ELECTRIC POWER UNIVERSITY
FACULTY OF CONTROL & AUTOMATION

ASSIGNMENT ON SPECIALIZED ENGLISH

Topic: Making A Study About DC Motor

Instructor: Ths. Mai Hoang Cong Minh
Group: 1
Student’s name:
Nguyen Hoang Anh - Code: 18810430122
Nguyen Le Truong An - Code: 18810430184
Le Tuan Anh - Code: 18810430164
Dang Van Dat - Code: 18810430179
Luong Quoc Dai - Code: 18810430183
Class: D13TDH&DKTBCN2

HaNoi,6/2021


Table of Contents
Introduction...................................................................................................................1
Chapter 1: General introduction of DC motor................................................................2
1, What is a DC Motor?.............................................................................................2
2, Classification of dc motors....................................................................................2
2.1, Separately Excited DC Motor.............................................................................3
2.2, Series excited DC motor......................................................................................4
2.3 Shunt excited DC Motors.....................................................................................5
2.4. Compound excited DC motor..............................................................................6
3, The advantage of a DC motor................................................................................7
4, Disadvantages of DC motor...................................................................................8

Chapter 2: Structure and working principle of DC motor..............................................9
1, Structure of a DC motor.........................................................................................9
1.1, Stator...................................................................................................................9
1.2, Rotor.................................................................................................................. 11
2, Working Principle of a DC Motor........................................................................14
2.1, Starting of DC Motors.......................................................................................17
2.2, Necessity of Starter for a DC Motor..................................................................18
2.3, Starting Methods of DC Motor................................................................19
Chapter 3 : Applications of DC Motors.......................................................................22
3.1, Applications of DC motor.................................................................................22
3.2, Applications of Series Excited DC motor.........................................................23
3.3, Applications of Shunt Excited DC motor..........................................................24
3.4, Applications of Compound Excited DC motor..................................................24
3.5, Applications of Separately Excited DC motor..................................................25
Chapter 4 : Conclusion................................................................................................26
References...................................................................................................................27


Table of Figures
Fig. 1.1: Separately Excited DC Motor..........................................................................3
Fig. 1.2: Series Excited DC Motor.................................................................................4
Fig. 1.3: Shunt Excited DC Motor.................................................................................5
Fig. 1.4: Compound Excited DC Motor.........................................................................6

Y
Fig. 2.1: Structure of DC Motor.....................................................................................9
Fig. 2.2: Yoke............................................................................................................... 11
Fig. 2.3: Field Winding................................................................................................11
Fig. 2.4: Armature Winding.........................................................................................12
Fig. 2.5: Armature core, Brush and Commutator.........................................................13

Fig. 2.6: Commutator...................................................................................................14
Fig. 2.7: Working principle of DC Motor....................................................................16
Fig. 2.8: Position of main field Fm and rotor field Fr..................................................17
Fig. 2.9: Motor Action.................................................................................................17
Fig. 2.10: Three-point shunt motor starter...................................................................20
Fig. 2.11: Four-Point DC motor starter........................................................................21


Introduction

Nowaday, the DC motor still occupies an important position in the automatic
control system of electric drive, it is widely used in the system that requires high
accuracy, wide adjustment area and complex regulation. complex. Along with the
progress of human civilization, we can witness the rapid development of both the scale
and the level of modern production. In that development we can also clearly recognize
and asserts that electricity and power-consuming machines play an indispensable role.
It is always one step ahead as a premise but also as a key to determining the success of
an industrial production system. There is not a country, a production that does not use
electricity and electric machines
Today, our country's economy is on a strong development trend towards
industrialization - modernization. And in modern production, DC machine is still
considered an important machine. It can be used as an electric motor, generator or in
other working conditions. Among them, electric motor is increasingly widely used in
industries from small to large. DC motor. has very good speed control effect, so the
machine is widely used in industries with high requirements for speed regulation such
as steel rolling, mining, transportation... Therefore, learning and applying DC motors
is an extremely necessary job for each of us engineers in particular and the automation
industry in general.

