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Topic: Starter of Engine Branch: Electrical Engineering: Prepared By:-Guided By

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Tanvi Khurana

This document discusses starters for electric motors. It begins by defining a starter as a device that controls electrical power to equipment like motors, allowing them to start and stop. It then discusses the necessity of starters for large motors to limit starting current. It describes two common types of starters - 3 point and 4 point starters. For 3 point starters, it explains their construction, working, and how they protect DC motors from high starting currents. For 4 point starters, it provides details on their additional construction point and coil, and how this allows the no voltage coil to hold the handle independently of changes in the field circuit.

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Topic: Starter of Engine Branch: Electrical Engineering: Prepared By:-Guided By

Uploaded by

Tanvi Khurana
98 views16 pages
This document discusses starters for electric motors. It begins by defining a starter as a device that controls electrical power to equipment like motors, allowing them to start and stop. It then discusses the necessity of starters for large motors to limit starting current. It describes two common types of starters - 3 point and 4 point starters. For 3 point starters, it explains their construction, working, and how they protect DC motors from high starting currents. For 4 point starters, it provides details on their additional construction point and coil, and how this allows the no voltage coil to hold the handle independently of changes in the field circuit.

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Starter of Engine

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Starter of Engine

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This document discusses starters for electric motors. It begins by defining a starter as a device that controls electrical power to equipment like motors, allowing them to start and stop. It then discusses the necessity of starters for large motors to limit starting current. It describes two common types of starters - 3 point and 4 point starters. For 3 point starters, it explains their construction, working, and how they protect DC motors from high starting currents. For 4 point starters, it provides details on their additional construction point and coil, and how this allows the no voltage coil to hold the handle independently of changes in the field circuit.

Copyright:

© All Rights Reserved
Download as PPT, PDF, TXT or read online from Scribd
Download as ppt, pdf, or txt
98 views16 pages

Topic: Starter of Engine Branch: Electrical Engineering: Prepared By:-Guided By

Uploaded by

Tanvi Khurana
This document discusses starters for electric motors. It begins by defining a starter as a device that controls electrical power to equipment like motors, allowing them to start and stop. It then discusses the necessity of starters for large motors to limit starting current. It describes two common types of starters - 3 point and 4 point starters. For 3 point starters, it explains their construction, working, and how they protect DC motors from high starting currents. For 4 point starters, it provides details on their additional construction point and coil, and how this allows the no voltage coil to hold the handle independently of changes in the field circuit.

Copyright:

© All Rights Reserved
Download as PPT, PDF, TXT or read online from Scribd
Download as ppt, pdf, or txt
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TOPIC : Starter of Engine

Branch : Electrical Engineering

Prepared By :-
GUIDED BY :-
Content

 What is Starter?

 Necessity of Starter.

 Types of Starter

 3 point Starter

 4 point Starter
What is Starter ?

 A Starter is a device that controls the use of electrical power to equipment,

usually a motor. As the name implies, starters "start" motors. They can also
stop them, reverse them, and protect them. Starters are made from two
building blocks, Contactors and Overload Protection.
 Contactors control the electric current to the motor. Their function is to repeatedly

establish and interrupt an electrical power circuit.

 Overload Protection protects motors from drawing too much current, overheating,

and from literally "burning out".


Necessity of Starter
 When the motor is at rest, there is, as yet, obviously no back e.m.f developed
in the armature.

“What if we apply the full supply voltage applied across stationary armature.”

 The necessity of starter is only there in big motors, in small motors starter is
not required. It does not result in any harm to the motor for the following
reasons :
 Such motors have a relatively higher armature resistance than large motors, hence
their starting current is not so high.
 Being small, they have low moment of inertia, hence they speed up quickly.
 The momentary large starting current taken by them is not sufficient to produce a
large disturbance in the voltage regulation of the supply lines.
3 Point Starter
 A 3 point starter in simple words is a device that helps in the starting and
running of a shunt wound DC motor or compound wound DC motor.

“Why DC shunt motor require this type of assistance of starter?”

 The current will be dangerously high at starting (as armature resistance Ra is


small) and hence its important that we make use of a device like the 3 point
starter to limit the starting current to an allowable lower value.
Construction

 Construction wise a starter is a variable resistance, integrated into number

of sections as shown in the figure.

 The contact points of these sections are called studs and are shown

separately as OFF, 1, 2,3,4,5, RUN.

 Other than that there are 3 main points, referred to as

a) 'L' Line terminal. (Connected to positive of supply.)

b) 'A' Armature terminal. (Connected to the armature winding.)

c) 'F' Field terminal. (Connected to the field winding.)

 And from there it gets the name 3 point starter.


