Construction of DC Motor

7/30/2019 Construction of DC Motor

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Principle of DC Motor

Current Motor works on the principal, when a current carrying conductor is placed in a

magnetic field, it experiences a torque and has a tendency to move. This is known as

motoring action. If the direction ofelectric currentin the wire is reversed, the direction of

rotation also reverses. When magnetic field and electric field interact they produce amechanical force, and based on that the working principle ofdc Direct motor established.

The direction of rotation of a this This DC ormotor is given by Flemings left hand rule,

which states that if the index finger, middle finger and thumb of your left hand are extended

mutually perpendicular to each other and if the index finger represents the direction of

magnetic field, middle finger indicates the direction ofelectric current, then the thumb

represents the direction in which force is experienced by the shaft of the dc motor

Construction of DC Motor

A DC Motorlike we all know is a device that deals in the conversion of Electrical energy to

mechanical energy and this is essentially brought about by two major parts require for

the construction of dc motor, namely

1) StatorThe static part that houses windings and receives the supply and the field

2) RotorThe rotating part that brings about the mechanical rotations.

Other than that there are several subsidiary parts namely the

3)Yoke of dc motor.

4)Poles of dc motor.

5)Field winding of dc motor.
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6)Armature winding of dc motor.

7)Commutator of dc motor.

8)Brushes of dc motor.

All these parts put together configures the total construction of a dc motor,

Now lets do a detailed discussion about all the essential parts of dc motor

Yoke of DC Motor
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The magnetic frame or theyoke 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 inner sophisticated parts of the motor and provide support to the

armature. It also supports the field system by housing the magnetic poles and field windings

of the dc motor.

Poles of dc motor

The magnetic poles of DC motor are structures fitted onto the inner wall of the yoke with

screws. The construction of magnetic poles basically comprises of two parts namely, the pole

core and the pole shoe stacked together under hydraulic pressure and then attached to the

yoke. These two structures are assigned for different purposes, the pole core is of small cross

sectional area and its function is to just hold the pole shoe over the yoke, whereas the poleshoe having a relatively larger cross-sectional area spreads the flux produced over the air gap

between the stator and rotor to reduce the loss due to reluctance. The pole shoe also carries

slots for the field windings that produce the field flux.


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Field Winding of DC Motor

The field winding of dc motor are made with field coils (copper wire) wound over the slots ofthe pole shoes in such a manner that when field current flows through it, then adjacent poles

have opposite polarity are produced. The field windings basically form an electromagnet, that

produces field flux within which the rotor armature of the dc motor rotates, and results in the

effective flux cutting.

Armature winding of DC Motor


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Armature winding of DC Motor

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, thats 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.

The armature core are provided with slots made of the same material as the core to which the

armature windings made with several turns of copper wire distributed uniformly over the ofsupply current and field.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 supply current and field

Lap Winding

In this case the number of parallel paths between conductors A is equal to the number of

poles P.


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i.e A = P

***An easy way of remembering it is by remembering the word LAP L A=P

Wave Winding

Here in this case, the number of parallel paths between conductors A is always equal to 2

irrespective of the number of poles.

Hence the machine designs are made accordingly.

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 windings

housed over a rotating structure through the brushes of dc motor.

Brushes of DC Motor

Thebrushes of dc motor are made with carbon or graphite structures, making sliding contact

over the rotating commutator. The brushes are used to relay theelectric currentfrom external

circuit to the rotating commutator form where it flows into the armature windings. 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
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Electrical motors are everywhere around us. Almost all the electromechanical movements we

see around us are caused either by an A.C. or a DC motor. Here we will be exploring this

kind of motors. This is a device that converts DC electrical energy to a mechanical energy.