1



Chapter 1: General introduction of DC motor
1, What is a DC Motor?
A DC motor is an electric motor that runs on direct current (DC) electricity.

A direct current (DC) motor is a type of electric machine that converts electrical
energy into mechanical energy. DC motors take electrical power through direct
current, and convert this energy into mechanical rotation.
DC motors use magnetic fields that occur from the electrical currents generated,
which powers the movement of a rotor fixed within the output shaft. The output
torque and speed depends upon both the electrical input and the design of the
motor.
2, Classification of dc motors
Classification of DC motors When considering DC motors as well as DC
generators, they are classified according to the way the motor is excited. Accordingly,
we have 4 commonly used DC motors:
+) Separately Excited DC Motor: Armature and excitation are supplied from two
separate sources.
+) Shunt Excited DC motor: The field winding is connected in parallel with the
armature.
+) Series Excited DC motor: The excitation winding is connected in series with the
armature.
+) Compound Excited DC motor: Consists of 2 excitation windings, one in parallel
with the armature and one in series with the armature.

2


2.1, Separately Excited DC Motor


Fig. 1.1: Separately Excited DC Motor

In separately excited DC motor, separate supply Provided for excitation of
both field coil and armature coil. The field coil is energized from a separate DC
voltage source and the armature coil is also energized from another source.
Armature voltage source may be variable but, independent constant DC voltage
is used for energizing the field coil. So, those coils are electrically isolated from
each other, and this connection is the specialty of this type of DC motor.
Both in shunt wound dc motor and separately excited dc motor field is
supplied from constant voltage so that the field current is constant. Therefore,
these two motors have similar speed -armature current and torque – armature
current characteristics. In this type of motor flux is assumed to be constant.
Speed of this type of dc shunt motor is controlled by the following methods:
- Field control methods
- Field rheostat control
- Armature control methods
Separately excited dc motors have industrial applications. They are often
used as actuators. This type of motors is used in trains and for automatic traction
purposes.

3


2.2, Series excited DC motor

Fig. 1.2: Series Excited DC Motor
Series DC motors are a group of self-excited DC motors in which the field coil is
connected in series to the armature winding and thus a higher current pass through it.
A series DC motor mission is to turn the electrical energy into mechanical one based

on electromagnetic law. In this process, the cooperation between the magnetic field
present around a current-carrying conductor and an outside field results in a rotational
motion on an output shaft.
A series DC motor converts electric energy to mechanic energy based on the
electromagnetic principle. In this type of DC motor, a power supply terminal sits at
one end of the armature and field coils. By applying the voltage, power starts in these
terminals and passes through armature and field windings. As conductors in this
winding are huge, they have minimal resistance. As a result, the motor gets a huge
supply of power from the terminals. By the flow of this large current in the armature
and field coils, a strong magnetic field is produced that creates a massive torque in the
shafts. This strong torque, in turn, spins the armature and produces the target mechanic
energy.
The series DC motors are used where high starting torque is required and
variations in speed are possible. For example – the series motors are used in the
traction system, cranes, air compressors, Vaccum Cleaner, Sewing machine, etc .

4


2.3 Shunt excited DC Motors

Fig. 1.3: Shunt Excited DC Motor

Shunt DC motors are a type of direct connection motors in which the shunt
field winding is only shunt to the armature winding and not the series field
winding. Shunt DC motors are also known as compound wound DC motors.
Shunt DC motors provide excellent speed regulation, as the shunt field can be
excited separately from the armature windings. This feature of shunt direct
connection motors provides simplified reversing controls.
Because of their self-regulating speed capabilities, shunt DC motors are

ideal for applications where precise speed control is required. Keep in mind,
however, that they cannot produce high starting torque, so the load at startup
must be small. Applications that meet these criteria and are suitable for shunt
DC motors include machines tool such as lathes and grinders, and industrial
equipment such as fans and compressors.

