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UNIT 2
DC MACHINES
2.1 Constructional Details of DC Generators
A dc generator is an electric machine which converts mechanical energy into direct current electricity. This
energy conversion is based on the principle of production of dynamically induced emf. This article
outlines basic construction and working of a DC generator.
Thus, a DC generator or a DC motor can be broadly termed as a DC machine. These basic constructional details
are also valid for the construction of a DC motor. Hence, let's call this point as construction of a DC
machine instead of just 'construction of a dc generator'.
The above figure shows the constructional details of a simple 4-pole DC machine. A DC machine consists two
basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.
1.
Yoke: The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not only
provides mechanical strength to the whole assembly but also carries the magnetic flux produced by the field
winding.
2.
Poles and pole shoes: Poles are joined to the yoke with the help of bolts or welding. They carry field
winding and pole shoes are fastened to them. Pole shoes serve two purposes; (i) they support field coils and
(ii) spread out the flux in air gap uniformly.
3.
Field winding: They are usually made of copper. Field coils are former wound and placed on each pole and
are connected in series. They are wound in such a way that, when energized, they form alternate North and
South poles.
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Armature core (rotor)
4.
Armature core: Armature core is the rotor of the machine. It is cylindrical in shape with slots to carry
armature winding. The armature is built up of thin laminated circular steel disks for reducing eddy current
losses. It may be provided with air ducts for the axial air flow for cooling purposes. Armature is keyed to
the shaft.
5.
Armature winding: It is usually a former wound copper coil which rests in armature slots. The armature
conductors are insulated from each other and also from the armature core. Armature winding can be wound
by one of the two methods; lap winding or wave winding. Double layer lap or wave windings are generally
used. A double layer winding means that each armature slot will carry two different coils.
6.
Commutator and brushes: Physical connection to the armature winding is made through a commutatorbrush arrangement. The function of a commutator, in a dc generator, is to collect the current generated in
armature conductors. Whereas, in case of a dc motor, commutator helps in providing current to the
armature conductors. A commutator consists of a set of copper segments which are insulated from each
other. The number of segments is equal to the number of armature coils. Each segment is connected to an
armature coil and the commutator is keyed to the shaft. Brushes are usually made from carbon or graphite.
They rest on commutator segments and slide on the segments when the commutator rotates keeping the
physical contact to collect or supply the current.
Commutator
2.2 Operating principles of DC Generators
Generator is a machine which produces EMF dynamically from a mechanical input.In other words it is a
machine which converts mechanical energy to electrical energy.DC generator is nothing but which gives DC
output voltage.
DC generator works on the principle of faraday's laws.It states that,
Whenever conductors cuts the magnetic flux (φ) EMF induces,the amount of EMF is directly proportional to
rate of change of flux linkages.
E=-dφ/dt
[-ve sign given by lenz's law]
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The above equation of faraday's law is the fundamental working principle of generator,
Synchronous generator etc. A DC generator produces direct power, AC generator produces alternating power.
To get better idea on working of generator you need to know about simple loop generator. Let us consider a
simple loop DC generator as shown in the figure with one turn coil and one pair of magnet to
produce magnetic flux.
So to get rate of change in flux (φ) linkages either conductors or magnetic field should be in motion. So for
our convenience assume magnets are stationary and coil is rotating with the help of any mechanical input. Now
consider the arrangement as below without any input it is in stationary position.
ABCD is the rectangular loop of conductor[coil] which is placed in magnetic field. working of simple loop
generator can be explained in 3 steps,
Note: Current & Voltages have same direction because voltage is cause of current. we can't find direction of
voltage simply by applying any rule so we take current direction as reference in DC generator working
Step-1
Rotate the coil ABCD in the clockwise direction with help of mechanical input say prime-mover, and it is
moved to right angled position,
As during this movement AB and CD sides of the coil cut the flux lines there will be an EMF induced in these
both of the sides (AB & BC) of the loop by faraday's law, Maximum flux is linked in this position because AB
and CD are parallel to magnetic field direction. Field strength at AB & CD are not same so dφ exist. As the
closed path exists in the coil current flows in the coil. Current direction can be found by Flemming's right hand
rule.
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By Flemming's right hand rule current flow we find current direction as from A to B ,C to D.
Step-2
Now coil is rotated to next right angled position, AB and CD are perpendicular to magnetic filed direction.