Detailed Description of a DC Motor

To understand the DC motor in details lets consider the diagram below, The direct current

motor represented by the circle in the center, on which is mounted the brushes, where we

connect the external terminals, from where supply voltage is given. On the mechanical

terminal we have a shaft coming out of the Motor, and connected to the armature, and the

armature-shaft is coupled to the mechanical load. On the supply terminals we represent the

armature resistance Ra in series. Now, let the input voltage E, is applied across the

brushes.Electric currentwhich flows through the rotor armature via brushes, in presence of

the magnetic field, produces a torque Tg

. Due to this torque Tg

the dc motor armature

rotates. As the armature conductors are carrying currents and the armature rotates inside the

stator magnetic field, it also produces an emf Eb in the manner very similar to that of a

generator. The generated Emf Eb is directed opposite to the supplied voltage and is known as

the back Emf, as it counters the forward voltage

The back emf like in case of a generator is represented byis
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Where, P = no of poles

= flux per pole

Z= No. of conductors

A = No. of parallel paths

and N is the speed of the DC Motor.

So from the above equation we can see Ebis proportional to speed N. That is

whenever a Direct Current Motor rotates, it results in the generation of back Emf. Now

lets represent the rotor speed by in rad/sec. So Ebis proportional to .

So when the speed of the motor is reduced by the application of load, Eb decreases.

Thus the voltage difference between supply voltage and back emf increases that means

E Eb increases. Due to this increased voltage difference, armature current will

increase and therefore torque and hence speed increases. Thus a DC Motor is capable

of maintaining the same speed under variable load.

Now armature current Ia is represented by

Now at starting,speed = 0 so at starting Eb = 0.

Now since the armature winding resistance Ra is small, this motor has a very high starting

current in the absence of back Emf. As a result we need to use a starter for starting a DC

Motor

Now as the motor continues to rotate, the back Emf starts being generated and gradually the

current decreases as the motor picks up speed


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Working or Operating Principle of DC Motor

ADC Motorin simple words is a device that converts direct current(electrical energy) intomechanical energy. Its 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

The very basicconstruction of a dc motorcontains a current carrying armature which is

connected to the supply end through commutator segments and brushes and 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 hand rule to determine the direction of force acting on the armature

conductors of dc motor of Flemings left.
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Flemings 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) mutua perpendicular to both the direction of

field and the current in the conductor.

For understanding the principle of DC motor we have to determine the magnitude of

the force, by considering the diagram below clear We know that when an infinitely

small charge dq is made to flow at a velocity v under the influence of an electric fieldE, and a magnetic field B, then the Lorentz Force dF experienced by the charge is

given by:-


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dF = dq(E + v X B)

For the operation of dc motor, considering E = 0

dF = dq v X B

i.e. its the cross product of dq v and magnetic field B.

or dF = dq (dL/dt) X B [v = dL/dt]

Where dL is the length of the conductor carrying charge q.

or dF = (dq/dt) dL X B

or dF = I dL X B [Since, current I = dq/dt]

or F = IL X B = ILB Sin

or F = BIL Sin

From the 1st diagram we can see that theconstruction of a dc motoris such that the

direction ofelectric currentthrough 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 andcurrentisconstant.

i.e.=90
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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

armatureconductor,Fl =BILSin90=BIL

Similarly force on the right hand side conductor F r= B( I)L.Sin90 = BIL

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, thetwo opposite forces producesa rotational force or a torquethat results in the rotation of the armature conductor.

Now lets examine the expression of torque when the armature turn crate an angle of

with its initial position. The torque produced is given by

Torque = force, tangential to the direction of armature rotation X distance.

or=Fcos.wor

= BIL w cos

Where 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 cos 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 . To explain the variation of torque and the principle behind rotation of the

motor let us do a stepwise analysis


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Step1

Initially considering the armature is in its starting point or reference position where the

angle

a=0.

=Bilwcos0=BILw

Since = 0, the term cos = 1, or the maximum value, hence torque at this position is

maximum given by = BILw. This high starting torque helps inovercomingtheinitialinertia of rest of the armature and sets it into rotation.


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Step 2:

Once the armature is set in motion, the angle between the actual position of the armatureand its reference initial position goes on increasing in the path of its rotation until it becomes

90 from its initial position. Consequently the term cos decreases and also the value of

torque.


