RGPV DIPLOMA EX305 UNIT III NOTES FOR DIPLOMA STUDENTS OF III SEM EX BRANCH

1. 1
READING MATERIAL FOR III SEM DIPLOMA EX
STUDENTS OF RGPV AFFILIATED COLLEGES
SUBJECT ELECTRICAL MACHINES I
Professor MD Dutt
Addl General Manager (Retd)
BHARAT HEAVY ELECTRICALS LIMITED
Professor(Retd) in EX Department
Bansal Institute of Science and Technology
KOKTA ANANAD NAGAR BHOPAL
Presently Head of The Department ( EX)
Shri Ram College Of Technology
Thuakheda BHOPAL
Sub Code BE 305 Subject Electrical Machines I
UNIT III D.C MOTORS

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RGPV DIPLOMA EX BRANCH
SEMESTER: THIRD SCHEME: Jul.09 COURSE CODE: 305 PAPER CODE: 6232
NAME OF COURSE: ELECTRICAL MACHINE – I
COMMON WITH PROGRAM (S): E01
Syllabus UNIT III
D. C. Motors -
Principle, production of back EMF, torque equation. Classification, characteristics and
applications of motors. D. C. motor starters speed control, losses and efficiency.
Brake test, Swinburne test. Simple numerical.
.
INDEX
S
No
Topic Page
1 Principle 3
2 Production of back EMF 3
3 Torque equation 4,5
4 Classification 6,7
5 Characteristics 7,8,9,10,11
6 Application of DC motors 11,12
7 DC motor starters speed control 12,13,14,15,16,17,18,19
8 Losses and efficiency 19,20,21,22,23
9 Break test 23

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10 Swinburne test 23,24,25,26
Basic principle of operation of a D.C. Motor
When a conductor carrying current is put in a magnetic field, a force is produced in
it. Let us consider one such conductor placed in a slot of armature and it is acted
upon the magnetic field from the north pole of the motor. By applying L.H.S it is
found that the conductor has tendency to move to the L.H.S. Since the conductor is
placed on the slot at circumference of rotor , The force acts in a tangential direction
of rotor, Thus a torque is developed on the rotor Similar torques are produced on
all the rotor conductors .Since the rotor is free to move, It starts rotating.
BACK EMF
When the motor of armature rotates, its conductors cut the magnetic flux,
Therefore the e.m.f. rotation Er is induced in them. In case of a motor, the e.m.f is
known as BACK E.M.F or counter e.m.f. The back e.m.f opposes the applied

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voltage , Since the back E.M.F is induced due to generating action it magnitude is
given
Eb = PNZΦ/60A
TORQUE OF A DC MACHINE
When the machine is operating as a motor the torque is transferred to the
shaft of the rotor and drives the mechanical load, The expression for the
torque is same for the generator or motor.
The Voltage equation of DC machine is
V= E+IaRa
Multiplying with Ia
We get
V Ia = EIa +Ia Ra
VIa = Electrical input to the armature
Ia Ra = copper loss in the armature
We also know that
Input = output + losses
EIa = Electrical equivalent of gross mechanical power developed by
armature
τav = average electromagnetic torque developed by armature in Newton
Mtrs (Nm)
Mechanical power developed by the armature

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Pm = ώτav = 2Пņτav
Pm = EIa = ώτav = 2Пņτav
But
E = nP ΦZ/A
Therefore
(nP ΦZ/A) Ia =2Пņτav
τav = (PZ/2П A ) X ΦIa
This is called Torque equation
For a given machine the value of PZ A are constant
Therefore PZ/2П A is also constant = k
τav = k ΦIa
EQUIVALENT
DIAGRAM
aaat
fff
RIEV
RIV
±±±±====
====

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CLASSIFICATION OF DC MOTORS
A DC motor whose field winding is supplied itself is called self
excited DC machine, A DC motor whose field winding is supplied
separately excited DC motor
In a self excited DC machine the field coils are connected in parallel
with armature winding .
DC motors are classified as follows:-
i) Shunt excited DC motor
ii) Series excited DC motor
iii) Compound DC motor
iv) Separately excited DC Motor
i)Shunt excited DC motor :- In a shunt excited DC motor the
field coil winding is connected parallel to the armature winding
Ish = V/ Rsh Ia = Il – Ish
Eb = V – Ia Ra -2Vb

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ii) SERIES EXCITED DC MOTOR
In case of series excited DC motor the line current passes through series field
winding and also armature current.
Ia = Il = Ise Eb = V – Ia(Ra +Rse) -2 Vb
COMPOUND DC MOTOR:- There are two type of compound DC motors
i) Cumulative compound motor
ii) Differential compound motor
In the compound wound DC motor the field is produced by the shunt
field as well as series field. Generally shunt field is stronger than
series field. When the series field assists the shunt field it is called
cumulative compound wound DC motor. When series field opposes
the shunt field the motor is called differentially wound DC motor.

