Download UNIT – 1 Explain the principle of operation of a DC Motor. ANS

UNIT – 1
1. Explain the principle of operation of a DC Motor.
ANS:
Principle of Operation of a Simple D.C Motor
A rectangular coil which is free to rotate about a fixed axis shown placed inside a
magnetic field produced by permanent magnets in fig.(2.3). A direct current is fed into the coil
via carbon brushes bearing on a commutator, which consists of a metal ring split into two
halves separated by insulation.
When current flows in the
coil a magnetic field is set up
around the coil which interacts with
the magnetic field produced by the
magnets. This causes a force (F) to
be exerted on the current-carrying
conductor which by Fleming's lefthand rule, is down wards between
point (A) and (B), up ward between
(C) and (D) for the current direction
shown. This causes a torque and
the coil rotates anticlockwise.
When the coil has turned through (90o) from the position shown in figure, the brushes
connected to the positive and negative terminals of supply make contact with different halves of
the commutator ring, thus reversing the direction of the current flow in the conductor. If the
current is not reversed and the coil rotates past this position the forces acting on it change
direction and it rotates in the opposite direction thus never making more than half a
revolution.
The current direction is reversed every time the coil swing through the vertical position
and thus the coil rotates anti-clockwise for as long as the current flows. This is the principle of
operation of a D.C motor which is thus a device that takes in electrical energy and converts it
into mechanical energy.
2. What is the significance of Back EMF.
ANS:
Significance of the Back e.m.f.
When the motor armature rotates, the conductors also rotate and hence cut the flux. In
accordance with the laws of electromagnetic induction, e,m.f. is induced in them whose
direction, as found by Fleming's Right-hand Rule, is in opposition to supplied voltage.
Because of its opposing direction, it is referred to as counter e.m.f. or back e.m.f. (ERbR). It
V  E b  I a Ra
will be seen that
Also Eb 

Ia 
V  Eb
Ra
Z N P
  Volts
60  A 
As pointed out above, Back e.m.f. depends, among other factors, upon the armature speed. If
speed is high, Eb is large, hence armature current (Ia), as seen from the above equation, is
small. If the speed is less, then (Eb) is less, hence more current flows which develops more
torque. So, we find that (Eb) acts like a governor i.e. it makes a motor self-regulating so that it
draws as much current as is just necessary.
3. Derive the Torque equation of a DC Motor.
ANS:
TORQUE EQUATION OF A D.C MACHINE
The torque in any D.C machine depends on three factor:
1. The flux (φ) in the machine.
2. The armature (or rotor) current (Ia) in the machine.
3. A constant depending on the construction of the machine.
The torque on the armature of a real machine is equal to the number of conductors (Z)
times the torque on each conductor. The torque in any single conductor under the pole faces
is.
T  r*F
F  B *l * I
T  r * B *l * I
If there are (a) current paths in the machine, then the
total armature current (Ia) is split among the (a) current
path, so the current in a single conductor is given by
I Cond . 
Ia
and the torque in a single conductor on the
a
motor may be expressed as
TCond . 
r . B. . I a
a
Since there are (Z) conductors, the total induce torque in a D.C machine rotor is:
Tind.. 
Z .r . B. . Ia
a
The flux per pole in this machine can be expressed as
  BAP 
B(2rl )
P
So the total induce torque can be re-expressed as
Tind.. 
Z . . I a  P 
 
