NAKAWA VTI

INFORMATION SHEET
Training: Vocational Diploma Electrical Engineering Code: VDEE2222
Course: Vocational Diploma Electrical Engineering

Subject: Industrial Electrical machines Prepared by: Mwase K Johnson

Title（ Details) : DC machines , (Components, Types of DC
generator and DC motor. Torque, Speed Date of Execution: 20/6/2024
control, Armature reaction and commutation)

Construction of DC Machines

An electromechanical device which can convert direct current (dc) electricity into
mechanical energy or mechanical energy into direct current (dc) electricity( is known as a DC
machine.

If the machine converts DC electrical energy into mechanical energy, it is known as DC motor. If the

machine converts mechanical energy into DC electrical energy, it is known as DC generator. Both DC

motor and DC generator have the similar construction. A typical DC machine consists of the

following major parts –

 Yoke or Frame
 Armature
 Field system
 Commutator
 Brushes
 Bearings

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1 . The yoke o r f r a m e

It is the outer f r a m e o f the DC machine. It is made up of such materials that have h i g h

permeability and high mechanical strength. In practice, the y o k e o f DC machine is made up of
cast steel.
The yoke or frame of the DC machine serves the following main purposes –

i. It protects the i n t e r n a l machine parts like armature, windings, field p o l e s , e t c . against

mechanical damages.

ii. The yoke houses the magnetic field system.

iii. It provides a low reluctance path to the working magnetic flux.

iv. . It supports the rotor or armature through bearings.

2. Armature

In DC machines (motor or generator), armature is a system of conductors or coils that can

r o t a t e freely on t h e supporting bearings. The w o r k i n g torque and EMF are developed
in coils of the armature. The armature consists of two main parts namely, armature core and
armature winding.

The armature core is a solid cylindrical structure, made up of high permeability thin silicon steel
laminations. On the outer p e r i p h e r y of the core slots are cut to carry the armature winding. The
a r m a t u r e winding is made up o f copper wires. The a r m a t u r e winding of DC machine is
generally former w o u n d .

Depending upon the end connections of the armature conductors, the armature winding may be of
two types namely lap winding and wave winding. The type of winding decides the voltage and
current rating of the machine. In case o f the l a p winding, the n u mb e r s o f p a r a l l e l p a t h s
( A ) f o r c u r r e n t t o f l o w a r e equal to the number of poles (P) in the machine. On the other
hand, for wave winding, the number of parallel paths (A) are equal to 2.
`

Fig- Armature Core

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Code: VDEE2222
INFORMATION SHEET

3. Field System

Field system is the part of a DC machine which produces the working magnetic flux in the
m a c h i n e . It is basically a system of electromagnets which is excited by a DC supply. In case of
DC machine, the field system is a stationary part of the machine and it is bolted to the yoke or
frame of t h e machine. There are three main parts of a field system in dc machines namely pole core,
pole shoes, and field coils.

The pole core is made up of thin steel laminations. One end of the pole core is bolted to the frame
and other end has pole shoe. The pole core carries the field winding.

The pole shoe is a projected part of the pole core and has a large area of cross- section. Pole
shoes help in spreading the magnetic f l u x uniformly in the air gap, and offers low reluctance path
to the magnetic flux. Also, it supports the field winding. The f i eld c o i l or winding is made up o f
copper wire. The field w i n d i n g is former wound and in serte d around the pole c o re . When field
windings are excited by DC supply, they become electromagnets and produce magnetic flux in the
machine.

4. Commutator

The c o mmu t a t o r is one o f the i m p o r t a n t parts of the D C machine. It is basically

mechanical rectifier. It is a cylindrical shaped device and is made up of copper. The outer
periphery of the commutator has V-shaped slots to carry commutator segments. Where, the
commutator segments are copper bars inserted in the slots. These segments are insulated from
each o the r by mica. The commutator is mounted on the shaft of the DC machine on one side of
the armature. The armature conductors are connected to the commutator segments with the help
of copper lugs.