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The torque in this case is given by = BILwcos which is less than BIL w when 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. = 90, and as a result the term cos

= 0.

The torque acting on the conductor at this position is given by.

= BILwcos90 = 0


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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 anglebetween the actual position of the armature and the initial plane again decreases and torque

starts acting on it again

Torque Equation of DC Motor

The term torque as best explained by Dr. Huge d Young is the quantitative measure of

the tendency of a force to cause a rotational motion, or to bring about a change in

rotational motion. It is in fact the moment of a force that produces or changes a

rotational motion.

The equation of torque is given by

= F.R.sin-(1)


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Where F is force in linear direction.

R is radius of the object being rotated,

and is the angle, the force F is making with R vector

and is the angle, the force F is making with R vector

Thedc motoras we all know is a rotational machine, and torque of dc motor is a

very important parameter in this concern, and its of utmost importance to understand

thetorque equation of dc motor for establishing its running characteristics. Referring

to the diagram beside, we can see, that if E is the supply voltage, Eb is the back emf

produced and Ia, Ra are the armature current and armature resistance respectively then

the voltage equation is given by.

E=Eb +Ia.Ra-(2)

But keeping in mind that our purpose is to derive thetorque equation of dc motor we
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multiply both sides of equation (2) by Ia.

EIa =Eb Ia +Ia2.Ra-(3)

Now Ia2.Ra is the power loss due to heating of the armature coil, and the true effective

mechanical power that is required to produce the desired torque of dc

To establish the torque equation, let us first consider the basic circuit diagram of a dc

motor, and its voltage equation.

machine is given by

Pm = Eb.Ia-(4)

The mechanical power Pm is related to the electromagnetic torque Tg as

Pm = Tg. -(5)

Where is speed in rad/sec.

Now equating equation (4) & (5) we get

Eb.Ia = Tg.

Eb=pzn/60A..(6)

Now for simplifying the torque equation of dc motor we substitute Where, P is no of poles

is flux per pole

Z is No. of conductors

A is No. of parallel paths

and N is the speed of the D.C. Motor.

W=2n/60.(7)

Substituting equation (6) and (7) in equation (4), we get:

Tg=PZIa/2A

The torque we so obtain, is known as the electromagnetic torque of dc motor, and

subtracting the mechanical and rotational losses from it we get the mechanical torque.

Tm = Tgmechanical losses.

This is the torque equation of dc motor. It can be furthersimplified
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as:

Tg = kaIa

Ka=PZ/2A

Which is constant for a particular machine and therefore the torque of dc motor varies

with only flux and armature current Ia

The Torque equation of a dc motor can also be explained considering the figure below.

Current/conductorIc =Ia /A

forceperconductor=fc =BLIa/A


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Now torque Tc = fc.r = BLIa.r/A

Tc=PIa/2A

Hence the total torque developed of a dc machine isit

Tg=PZIa/2A

This torque equation of dc motor can be further simplified as:

Tg = kaIa

Ka=PZ/2A

Which is constant for a particular machine and therefore the torque of dc motor varies

with only flux and armature current Ia

Types of DC Motor Separately Excited Shunt Series Compound

DC Motor

Thedirect current motoror thedc motorhas a lot of application in todays field of

engineering and technology. Starting from an Electric shaver to parts of automobiles, in

all small or medium sized motoring applicationsDC motorscome handy. And because

of its wide range of application different functional types of dc motor are available in

the market for specific requirements.

The types of DC motor can be listed as follows

DC Motor Separately Excited DC Motor Self Excited DC MotorShunt Wound DC Motor

Series Wound DC Motor

Compound Wound DC Motor

Cumulative Compound DC Motor

Short Shunt DC Motor

Long Shunt DC Motor

Differential Compound DC Motor


Long Shunt DC Motor

Permanent Magnet DC Motor
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Now lets do a detailed discussion about all the essential types of dc motor.