Magnetic field strength at AB & CD are same. So difference between the field strength is zero i.e., dφ almost
zero. Hence induced emf, current also zero.
Step-3
Now coil is rotated to next right angled position, AB and CD are parallel to magnetic filed direction.So
maximum amount of EMF will induce[Like Step-1].But the current direction reverses.By flemming's right hand
rule current flow from B to A ,D to C.
This is the working principle of any generator, but from step-1 & step-2 we can observe that, current getting
reversed that we don't want in DC generator to eliminate alternating current we use commutator. Commutator is
an arrangement which converts AC to DC, in this way DC generator works produces DC voltages. In practical
DC generator there will no of conducts and poles[magnets] to produce required amount of EMF.
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DC generator Output Wave
After completion of half rotation, the direction of current in the particular conductor will be reversed. Hence,
the direction of current in every armature conductor will be alternating after a half rotation. With a split ring
commutator, connections of the armature conductors also gets reversed when the current reversal occurs. And
therefore, we get unidirectional current at the terminals.
What is the function of a commutator?
The function of commutator is to collect alternating current induced in armature conductor and convert it into
unidirectional current.
Constructionally it is cylindrical in shape built with hard drawn high conductivity copper segments. In figure
two segments E and F are shown. They are separated by a thin layer of mica insulation usually of V—shape.
Two brushes X and Y are mounted on the segments.
In figure, the first half rotation i.e., from 0 to 180° the coil position, with the segments are shown. The direction
of current flow is from point A to B.
In the next half rotation as in figure, the segments are interchanged. Thus, the direction of current in this case is
from B to A, similarly though the segments. i.e., 0 to 180°. is from A to B and 180 to 360° is from B to A.
By considering the total revolution 0 to 180° is taken as positive and 180 to 360° as negative. This current is
passed through the load resistance R in the external circuit and develops the unidirectional or DC voltage.
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2.3 EMF equation of DC Generators
Let
P = Number of poles of the generator
Φ = Flux produced by each pole in webers (Wb)
N = Speed of armature in r.p.m.
Z = Total Number of Armature Conductors
A = Number of parallel paths in which the 'Z' number of conductors are divided
So
A = P for lap type of winding
A = 2 for wave type of winding
Now e.m.f. gets induced in the conductor according to Faraday's law of electromagnetic induction. Hence
average value of e.m.f. induced in each armature conductor is,
e = Rate of cutting the flux = dΦ/dt
Now consider one revolution of conductor. In one revolution, conductor will cut total flux produced by all
the poles i.e. Φ x P. While time required to complete one revolution is 60/N seconds as speed is N r.p.m.
This is the e.m.f. induced in one conductor. Now the conductors in one parallel path are always in series.
There are total Z conductor with A parallel paths, hence Z/A number of conductors are always in series and
e.m.f. remains same across all the parallel paths.
... Total e.m.f. can be expressed as,
This is nothing but the e.m.f. equation of a d.c. generator.
Example : A 4 pole, lap wound, d.c. generator has a useful flux of 0.07 Wb per pole. Calculate the generated
e.m.f. when it is rotated at a speed of 900 r.p.m. with the help of prime mover. Armature consists of 440 number
of conductors. Also calculate the generated e.m.f. if lap wound armature is replaced by wave wound armature.
Solution :
P = 4 Z = 440 Φ = 0.07 Wb and N = 900 r.p.m.
i) For lap wound,
ii) For wave wound
A=P=4
A=2
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2.4 Types of DC Generators
2.5 Characteristics of DC Generators
Generally, following three characteristics of DC Generator are taken into considerations: (i) Open Circuit
Characteristic (O.C.C.), (ii) Internal or Total Characteristic and (iii) External Characteristic.
These characteristics of DC generators are explained below.
1. Open Circuit Characteristic (O.C.C.) (E0/If)
Open circuit characteristic is also known as magnetic characteristic or no-load saturation characteristic.
This characteristic shows the relation between generated emf at no load (E 0) and the field current (If) at a given
fixed speed. The O.C.C. curve is just the magnetization curve and it is practically similar for all type of
generators. The data for O.C.C. curve is obtained by operating the generator at no load and keeping a constant
speed. Field current is gradually increased and the corresponding terminal voltage is recorded. The connection
arrangement to obtain O.C.C. curve is as shown in the figure below. For shunt or series excited generators, the
field winding is disconnected from the machine and connected across an external supply.
