INTRODUCTION
1.1Background and objectives
1.1.1 Background
DC injection braking is a method of slowing AC electric motors. A DC voltage is injected into
the winding of the AC motor after the AC voltage is disconnected, providing braking force to the
rotor. A DC voltage is applied to the motor windings, creating a stationary magnetic field which
applies a static torque to the rotor. This slows and eventually halts the rotor completely. As long
as the DC voltage is applied to the windings, the rotor will be held in position and resistant to
any attempt to spin it. The higher the voltage that is applied, the stronger the braking force and
holding power. DC injection brakes can also be used for emergency braking, when the motor
needs to be stopped immediately for whatever reason.
Transformer is used for step down the voltage and low voltage is further rectifying by bridge
rectifier for DC supply. Timer switch is used for disconnect the dc supply after certain time. In
DC injection braking system DC should not be injected for a long time (i.e. more than necessary)
as this will damage the stator coil of motor.
1.1.2 Objectives
The main objectives of this project are:
1) To rapidly and safely stop an induction motor.
2) To reduce the problem caused during mechanical brake.
3) To deliver high levels of torque to stop the motor.
4) To save the time if the motor must be stopped and restarted multiple times during the
course of a work day.

8. 1.2Application
The main application of this project is in industries where motor needs to stop and starts
frequently and during accident this can protect from serious injure. Application of this project is
in industries like:-
1) Rolling mills
2) Cut-off saws
3) Rubber mills
4) Rock crushers etc

9. 1.3System Overview
1.3.1 Block Diagram
Figure 1: Block Diagram
3-Phase supply
Main contactor
3-Phase Motor
Transformer
Bridge Rectifier
Timer Switch
Contactor for DC
Supply
Running
Indicator
Brake
Indicator
Single phase supply

10. 1.3.2 Block Diagram Description
Figure: 1 shows the block diagram of our project. Three phase supply of 400 V 50 Hz is given to
the 3-Phase Induction motor through main contactor . Single phase supply is tapped from 3 phase
supply for operation of contactor, timer and single phase transformer. An AC supply voltage of
230 V 50 Hz is stepped down by transformer to 110 V. The secondary output is rectified by a
full-wave bridge rectifier.
Rectified DC is connected to any one winding of motor passing through contactor as shown in
block diagram. For the operation of contactor for DC supply, single phase supply is given by
passing through timer switch. Timer switch is connected in NC contact. The purpose of using
timer switch is to disconnect DC supply after braking is done. Two indicators are used in our
project for indication of running and braking of motor. Running indicator is connected parallel to
main contactor and braking indicator is connected parallel to contactor for DC supply as shown
in figure: 1.

11. 1.4 Methodology
First, we surveyed for the detailed theory and principles used in our project. Secondly, we
surveyed about the components used and their availability. The power supply and the braking
circuit of the project have been fabricated on plywood board. Testing and repairing was done and
finally braking was done on induction motor.
FINAL TESTING
FABRICATION
PROJECT SELECTION
LITERATURE SURVEY
MARKET SURVEY
DESIGNING
ASSEMBLING
TESTING AND
MAINTAINING
Figure 2: Methodology

12. CHAPTER 2
TECHNOLOGY AND LITERATURE SURVEY
The project DC Injection Braking Of Induction Motor aims at braking of a motor by
disconnecting AC supply and injecting DC supply. Brief descriptions of components used in this
project are given below.
2.1 Contactor
A contactor is an electrically controlled switch used for switching an electrical power circuit,
similar to relay expects with high current ratings. A contactor is typically controlled by a circuit
which has a much lower power level than the switched circuit, such as a 230 V coil
electromagnet controlling a 400 V motor switch. Contactor is a NO switched at normal condition
when its coil gets energized then switch gets contacts. Contactor also contain auxiliary contact
(i.e. NO and NC).
Two contactors are used in this project. One for main three phase supply for motor and another
for DC supply.
Figure 3: Contactor