2  a 
The induce torque equations given above are only approximations, because not all the
conductors in the machine are under the pole faces at any given time.
4. What are the types of DC Motors.
ANS:
Types of D.C Motors
(a) Permanent-Magnet D.C Motor
The permanent-magnet D.C motor shown in fig, is construction in the same manner as
its D.C generator counterpart.
When this type of motor is used, the D.C power supply is
connected directly to the armature conductors through the brush to
commutator assembly. The magnetic field is produced by permanent
magnets mounted on the stator. The permanent-magnet motor has
several advantages over conventional types of D.C motors. The advantage
is a reduced operational cost, and The direction of rotation of a
permanent magnet motor can be reversed by reversing the two power
lines. The speed (c/s) of the permanent-magnet motor are similar to
those of the shunt wound D.C motor.
(b) Shunt- Wound D.C Motor
Shunt-wound D.C motor are more commonly used than any other type of D.C motor. As
shown in figure(2.6), the shunt-wound D.C motor has field coils
connected in parallel with its armature. This type of D.C motor
has field coils that are wound of many turns of small diameter
wire and have a relatively high resistance. Since the field is a
high-resistant parallel path of the circuit of the shunt motor, a
small amount of current flows through the field. A strong
electromagnetic field is produced because of the many turns of
wire that form the field windings. Since the field current has little
effect on the strength of the field, motor speed is not affected
appreciably by variation in load current.
V = Eb + IaRa
I = Ia + If
Because of its good speed regulation, and its ease of speed control, the D.C shunt motor is
commonly used for industrial applications.
(c) Series-Wound D.C Motor
In the series-wound motor the field winding is in series with the armature across the
supply as shown in fig.(2.7). There is only one path for current to flow from the D.C voltage
source. Therefore, the field is wound of relatively few turns of
large-diameter wire, giving the field a low resistance.
Changes in load applied to the motor shaft cause change in
current through the field. If the mechanical load increase, the
current also increase. The increased current creates a
stronger magnetic field. The speed of a series motor varies from
very fast at no load, to very slow at heavy loads. Since large
currents may flow through the low resistance field, the series
motor produces a high torque output. Series motors are used
when heavy loads must be moved, and speed regulation is not
important. A typical application is automobile starter motors.
V = Eb + I(Ra + Rf)
I = Ia = Ise
(d) Compound-Wound D.C Motor
The compound-wound D.C motor, has two sets of field windings, one in series with the
armature and one in parallel. This motor combines the desirable characteristics of the seriesand shunt- wound motors. It has high torque similar to that of a series-wound motor, along
with good speed regulation similar
to that of a shunt motor.
Therefore, when good torque and
good speed regulation are needed,
the compound-wound D.C motor
can be used. There are two
common types of compound
motor connection, the longshunt connection and shortshunt connection, as shown in
fig.(2.8). And there are two different types of compound motors in common use, they are the
cumulative compound motor and the differential compound motor.
5. Write the Efficiency equation for a DC Motor.
ANS:
The efficiency of a D.C. motor
The efficiency of a D.C. machine is given by.
Efficiency, 
Output Power
X 100%
Input Power
Also , Total Losses  I a2 Ra  I f V  C ( for Shunt Motor)
And Total Losses  I 2 R  C ( for Series Motor)
where C is the sum of the iron, friction and windage losses,
R is the total resistance for series motor (R = Ra + Rf)
for a motor, the input power=VI and the output power=VI-losses
hence,
 VI  I a2 Ra  I f V  C 
 100%
 


VI


 VI  I 2 R  C 
 100%


VI


 
for Shunt Motor
for Series Motor
The efficiency of a motor is a maximum when the load is such that
I a2 . Ra  I f .V  C ( for Shunt Motor )
I 2 .R  C
( for Series Motor )
6. Draw the Motor Characteristics of a DC MOTOR with neat sketches.
ANS:
MOTORS CHARACTERISTICS
The characteristic curves of a motor are those curve which shown relationship between the
following quantities:
1. Torque and armature current i.e. (T/Ia) characteristic.
2. Speed and armature current (n/Ia) characteristic.
3. Speed and torque (n/T) characteristic.
CHARACTERISTIC OF SHUNT-WOUND MOTOR
The field current Ish is constant since the field winding is directly connected to the supply
voltage V which is assumed to be constant. Hence, the flux in a shunt motor is approximately
constant.
1. (T/Ia) Characteristic
The theoretical torque/ armature current (c/s) can be
derived from the expression a T ∝φ .I for a shunt-wound
motor, the field winding is connected in parallel with the
armature circuit and thus the applied voltage gives a
constant field current, i.e. a shunt-wound motor is a
constant flux machine. Since (φ ) is constant, it follows that
a T ∝ I , and the (c/s) is as shown in fig.
Shunt motor should not be started on
heavy load
2. (n / Ia) Characteristic
For a shunt motor, V, φ and Ra are constants, hence as armature current (Ia) increases, Ia Ra
increase and (V-Ia Ra) decrease, and the speed is proportional to a quantity, which is
decreasing and is shown in fig.
n
Eb

.As the load on the shaft of the motor increases, (Ia)
increases and the speed drops slightly. In practice, the speed
falls by about (10%) between no-load and full-load on many
D.C shunt-wound motors. Due to this relatively small drop
in speed, the D.C shunt-wound motor is taken as basically
being a constant-speed machine.
From equation :
V = E + I .R
E=V−IR
Eb ∝ φ . hence φ is constant then n 
V  I a R a 