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Now, from the emf equation of dc generator, we know that Eg = kɸ. Hence, the generated emf should be
directly proportional to field flux (and hence, also directly proportional to the field current). However, even
when the field current is zero, some amount of emf is generated (represented by OA in the figure below). This
initially induced emf is due to the fact that there exists some residual magnetism in the field poles. Due to the
residual magnetism, a small initial emf is induced in the armature. This initially induced emf aids the existing
residual flux, and hence, increasing the overall field flux. This consequently increases the induced emf. Thus,
O.C.C. follows a straight line. However, as the flux density increases, the poles get saturated and the ɸ becomes
practically constant. Thus, even we increase the If further, ɸ remains constant and hence, Eg also remains
constant. Hence, the O.C.C. curve looks like the B-H characteristic.
The above figure shows a typical no-load saturation curve or open circuit characteristics for all types of DC generators.
2. Internal Or Total Characteristic (E/Ia)
An internal characteristic curve shows the relation between the on-load generated emf (Eg) and the armature
current (Ia). The on-load generated emf Eg is always less than E0 due to the armature reaction. Eg can be
determined by subtracting the drop due to demagnetizing effect of armature reaction from no-load voltage E0.
Therefore,
internal
characteristic
curve
lies
below
the
O.C.C.
curve.
3. External Characteristic (V/IL)
An external characteristic curve shows the relation between terminal voltage (V) and the load current (I L).
Terminal voltage V is less than the generated emf Eg due to voltage drop in the armature circuit. Therefore,
external characteristic curve lies below the internal characteristic curve. External characteristics are very
important to determine the suitability of a generator for a given purpose. Therefore, this type of characteristic is
sometimes
also
called
as performance
characteristic or load
characteristic.
S
Internal
and
external
characteristic
curves
are
shown
below
for
each type
of
generator.
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Characteristics Of Separately Excited DC Generator
If there is no armature reaction and armature voltage drop, the voltage will remain constant for any load current.
Thus, the straight line AB in above figure represents the no-load voltage vs. load current I L. Due to
the demagnetizing effect of armature reaction, the on-load generated emf is less than the no-load voltage. The
curve AC represents the on-load generated emf Eg vs. load current IL i.e. internal characteristic (as Ia = IL for a
separately excited dc generator). Also, the terminal voltage is lesser due to ohmic drop occurring in the armature
and brushes. The curve AD represents the terminal voltage vs. load current i.e. external characteristic.
Characteristics Of DC Shunt Generator
To determine the internal and external load characteristics of a DC shunt generator the machine is allowed to
build up its voltage before applying any external load. To build up voltage of a shunt generator, the generator is
driven at the rated speed by a prime mover. Initial voltage is induced due to residual magnetism in the field
poles. The generator builds up its voltage as explained by the O.C.C. curve. When the generator has built up the
voltage, it is gradually loaded with resistive load and readings are taken at suitable intervals. Connection
arrangement is as shown in the figure below.
Unlike, separately excited DC generator, here, IL≠Ia. For a shunt generator, Ia=IL+If. Hence, the internal
characteristic can be easily transmitted to Eg vs. I L by subtracting the correct value of If from Ia.
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During a normal running condition, when load resistance is decreased, the load current increases. But, as we go
on decreasing the load resistance, terminal voltage also falls. So, load resistance can be decreased up to a certain
limit, after which the terminal voltage drastically decreases due to excessive armature reaction at very high
armature current and increased I2R losses. Hence, beyond this limit any further decrease in load resistance
results in decreasing load current. Consequently, the external characteristic curve turns back as shown by dotted
line
in
the
above
figure.
Characteristics Of DC Series Generator
The curve AB in above figure identical to open circuit characteristic (O.C.C.) curve. This is because in DC
series generators field winding is connected in series with armature and load. Hence, here load current is similar
to field current (i.e. IL=If). The curve OC and OD represent internal and external characteristic respectively. In a
DC series generator, terminal voltage increases with the load current. This is because, as the load current
increases, field current also increases. However, beyond a certain limit, terminal voltage starts decreasing with
increase in load. This is due to excessive demagnetizing effects of the armature reaction.
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Characteristics Of DC Compound Generator
The above figure shows the external characteristics of DC compound generators. If series winding amp-turns are
adjusted so that, increase in load current causes increase in terminal voltage then the generator is called to be
over compounded. The external characteristic for over compounded generator is shown by the curve AB in
above
figure.