5


2.4. Compound excited DC motor

Fig. 1.4: Compound Excited DC Motor

Compound DC motors or compound wound motors have both series and
shunt field windings. These types of motors offer good starting torque, but they
may have control problems in variable speed drive applications. These motors
can be connected to two arrangements: cumulatively and differentially. The
cumulative type connects the series field to aid the shunt field, providing higher
starting torque but less speed regulation. The differential type has good speed
regulation and usually operates at a constant speed.
Because of the capabilities of the compound motors to provide high starting
torque and also speed regulation and control it has many applications in different
areas. Below we listed some of the applications of a compound DC motor:
- Compound motors due to their ability to perform better on heavy load changes
are used in elevators.
- Due to their high starting torque and better speed control for pressure
variations, they are used in shears and punches.
- This kind of motors because of the high starting torque and heavy-duty load is
used in steel rolling mills.
- Again due to the capacity of driving heavy loads, they are used in the printing

press and cutting machines.
- They are also used in stamping presses to provide high starting torque.
- Their good speed control and high starting torque make them a great choice to
be used in mixers.
6


The small DC machines whose ratings are in fractional kilowatt are mainly used
as control device such in techno generators for speed sensing and in servo
motors for positioning and tracking.
3, The advantage of a DC motor
The advantage of a DC motor is that it can be used as an electric motor or
generator in different working conditions. But the biggest advantage of DC motors is
speed regulation and overload capacity. If the asynchronous motor itself cannot meet
the requirements, or if it is, the cost of associated converters (such as frequency
converters....) is very expensive, then the DC motor does not. Wide and precise
adjustments can be made, but the power circuit structure, the control circuit is simpler,
and the quality is high.
Different sizes of DC motor parts will create different DC motors, suitable for
different needs. As mentioned before, small ones can be used in toys, tools, and home
appliances and larger ones are used in the elevator and hoists and propulsion of electric
vehicles. Although AC motors decreased the selling of DC motors on account of
simple generation and transmission with fewer losses to long distances, needing less
maintenance and can be operated in explosive atmospheres, DCs still are utilized in
where ACs can’t fulfill the needs. DCs have their own unique features and importance
in industries that make up for lots of other advantages that AC motors have over them.
DC motors are suitable for low-speed torque, or when having an adjustable speed
and constant is necessary. In other words, with DC motors the speed can be controlled
over a wide range. That means they offer a wide range of speed control both below and
above the rated speed. This feature of DC motors can be got in shunt types. By

armature controlling and field controlling, you can enjoy this unique advantage of DC
motors over AC motors. Moreover, DCs have a very high and strong starting torque
compared to normal operating torque. Therefore, DCs are used in electric trains and
cranes having overwhelming burdens in the beginning conditions. In addition to the
above-mentioned advantages, DC motors have smaller converters and drives as well as
higher motor power density. Not to mention that they have full torque at zero speed!
Being around on the market for more than 140 years, DC motors are often more
affordable than AC motors and have a simpler and more efficient design. Plus, their
maintenance is easy and takes little to no time. If you redesign your current installation
to use an AC motor, it will cost way more than just simply replacing the DC motor
inside the installation. So, you not only repair your system by installing a new unit
inside but save lots of money. Needless to said that such small replacement saves time
7


as well and happens quickly without wasting your time. Need more advantages to fall
in love with DC motor parts and structure?
4, Disadvantages of DC motor
Disadvantages are high maintenance and low life-span for high intensity uses.
Maintenance involves regularly replacing the brushes and springs which carry the
electric current, as well as cleaning or replacing the commutator. These components
are necessary for transferring electrical power from outside the motor to the spinning
wire windings of the rotor inside the motor
Sparks generated on the commutator and brushes will be dangerous in an explosive
atmosphere.
Expensive price, not high capacity.