The commutator performs the following two major functions –

 In a DC generator, it collects the current from the armature conductor. In a DC

motor, it supplies the current to the armature conductors.

 It converts the alternating current of the armature into unidirectional current in the
external circuit with the help of brushes, and vice-versa.

Fig- Commutator

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5. Brushes

Brushes are used to make an electrical connection with the rotating commutator. These
collect (or supply) current from (or to) the moving commutator. Brushes are usually made up
of carbon. They are housed in brush holders and are in contact with the commutator surface
with the help of spring pressure.

6. Bearings

Bearings are used in the DC machine to reduce the frictional losses. Thus, the main function of
bearings in the DC machine is to support the machine shaft with minimum friction. In DC
machines, ball bearings or roller bearings are commonly used.

D.C. Motors
Introduction

For industrial drives, d.c. motors are as popular as 3-phase induction motors. It is advantageous
to convert alternating current into direct current in order to use d.c. motors. The reason is that
speed/torque characteristics of d.c. motors are much more superior to that of a.c. motors.
Basically, there is no constructional difference between a d.c. motor and a d.c. generator. The
same d.c. machine can be run as a generator or motor.

Like d.c. generators, d.c. motors are also of three types: - series-wound, shunt-wound and compound-
wound. The use of a particular motor depends upon the mechanical load it has to drive.

D.C. Motor Principle

A machine that converts d.c. power into mechanical power is known as a d.c. motor. Its operation is
based on the principle that when a current carrying conductor is placed in a magnetic field, the
conductor experiences a mechanical force. The direction of this force is given by Fleming’s left hand
rule and magnitude is given by;
F = BIl (Newton)
Where, B stands for magnetic field (Wb/m2), I stand for current (A)
L stands for length of the coil (m).

This is said to be the motoring action. When t h e electric current direction is changed, rotation
also changes its direction. Here, mechanical force is produced on the interaction between
electric field and magnetic field.
Fleming’s left hand rule gives the rule behind the motor’s direction of rotation. According to this
rule, left hand’s index finger, middle finger and thumb are held like mutually perpendicular to
one other, where magnetic field direction is represented by the index finger, electric current
direction is represented by middle finger and thumb shows the direction of the force
that is experienced by the DC motor’s shaft.

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Working of D.C. Motor

Consider a part of a multipolar d.c. motor as shown in Fig .4. When the terminals of the motor are connected
to an external source of d.c. supply:
(i) the field magnets are excited developing alternate N and S poles;
(ii) the armature conductors carry currents. All conductors under N-pole carry currents in one direction
while all the conductors under S-pole carry currents in the opposite direction.
Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry
currents out of the plane of the paper as shown in Fig.4. Since each armature conductor is carrying current
and is placed in the magnetic field, mechanical force acts on it and applying Fleming’s left hand rule, it
is clear that force on each conductor is tending to rotate the armature in anticlockwise direction. All these
forces add together to produce a driving torque which sets the armature rotating. When the
conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the
same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction
of force on the conductor remains the same.

Fig 4

Back-e.m.f or Counter – voltage

A coil is placed in a magnetic field with flux density B. A DC voltage source is connected to the
two ends of the coil which lets current I to flow through it. Due to the interaction between the
electric current and magnetic field, coil experiences a force or moment on the both the sides. Coil

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starts moving in the direction of force. In DC motor, rotor is wound with several numbers of coils
which rotates due to the force. With the current or the magnetic field increasing, the force
also increases thereby the coil moves faster. Torque is also produced at the same time while
the coils are moving.
Each time when the coil rotates flux linked with it changes thereby an e.m.f is induced. This
voltage, induced e.m.f, opposes the voltage which causes current to flow in the conductor and
is termed as Back-e.m.f or Counter-voltage. The current that flows through the armature
depends on the difference between the counter-voltage and applied voltage. According to
Lenz’s law, the current by the counter-voltage opposes the cause of its origin which ends up with
the slowdown of rotor. Gradually, rotor slowdown to the value just enough for the force value
produced by the magnetic field (F= BIL) to become equal to the load force that is being applied
to the shaft. From now on, the system carries out with a constant velocity.