If series winding amp-turns are adjusted so that, the terminal voltage remains constant even the load current is
increased, then the generator is called to be flat compounded. The external characteristic for a flat compounded
generator
is
shown
by
the
curve
AC.
If the series winding has lesser number of turns than that would be required to be flat compounded, then the
generator is called to be under compounded. The external characteristics for an under compounded generator are
shown by the curve AD.
2.6 DC Motor
When a machine converts electric energy into mechanical energy, it is called a motor. There is no
fundamental difference in either the construction or the operation of the two machines. In fact, the same
machine may be used as a motor or a generator.
Operating Principle: A DC motor in simple words is a device that converts direct current(electrical energy)
into mechanical energy. It’s of vital importance for the industry today, and is equally important for engineers to
look into the working principle of DC motor in details that has been discussed in this article. In order to
understand the operating principle of DC motor we need to first look into its constructional feature.
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The very basic construction of a DC motor contains a current carrying armature which is connected to the
supply end through commutator segments and brushes it is placed within the north south poles of a permanent or
an electro-magnet as shown in the diagram below.
Now to go into the details of the operating principle of DC motor its important that we have a clear
understanding of Fleming’s left hand rule to determine the direction of force acting on the armature conductors
of DC motor.
Fleming’s left hand rule says that if we extend the index finger, middle finger and thumb of our left hand in such
a way that the current carrying conductor is placed in a magnetic field (represented by the index finger) is
perpendicular to the direction of current (represented by the middle finger), then the conductor experiences a
force in the direction (represented by the thumb) mutually perpendicular to both the direction of field and the
current in the conductor.
For clear understanding the principle of DC motor we have to determine the magnitude of the force, by
considering the diagram below. We know that when an infinitely small charge dq is made to flow at a velocity
‘v’ under the influence of an electric field E, and a magnetic field B, then the Lorentz Force dF experienced by
the charge is given by:-
For the operation of DC motor, considering E = 0
i.e. it’s the cross product of dq v and magnetic field B.
Where dL is the length of the conductor carrying charge q.
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From the 1st diagram we can see that the construction of a DC motor is such that the direction of current through
the armature conductor at all instance is perpendicular to the field. Hence the force acts on the armature
conductor in the direction perpendicular to the both uniform field and current is constant.
So if we take the current in the left hand side of the armature conductor to be I, and current at right hand side of
the armature conductor to be − I, because they are flowing in the opposite direction with respect to each other.
Then the force on the left hand side armature conductor,
Similarly force on the right hand side conductor
∴ we can see that at that position the force on either side is equal in magnitude but opposite in direction. And
since the two conductors are separated by some distance w = width of the armature turn, the two opposite forces
produces a rotational force or a torque that results in the rotation of the armature conductor. Now let's examine
the expression of torque when the armature turn crate an angle of α with its initial position. The torque produced
is given by,
Where, α is the angle between the plane of the armature turn and the plane of reference or the initial position of
the armature which is here along the direction of magnetic field. The presence of the term cosα in the torque
equation very well signifies that unlike force the torque at all position is not the same. It in fact varies with the
variation of the angle α. To explain the variation of torque and the principle behind rotation of the motor let us
do a step wise analysis.
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Step 1: Initially considering the armature is in its starting point or reference position where the angle α = 0.
Since, α = 0, the term cos α = 1, or the maximum value, hence torque at this position is maximum given by τ =
BILw. This high starting torque helps in overcoming the initial inertia of rest of the armature and sets it into
rotation.
Step 2: Once the armature is set in motion, the angle α between the actual position of the armature and its
reference initial position goes on increasing in the path of its rotation until it becomes 90° from its initial
position. Consequently the term cosα decreases and also the value of torque. The torque in this case is given by
τ = BILwcosα which is less than BIL w when α is greater than 0°.
Step 3: In the path of the rotation of the armature a point is reached where the actual position of the rotor is
exactly perpendicular to its initial position, i.e. α = 90°, and as a result the term cosα = 0. The torque acting on
the conductor at this position is given by,
i.e. virtually no rotating torque acts on the armature at this instance. But still the armature does not come to a
standstill, this is because of the fact that the operation of DC motor has been engineered in such a way that the
inertia of motion at this point is just enough to overcome this point of null torque. Once the rotor crosses over
this position the angle between the actual position of the armature and the initial plane again decreases and
torque starts acting on it again.