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Cumulative compound dc motor Differential compound DC motor
iii)Separately Excited DC Motor :- In separately excited DC motor there are
two sources of DC supply is required one for Armature and other one is
for field winding . The field windings are made from thin wires with
more number of turns because the field current in this case is very small.
OPERATING CHARACTERISTICS OF DC MOTORS
In both cases of shunt or separately excited D.C motor’s . the field is supplied from
a constant voltage so that the field current is constant. It is graph between two
dependent quantities.
SPEED ARMATURE CURRENT CHARECTERISTICS:- In a shunt motor,
Ish =V÷Rsh, if V is constant Rsh will also be constant, Hence the flux is constant
but at no load the flux decreases slightly due to armature reaction. If the effect of
armature Reaction is neglected the flux will remain constant. The motor speed is
given by
N= V-Ia RA
Φ
If Φ
Is constant
N is proportional to V-Ia RA

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This is the equation of straight line with a negative slope. That is the speed N of
motor decreases linearly with increase of current. Since the IaRa at full load is
very small compare to V the drop in speed from noload to full load is very small.
For all practical purposes the shunt motor is taken a constant speed motor.
TORQUE / ARMATURE CURRENT CHARECTERISTICS:- IF the flux Φ is
constant in a shunt motor, the torque would increase linearly with armature current
Ia, However for larger Ia , the net flux decreases due to the demagnetizing effect of
armature reaction. In view of this , the torque current characteristics deviates from
straight line as illustrated here below
τav = k ΦIa
SPEED TORQUE CHARECTERISTICS::- The speed torque characteristics is
also called the mechanical characteristics and under steady state conditions it can
be obtained as follows

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ωm = Vt – Ia Ra
KaΦ
It is seen from the characteristics that with increase in torque the speed drops.
DC SERIES MOTOR
SPEED CURRENT CHARECTERISTICS::- If the armature reaction and
saturation is neglected, the main flux is directly proportional to the armature
current Ia , therefore it can be written as
Φ K Ia where K is constant
ωm o = Vt/ Ka Φ = Vt/Ka K Ia
Since ωm o is inversely proportional to Ia, the no load speed of series motor
becomes dangerously due to small no load current . In view of this, the series
motor must always be started and operate under load mechanically coupled with it.

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TORQUE CURRENT CHARECTERISTICS::- We know that Φ = KIa
which shows that the torque is proportional to the Ia and therefore Torque current
characteristics is PARABOLA
SPEED TORQUE CHARECTERISTICS::- The speed verses Torque curve is
HYPERBOLA. It is seen from the curve that the increase in Torque τe does not
change the speed drop drastically
DC COMPOUND MOTORS:-
SPEED CURRENT CHARECTERISTICS::- With increase in Ia and Φse
increases. With increase in Ia the speed drops at faster rate in a cumulative
compound motor.

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TORQUE CURRENT CHARECTERISTICS At no load the value of Ia = 0,
Φse=0 and τe=0. As the armature current rises with load the shunt field Φsh
remains almost constant but series field Φse rises, as a result motor torque τe
SPEED TORQUE CHARECTERISTICS With the increase in motor torque τe
, armature current rises, with this field flux Φse rises, Consequently speed drops in
cumulatively compound DC motor.
APPLICATION OF DC MACHINES:-
Presently the DC generators are overtaken by AC to DC rectifiers which are
static equipments. Thus DC generator are superseded by rectified ac supply.
The main application of DC machines are follows
SERIES MOTOR:-
These motors are required where high starting torque is required and speed
can vary like traction, cranes.
SHUNT MOTOR :-
These motors are used where constant speed is required and starting
condition are not severe for example, Lifts, Fans, Blowers, Lathe etc.