Practical application of Back-e.m.f or Counter e.m.f

When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors
move through the magnetic field and hence e.m.f. is induced in them as in a generator The induced e.m.f.
acts in opposite direction to the applied voltage V(Lenz’s law) and in known as back or counter e.m.f. Eb.
The back e.m.f. Eb(= P ZN/60 A) V is always less than the applied voltage V, although this difference is small
when the motor is running under normal conditions.

Fig 4a

Consider a shunt wound motor shown in Fig. 4a. When d.c. voltage V is applied across the motor terminals,
the field magnets are excited and armature conductors are supplied with current. Therefore, driving torque acts
on the armature which begins to rotate. As the armature rotates, back e.m.f. Eb is induced which opposes
the applied voltage V. The applied voltage V has to force current through the armature against the back
e.m.f. Eb.
The electric work done in overcoming and causing the current to flow against Eb is converted into
mechanical energy developed in the armature. It follows, therefore, that energy conversion in a d.c. motor is
only possible due to the production of back e.m.f. Eb.
Net Voltage across armature circuit = V - Eb
If Ra is the armature circuit resistance, then, I a = V – Eb
Ra
Since V and Ra are usually fixed, the value of Eb will determine the current drawn by the motor. If
the speed of the motor is high, then back e.m.f. Eb (= P ZN/60 A) is large and hence the motor will draw less
armature current and vice- versa.

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The significance of back-e.m.f lies in making DC motor a self-regulating machine, which

enables the armature current to be drawn by the motor as much as it is required to create the
torque that is required by the load. The armature current flow in a DC motor is regulated by the
back e.m.f which automatically keeps on changing the current in armature so as to meet the load
requirements. Armature current I a = V – Eb
Ra
(i) When the motor is running on no load, small torque is required to overcome the friction and
windage losses. Therefore, the armature current Ia is small and the back e.m.f. is nearly equal to
the applied voltage.
(ii) If the motor is suddenly loaded, the first effect is to cause the armature to slow down. Therefore, the
speed at which the armature conductors move through the field is reduced and hence the back e.m.f.
Eb falls. The decreased back e.m.f. allows a larger current to flow through the armature and larger
current means increased driving torque. Thus, the driving torque increases as the motor slows down. The
motor will stop slowing down when the armature current is just sufficient to produce the increased torque
required by the load.

(iii) If the load on the motor is decreased, the driving torque is momentarily in excess of the requirement
so that armature is accelerated. As the armature speed increases, the back e.m.f. Eb also increases
and causes the armature current Ia to decrease. The motor will stop accelerating when the
armature current is just sufficient to produce the reduced torque required by the load.

It follows, therefore, that back e.m.f. in a d.c. motor regulates the flow of armature current i.e., it
automatically changes the armature current to meet the load requirement.

Voltage Equation of D.C. Motor

Let in a d.c. motor above:
V = applied voltage
Eb = back e.m.f.
Ra = armature resistance
Ia = armature current

Since back e.m.f. Eb acts in opposition to the

I a = V – Eb
Ra

V = Eb + I a R a This is known as voltage equation of the d.c. motor.

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a
Power Equation

If Eq.(i) above is multiplied by Ia throughout, we get

V Ia = Eb Ia + I a2 R a

This is known as power equation of the d.c. motor.

VIa = electric power supplied to armature (armature input) EbIa = power developed by
armature (armature output)
I a2 R a = electric power wasted in armature (armature Cu loss)
Thus out of the armature input, a small portion (about 5%) is wasted as I a2 R a
and the remaining portion EbIa is converted into mechanical power within the armature.