8



Chapter 2: Structure and working principle of DC motor

Before understanding the working of DC motor, first, we have to know
about its construction. rotor and stator are the two main parts of the DC motor.
Rotor is the rotating part, and the stator is their stationary part. The armature coil
is connected to the DC supply.
The armature coil consists the commutators and brushes. The commutators
convert the AC induced in the armature into DC and the brushes transfer the
current from rotating part of the motor to the stationary external load. The
armature is placed between the north and south pole of the permanent or
electromagnet.
1, Structure of a DC motor

Fig. 2.1: Structure of DC Motor
The DC motor can be divided into two main parts: the stationary part (stator) and the
dynamic part (Rotor).
1.1, Stator
This is the static part that holds the field windings, receives the power supply and
forms the outer part of the motor. The stator of a DC motor consists of two or more

9


pieces of permanent magnet and pole. Here, a coil is wound into a magnetic
component to form the stator.
A stator is one of the DC motors parts that is, as the name suggests, a static unit
containing the field windings. The stator is the DC Motor part that receives the supply.
1.1.1, Main magnetic pole
The part that generates the magnetic field, including the magnetic iron core and
the field winding wire outside the iron core. The magnetic pole core is made of 0.5 to

1mm thick sheet of electrical engineering steel or carbon steel, which is pressed and
spread tightly. In small electric motors, block steel can be used. The magnetic pole is
fastened to the case by bolts. The field winding is wrapped with insulated copper wire
and each coil is thoroughly insulated into a block, impregnated with insulating paint
before being placed on the magnetic poles. The field windings placed on these
magnetic poles are connected in series
1.1.2, Secondary magnetic pole
The auxiliary magnetic pole is placed on the main magnetic pole. The steel core
of the auxiliary magnetic pole is usually made of block steel and on the body of the
auxiliary magnetic pole, there is a winding wire that is similar to that of the main
magnetic pole. The auxiliary magnetic pole is attached to the case by bolts.
1.1.3, Magnetic shaft: is used as a magnetic circuit connecting the magnetic poles,
and at the same time as the case. In small and medium electric motors, thick steel is
often used to bend and weld, in large electric machines, cast steel is often used.
Sometimes in small electric motors, cast iron is used as the body
1.1.4, Yoke of DC Motor
The magnetic frame or the yoke of DC motor made up of cast iron or steel and
forms an integral part of the stator or the static part of the motor.
Its main function is to form a protective covering over the sophisticated inner parts of
the motor and provide support to the armature. It also supports the field system by
housing the magnetic poles and field winding of the DC motor.

10


Fig. 2.2: Yoke
1.1.5, Field Winding
Field winding: wrapped with insulated copper wire and each coil is carefully
insulated into a block, impregnated with insulating paint before being placed on the
magnetic pole.


Fig. 2.3: Field Winding
1.1.6, Machine Cover
To protect the machine from falling objects that damage the winding and to protect
people from electrical contact. In small and medium-sized electric machines, the lid
also acts as a bearing holder. In this case the lid is usually made of cast iron.
1.2, Rotor
A rotor is the rotating part of the DC machine that brings about the mechanical
rotations.
This is the inner rotating part which carries out the mechanical rotations. This part
consists of windings connected to the external supply circuit via commutators.
Ferromagnetic materials are used in the construction of stator and rotor and an air-gap
separates the parts.
11


The part that generates the electromotive force includes: The magnetic circuit is
made of ferromagnetic materials (technical steel sheets) folded together. On the
magnetic circuit there are grooves to nest the armature winding. Armature winding:
Consists of many wires connected together according to a certain rule. Each coil
consists of many turns of wire, the ends of the wire are connected to copper plates
called commutators, which are insulated from each other and isolated from the shaft
called commutator or commutator. The hinges on the commutator are a pair of
charcoal made of graphite and are attached to the commutator wall by springs.
1.2.1, Armature Winding of DC Motor
The armature winding of DC motor is attached to the rotor, or the rotating part of
the machine, and as a result is subjected to altering magnetic field in the path of its
rotation which directly results in magnetic losses. For this reason the rotor is made of
armature core, that’s made with several low-hysteresis silicon steel lamination, to
reduce the magnetic losses like hysteresis and eddy current loss respectively. These

laminated steel sheets are stacked together to form the cylindrical structure of the
armature core
Slots are found in the armature core which is made of the same material as that of
core on to which the windings of copper wire with several turns are distributed over
the whole periphery of the armature core uniformly. Fibrous wedges are used to shut
the slot openings so as to avoid the conductor from moving out caused due to the high
amount of centrifugal force that is produced during the armature rotation in the
presence of magnetic field and supply current.