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2.8 Characteristics of DC Motor
Generally, three characteristic curves are considered important for DC motors which are, (i) Torque vs. armature
current, (ii) Speed vs. armature current and (iii) Speed vs. torque. These are explained below for each type of
DC motor. These characteristics are determined by keeping the following two relations in mind.
Ta ∝ ɸ.Ia and N ∝ Eb/ɸ
These above equations can be studied at - emf and torque equation of dc machine. For a DC motor, magnitude
of the back emf is given by the same emf equation of a dc generator i.e. E b = PɸNZ / 60A. For a machine, P, Z
and
A
are
constant,
therefore,
N
∝
Eb/ɸ
Characteristics Of DC Series Motors
Torque Vs. Armature Current (Ta-Ia)
This characteristic is also known as electrical characteristic. We know that torque is directly proportional to
the product of armature current and field flux, T a ∝ ɸ.Ia. In DC series motors, field winding is connected in
series with the armature, i.e. Ia = If. Therefore, before magnetic saturation of the field, flux ɸ is directly
proportional to Ia. Hence, before magnetic saturation Ta α Ia 2. Therefore, the Ta-Ia curve is parabola for smaller
values of Ia.
After magnetic saturation of the field poles, flux ɸ is independent of armature current Ia. Therefore, the torque
varies proportionally to Ia only, T ∝ Ia.Therefore, after magnetic saturation, Ta-Ia curve becomes a straight line.
The shaft torque (Tsh) is less than armature torque (Ta) due to stray losses. Hence, the curve Tsh vs Ia lies
slightly lower.
In DC series motors, (prior to magnetic saturation) torque increases as the square of armature current, these
motors are used where high starting torque is required.
Speed Vs. Armature Current (N-Ia)
We know the relation, N ∝ Eb/ɸ
For small load current (and hence for small armature current) change in back emf Eb is small and it may be
neglected. Hence, for small currents speed is inversely proportional to ɸ. As we know, flux is directly
proportional to Ia, speed is inversely proportional to Ia. Therefore, when armature current is very small the
speed becomes dangerously high. That is why a series motor should never be started without some
mechanical load.
But, at heavy loads, armature current Ia is large. And hence, speed is low which results in decreased back emf
Eb.
Due
to
decreased
Eb,
more
armature
current
is
allowed.
Speed Vs. Torque (N-Ta)
This characteristic is also called as mechanical characteristic. From the above two characteristics of DC
series motor, it can be found that when speed is high, torque is low and vice versa.
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Characteristics Of DC Shunt Motors
Torque Vs. Armature Current (Ta-Ia)
In case of DC shunt motors, we can assume the field flux ɸ to be constant. Though at heavy loads, ɸ decreases
in a small amount due to increased armature reaction. As we are neglecting the change in the flux ɸ, we can say
that torque is proportional to armature current. Hence, the Ta-Ia characteristic for a dc shunt motor will be a
straight
line
through
the
origin.
Since heavy starting load needs heavy starting current, shunt motor should never be started on a heavy load.
Speed Vs. Armature Current (N-Ia)
As flux ɸ is assumed to be constant, we can say N ∝ Eb. But, as back emf is also almost constant, the speed
should remain constant. But practically, ɸ as well as Eb decreases with increase in load. Back emf Eb decreases
slightly more than ɸ, therefore, the speed decreases slightly. Generally, the speed decreases only by 5 to 15% of
full load speed. Therefore, a shunt motor can be assumed as a constant speed motor. In speed vs. armature
current characteristic in the following figure, the straight horizontal line represents the ideal characteristic and
the actual characteristic is shown by the dotted line.
Characteristics Of DC Compound Motor
DC compound motors have both series as well as shunt winding. In a compound motor, if series and shunt
windings are connected such that series flux is in direction as that of the shunt flux then the motor is said to be
cumulatively compounded. And if the series flux is opposite to the direction of the shunt flux, then the motor is
said to be differentially compounded. Characteristics of both these compound motors are explained below.
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(a) Cumulative compound motor
Cumulative compound motors are used where series characteristics are required but the load is likely to be
removed completely. Series winding takes care of the heavy load, whereas the shunt winding prevents the motor
from running at dangerously high speed when the load is suddenly removed. These motors have generally
employed a flywheel, where sudden and temporary loads are applied like in rolling mills.