Condition For Maximum Power

The mechanical power developed by the motor is Pm = EbIa

Now Pm = EbIa = V Ia - I a2 R a

Since, V and Ra are fixed, power developed by the motor depends upon armature current. For
maximum power, dPm/dIa should be zero.
Hence mechanical power developed by the motor is maximum when back e.m.f. is equal to half the applied
voltage.

Limitations

In practice, we never aim at achieving maximum power due to the following reasons:
(i) The armature current under this condition is very large—much excess of rated current of the machine.
(ii) Half of the input power is wasted in the armature circuit. In fact, if we take into account other losses
(iron and mechanical), the efficiency will be well below 50%.

Losses of a DC Motor

DC motor comes cross a number of losses like:

1. Copper Loss: This consists of Armature Cu loss, Field Cu loss and loss due to the
resistance by the brush contact.

2. Mechanical Loss: This consists of Friction losses and Windage losses.

3. Iron Loss: This consists of Hysteresis loss and Eddy current loss.

Copper Loss: These losses occur due to the current flowing through the windings, mainly
armature and field windings. Armature Copper loss is the loss found in the armature circuit.
The armature copper loss is a function of time as the armature current value is determined by
load. Field Copper loss is loss found in the field circuit. Field copper loss depends on field
circuit resistance and thus remains constant with no variation in circuit resistance. Brush
contact loss is due to the resistance offered by brush contacts.

Mechanical Loss: Since there is moving parts and so as the machine runs there is lot of
frictional forces to overcome with large expenditure of valuable energy and results in the
heating up of rubbed parts. Mechanical losses are independent of the load and depend
on the speed which makes it difficult to estimate by calculations directly whereas this can be
measured.

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Iron Loss: The armature core made of iron is continuously rotating in the magnetic field
which creates losses in the core. Therefore, these losses are also known as core losses.
Armature core which undergoes reversal of magnetization causes hysteresis loss. This type
of loss depends on the iron used in the manufacture of core, frequency at which
magnetic reversals occur and the flux density amount.

The back-e.m.f induced in the core is small allowing more current, called the eddy current, to flow
through it due to lower resistance offered by core. There is power loss due to this current known
as eddy current loss.

Types of D.C. Motors

Like d . c generators, there a r e three ty p e s o f d.c. motors characterized by the connections of field
winding in relation to the armature :

I. Series-wound motor

In which the field winding is connected in series with the armature. Therefore, series field winding
carries the armature current. Since the current passing through a series field winding is the same as the
armature current, series field windings must be designed with much fewer turns than shunt armature
windings for the same m.m.f. Therefore, a series field winding has a relatively small number of turns of thick
wire that makes it possess a low resistance.

Fig 1 Series-wound motor

II Shunt-wound motor
In which the field winding is connected in parallel with the armature. The current through the shunt
field winding is not the same as the armature current. Shunt field windings are designed to produce the
necessary m.m.f. by means of a relatively large number of turns of wire having high resistance.
Therefore, shunt field current is relatively small compared with the armature current.

Fig 2 Shunt-wound motor

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III Compound-wound motor

Which has two field windings; one connected in parallel with the armature and the other in series
with the armature. There are two types of compound motor connections (like d.c. generators). When
the shunt field winding is directly connected across the armature terminals Fig 3 it is called short-
shunt connection.

When the shunt winding is so connected that it shunts the series combination of armature and series
field Fig. 4 it is called long-shunt connection.

Fig 3

Fig 4

The compound machines (generators or motors) are always designed so that the flux produced by shunt
field winding is considerably larger than the flux produced by the series field winding. Therefore,
shunt field in compound machines is the basic dominant factor in the production of the magnetic field in
the machine.

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IV Externally-excited DC motor or Separately - excited DC motor

This type of DC motor is constructed such that the field is not connected to the armature. 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.

Fig 5

V Permanent Magnet DC Motor

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.