Fig. 2.4: Armature Winding
1.2.2, Armature core of DC motor
The armature core is provided with slots made of the same material as the core to
which the armature winding made with several turns of copper wire distributed
12


uniformly over the entire periphery of the core. The slot openings a shut with fibrous
wedges to prevent the conductor from plying out due to the high centrifugal force
produced during the rotation of the armature, in presence of supply current and field.
Used for magnetic conduction, usually 0.5mm thick electrical engineering steel
sheets coated with thin insulation on both sides and then pressed tightly to reduce
losses caused by eddy currents. On the steel foil, there is a groove shape stamping so
that after pressing it, the wire is put in. In the medium and higher engines, people also
stamp the ventilation holes so that when pressed into the iron core, it is possible to
create axial ventilation holes.

Fig. 2.5: Armature core, Brush and Commutator
1.2.3, Brushes of DC Motor
The brushed DC motor generates torque directly from DC power supplied to the
motor by using internal commutation, stationary permanent magnets, and rotating

electrical magnets
The brushes of DC motor are made with carbon or graphite structures, making
sliding contact over the rotating commutator. The brushes are used to relay the current
from external circuit to the rotating commutator form where it flows into the armature
winding. So, the commutator and brush unit of the DC motor is concerned with
transmitting the power from the static electrical circuit to the mechanically rotating
region or the rotor.
1.2.4, Commutator of DC Motor
The commutator of DC motor is a cylindrical structure made up of copper
segments stacked together, but insulated from each other by mica. Its main function as
far as the DC motor is concerned is to commute or relay the supply current from the
mains to the armature winding housed over a rotating structure through the brushes of
DC motor.
13


The commutator is a split ring made up of Copper segments, Commutator is
another DC motor part. The operating system of a DCs is based on the interaction of
the two magnetic fields of rotating armature and a fixed stator. As the north pole of the
armature is attracted to the south pole of the stator and south pole of armature is
attracted to the north pole of the stator, a force is produced on the armature which
makes it to turn. the process in which the field in the armature windings is switched to
produce constant torque in one direction is called Commutation. the commutator is a
device connected to the armature enabling this switching of current. Different
segments of its cylindrical structure are insulated from each other by Mica. The
commutator is designed to commute the supply current to the armature winding from
the mains. The commutator passes through the brushes of the DC motor.
The basic purpose of commutation is to certify that the torque acting on the
armature is always in the same direction. Naturally, the generated voltage in the
armature is alternating, the commutator converts it to the direct current. To control the

direction the electromagnetic fields are pointing to, the commutator turns the coils on
and off. On one side of the coil, the electricity should always flow away, and on the
other side, electricity should always flow towards. This ensures that the torque is
always produced in the same direction.

Fig. 2.6: Commutator

1.2.5, Shaft
Shaft: The part on which the armature iron core, commutator, propeller, ball
bearing... The machine shaft is usually made of good carbon steel.
2, Working Principle of a DC Motor
The operation of a DC motor is based on the principle that when a current carrying
conductor is placed in a magnetic field, a mechanical force is experienced by it. The
14


direction of this force is determined by Fleming’s Left-Hand Rule and its magnitude is
given by the relation:
F = BIL Newton
 Fleming’s left-hand rule:

Fig.: Fleming’s left hand rule
Fleming’s left-hand rule is use to determine direction of force acting on the
armature conductor of DC motor.
Statement: “If you keep your left hand in a way that the thumb, first finger and middle
finger are at right angle to each other” as shown in fig: “first finger represent direction
of magnetic field, middle finger represent direction of the current in conductor, thumb
will represent direction of force act on the conductor”.
For simplicity, consider only one coil of the armature placed in the magnetic field
produced by a bipolar machine [see Fig. 2.7a]. When DC supply is connected to the

coil, current flows through it which sets up its own field as shown in Fig. 2.7b. By the
interaction of the two fields (i.e., field produced by the main poles and the coil), a
resultant field is set up as shown in Fig. 2.7c. The tendency of this is to come to its
original position i.e., in straight line due to which force is exerted on the two coil sides
and torque develops which rotates the coil.