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Separately Excited DC Motor

As the name suggests, in case of a separately excited DC Motor the supply is given

separately to the field and armature windings. The main distinguishing fact in these types

of dc motor is that, the armature current does not flow through the field windings, as the

field winding is energized from a separate external source of dc current as shown in the

figure beside.

From thetorque equation of dc motorwe know Tg = Ka Ia So the torque in this case can be

varied by varying field flux , independent of the armature current I a.

Permanent Magnet DC Motor

The permanent magnet DC motor consists of an armature winding as in case of an

usual motor, but does not necessarily contain the field windings. The construction of

these types of DC motor are such that, radially magnetized permanent magnets are

mounted on the inner periphery of the stator core to produce the field flux. The rotor on

the other hand has a conventional dc armature with commutator segments and brushes.

The diagrammatic representation of a permanent magnet dc motor is given below.
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Thetorque equation of dc motorsuggests Tg = Ka Ia. Here is always constant, as

permanent magnets of required flux density are chosen at the time of construction and

cant be changed there after.

for a permanent magnet dc motor Tg = Ka1 Ia

Where Ka1 = Ka. which is another constant. In this case the torque of DC Motorcan only

be changed by controlling armature supply.

Self Excited DC Motor

In case of self excited dc motor, the field winding is connected either in series or in parallel or

partly in series, partly in parallel to the armature winding, and on this basis its further classified

as:

i)Shunt Wound DC Motor

ii)Series Wound DC Motor

iii)Compound Wound DC Motor

Lets now go into the details of these types of self excited dc motor

Shunt Wound DC Motor
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In case of a shunt wound dc motor or more specificallyshunt wound self excited dc motor, the

field windings are exposed to the entire terminal voltage as they are connected in parallel to the

armature winding as shown in the figure below.

To understand the charecterestic of these types of DC motor, lets consider the basic voltage

equation given by

E=Eb +Ia Ra(1)

[Where E, Eb, Ia, Ra are the supply voltage, back emf, armature current and armature

resistance respectively]

Now Eb =ka (2)

[since back emf increases with flux and angular speed ]

Now substituting Eb from equation (2) to equation (1) we get,

E = ka + Ia Ra

Thetorque equation of a dc motorresembles Tg = KaIa(4)

Substituting the torque equation in equation (3) we get

This is similar to the equation of a straight line, and we can graphically representing the

torque speed characteristic of a shunt wound self excited dc motor as
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The shunt wound dc motor is a constant speed motor, as the speed does not vary here with the

variation of mechanical load on the output.

Series Wound DC Motor

In case of a series wound self excited dc motor or simply series wound dc motor, the entirearmature current flows through the field winding as its connected in series to the armature

winding. The series wound self excited dc motor is diagrammatically represented below for clear

understanding.


27/49

Now to determint the torque speed characteristic of these types of DC motor, lets get to the

torque speed equation.

From the circuit diagram we can see that the voltage equation gets modified to

E = Eb + Ia (Ra + Rs)(5)

Where as back emf remains Eb = ka

Neglecting saturation we get = K1.If = K1.Ia

[ since field current = armature current]

Eb = ka K1Ia = KsIa(6)

From equation (5) & (6)

From this equation we obtain the torque speed characteristic as In a series wound dc motor, the

speed varies with load. And operation wise this is its main difference from a shunt wound dc

motor.

Compound Wound DC Motor

The compound excitation characteristic in adc motorcan be obtained by combining the

operational characteristic of both the shunt andseries excited dc motor. The compound

wound self excited dc motor or simplycompound wound dc motoressentially contains
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the field winding connected both in series and in parallel to the armature winding as shown

in the figure below:

The excitation of compound wound dc motor can be of two types depending on the nature

of compounding.