13. 2.2Transformer
Transformer is an electrical component that works on the principal of Faraday’s law of induction
by converting electrical energy from one value to another where power and frequency remain
constant. Transformer does this by linking together two or more electrical circuits using a
common oscillating magnetic circuit which is produced by the transformer itself.
Transformer is used for step down 230 V into 110 V in this project. The purpose of stepped
down is for rectification. Rated current is of 4 amp.
Figure 4: Transformer

14. 2.3 Timer Switch
Timer switch is a timer that operates an electric switch controlled by the timing mechanism. It consist of
supply terminal, comment point, NO and NC contact. Supply power is given from the supply terminal A1 and
A2. The incoming line is connected to common point and as required we can connect outgoing line to NO or
NC point. Required time can be set from the selector. At normal condition common point is in contact with NC
point but after set time reached the common point gets contact with NO point and that is how timer switch
works.
In this project timer switch is used for disconnecting the DC supply from the motor after brake is done. The
timer switch operates the contactor which is use for DC supply. Contactor is connected to NC point of the
timer switch. When timer switch gets supply then the contactor too gets supply and after set time reached then
the contactor gets disconnected.
Figure 5: Timer Switch

15. 2.4 NO NC Switch
NO is normally open i.e. the contacts are normally open and close when the switch is
actuated.NC is normally closed i.e. the contacts are normally closed and open when the switch
is actuated.NO NC is generally used to describe contactors and manual switches like
emergency stop buttons. It means that there is one pair of normally closed and one pair of
normally open contacts with their own terminals i.e. there will be four terminals. NO terminal
is used for starting the motor and NC is used to disconnect the supply (i.e. to stop the motor).
In this project two switches are used. One for starting the motor by providing supply to
contactor and another for disconnecting the supply for contactor which disconnect the motor
supply. For NO switch holding patch should be provide so that supply doesn’t get disconnect
after releasing the button. Auxiliary contact of the contactor provides the holding path.
Figure 6: NO NC Switch

16. 2.5 Bridge Rectifier
This type of single phase rectifier uses four individual rectifying diodes connected in a closed
loop “bridge” configuration to produce the desired output. The main advantage of this bridge
circuit is that it does not require a special center tapped transformer, thereby reducing its size and
cost. The single secondary winding is connected to one side of the diode bridge network and the
load to the other side.
The four diodes labeled D1 to D4 are arranged in “series pairs” with only two diodes conducting
current during each half cycle. During the positive half cycle of the supply,
diodes D3 and D2 conduct in series while diodes D3 and D4 are reverse biased and the current
flows through the load. The purpose of rectifying is to get DC output for braking of motor. The
DC output obtained for this project is 100 V.
Figure 7: Bridge Rectifier

17. 2.6 Induction Motor
Figure 8: 3-Phase IM
An induction motor, 3 phase induction motor or asynchronous motor is an AC electric motor in
which the electric current in the rotor needed to produce torque is obtained by electromagnetic
induction from the magnetic field of the stator winding. An induction motor can therefore be
made without electrical connections to the rotor. An induction motor's rotor can be either wound
type or squirrel-cage type.
In this project IM of 2 HP is used for braking which runs on 380 V 50 Hz having 3.7 Amp rated
current. Speed of this motor is 1450 rpm.

18. 2.7 Indicating Bulb
Indicating bulb is used for indicate while running and braking of IM. Green light glow while
running and red light indicate that motor is brake. Green bulb is connected in parallel with main
contactor and red light is connected in parallel with the contactor which is used for DC supply
for braking purpose. The bulb is of 5 watt each.
Figure 9: Indicating Bulb

19. CHAPTER 3
SYSTEM ANALYSIS AND EXPERIMENT
Our project consists of following circuit:
 DOL stator circuit
 Rectifier circuit
 Contactor with timer circuit
3.1 DOL stator circuit
Different starting methods are employed for starting induction motors because Induction
Motor draws more starting current during starting. To prevent damage to the windings due to
the high starting current flow, we employ different types of starters.
The simplest form of motor starter for the induction motor is the Direct on Line starter. The
DOL starter consist a MCCB, Contactor and an overload relay for protection. Typically, the
contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the
contactor is used, across the start button, as a hold in contact. i.e. the contactor is electrically
latched closed while the motor is operating.
Motor
Figure 10: DOL stator circuit