3. (n / T) Characteristic
The theoretical speed/ torque Characteristic can be deduced
from (1) and (2) above and is shown in fig.(2.11). Eb  K  
T  K  Ia

K  V 
E b  V  I a R a 
T
Ra
K


V
T

Ra
K  K  2
This equation is just a straight line with a negative slope.
CHARACTERISTIC OF SERIES-WOUND MOTOR
Note that current passing through the field winding is the same as that in the armature. If the
mechanical load on the motor increases, the armature current also increases. Hence, the flux
in a series motor increases with the increase in armature current and vice-versa.
1. (T/Ia) Characteristic
In a series motor, the armature current flows in the field
winding and is equal to the supply current (I). The torque a
T ∝φ .Ia over a limited range, before magnetic saturation
of the magnetic circuit of the motor is reached. Thus (φ ∝ I
) and (T ∝ I 2 ). Hence, (T/Ia) curve is a parabola as shown
in fig.(2.12). After magnetic saturation, φ almost becomes
a constant and (T ∝ I ), so the characteristic becomes a
straight line.
This means that starting torque of a d.c. series motor
will be very high as compared to a shunt motor (where that
2. (n / Ia) Characteristic
In a series motor, Ia = I and below the magnetic saturation
level, φ ∝ I . Thus
n
V  IR
when ( R ) is the combined
I
resistance of the series field and armature circuit.
Since (I.R ) is small compared with (V), then an approximate
relationship for the speed is n
Hence,
n

V
since (V) is constant.
I
1
I
Speed varies inversely as armature current as shown in fig. The high speed at small values of
current indicates that this type of motor must not be run on very light loads and invariably.
Such motors are permanently coupled to their loads.
Thus, upto magnetic saturation, the N/Ia curve follows the hyperbolic path as shown in Fig.
After saturation, the flux becomes constant and so does the speed.
3. (n / T) Characteristic
The theoretical speed/ torque (c/s) may be derived
from (1) and (2) above by obtaining the torque and
speed for various values of current and plotting the
co-ordinates on the speed/torque (c/s). The serieswound motor has a large torque when the current is
large on starting. A typical speed/ torque (c/s) is
shown in fig.
Note. The minimum load on a d.c. series motor
should be great enough to keep
the speed within limits. If the speed becomes
dangerously high, then motor must be disconnected
from the supply.
CHARACTERISTIC OF COMPOUND-WOUND MOTOR
A compound-wound motor has both a series and a shunt field winding, (i.e. one winding in
series and one in parallel with the armature circuit), by varying the number of turns on the
series and shunt windings and the directions of the magnetic fields produced by these
windings (assisting or opposing), families of (c/s) may be obtained to suit almost all
applications. There are two common types of compound
motor connection, the long-shunt connection and
short-shunt connection. And there are two different
types of compound motors in common use, they are the
cumulative compound motor and the differential
compound motor. In the cumulative compound motor,
the field produced by the series winding aids the field
produced by the shunt winding. The speed of this motor
falls more rapidly with increasing current than does
that of the shunt motor because the field increases. In
the differential compound motor, the flux from the
series winding opposes the flux from the shunt winding.
The field flux, therefore, decreases with increasing load current. Because the flux decreases,
the speed may increases with increasing load. Depending on the ratio of the series-to-shunt
field ampere-turns, the motor speed may increases very rapidly.
The torque-speed (c/s) of a cumulatively compound
D.C motor
In the cumulative compounded D.C. motor, there is a
component of flux which is constant and another
component which is proportional to its armature
current (and thus to its load). Therefore, the
cumulatively compounded motor has a higher starting
torque than a shunt motor (whose flux is constant) but
a lower starting torque than a series motor (whose
entire flux is proportional to armature current). At light
loads, the series field has a very small effect, so the motor behaves approximately as a shunt
D.C. motor. As the load gets very large, the series flux becomes quite important and the torquespeed curve begins to look like a series motor's (c/s). A comparison of the torque-speed (c/s) of
each of these types of machines is shown in figure (2.16).
The torque-speed (c/s) of a differentially compound D.C motor
In a differentially compound D.C. motor, the shunt
magneto motive force and series magneto motive
force subtract from each other. This means that as
the load on the motor increases, Ia increases and
the flux in the motor decreases. But as the flux
decreases, the speed of the motor increases. This
speed increases causes anther increases in load,
which further increases Ia, further decreasing the
flux, and increasing the speed again. The result is
that a differentially compounded motor is unstable
and tends to run away. It is so bad that a
differentially compounded motor is unsuitable for
any application.
7. Mention the Applications of DC Motors.
ANS:
APPLICATIONS OF D.C. MOTORS
1. Shunt motors
The characteristics of a shunt motor reveal that it is an approximately constant speed motor. It
is, therefore, used
(i) where the speed is required to remain almost constant from no-load to full-load
(ii) where the load has 10 be driven at a number of speeds and any one of which is required to
remain nearly constant.
Industrial use: Lathes, drills, boring mills, shapers, spinning and weaving machines etc.
2. Series motors
It is a variable speed motor i.e., speed is low at high torque and vice-versa. However, at light or
no-load, the motor tends to attain dangerously high speed. The motor has a high starting
torque. It is, therefore, used
(i) where large starting torque is required e.g., in elevators and electric Traction
(ii) where the load is subjected to heavy fluctuations and the speed is automatically required to
reduce at high torques and vice-versa.
Industrial use: Electric traction, cranes, elevators, air compressors, vacuum cleaners, hair
drier, sewing machines etc.
3. Compound motors
Differential-compound motors are rarely used because of their poor torque characteristics.
However, cumulative-compound motors are used where a fairly constant speed is required with
irregular loads or suddenly applied heavy loads.
Industrial use: Presses, shears, reciprocating machines etc.
8. Write briefly about 3-Point Starter.
ANS:
3 POINT STARTER
A 3 point starter in simple words is a device that helps in the starting and running of a
shunt wound DC motor or compound wound DC motor. Now the question is why these types of
DC motors require the assistance of the starter in the first case. The only explanation to that is
given by the presence of back emf Eb, which plays a critical role in governing the operation of
the motor. The back emf, develops as the motor armature starts to rotate in presence of the
magnetic field, by generating action and counters the supply voltage. This also essentially
means, that the back emf at the starting is zero, and develops gradually as the motor gathers
speed.
The general motor emf equation
V  Eb  I a Ra
at starting is modified to V = Ia.Ra as at starting Eb = 0.
Ia 
V
Ra
Thus we can well understand from the above equation that the current will be dangerously
high at starting (as armature resistance Ra is small) and hence its important that we make use
of a device like the 3 point starter to limit the starting current to an allowable lower value.