Cumulative Compound DC Motor

When the shunt field flux assists the main field flux, produced by the main field connected

in series to the armature winding then its called cumulative compound dc motor.

total= series+ shunt

Differential compound dc motoR

In case of a differentially compounded self excited dc motor i.e. differential compound dc

motor, the arrangement of shunt and series winding is such that the field flux produced by the

shunt field winding diminishes the effect of flux by the main series field winding.

total= seriesshunt

The net flux produced in this case is lesser than the original flux and hence does not findmuch of a practical application.


29/49

The compounding characteristic of the self excited dc motor is shown in the figure below.

Both the cumulative compound and differential compound dc motor can either be of short

shunt or long shunt type depending on the nature of arrangement.


If the shunt field winding is only parallel to the armature winding and not the series field

winding then its known as short shunt dc motor or more specifically short shunt type

compound wound dc motor.

Long Shunt DC Motor

If the shunt field winding is parallel to both the armature winding and the series field

winding then its known as long shunt typecompounded wound dc motoror simply long

shunt dc motor.

Short shunt and long shunt type motors have been shown in the diagram below.
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Shunt Wound DC Motor

The shunt wound dc motor falls under the category of self excited dc motors, where the

field windings are shunted to, or are connected in parallel to the armature winding of the

motor, as its name is suggestive of. And for this reason both the armature winding and the

field windings are exposed to the same supply voltage, though there are separate branches

for the flow of armature current and the field current as shown in the figure of dc shunt

motor below.

Voltage and current equation of a shunt wound dc motor

Let us now consider the voltage and current being supplied from the electrical terminal to the

motor be given by E and Itotal respectively. This supply current in case of theshunt wound dc

motor is split up into 2 parts. Ia, flowing through the armature winding of resistance Raand

Ish flowing through the field winding of resistance Rsh. The voltage across both windings remains

the same.

From there we can write Itotal = Ia + Ish


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E=Eb +Ia Ra

Or E = Eb + (Itotal Ish).Ra

Now in general practice, when the motor is in its running condition, and supply voltage is

constant the shunt field current given by,

But we know Ish

i.e. field flux is proportional to filed current Ish

Thus the field flux remains more or less constant and for this reason a shunt wound dc

motor is called a constant flux motor.

Construction of a shunt wound dc motor

The construction of a dc shunt motor is pretty similar to other types of dc motor, as shown in the

figure below.


33/49

Just that there is one distinguishable feature in its designing which can be explained by taking

into consideration, the torque generated by the motor. To produce a high torque,

i) The armature winding must be exposed to an amount ofelectric currentthats much higher than

the field windings current, as the torque is proportional to the armature current.

ii) The field winding must be wound with many turns to increase the flux linkage, as flux linkage

between the field and armature winding is also proportional to the torque.

Keeping these two above mentioned criterion in mind a dc shunt motor has been designed in a

way, that the field winding possess much higher number of turns to increase net flux linkage and

are lesser in diameter of conductor to increase resistance(reduce current flow) compared to the

armature winding of the dc motor. And this is how a shunt wound dc motor is visibly

distinguishable in static condition from thedc series motor(having thicker field coils) of the self

excited type motors category.

Self-Speed regulation of a shunt wound dc motor

A very important and interesting fact about the dc shunt motor, is in its ability to self regulate

its speed on application of load to the shaft of the rotor terminals. This essentially means that

on switching the motor running condition from no load toloaded, surprisingly there is no

considerable change in speed of running, as would be expected in the absence of any speed

regulating modifications from outside. Let us see how?

Let us do a step-wise analysis to understand it better.

1) Initially considering the motor to be running underno load or lightly loaded condition at a

speed of N rpm.

2) On adding a load to the shaft, the motor does slow down initially, but this is where the

concept of self regulation comes into the picture.

3) At the very onset of load introduction to a shunt wound dc motor, the speed definitely

reduces, and along with speed also reduces the back emf, Eb. Since Eb N, given by

This can be graphically explained below
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4) This reduction in the counter emf or the back emf Eb results in the increase of the net

voltage. As net voltage Enet = E Eb . Since supply voltage E remains constant.