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COMPOUND MOTORS
These motors are used where high starting torque and fairly constant speed
is required, presses, shears, conveyors, elevators, rolling machines.
Small DC machines ( In fractional KW ) are used primarily as control device
like tacho generator for speed sensing and servomotor for positioning and
tracking.
STARTING OF DC MOTORS:-
At the time of starting the motor speed is zero. Therefore back emf is also zero.
Vt = Ea +Ia Ra Ea = 0
Vt = 0 + IaRa for shunt motor
Vt = 0+ Ia (Ra +Rs) for series motor and compound motor
With the rated voltage is applied the starting armature current is
= Vt/Ra for shunt motor , = Vt/ Ra+Rs for series motor and compound motor
Since the value of Ra and Rs is very small the motor draws large starting armature
current from supply means.
A 10Kw ,250V shunt motor having Ώ armature resistance 0.2 Ώ wiil draw
250/0.2 =1250A where as the rate current is 40A. Such heavy in rush of currents
may result the following
1. Detrimental sparking at commutator
2. Damage of armature winding and detritions of insulation due to overheating
3. High starting torque and quick acceleration which may damage the rotating
parts of the rotor

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4. Large dips in supply voltage.
In view of this , the armature current must be limited to a value that can be
commutated safely, by inserting a suitable external resistance in the armature
circuit. As the motor accelerates the back EMF is generated in the armature
and reduces the armature current to a small value. The external resistance
inserted must be gradually reduced as the rotor accelerates.
SHUNT AND COMPOUND STARTER’s A Primary function of starter is
to limit starting current in the armature circuit during starting or
accelerating time. The simplest type of starter is however modified to
include few protective devices such as over current release, no volt release
etc.
THREE POINT STARTERS FOR DC SHUNT MOTOR
The handle H is kept at OFF position by spring S . For starting the motor ,
the handle H is moved manually and when makes contact with the resistance
stud 1 it is in the START position. In position the field winding receives the
full supply voltage., but the armature current is limited by the graded
resistance R = ( R1+R2+R3+R4). The starter handle is then gradually moved
from stud to stud , allowing the speed of the motor to build up until it
reaches the RUN position. In this position (a) the motor attains the full speed
, (b) The supply is directly across the both winding of motor(c) The
resistance R is completely cut out. The handle H is held in RUN position by
an electromagnet energized by a no volt

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Coil NVC. The no no volt trip coil is connected in series with field winding
of the motor. In the event of switching off, or when the supply voltage falls
below a predetermined value, or complete failure of supply while the motor
is running. NVC is de energized, This results in release of the handle. .
which is then pulled back to the OFF position. By the action of spring. The
current to the motor is cut off, and the motor is not started without
resistance R in the armature circuit.. The NVC also provides protection
against an open circuit in the field winding. The NVC is called no-volt or
under voltage protection of motor. Without this protection, the supply
voltage might be restored with the handle in the RUN position,
Consequently full line voltage may be applied directly to the armature
resulting in a very large current.
The other protective device incorporated in the starter is overload protection.
Overload protection is provide by overload trip coil OLC and the NVC.
The overload coil is a small electromagnet. It carries the armature and for
normal values of armature current the magnetic pull of OLC is insufficient
to attract the strip P When the armature current exceeds the normal rated
value ( that the motor is overloaded) P is attracted by the electromagnet of
OLC and closes the contact aa, Thus NVC is short circuited , This results in
release of handle H which returns to OFF position and the motor supply is
cut off.
DRAW BACK OF THREE POINT STARTER The motor with large
speed variation with armature voltage control suffers . To increase the speed
of the motor the field is to be decreased, therefore the current through the
shunt field is reduced. The field current may become very low. The very low
field current may not develop sufficient electromagnetic hold to over come
the force of the spring

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.
FOUR POINT STARTER The schematic connection diagram is given
herewith. The basic difference in the circuit of Four Point starter compare to
3 point starter is that , The holding coil is removed from the shunt field
circuit and is connected directly across the line with a current limiting
resistance r in series. Such arrangements form three parallel circuits.
1) Armature, starting resistance and overload release.
2) A variable resistance and shunt field winding
3) Holding coil and current limiting resistance.
With this arrangement, a change in field current for variation of speed of
the motor, does not affect the current through the holding coil, because
the two circuits are independent of each other.
Now a days automatic push button starters are used. In such starters the
ON push button is preset at the time of starting limiting the armature
current. The resistors are gradually disconnected by an automatic
controlling arrangement until full line voltage is available to the armature
circuit. By the pressing of OFF button, the circuit is disconnected.