Construction of 3 Point Starter
Construction wise a starter is a variable resistance, integrated into number of sections as
shown in the figure beside. The contact points of these sections are called studs and are shown
separately as OFF, 1, 2,3,4,5, RUN. Other than that there are 3 main points, referred to as
1. 'L' Line terminal. (Connected to positive of supply.)
2. 'A' Armature terminal. (Connected to the armature winding.)
3. 'F' Field terminal. (Connected to the field winding.)
And from there it gets the name 3 point starter.
Now studying the construction of
3 point starter in further details
reveals that, the point 'L' is
connected to an electromagnet
called overload release (OLR) as
shown in the figure. The other
end of 'OLR' is connected to the
lower end of conducting lever of
starter handle where a spring is
also attached with it and the
starter handle contains also a soft
iron piece housed on it. This
handle is free to move to the
other side RUN against the force
of the spring. This spring brings
back the handle to its original
OFF position under the influence
of its own force. Another parallel
path is derived from the stud '1',
given
to
the
another
electromagnet called No Volt Coil
(NVC) which is further connected
to terminal 'F'. The starting resistance at starting is entirely in series with the armature. The
OLR and NVC acts as the two protecting devices of the starter.
Working of Three Point Starter
Having studied its construction, let us now go into the working of the 3 point starter. To
start with the handle is in the OFF position when the supply to the DC motor is switched on.
Then handle is slowly moved against the spring force to make a contact with stud No. 1. At this
point, field winding of the shunt or the compound motor gets supply through the parallel path
provided to starting resistance, through No Voltage Coil. While entire starting resistance comes
in series with the armature. The high starting armature current thus gets limited as the
current equation at this stage becomes Ia = E/(Ra+Rst). As the handle is moved further, it goes
on making contact with studs 2, 3, 4 etc., thus gradually cutting off the series resistance from
the armature circuit as the motor gathers speed. Finally when the starter handle is in 'RUN'
position, the entire starting resistance is eliminated and the motor runs with normal speed.
This is because back emf is developed consequently with speed to counter the supply voltage
and reduce the armature current. So the external electrical resistance is not required anymore,
and is removed for optimum operation. The handle is moved manually from OFF to the RUN
position with development of speed.
Working of NO VOLTAGE COIL of 3 Point Starter
The supply to the field winding is derived through no voltage coil. So when field current flows,
the NVC is magnetized. Now when the handle is in the 'RUN' position, soft iron piece connected
to the handle and gets attracted by the magnetic force produced by NVC, because of flow of
current through it. The NVC is designed in such a way that it holds the handle in 'RUN'
position against the force of the spring as long as supply is given to the motor. Thus NVC holds
the handle in the 'RUN' position and hence also called hold on coil.
Now when there is any kind of supply failure, the current flow through NVC is affected and it
immediately looses its magnetic property and is unable to keep the soft iron piece on the
handle, attracted. At this point under the action of the spring force, the handle comes back to
OFF position, opening the circuit and thus switching off the motor. So due to the combination
of NVC and the spring, the starter handle always comes back to OFF position whenever there is
any supply problems. Thus it also acts as a protective device safeguarding the motor from any
kind of abnormality.
9. Explain the working principle of 4-point starter in detail. And also justify why 4-point starter is
more advantages than 3-point starter.
ANS:
WORKING PRINCIPLE OF FOUR POINT STARTER
The 4 point starter like in the case of a 3 point starter also acts as a protective device that
helps in safeguarding the armature of the shunt or compound excited dc motor against the
high starting current produced in the absence of back emf at starting.
The 4 point starter has a lot of constructional and functional similarity to a three point starter,
but this special device has an additional point and a coil in its construction, which naturally
brings about some difference in its functionality, though the basic operational characteristic
remains the same.
Now to go into the details of operation of 4 point starter, lets have a look at its constructional
diagram, and figure out its point of difference with a 3 point starter.
Construction and Operation of Four Point Starter
A 4 point starter as the name suggests has 4 main operational points, namely
1. 'L' Line terminal. (Connected to positive of supply.)