Continue….
 The point 'L' is connected to an electromagnet called overload release
(OLR) as shown in the figure.
 The other end of 'OLR' is connected to the lower end of conducting lever of
starter handle where a spring is also attached with it and the starter handle
contains also a soft iron piece housed on it.
 Handle is free to move to the other side RUN against the force of the spring,
spring brings back the handle to its original OFF position under the
influence of its own force.
 Another parallel path is derived from the stud '1', given to the another
electromagnet called No Volt Coil (NVC) which is further connected to
terminal 'F'.
 The starting resistance at starting is entirely in series with the armature. The
OLR and NVC acts as the two protecting devices of the starter.
Working
To start with the handle is in the OFF position when the supply to the DC
motor is switched on. Then handle is slowly moved against the spring force to
make a contact with stud No. 1.
At this point, field winding of the shunt or the compound motor gets supply
through the parallel path provided to starting resistance, through No Voltage
Coil. While entire starting resistance comes in series with the armature.
The high starting armature current thus gets limited as the current equation
at this stage becomes Ia = E/(Ra+Rst).
 As the handle is moved further, it goes on making contact with studs 2, 3, 4
etc., thus gradually cutting off the series resistance from the armature circuit
as the motor gathers speed.
Finally when the starter handle is in 'RUN' position, the entire starting
resistance is eliminated and the motor runs with normal speed.

“Now the obvious question is once the handle is taken to the RUN position how
is it supposed to stay there, as long as motor is running ?”
Continue….
 The supply to the field winding is derived through no voltage coil. So when
field current flows, the NVC is magnetized.
 Now when the handle is in the 'RUN' position, soft iron piece connected to
the handle and gets attracted by the magnetic force produced by NVC,
because of flow of current through it.
 The NVC is designed in such a way that it holds the handle in 'RUN' position
against the force of the spring as long as supply is given to the motor. Thus
NVC holds the handle in the 'RUN' position and hence also called hold on
coil.
 Now when there is any kind of supply failure, the current flow through NVC is
affected and it immediately looses its magnetic property and is unable to keep
the soft iron piece on the handle, attracted.
 At this point under the action of the spring force, the handle comes back to
OFF position, opening the circuit and thus switching off the motor. So due to
the combination of NVC and the spring, the starter handle always comes
back to OFF position whenever there is any supply problems.
 Thus it also acts as a protective device safeguarding the motor from any kind
of abnormality.
4 Point Starter

 The 4 point starter like in the case of a 3 point starter also acts as a protective

device that helps in safeguarding the armature of the shunt or compound


excited dc motor against the high starting current produced in the absence
of back emf at starting.

 The 4 point starter has a lot of constructional and functional similarity to a

three point starter, but this special device has an additional point and a coil in
its construction, which naturally brings about some difference in its
functionality, though the basic operational characteristic remains the same.
Construction
 A 4 point starter as the name suggests has 4 main operational points, namely
1. 'L' Line terminal. (Connected to positive of supply.)
2. 'A' Armature terminal. (Connected to the armature winding.)
3. 'F' Field terminal. (Connected to the field winding.)
Like in the case of the 3 point starter, and in addition to it there is,
4. A 4th point N. (Connected to the No Voltage Coil)

 The remarkable difference in case of a 4 point starter is that the No Voltage


Coil is connected independently across the supply through the fourth terminal
called 'N' in addition to the 'L', 'F' and 'A'.
 As a direct consequence of that, any change in the field supply current does
not bring about any difference in the performance of the NVC. Thus it must be
ensured that no voltage coil always produce a force which is strong enough
to hold the handle in its 'RUN' position, against force of the spring, under all
the operational conditions.
Continue….
 Such a current
is adjusted
through No
Voltage Coil
with the help of
fixed resistance
R connected in
series with the
NVC using
fourth point 'N'
as shown in the
figure above.
Continue….
 Apart from this above mentioned fact, the 4 point and 3 point starters are
similar in all other ways like possessing is a variable resistance, integrated
into number of sections as shown in the figure above. The contact points of
these sections are called studs and are shown separately as OFF, 1, 2, 3, 4,
5, RUN, over which the handle is free to be maneuvered manually to regulate
the starting current with gathering speed.
Working
 Considering that supply is given and the handle is taken stud No.1, then the
circuit is complete and line current that starts flowing through the starter. In
this situation we can see that the current will be divided into 3 parts, flowing
through 3 different points.
i) 1 part flows through the starting resistance (R1+ R2+ R3…..) and then to
the armature.
ii) A 2nd part flowing through the field winding F.
iii) And a 3rd part flowing through the no voltage coil in series with the
protective resistance R.
 Any change in the shunt field circuit does not bring about any change in the
no voltage coil as the two circuits are independent of each other.
 This essentially means that the electromagnet pull subjected upon the soft
iron bar of the handle by the no voltage coil at all points of time should be
high enough to keep the handle at its RUN position, or rather prevent the
spring force from restoring the handle at its original OFF position, irrespective
of how the field rheostat is adjusted.
Thank You

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