Speed of a D.C. Motor

The field of a DC motor is varied using external devices, usually field resistors. For a constant applied
voltage to the field (E), as the resistance of the field (R f) is lowered, the amount of current flow through the

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field (If) increases as shown by Ohm's law in Equation 1

…………………………………………………………………..1

Increase in field current will cause field flux (Φf) to increase. Conversely, if the resistance of the field is
increased, field flux will decrease. If the field flux of a DC motor is decreased, the motor speed will increase.

The reduction of field strength reduces the CEMF ( Back e.m.f) of the motor, since fewer lines of flux are
being cut by the armature conductors,
as shown in Equation 2

………………………………………………………………..2

A reduction of counter EMF allows an increase in armature current as shown in

Equation 3

……………………………………………………………………….3

This increase in armature current causes a larger torque to be developed; the increase
in armature current more than offsets the decrease in field flux as shown in Equation 4

……………………………………………………………………………4

This increased torque causes the motor to increase in speed. as shown in Equation 5

……….………………………………………………………………………………5
This increase in speed will then proportionately increase the CEMF. The speed and CEMF will continue
to increase until the armature current and torque are reduced to values just large enough to supply
the load at a new constant speed.

Summary
 The function of torque in a DC motor is to provide the mechanical output to drive the piece of
Equipment that the DC motor is attached to.
 Torque is developed in a DC motor by the armature (current-carrying conductor) being present in the
motor field (magnetic field).

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 CEMF (or Back-e.m.f) is developed in a DC motor by the armature (conductor) rotating (relative
motion) in the field of the motor (magnetic field).

 The function of the voltage that is developed in a DC motor (CEMF) opposes the applied voltage and
results in the lowering of armature current
.
Speed Control of D.C. Motors

From the D. C. motor equation given by V = Eb + I a R a or

Eb = V - I a R a ………………………………………………………………………………………………………………….1

Also Eb is generated e.m.f generally referred to as back e.m.f ( or Counter e. m. f (CEMF) whose

magnitude is given by (2Z) /c * (p Φ N) /60 = 1 Φ N since K1 = (2Zp)/60c is constant for any particular
machine. Where Z = total number of armature conductors, and c = number of parallel paths in the armature,
p=pair poles, N= speed of machine in r.p.m

Lap wound machine c=2p; a wave wound machine c= 2

Therefore Eb = K1 Φ N …………………………………………………………………….2

i.e. K1 Φ N = V - I a R a

Therefore N = (V - I a R a) / K1 Φ rev. per. min…………………………………………3

Speed N α (V - I a R a)) / Φ α Eb/ Φ ………………………………………………………4

Therefore Speed α (back e.m.f) / magnetic flux

The above equation 3 shows that the speed depends upon the supply voltage V, the armature circuit
resistance Ra,(by adding resistance to the armature circuit (increase resistance) and the field flux Ф, which is
produced by the field current. In practice, the variation of these three factors is used for speed control.
There are three possible methods of speed control of d.c. motor or varying the speed

(i) Resistance variation i n the a r m a t u r e circuit: This method is called armature resistance control
or Rheostat control.

Figure: (a) Speed control of a d.c. Shunt motor by armature resistance control. (b) Speed
control of a D.C. Series motor by armature resistance control.

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(ii) Variation of field flux Ф: This method is called field flux control.

Since the field current produces the flux,, if we control the field current then the speed can be
controlled. In the shunt motor, speed can be controlled by connecting a variable resistor Rc in series
with the shunt field winding. In the F ig (a) above resistor, Rc is called the shunt field regulator.

A variable r e s i s t a n c e Rd is connected in parallel with the series field winding fig (b). The resistor
connected in parallel is called the diverter. A portion of the main current is diverted through Rd.

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Code: VDEE2222
INFORMATION SHEET

(iii) . Variation of the applied voltage. This method is also called armature voltage control

We can control the speed of the D.C. motors by varying the applied voltage to the armature. Ward-
Leonard system of speed control works on this principle of armature voltage control. Fig 5. In this
system, M is the main dc motor whose speed is to be controlled, and G is a separately
excited dc generator. The generator G is driven by a 3- phase driving motor which may be an
induction motor or asynchronous motor. The combination of ac driving motor and the dc
generator is called the motor- generator (M-G) set.