2. 'A' Armature terminal. (Connected to the armature winding.)
3. 'F' Field terminal. (Connected to the field winding.)
4. A 4th point N. (Connected to the No Voltage Coil)
The remarkable difference in case of a 4 point starter is that the No Voltage Coil is connected
independently across the supply through the fourth terminal called 'N' in addition to the 'L', 'F'
and 'A'. As a direct consequence of that, any change in the field supply current does not bring
about any difference in the performance of the NVC. Thus it must be ensured that no voltage
coil always produce a force which is strong enough to hold the handle in its 'RUN' position,
against force of the spring, under all the operational conditions. Such a current is adjusted
through No Voltage Coil with the help of fixed resistance R connected in series with the NVC
using fourth point 'N' as shown in the figure above.
Apart from this
above
mentioned fact,
the 4 point and
3 point starters
are similar in
all other ways
like possessing
is a variable
resistance,
integrated into
number
of
sections
as
shown in the
figure
above.
The
contact
points of these
sections
are
called
studs
and are shown
separately
as
OFF, 1, 2, 3, 4,
5, RUN, over
which the handle is free to be maneuvered manually to regulate the starting current with
gathering speed.
Now to understand its way of operating lets have a closer look at the diagram given above.
Considering that supply is given and the handle is taken stud No.1, then the circuit is
complete and line current that starts flowing through the starter. In this situation we can see
that the current will be divided into 3 parts, flowing through 3 different points.
i) 1 part flows through the starting resistance (R1+ R2+ R3…..) and then to the armature.
ii) A 2nd part flowing through the field winding F.
iii) And a 3rd part flowing through the no voltage coil in series with the protective resistance R.
So the point to be noted here is that with this particular arrangement any change in the shunt
field circuit does not bring about any change in the no voltage coil as the two circuits are
independent of each other. This essentially means that the electromagnet pull subjected upon
the soft iron bar of the handle by the no voltage coil at all points of time should be high enough
to keep the handle at its RUN position, or rather prevent the spring force from restoring the
handle at its original OFF position, irrespective of how the field rheostat is adjusted.
This marks the operational difference between a 4 point starter and a 3 point starter. As
otherwise both are almost similar and are used for limiting the starting current to a shunt
wound DC motor or compound wound DC motor, and thus act as a protective device.
10. Explain Swinburne’s Test in detail.
ANS:
SWINBURNES TEST OF DC MACHINE:
This method is an indirect method of testing a dc machine. It is named after Sir James
Swinburne. Swinburne's test is the most commonly used and simplest method of testing of
shunt and compound wound dc machines which have constant flux. In this test the efficiency
of the machine at any load is pre-determined. We can run the machine as a motor or as a
generator. In this method of testing no load losses
are measured separately and eventually we can
determine the efficiency.
The circuit connection for Swinburne's test is
shown in figure. The speed of the machine is
adjusted to the rated speed with the help of the
shunt regulator R as shown in figure.
CALCULATION OF EFFICIENCY
Let, I0 is the no load current (it can be measured
by ammeter A1)
Ish is the shunt field current (it can be measured
by ammeter A2)
Then, no load armature current = (I0 - Ish)
Also let, V is the supply voltage. Therefore, No load power input = VI 0watts.
In Swinburne's test no load power input is only required to supply the losses.
The losses occur in the machine mainly are:
 Iron losses in the core
 Friction and windings losses
 Armature copper loss.
Since the no load mechanical output of the machine is zero in Swinburne's test, the no load
input power is only used to supply the losses.
The value of armature copper loss = (I 0 - Ish)2 Ra
Here, Ra is the armature resistance.
Now, to get the constant losses we have to subtract the armature copper loss from the no load
power input. Then,
Constant losses WC = VI0 -(I0 - Ish)2 Ra
After calculating the no load constant losses now we can determine the efficiency at any load.
Let, I is the load current at which we have to calculate the efficiency of the machine.
Then, armature current (Ia) will be (I - Ish), when the machine is motoring.
And
Ia = (I + Ish), when the machine is generating.
Calculation of Efficiency When the Machine is Motoring on Load
Power input = VI
Armature copper loss, PCU = I2 Ra = (I - Ish)2Ra
Constant losses, WC = VI0 -(I0 - Ish)2 Ra
Total losses = PCU + WC
∴ Efficiency of the motor:
M 
Input  Losses V I  PCU  WC 
Output