20. 3.2 Rectifier Circuit
A Full wave rectifier is a circuit arrangement which makes use of both half cycles of input
alternating current (AC) and converts them to direct current (DC). A half wave rectifier makes
use of only one half cycle of the input alternating current. Thus a full wave rectifier is much
more efficient (double+) than a half wave rectifier. This process of converting both half cycles of
the input supply (alternating current) to direct current (DC) is termed full wave rectification.
Above figure shows the circuit arrangement of bridge rectifier which is used in this project. Here
in this project we stepped down 230 V into 110 V and with the help of bridge rectifier we
convert 110 V AC into DC for braking purpose.
Tomotorwinding
Figure 11: Rectifier circuit

21. 3.3 Contactor with Timer Switch
+ - N P
C T
Above figure shows the circuit diagram for operation of contactor with timer switch. In this
project we use timer switch for disconnect of contactor from providing DC supply to winding.
Providing DC supply for a long time creates heat on winding and may get damaged.
In this project contactor is connected with NC on timer switch which means it gets supply on
normal condition. When set time reached its value then timer switch trip to NO contact and make
open circuit for contactor and that is how this circuit works.
NC NO
Figure 12: Contactor with timer switch

22. CHAPTER 4
PRODUCT DESCRIPTION AND PERFORMANCE
As discussed in chapter 3 we have used several circuits for the running of this project which are
listed below Once again.
 DOL stator circuit
 Rectifier circuit
 Contactor with timer circuit
The supply for operation of Induction motor required 3 phase 50 Hz and was provided using
DOL stator. 220 V was stepped down to 110V with the help of step down transformer. The
output obtained at secondary side of transformer was rectified with the help of full wave bridge
rectifier. The DC obtained from bridge rectifier was connected to one winding of motor passing
through the contactor.
Contactor use for DC supply was controlled with timer switch. Timer switch gets supply when
main contactor gets disconnect. This was done by connection of timer switch with the NO
contact of same switch which was made connection with NC contact of main contactor. For
holding contact for this timer the terminal of NO contact was connected with auxiliary contact of
second contactor as shown in circuit diagram below. With this when we press button for stop the
main contactor gets de-energized and timer as well and second contactor gets supply with this
AC supply gets disconnect and DC supply gets into one winding of Motor. This is how this
project works and motor gets brake and so called DC injection braking. Timer should be set to 2-
4 sec so that it will disconnect DC supply from winding.
The output obtained from this project is very efficient. The motor stops within a second with this
project whereas without using DC injection braking the motor takes 20 sec to be in complete
rest. That’s how this project saves time.

23. Figure 13: Circuit Diagram

24. Figure 14: DC injection braking of IM

25. CHAPTER 5
DISCUSSION AND CONCLUSION
5.1 Work Accomplished
Following are the work done to complete this project:-
 Designed the power supply and braking circuit.
 Assembled all components and connection was done on plywood according to circuit.
 Tested the circuit without connection of motor.
 Tested the project with motor and get succeed to brake perfectly.
 Finishing was given by painting on plywood and providing stand to the panel board.
5.2 Problem Faced
Following were the problems faced during this project:-
 Unavailability of components on lab.
 Unavailability of some components on market.
 Problems while exchanging the component with different rating of same type from where we
brought.

26. CONCLUSION
This project has helped us to learn the practical aspects of the knowledge we gained so far
through different courses. The practical aspect of the theoritical component is learned along with
the confidence to connect the complex circuit in board. We learnt a lot about electric braking
system, its advantage over mechanical braking and many more . The technical and practical field
knowledge required in handling project is achieved. Thus working in this project has been an
assets for us and we have been benefited from the experience gained during its completion.

27. References
Following references are taken for this project
(1) www.wikipedia.com
(2) www.electricalforyou.com
(3) www.easyelectrical.com
(4) Text book of electrical engineering by Bl theraja (Reprinted in 2008)
(5) Lectures notes