5) As a result of this increased amount of net voltage, the armature current increases and

consequently the torque increases.

Since, Ia given by

The change in armature current and torque on supplying load is graphically shown below


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6) This increase in the amount of torque increases the speed and thus compensating for the

speed loss on loading. Thus the final speed characteristic of a dc shunt motor, looks like.

From there we can well understand this special ability of the shunt wound dc motor to

regulate its speed by itself on loading and thus its rightly called the constant flux or

constant speed motor. Because of which it finds wide spread industrial application where

ever constant speed operation is required.


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Series Wound DC Motor or DC Series Motor

A series wound dc motor like in the case ofshunt wound dc motororcompound wound dc

motorfalls under the category of self-exciteddc motors, and it gets its name from the fact that

the field winding in this case is connected internally in series to the armature winding. Thus

the field windings are exposed to the entire armature current unlike in the case of a shunt

motor

Construction

Construction wise a this motor is similar to any othertypes of dc motorsin almost all aspects.

It consists of all the fundamental components like the stator housing the field windings or the

rotor carrying the armature conductors, and the other vital parts like the commutator or the

brush segments all attached in the proper sequence as in the case of a genericDC motor.

Yet if we are to take a close look into the wiring of the field and armature coils of thisdc

motor, its clearly distinguishable from the other members of this type. To understand that let

us revert back into the above mentioned basic fact, that the this motor has field coil connected

in series to the armature winding. For this reason relatively higher current flows through the

field coils, and its designed accordingly as mentioned below.

The field coils of dc series motor are wound with relatively fewer turns as theelectric

currentthrough the field is its armature current and hence for required mmf less numbers of

turns are required.
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ii) The wire is heavier, as the diameter is considerable increased to provide minimumresistance to the flow of full armature current.

In spite of the above mentioned differences, about having fewer coil turns the runningof thisdc motorremains unaffected, as the current through the field is reasonably high

to produce a field strong enough for generating the required amount of torque. To

understand that better lets look into the voltage and current equation of dc seriesmotor.

Voltage and current equation

The electrical layout of a typical series wound dc motor is shown in the diagram below Let thesupply voltage and current given to the electrical port of the motor be given by E and

Itotal respectively.

Since the entire supply current flows through both the armature and field conductor.

Itotal = Ise = Ia

Where Ise is the series current in the field coil and Ia is the armature current.

Now form the basic voltage equation of the dc motor.

E = Eb + Ise Rse + Ia Ra

Where Eb is the back emf.

Rse is the series coil resistance & Ra is the armature resistance.

Since Ise = Ia, we can write.

E = Eb + Ia( Ra + Rse)

This is the basic voltage equation of a series wound dc motor. Another interesting fact about the

dc series motor worth noting is that, the field flux like in the case of any other dc motor is

proportional to field current.

i.e. Ise

But since here Ise = Ia = Itotal

Ise Ia
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i.e. the field flux is proportional to the entire armature current or the total supply current. And for

this reason, the flux produced in this motor is strong enough to produce sufficient torque, even

with the bare minimum number of turns it has in the field coil.

Speed & Torque of series dc motor

A series wound motors has linear relationship existing between the field current and the

amount of torque produced. i.e. torque is directly proportional to current over the entire range

of the graph. As in this case relatively higher current flows through the heavy series field

windings with thicker diameter, the electromagnetic torque produced here is much higher

than normal. This high electromagnetic torque produces motor speed, strong enough to lift

heavy load overcoming its initial inertial of rest. And for this particular reason the motor

becomes extremely essential as starter motors for most industrial applications dealing in

heavy mechanical load like huge cranes or large metal chunks etc. Series motors are

generally operated for a very small duration, about only a few seconds, just for the purpose of

starting. Because if its run for too long, the high series current might burn out the series field

coils thus leaving the motor useless.

Speed regulation of series wound dc motor

Unlike in the case of a shunt motor, the dc series motor has very poor speed regulation. i.e.

the series motor is unable to maintain its speed on addition of external load to the shaft. Let

us see why?