Input
Input
VI
CALCULATION OF EFFICIENCY WHEN THE MACHINE IS GENERATING ON LOAD
Power input = VI
Armature copper loss, PCU = I2 Ra = (I + Ish)2 Ra
Constant losses, WC = VI0 - (I0 - Ish)2 Ra
Total losses = PCU + WC
∴ Efficiency of the generator:
G 






VI
Output
Output


Input
Output  Losses VI  PCU  WC 
ADVANTAGES OF SWINBURNE'S TEST
The main advantages of this test are :
This test is very convenient and economical as it is required very less power from supply to
perform the test.
Since constant losses are known, efficiency of Swinburne's test can be pre-determined at any
load.
DISADVANTAGES OF SWINBURNE'S TEST
The main disadvantages of this test are :
Iron loss is neglected though there is change in iron loss from no load to full load due to
armature reaction.
We cannot be sure about the satisfactory commutation on loaded condition because the test is
done on no-load.
We can’t measure the temperature rise when the machine is loaded. Power losses can vary with
the temperature.
In dc series motors, the Swinburne’s test cannot be done to find its efficiency as it is a no load
test.
11. Explain the Speed control Techniques for a DC Motor with neat diagrams.
ANS:
SPEED CONTROL OF D.C. MOTORS
Although a far greater percentage of electric motors in service are a.c. motors, the d.c.
motor is of considerable industrial importance. The principal advantage of a d.c. motor is that
its speed can be changed over a wide range by a variety of simple methods. Such a fine speed
control is generally not possible with a.c. motors. In fact, fine speed control is one of the
reasons for the strong competitive position of d.c. motors in the modem industrial applications.
The speed of a d.c. motor is given by:
N 
EB

or
Where R = Ra
R = Ra + Rse

N k
V
 Ia R


rpm
for SHUNT motor
for SERIES motor
Therefore, they are three main methods of controlling the speed of a D.C. motor, namely:
By varying the flux per pole (f). This is known as FLUX CONTROL METHOD.

By varying the resistance in the armature circuit. This is known as ARMATURE CONTROL
METHOD.