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SPEED CONTROL OF DC MOTORS
The speed of DC motor depends on the following equation:-
N = ( V-IaRa)/ Φ
From the above equation it is clear that the speed is dependent on supply voltage V
armature resistance and field flux Φ which is produced by field current. These
three factors are used for controlling speed of a DC motor.
1 Variation of armature resistance in the armature circuit.
2 Variation of field flux Φ
3 Variation of armature voltage.
ARMATURE RESISTANCE CONTROL :-In this method a variable series resistor
Re is connected in the armature circuit. In the case of shunt motor the field is

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directly connected to the supply and therefore flux Φ Is not affected by this. In the
case of series motor the flux gets affected due to change in Ia and RA since Re
resistor carries full load armature current it must be designed keeping into this
consideration
Disadvantages:-
i) A large amount of power is wasted in external resistance Re
ii) Control is limited to give speeds below the normal rated speed.
iii)For a given value of Re the speed reduction is not constant but varies with
motor load.
VARIATION OF FIELD FLUX Φ
In a shunt motor this is done by connecting a variable resister Re in series with
field winding. The resister Re is called the field regulator.. the Re reduces the field
current by virtue of this the Φ reduces, decrease in flux causes increase in speed.
Increase in flux causes decrease in speed. By this method the speed can be attained
above normal rated speed.
The variation in field current is done by two methods
i) By connecting a variable resistor parallel to the field winding. This is called
divertor method
ii) The second method is tapped field control.

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ADVANTAGES
I) This is very easy and convenient method
II) Since the field current Ish is very small , the power loss in shunt field is also
very small
ARMATURE VOLTAGE CONTROL Ward Leonard system of speed control is
based on tis method. In this system M is the motor whose speed is to be controlled
and G is the separately excite DC generator, The generator is driven by 3 Phase
induction motor or synchronous motor. The combination of ac motor and DC
generator is called MG Set.
By changing generator field current the generated voltage is changed this voltage
increases the speed of the motor. With the armature voltage control constant torque
variable speed is obtained.

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ADVANTAGES OF WARD LEONARD DERIVES
1) Smooth control of DC motor on both directions is possible
2) It has inherent regenerative breaking capacity
3) When load is intermittent like rolling mills, IM is used with flywheel to take
care of intermittent loading, in case of WARD LEONARD this is possible
without flywheel.
4) By using over excited Synchronous motor a drive the system power factor
can be improved
DISADVANTAGES OF WARD LEONARD DERIVES
1) Higher initial cost due to MG set
2) Larger size and weight, require more floor area
3) Frequent maintenance and produces more noise.
4) Lower efficiency and higher total losses.

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SOLID STATE CONTROL
Rotating MG sets are now a days replaced with solid state convertors to control
the speed of DC motors, The convertors are controlled rectifiers or choppers.
In case of AC supply, controlled rectifiers are used to convert fixed ac supply
voltage into variable D voltage.
When the supply is D.C Choppers are used to obtain variable DC voltage from the
fixed DC voltage supply.
LOSSES IN A DC MACHINES and efficiency
Following are the losses in the DC machines
1) Electrical or copper loss ( I²R Losses)
2) Core losses or Iron losses
3) Brush Losses
4) Mechanical losses
5) Stray load losses
ELECTRICAL LOSSES:- Windings having resistance consumes certain
losses, these are termed as copper losses because mostly windings are made
of copper.
i) Armature copper loss Ia²Ra ( Ia is armature current)
ii) Shunt field copper loss Ish²Rsh
iii) Copper loss in the series field Ise²Rse
iv) Copper loss in the interpole winding which are in series with armature
Ia²Ri

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v) Compound machines both series field and shunt field copper losses
are also there
vi) Copper losses are there in the compensating winding
CORE LOSES:-
The core losses are the hysteresis losses and Eddy current losses.
Since the machine usually operates at constant flux density and speed,
these losses are almost constant. These losses are about 20% of Full
load losses.
BRUSH LOSES:- There is a power loss at the brush contact with
commutator and the carbon brushes. This loss can me measured by the
voltage drop at the brush contact and armature current.
Pbd = Vbd Ia
The voltage drop is more or less remains constant over a wide range
of Ia and it is assumed 2V ( approx)
MECHANICAL LOSSES:- The losses associated with mechanical
effect are called mechanical losses. These consists of friction losses at
bearing and windage losses ( fan losses) . the fans are used to take
away the heat produced due to I²R losses and iron losses inside the
machine.
STRAY LOAD LOSSES:- These are miscellaneous losses which are
due to the following reasons:-
1) Distortion of flux due to armature reaction