When mechanical load is added to the shaft at any instance, the speed automatically

reduces whatever be the type of motor. But the term speed regulation refers to the ability of

the motor to bring back the reduced speed to its original previous value within reasonable

amount of time. But this motor is highly incapable of doing that as with reduction in speed

N on addition of load, the back emf given by,


39/49

Also reduces as Eb N

This decrease in back Emf Eb , increases the net

voltage E- Eb, and consequently the series field current

Ise =E-Eb/(Ra +Rse)

increases. The value of series current through the field coil becomes so

high that it tends to saturate of the magnetic core of the field.

As a result the magnetic flux linking the coils increases at a

much slower rate compared to the increase in current beyond the

saturation region as shown in the figure below. The weak magnetic field produced as a

consequence is unable to provide for the necessary amount of force to bring back the

speed at its previous value before application of load.

So keeping all the above mentioned facts in mind, a series wound dc motor is most

applicable as a starting motor for industrial applications.

Compound Wound DC Motor or DC Compound Motor


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A compound wound dc motor or rather a dc compound motor falls under the category of

self excited motors, and is made up of both series the field coils S 1 S2 and shunt field coils

F1 F2 connected to the armature windings as shown in the figure below.

Both the field coils provide for the required amount of magnetic flux, that links with the

armature coil and brings about the torque necessary to facilitate rotation at desired speed.

As we can understand, a compound wound dc motor is basically formed by the

amalgamation of ashunt wound dc motorandseries wound dc motorto achieve the better off

properties of both these types. Like ashunt wound dc motoris bestowed with an extremely

efficient speed regulation characteristic, whereas thedc series motorhas high starting torque.

So the compound wound dc motor reaches a compromise in terms of both this features and

has a good combination of proper speed regulation and high starting toque. Though its staring

torque is not as high as in case ofdc motor, nor is its speed regulation as good as ashunt dcmotor. Overall characteristics ofdc shunt motorfalls somewhere in between these 2 extreme

limits.

Types of Compound wound dc motor

The compound wound dc motor can further be subdivided into 2 major types on the basis of its

field winding connection with respect to the armature winding, and they are:

Long shunt compound wound dc motor

In case of long shunt compound wound dc motor, the shunt field winding is connected in parallel

across the series combination of both the armature and series field coil, as shown in the diagram

below.

Voltage and current equation of long shunt compound wound dc

motor
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Let E and Itotal be the total supply voltage and current supplied to the input terminals of the

motor. And Ia , Ise , Ish be the values of current flowing through armature resistance Ra , series

winding resistance Rse and shunt winding resistance Rsh respectively.

Now we know in shunt motor, Itotal = Ia + Ish

And in series motor Ia =Ise

the current equation of a compound wound dc motor is given by

the current equation of a compound wound dc motor is given by

Itotal = Ise + Ish(1)

And its voltage equation is

E = Eb + Ia( Ra + Rse)

Short shunt compound wound dc motor

In case of short shunt compound wound dc motor, the shunt field winding is connected in parallel

across the armature winding only. And series field coil is exposed to the entire supply current,

before being split up into armature and shunt field current as shown in the diagram below.

Voltage and current equation of short shunt compound wound dc

motor

Here also let, E and Itotal be the total supply voltage and current supplied to the input terminals of

the motor. And Ia , Ise , Ish be the values of current flowing through armature resistance Ra , series

winding resistance Rse and shunt winding resistance Rsh respectively.


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But from the diagram above we can see

Itotal =Ise-(2)

Since the entire supply current flows through the series field winding

Since the entire supply current flows through the series field winding.

And like in the case of a dc shunt motor Itotal = Ia + Ish(3)

Equation (2) and (3) gives the current equation of a short shunt compound wound dc motor.