(b) Differential compound motor
Since in differential field motors, series flux opposes shunt flux, the total flux decreases with increase in load.
Due to this, the speed remains almost constant or even it may increase slightly with increase in load (N ∝ Eb/ɸ).
Differential compound motors are not commonly used, but they find limited applications in experimental and
research work.
2.8 Starting of DC Motor
Basic operational voltage equation of a DC motor is given as
E = Eb + IaRa and hence, Ia = (E - Eb) / Ra
Now, when the motor is at rest, obviously, the back emf Eb = 0. Hence, armature current at the moment of
starting can be given as Ia = E / Ra. In practical DC machines, armature resistance is basically very low,
generally about 0.5 Ω. Therefore, a large current flows through the armature during starting. This current is
large enough to damage the armature circuit.
Due to this excessive starting current -
1.
2.
the fuses may blow out and the armature winding and/or commutator brush arrangement may get damaged.
very high starting torque will be produced (as torque is directly proportional to the armature current), and
this high starting torque may cause huge centrifugal force which may throw off the armature winding.
3. other loads connected to the same source may experience a dip in the terminal voltage.
A large DC motor will pick up speed rather slowly due to its large rotor inertia. Hence, building up the back emf
slowly causing the level of high starting current maintained for quite some time. This may cause severe damage.
To avoid this, a suitable DC motor starter must be used. Very small dc motors, however, may be started
directly by connecting them to the supply with the help of a contactor or a switch. It does not result in any harm
because they gather speed quickly due to small rotor inertia. In this case, the large starting current will die down
quickly because of the fast rise in the back emf.
DC Motor Starters
To avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. So, a DC
motor is started by using a starter. There are various types of dc motor starters, such as 3 point starter, 4 point
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starter, no-load release coil starter, thyristor controller starter etc. The basic concept behind every DC motor
starter is adding external resistance to the armature winding during starting.
From the followings, 3 point starters and 4 point starters are used for starting shunt wound motors and
compound wound motors.
3 Point Starter
The
internal
wiring
of
a3
point
starter is
as
shown
in
the
figure.
When the connected dc motor is to be started, the lever is turned gradually to the right. When the lever touches
point 1, the field winding gets directly connected across the supply, and the armature winding gets connected
with resistances R1 to R5 in series. During starting, full resistance is added in series with the armature winding.
Then, as the lever is moved further, the resistance is gradually is cut out from the armature circuit. Now, as the
lever reaches to position 6, all the resistance is cut out from the armature circuit and armature gets directly
connected across the supply. The electromagnet 'E' (no voltage coil) holds the lever at this position. This
electromagnet
releases
the
lever
when
there
is
no
(or
low)
supply
voltage.
It can be seen that, when the arm is moved from the position 1 to the last position, the starter resistance gets
added in series with the field winding. But, as the value of starter resistance is very small as compared to the
shunt resistance, the decrease in shunt field current may be negligible. However, to overcome this drawback a
brass or copper arc may be employed within a 3 point starter which makes a connection between the moving
arm and the field winding, as shown in the figure of 4 point starter below.
When the motor is overloaded beyond a predefined value, 'overcurrent release electromagnet' D gets activated,
which short-circuits electromagnet E and, hence, releases the lever and the motor is turned off.
4 Point Starter
The main difference between a 3 point starter and a 4 point starter is that the no voltage coil (electromagnet
E) is not connected in series with the field coil. The field winding gets directly connected to the supply, as the
lever moves touching the brass arc (the arc below the resistance studs). The no voltage coil (or Hold-on coil) is
connected with a current limiting resistance Rh. This arrangement ensures that any change of current in the
shunt field does not affect the current through hold-on coil at all. This means, electromagnetic pull of the holdon coil will always be sufficient so that the spring does not unnecessarily restore the lever to the off position. A
4 point starter is used where field current is to be adjusted by means of a field rheostat for the purpose
of operating the motor above rated speed by reducing the field current.
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DC Series Motor Starter
Construction of DC series motor starters is very basic as shown in the figure. The start arm is simply moved
towards right to start the motor. Thus, maximum resistance is connected in series with the armature during
starting and then gradually decreased as the start arm moves towards right. This starter is sometimes also called
as a 2 point starter. The no load release coil holds the start arm to the run position and leaves it when the
voltage is lost.