Fig 5 Ward-Leonard drive

Armature Reaction in DC Machine

In a D C machine, the carbon brushes are always placed at the magnetic neutral axis. In no load condition,
the magnetic neutral axis coincides with the geometrical neutral axis.
The effect of magnetic field set up by armature current on the distribution of flux under main poles of
a DC machine is called Armature Re ac tion . The armature magnetic field has two effects:
(i) It demagnetizes or weakens the main flux and

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(ii) It cross-magnetizes or distorts it. Fig 1 shows the flux distribution of a bipolar generator when
there is no current in the armature conductors. The brushes are touching the commutator segments, it is seen
that: (a) the flux is distributed symmetrically with respect to the polar axis, which is the line joining the
centres of NS poles. (b) The magnetic neutral axis (M.N.A.) coincides with the geometrical neutral axis
(G.N.A.). Magnetic neutral axis may be defined as the axis along which no e.m.f is produced in the
armature conductors because they move parallel to the lines of flux. Or M.N.A. is the axis which is
perpendicular to the flux passing through the armature.

Fig 1

Brushes are always placed along M.N.A. Hence, M.N.A. is also called ‘axis of commutation’ because
reversal of current in armature conductors takes place across this axis.
Fig 2 shows the field (or flux) set up by the armature conductors alone when carrying current, the field
coils being unexcited. The current direction is downwards in conductors under N-pole and upwards in those
under S-pole.

Fig 2

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To reduce the adverse effects of Armature reaction:

a) Brush Shift
In case of a D . C . Generator, with b r u s h e s a l o n g G . N . A . and no commutating poles used, the brushes
must be shifted in the direction of rotation (forward lead) for satisfactory commutation. However, in case
of a D.C. motor, the brushes are given a negative lead i.e., they are shifted against the direction of rotation.
Brush shift has serious limitations, so the brushes have to be shifted to a new position every time the
load changes or the direction of rotation changes or the mode of operation changes. In view of this, brush
shift is limited only to very small machines.
b) By using commutating poles (compoles), a D.C. machine can be operated with fixed brush positions
for all conditions of load. Since commutating poles windings carry the armature current, then, when a
machine changes from generator to motor (with consequent reversal of current), the polarities of
commutating poles must be of opposite sign.
c) By increasing the Air Gap
Along air gap considerably increases the reluctance of the magnetic circuit, require greater number of
ampere- turns in the field to produce necessary flux in the air gap this lead to less distortion of the main field.
d) Compensating Windings
These windings are coils wound between the two adjacent poles in series with the armature. These windings
are embedded in slots in the pole shoes and are connected in series with armature in such a way that the
current in them flows in opposite direction to that flowing in armature conductors directly below the
pole shoes Fig 7. Compensating winding must provide sufficient m.m.f so as to counterbalance the
armature m.m.f. Compensating w i n d i n g s are used for large direct current machines which are subjected to
large fluctuations in load i.e. rolling mill motors and turbo-generators etc. Their function is to neutralize the
cross magnetizing effect of armature reaction. Thereby further saving the machine against flashover around
the whole commutator thereby avoiding short circuiting the whole armature.

Commutation:

Commutation is the action of transferring current from the armature conductors through the
commutator segments and brushes to the external circuit for the d.c. Generator and or vice versa for
the d.c. Motor

The currents induced in armature conductors of a d.c. generator are alternating. These currents flow
in one direction when armature conductors are under N-pole and in the opposite direction when they
are under S-pole. As conductors pass out of the influence of N-pole and enter that of S-pole, the current in
them is reversed. This reversal of current takes place along magnetic neutral axis or brush axis i.e. when
the brush spans and hence short circuits that particular coil undergoing reversal of current through it.
This process by which current in the short-circuited coil is reversed while it crosses the M.N.A. is
called commutation