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2) Short circuit currents in the coils due to commutation.
These losses are difficult to find out, However they are taken as 1
% of full load power output.
EFFICIENCY
InputPower
Losses
InputPower
LossesInputPower
InputPower
OutputPower
−−−−====
−−−−
====
====ηηηη
1

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Let us assume that the
R = Total resistance
I = Output current
Ish = Current through the shunt field
Ia armature current I +Ish
V is the terminal voltage
Power loss in the shunt field
= V Ish
Mechanical Losses = Friction losses at bearings+ friction losses at commutator +
windage losses
Stray losses Core losses mechanical losses and shunt field copper losses are
considered as combined fixed losses.
ή = Output / Input
= VI / ( VI + Ia²Rat +Pk =VbdIa
Ia = I + Ish
Since Ish compare to I is very small we can consider
Ia ≡ I

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ή = VI/ ( VI+ I²Rat +Vbd I +Pk)
LOAD for Maximum efficiency
Ifl = Full load current at maximum efficiency
Im = current at maximum efficiency
Im ² Rat = Pk
Im ² = Pk/ Rat
Current at Maximum ή = F.L Current X( Pk/F.L Copper loss) ²
TESTING OF DC MACHINES:-
Testing of Dc machines can be done by following three methods:-
1) Direct testing
2) Swinburne’s Test
DIRECT TESTING :- This method is suitable for small machines. In direct testing
method DC machine is subjected to rated load and the entire out put is wasted.
The ration of out put power to the input power gives the efficiency.
For DC generators the out put power is wasted in resistors, the out put voltage and
current gives the out put power.

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For Dc motors break test is carried out . the belt tightening hand wheels H1 and
H2 help in So adjusting the load on the pulley. S1 S2 are the spring balance. These
spring balances are calibrated in Kg. The motor out put is given by
=ω(S1 –S2)r X 9081 Watts
S1 S2 are the spring balance tight side and slack side readings, r is the effective
radius of the pulley in meters = (½ outside pulley diameter + ½ belt thickness)
ω is the ( 2Пņ) is the motor speed in rad/sec
If Vt is the motor terminal voltage and Il is the line current, then power input to the
motor VtIl watts
The efficiency
ή % = {ω ( S1-S2) 9.81 X100 }/ VtIl
For series motor the break should be tight enough before motor is switched on
Disadvantages
1) The spring balance are not steady
2) The friction torque , at particular setting of hand wheel H1 and H2 does not
remain constant.

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SWINBERN’s TEST
The machine is run as a motor at rated voltage and speed. Connection diagram is as
follows
Let V = supply voltage
I0 = No load current
Ish = Shunt Field current
Therefore No load armature current Ia0 = Io- Ish
NO load input VIo
No load power input supplies the following losses
i) Iron loss in the core
ii) Friction losses at bearing and commutator
iii) Windage losses
iv) Armature cupper loss at no load
When the machine is loaded the resistance of the armature and field
resistance changes due to temperature rise and due to I²R losses. Let the
resistance at the room temperature tOc is made by passing current
through armature and field from a low voltage DC supply. Let the rise be
50 degree the hot resistance
Rt1 = R0 + α0t1
R(t1=50) = R0{1 + α0 (t1+5050)}
α0 = temperature coefficient of resistance at zero degree temperature

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Stray losses = iron loss +friction loss + windage loss = input at no load
=VI0 – Pf –Pa0 = Ps
Pc = no load input – no load armature copper loss
Pc = Ps +Pf
By knowing the constant losses of the machine its efficiency can be
calculated at any other load.
Let I be the load current
Motor input VI
Armature cupper loss = Ia²Ra +Pc
ή = {VI – (Ia²Ra +Pc)}/ VI
Efficiency when running as generator
Ia = I + Ish
Out put = VI

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ή gen = VI/ { VI +( I+Ish) ²Ra +Pc}
ADVANTAGES
i) It is convenient and economical, since power requirement is very less
ii) The efficiency at any load can be determined as the constant losses are
known
DISADVANTAGES
i) This cannot be done on series machines.
ii) No account is taken for iron loss from no load to full load. At full
load due to the armature reaction iron losses increases.