15


a, Main field

b, Field due to current carrying coil

c, Resultant field
Fig. 2.7: Working principle of DC Motor
Alternately, it can be said that the main poles produce a field Fm. Its direction is
marked in Fig. 2.8. When current is supplied to the coil (armature conductors), it
produces its own field marked as Fr. This field tries to come in line with the main field
and an electromagnetic torque develops in clockwise direction as marked in Fig. 2.8.
In actual machine, a large number of conductors are placed on the armature. All
the conductors, placed under the influence of one pole (say, North pole) carry the
current in one direction (outward). Whereas, the other conductors placed under the
influence of other pole i.e., south pole, carry the current in opposite direction as shown
in Fig. 2.9. A resultant rotor field is produced. Its direction is marked by the arrowhead Fr. This rotor field Fr tries to come in line with the main field Fm and torque (Te)
develops. Thus, rotor rotates.

16


Fig. 2.8: Position of main field Fm and rotor field Fr


Fig. 2.9: Motor Action
It can be seen that to obtain a continuous torque, the direction of flow of current in
each conductor or coil side must be reversed when it passes through the magnetic
neutral axis (MNA). This is achieved with the help of a commutator.
Function of a Commutator
The function of a commutator in DC motors is to reverse the direction of flow of
current in each armature conductor when it passes through the M.N.A. to obtain
continuous torque.
2.1, Starting of DC Motors
To start a DC motor, when it is switched–ON to the supply with full rated voltage, it
draws heavy current during starting period (more than its rated value). This excessive
current overheats the armature winding and may even damage the winding insulation.
Therefore, during starting period a resistance called starter in connected in series with
the armature circuit to limit the starting current.

17


2.2, Necessity of Starter for a DC Motor
The starting of DC motor is somewhat different from the starting of all other types of
electrical motors. This difference is credited to the fact that a DC motor unlike other
types of motor has a very high starting current that has the potential of damaging the
internal circuit of the DC motor if not restricted to some limited value. This limitation
to the starting current of DC motor is brought about by means of the starter. Thus the
distinguishing fact about the starting methods of DC motor is that it is facilitated by
means of a starter. Or rather a device containing a variable resistance connected in
series to the armature winding so as to limit the starting current of DC motor to a
desired optimum value taking into consideration the safety aspect of the motor.
E b  V  I aR a


or

I aRa  V  E b

Where, E is the supply voltage, Ia is the armature current, Ra is the armature resistance
The armature current is given by the relation

Ia 

V  Eb
Ra

When the motor is at rest, the induced emf Eb in the armature is zero (Eb v N).
Consequently, if full voltage is applied across the motor terminals, the armature will
Ia 

V
Ra

draw heavy current (
) because armature resistance is relatively small. This
heavy starting current has the following effects:
(i)

It will blow out the fuses and prior to that it may damage the insulation of
armature winding due to excessive heating effect if starting period is more.

(ii)


(ii) Excessive voltage drop will occur in the lines to which the motor is
connected. Thus, the operation of the appliances connected to the same line
may be impaired and in some cases they may refuse to work.

To avoid this heavy current at start, a variable resistance is connected in series with the
armature, called a starting resistance or starter, and thus the armature current is limited

to safe value

is reduced

(I a 

(I a 

V
)
Ra  R

V  Eb
)
Ra  R

.Once the motor picks up speed, emf is built up and current

. After that the starting resistance is gradually reduced.
18


Ultimately, whole of the resistance is taken out of circuit when the motor attains