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Methods of Improving Commutation:

There are two practical ways of improving commutation by reducing bad commutation or sparking
at the brushes i.e. of making current reversal in the short-circuited coil as sparkless as possible. These
methods are known as (i) resistance commutation and (ii) e.m.f. commutation (which is done with the help of
either brush lead or interpoles, usually the later).
(i) Resistance Commutation:
The fact that carbon brushes have a high contact resistance assists commutation. This method of improving
commutation consists of replacing low-resistance Cu brushes by comparatively high-resistance carbon
brushes. The use of carbon brushes does assist in reducing the sparking.
The additional advantages of carbon brushes are that (i) they are to some degree self- lubricating
and polish the commutator and (ii) should sparking occur, they would damage the commutator less
than when Cu brushes are used. But some of their minor disadvantages are: (i) Due to their high contact
resistance (which is beneficial to sparkless commutation) a loss of approximately 2 volt is caused. Hence,
they are not much suitable for small machines where this voltage forms an appreciable percentage loss.
Owing to this large loss, the commutator has to be made somewhat larger than with Cu brushes in order to
dissipate heat efficiently without greater rise of temperature. (iii) because of their lower current density
than Cu they need larger brush holders.
(ii) EMF Commutation:
Since the reactance voltage is the main cause of bad commutation. It is necessary to
neutralize its effect. In this method, arrangement is made to neutralize the reactance voltage by
producing a reversing e.m.f in the short-circuited coil under commutation. This reversing emf, as the
name shows, is an e.m.f in opposition to the reactance voltage and if its value is made equal to the
latter, it will completely wipe it off, thereby producing quick reversal of current in the short-
circuited coil which will result in sparkless commutation. The usual way is by using interpoles.
(iii) Interpoles of Compoles: These are small poles fixed to the yoke and spaced in between the
main poles. They are wound with comparatively few heavy gauge Cu wire turns and are connected in
series with the armature so that they carry full armature current. Their polarity, in the case of a generator,
is the same as that of the main pole ahead in the direction of rotation whereas in a motor the interpoles
polarity is opposite the next main pole in the direction of rotation. The function of interpoles is two-fold:
 As their polarity is the same as that of the main pole ahead, they induce an e.m.f in the coil (under
commutation) which helps the reversal of current. The e.m.f induced by the compoles is known as
commutating or reversing e.m.f. The commutating e.m.f neutralizes the reactance e.m.f thereby making
commutation sparkless.
 Another function of the interpoles is to neutralize the cross-magnetizing effect of armature reaction.
Hence, brushes are not to be shifted from the original position.

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Fig 6 D.C generator

Fig 7 Compensating winding

Troubles in D.C. Motors

Several troubles may arise in a d.c. motor and a few of them are:
1. Failure to start

This may be due to (i) ground fault (ii) open or short-circuit fault (iii) wrong connections (iv) too low
supply voltage (v) frozen bearing or (vi) excessive load.

2. Sparking at brushes
This may be due to (i) troubles in brushes (ii) troubles in commutator
(iii) troubles in armature or (iv) excessive load.

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(i) Brush troubles may arise due to insufficient contact surface; too short a brush, too little spring
tension or wrong brush setting.
(ii) Commutator troubles may be due to dirt on the commutator, high mica, rough surface or
eccentricity.
(iii) Armature troubles ma y be d u e t o an open armature coil. An open armature coil will cause
sparking each time the open coil passes the brush. The location of this open coil is noticeable
by a burnt line between segments connecting the coil.

3. Vibrations and pounding noises

These maybe due to (i) worn bearings (ii) loose parts (iii) rotating parts hitting stationary parts (iv)
armature unbalanced (v) misalignment of machine (vi) loose coupling etc.

4. Overheating

The overheating of motor may be due to (i) overloads (ii) sparking at the brushes (iii) short-circuited
armature or field coils (iv) too frequent starts or reversals (v) poor ventilation (vi) incorrect voltage.

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