Now for equating the voltage equation, we applyKirchoffs lawto the circuit and get

E=Eb +Ia Ra +Ise Rse

E = Eb + Ia Ra + Itotal Rse

Apart from the above mentioned classification, a compound wound dc motor can further be sub

divided into 2 types depending upon excitation or the nature of compounding. i.e.

But

since Ise

=Itotal

Thus the final voltage equation can be written as

Cumulative compounding of dc motor

A compound wound dc motor is said to be cumulatively compounded when the shunt field flux

produced by the shunt winding assists or enhances the effect of main field flux, produced by the

serieswinding.
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total= series+ shunt

Differential compounding of dc motor

Similarly a compound wound dc motor is said to be differentially compounded when the flux due

to the shunt field winding diminishes the effect of the main series winding. This particular trait is

not really desirable, and hence does not find much of a practical application.

total= seriesshunt

The net flux produced in this case is lesser than the original flux and hence does not find much of

a practical application.

The compounding characteristic of the self excited dc motor is shown in the figure below.

Motor Starting of DC

The starting of DC motor is somewhat different from the starting of all other types

ofelectrical motors. This difference is credited to the fact that a dc motorunlike other

types of motor has a very high starting current that has the potential of damaging the

internal circuit of the armature winding ofdc motorif not restricted to some limited value.

This limitation to thestarting current of dc motor is brought about by means of the
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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 variableresistance

connected in seriesto 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.

Now the immediate question in Why the dc motor has such high starting current ?

To give an explanation to the above mentioned question let us take into consideration

the basic operational voltage equation of the dc motor given by,

E = Eb + Ia Ra

Where E is the supply voltage, Ia is the armature current, Ra is the armature resistance. And theback emf is given by Eb

Now the back emf, in case of adc motor, is very similar to the generated emf of a dc generator as

its produced by the rotational motion of the current carrying armature conductor in presence of

the field. This back emf of dc motor is given by

and has a major role to play in case of the starting of dc motor.

From this equation we can see that Eb is directly proportional to the speed N of the motor.

Now since at starting N = 0, Eb is also ZERO, and under this circumstance the voltage

equation is modified to

E = 0 + Ia Ra
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For all practical practices to obtain optimum operation of the motor the armature resistance is

kept very small usually of the order of 0.5 and the bare minimum supply voltage being 220

volts. Even under these circumstance the starting current, Ia is as high as 220/0.5 amp = 440

amp.

Very high electromagnetic starting torque of DC motor is produced by virtue of the high

starting current, which has the potential of producing huge centrifugal force capable of flying

off the rotor winding from the slots.

Such high starting current of dc motor creates two major problems.

1) Firstly, current of the order of 400 amp has the potential of damaging the internal

circuit of the armature winding of dc motor at the very onset.

2) Secondly, since thetorque equation of dc motoris given by

Starting Methods of DC Motor

As a direct consequence of the two above mentioned facts i.e high starting current and high

starting torque of DC motor, the entire motoring system can undergo a total disarray andlead towards into an engineering massacre and non-functionality. To prevent such an
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incidence from occurring several starting methods of dc motor has been adopted. The main

principal of this being the addition of external resistance Rextto the armature winding, so as

to increase the effective resistance to Ra + Rext, thus limiting the armature current to the

rated value. The new value of starting armature current is desirably low and is given by

Now as the motor continues to run and gather speed, the back emf successively develops

and increases, count

At this moment to maintain the armature current to its rated value, Rext is progressively

decreased unless its made zero, when the back emf produced is at its maximum. This

regulation of the external resistance in case of the starting of dc motor is facilitated by

means of the starter.

Starters can be of several types and requires a great deal of explanation and some intricate

level understanding. But on a brief over-view the main types of starters used in the industry

today can be illustrated as:-

1)3 point starter.

2)4 point starter.

Used for the starting ofshunt wound dc motorandcompound wound dc motor.
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3)Series wound dc motors starter using no load release coil.

All of these play a very significant role in limiting starting current of dc motor for proper

starting and running of the dc motor, and are described vividly under their respective sub-

headings.
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Construction of DC Motor

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