normal speed.
Another important feature of a starter is that it contains protective devices such as
overload protection coil (or relay) which provides necessary protection to the motor
against over loading and no-volt release coil.
2.3, Starting Methods of DC Motor
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 starter, no-load release coil starter,
thyristor controller starter etc.
2.3.1, Three-Point Shunt motor Starter
The schematic connection diagram of a shunt motor starter is shown in Fig. 2.10. It
consists of starting resistance R divided into several sections. The tapping points of
starting resistance are connected to number of studs. The last stud of the starting
resistance is connected to terminal A to which one terminal of the armature is
connected. The + ve supply line is connected to the line terminal L through main
switch. From line terminal, supply is connected to the starting lever SL through over
load release coil OLRC. A spring S is placed over the lever to bring it to the off
position, when supply goes off. A soft iron piece SI is attached with the starting lever
which is pulled by the no volt release coil under normal running condition. The far end
of the brass strip BS is connected to the terminal Z through a no volt release coil
NVRC. One end of the shunt field winding in connected to Z terminal of the starter.
An iron piece is lifted by OLRC under abnormal condition to short circuit the no-volt
release coil. The negative supply line is connected directly to the other ends of shunt
field winding and armature of the DC shunt motor.

19


Fig. 2.10: Three-point shunt motor starter
Operation

First of all the main switch is closed with starting lever resting in off position. The
handle is then turned clockwise to the first stud and brass strip. As soon as it comes in
contact with first stud, whole of the starting resistance R is inserted in series with the
armature and the field winding is directly connected across the supply through brass
strip. As the handle is turned further the starting resistance is cut out of the armature
circuit in steps and finally entire starting resistance is cut out of armature circuit.

2.3.2, Four-point Starter
For speed control of DC shunt or compound motors, a rheostat (variable resistor Rh) is
connected in series with the field winding, as shown in Fig. 2.11. In this case, if a
three-point starter is used and the value of Rh is so adjusted that the current flowing
through the shunt field winding is very small. It may be seen that the same current
flows through the no-volt release coil, then the magnetic strength of the coil may be
insufficient to hold the plunger at its ON position. This is an undesirable feature of a
three-point starter. This feature makes a three-point starter unsuitable for such
applications. Accordingly, a four-point starter is designed, as shown in Fig. 2.11, in
which the current flowing through the no-volt release coil is made independent of the
20


shunt field circuit. Figure 2.11a shows a four-point starter used with a shunt wound
motor whereas Fig. 2.11b shows a starter used with a compound wound machine.

a, Four-Point Shunt motor starter

b, Four-Point Shunt motor starter

Fig. 2.11: Four-Point DC motor starter
Operation
The working of a four-point starter is similar to a three-point starter with slight

changes.In this case, when the plunger touches the first stud, the line current is divided
into the following three parts:
(i)

First part passes through starting resistance and armature (as well as in

(ii)

series field for compound motors).
Second part passes through the field winding (and speed control resistance if

(iii)

applied) and
The third part passes through no-volt release coil and protective resistance
connected in series with the coil.

21


Chapter 3 : Applications of DC Motors

In the previous chapter, we went to learn the structure and working principle of DC
motor. In this section, we will introduce the applications of DC motors commonly used
today
3.1, Applications of DC motor
Generally, because of some specific advantages of each type of DC motors, there
are various uses of them. At home, small ones are used in tools, toys and many
household appliances. Some other applications of DCs include conveyors and
turntables and in industries, huge DCs usages consist of braking and reversing

applications. We tried to bring some specific examples as the DCs applications:
3.1.1, Pumps
Hydraulic pumps as an essential industrial tool are used in almost all industries
such as construction, mining, manufacturing and steel. DC motors because of their
variable speed control and also excellent starting torque are used to empower these
kinds of pumps. Most of the time lower-cost brushless DCs are used in pumps which
make it far easier to maintain on such a large industrial scope.
3.1.2, Toys
Due to the fact that small DC motors are easy to use and considerably rugged, they
are the best choice of manufacturer and hobbyists for children’s toys such as remotecontrol cars and trains. Toys that require different range of speed and types of
movements need a motor with a wide variety of voltages. Manufacturers find all of
these specifications in DCs.
3.1.3, Electric Cars
Another application for DCs is in electric cars. DC motors because of their energy
efficiency and durability are one of most favorite options for electric vehicles.
Moreover, many hobby use DCs because of their great and higher starting torque
particularly series-wound motors, and their variable speeds with voltage input.
3.1.4, Robots
For many hobbyist and engineering robots are any electromechanical devices
designed to do one or more specific tasks. To activate things like tracks, arm or
cameras, DC motors are one of the most achievable and reasonable choices with lower
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