By varying the applied voltage V. This is known as VOLTAGE CONTROL METHOD.
The first method (i.e. flux control method) is frequently used because it is simple and
inexpensive.
1. Flux control method
It is based on the fact that by varying the flux f, the
motor speed (N 1/Ф) can
be changed and hence the name flux control method. In
this method, a variable resistance (known as shunt field
rheostat) is placed in series with shunt field winding as
shown in Fig.
The shunt field rheostat reduces the shunt field current
Ish and hence the flux Ф. Therefore, we can only raise
the speed of the motor above the normal speed.
Generally, this method permits to increase the speed in
the ratio 3:1. Wider speed ranges tend to produce
instability and poor commutation.



Advantages
This is an easy and convenient method.
It is an inexpensive method since very little power is
wasted in the shunt field rheostat due to relatively small
value of Ish.
The speed control exercised by this method is
independent of load on the machine.


Disadvantages
Only speeds higher than the normal speed can be obtained since the total field circuit
resistance cannot be reduced below Rsh—the shunt field winding resistance.
There is a limit to the maximum speed obtainable by this method. It is because if the flux is too
much weakened, commutation becomes poorer.
Note. The field of a shunt motor in operation should never be opened because its speed will
increase to an extremely high value.
2. Armature control method
This method is based on the fact that by varying the
voltage available across the armature, the back e.m.f
and hence the speed of the motor can be changed.
This is done by inserting a variable resistance RC
(known as controller resistance) in series with the
armature as shown in Fig.
N  V  I A R A  RC 
Where RC is the Controller Resistance.
Due to voltage drop in the controller resistance, the
back e.m.f. (Eb) is decreased. Since N
Eb, the
speed of the motor is reduced. The highest speed
obtainable is that corresponding to RC = 0 i.e.,
normal speed. Hence, this method can only provide
speeds below the normal speed.




Disadvantages
A large amount of power is wasted in the controller
resistance since it carries full armature current Ia.
The speed varies widely with load since the speed
depends upon the voltage drop in the controller resistance and hence on the armature current
demanded by the load.
The output and efficiency of the motor are reduced.
This method results in poor speed regulation.
Due to above disadvantages, this method is seldom used to control tie speed of shunt motors.
Note. The armature control method is a very common method for the speed control of d.c.
series motors. The disadvantage of poor speed regulation is not important in a series motor
which is used only where varying speed service is required.
3. Voltage control method
In this method, the voltage source supplying the field current is different from that which
supplies the armature. This method avoids the disadvantages of poor speed regulation and low
efficiency as in armature control method. However, it is quite expensive. Therefore, this method
of speed control is employed for large size motors where efficiency is of great importance.
(i) Multiple voltage control. In this method, the shunt field of the motor is connected
permanently across a-fixed voltage source. The armature can be connected across several
different voltages through a suitable switchgear. In this way, voltage applied across the
armature can be changed. The speed will be approximately proportional to the voltage applied
across the armature. Intermediate speeds can be obtained by means of a shunt field regulator.
(ii) Ward-Leonard system. In this method, the adjustable voltage for the armature is obtained
from an adjustable-voltage generator while the field circuit is supplied from a separate source.
This is illustrated in Fig. The armature of the shunt motor M (whose speed is to be controlled)
is connected directly to a d.c. generator G driven by a constant-speed a.c. motor A. The field of
the shunt motor is supplied from a constant-voltage exciter E. The field of the generator G is
also supplied from the exciter E. The voltage of the generator G can be varied by means of its
field regulator. By reversing the field current of generator G by controller FC, the voltage
applied to the motor may be reversed. Sometimes, a field regulator is included in the field
circuit of shunt motor M for additional speed adjustment. With this method, the motor may be
operated at any speed upto its maximum speed.



Advantages
The speed of the motor can be adjusted through a wide range without resistance losses which
results in high efficiency.
The motor can be brought to a standstill quickly, simply by rapidly reducing the voltage of
generator G. When the generator voltage is reduced below the back e.m.f. of the motor, this
back e.m.f. sends current through the generator armature, establishing dynamic braking.
While this takes place, the generator G operates as a motor driving motor A which returns
power to the line.
This method is used for the speed control of large motors when a d.c. supply is not available.
The dis-advantage of the method is that a special motor-generator set is required for each
motor and the losses in this set are high if the motor is operating under light loads for long
periods.