working principle of dc motor

APresentation On

Working Principle Of D.C. Motors

ByVIKAS MITTAL

1

MOTOR

Definition of Motor

Machine that converts other forms of energy into mechanical energy and so imparts motion is called a motor.

2

MOTORDifferent types of Motors

Following are some types of motor:

Pneumatic motor : It converts pneumatic energy into mechanical energy.

Hydraulic motor :It converts kinetic energy of fluid into

mechanical energy.

3

MOTORDifferent types of Motors

Engine:It converts thermal energy into mechanical

energy.

Electric motor: It converts electrical energy into mechanical energy.

Turbine: It converts kinetic energy of water / Gas / Steam into mechanical energy. 4

ELECTRIC MOTOR

Machine that converts electrical energy into mechanical energy is called an electric motor.

5

ELECTRIC MOTOR

TYPES OF ELECTRIC MOTORSDepending upon the source of electrical energy,

there are two types of electric motors:

A.C. Motors : Motor that converts alternating current energy

into mechanical energy is called A.C. motor.

D.C. Motors : Motor that converts direct current energy into

mechanical energy is called D.C. motor.

6

DIRECT CURRENT MOTOR

CONSTRUCTION OF A DC MOTOR

DC Motor Consists of Four Main Parts

1. Field Magnets 2. Armature 3. Commutator 4. Brush and Brush Gears

7


PHOTOGRAPHIC VIEW OF A DC MOTOR

8


1. Field system The function of the field system is to

create a uniform magnetic field within which the armature rotates. Electro-magnets are preferred in comparison with permanent magnets on accounts of its greater magnetic effect and its field strength regulation, which can be achieved by controlling the magnetising current.

9


Field magnet consists of four parts given below :

1. Yoke or Frame 2. Pole cores 3. Pole shoes 4. Magnetising coils. 5. Interpoles

PHOTOGRAPHIC VIEW OF STATOR OF A DC MOTOR10


PHOTOGRAPHIC VIEW OF A FIELD COIL OF DC MOTOR

11


2. ARMATURE

It is a rotating part of a dc machine and is built up in a cylindrical or drum shape. The purpose of armature is to rotate the conductors in the uniform magnetic field. It consists of coils of insulated wires wound around an iron core. In addition , the most important function of the armature is to provide a path of very low reluctance to the magnetic flux. The armature core is made up of high permeability silicon steel stampings, each stamping, being separated from its neighbouring one by thin coating of varnish as insulation.

Armature continues…….12


Armature continues…….

The use of high grade steel is made (a) to keep hysteresis loss low, which is due to cyclic change of magnetisation caused by the rotation of core in the magnetic field and (b) to reduce eddy current in the core which are induced by the rotation of the core in the magnetic field. By using stampings or laminations, the path of the eddy current is cut into several units. Each lamination is about 0.3 to 0.6 mm thick.

13


PHOTOGRAPHIC VIEW OF AN ARMATURE OF DC MOTOR

14


3. COMMUTATOR The commutator is a form of rotating switchplaced between the armature and the external supplysource and so arrangedthat it will reverse thedirection of the currentflowing through the armature conductors during each rotation of the armature in case of motor.

commutator continues……

PHOTOGRAPHIC VIEW OF COMMUTATOR 15



It is a very important part of a dc machine and serves the following purpose:

1. It provides the electrical connections between the rotating armature coils and the stationary external circuit.

2. As the armature rotates, it performs a switching action to change the direction of flow of current in armature conductors so that the armature may be able to run in the same direction

3. It also keeps the rotor or armature mmf stationary in space.

commutator continues……16



The commutator is essentially of cylindrical structure and is built up of wedge shaped segments of high conductivity hard drawn copper or drop forged copper .These segments are insulated from each other by thin layers of mica ( usually of 0.5 to 1.0mm thickness) or micanite. The segments are held together by means of two V-shaped rings that fit into the V- grooves cut into the segments.


17



The winding ends are soldered with copper lugs or risers. The risers have air space between them so that air is drawn across the the commutator thereby keeping the commutator cool.


18



The commutator is pressed on to the armature shaft, and the outer periphery is then machined to provide smooth surface with which a stationary carbon (or graphite or copper) brush can maintain continuous contact at the armature and commutator rotate. Great care is taken in building the commutator because even slight eccentricity will cause the brushes to bounce, causing undue sparking.

19


4. BRUSHES AND BRUSH GEARS

The function of brushes is send current from external supply source to armature conductor i.e. armature winding. Brushes are rectangular in shape and rest on the commutator. Brushes are manufactured in variety of compositions and degrees of hardness to suit the commutation requirement. They may be classified roughly as carbon, carbon graphite, graphite, metal graphite.

20


PHOTOGRAPHIC VIEW OF BRUSH AND BRUSH GEARS SYSTEM OF DC MOTOR

21


WORKING PRINCIPLE:

The principle upon which a dc motor works is - If a current carrying conductor is placed in a magnetic field, mechanical force is experienced on the conductor, the direction of which is given by Fleming’s left hand rule (also called motor rule) and hence the the conductor moves in the direction of force.

22


The magnitude of the mechanical force experienced on the conductor is given by

F = B.I.L newtons

Where, B is the flux density in teslas ( Wb/m2 ) I is the current flowing through conductor in amperes L is the is length of conductor in meters.

In general if the conductor lies at an angle θ with a magnetic field of flux density B webers/meter2 , the mechanical force experienced on a conductor is given by

F = B.I.L. sin θ newtons

23


FLEMING’S LEFT HAND RULE:

This rule states that if the thumb, forefinger and middle finger of the left hand are stretched in such a way that they are at right angle to each other mutually and forefinger points towards the direction of magnetic field, the middle finger towards the direction of flow of current then the thumb will point the direction of force acting on the conductor.

24


MOTION

MAGNETICFIELD

DIRECTION OF CURRENT

25


FORCE ON A CURRENT CARRYING CONDUCTOR LYING IN THE MAGNETIC FIELD:

To understand the force on a current carrying conductor lying in the magnetic field, let us see the following three figures:Figure (a) shows the uniform magnetic field between the two opposite poles

SN

Fig.(a) Main magnetic field 26


In figure (b) the cross section of a conductor carrying current in inward direction placed between two magnets, the field being temporarily removed, is shown. By applying the right hand thumb rule, the direction of the field around the conductor is found to be clockwise

Field due to the current in the conductor

Fig (b) N S

27


If the current carrying conductor shown in figure (b) is placed in the magnetic field shown in figure (a), the resultant magnetic field would be similar to that shown in figure (c)

FORCEFIG. (c)

N S

28


The lines of force above the conductor are strengthened, since they are in the same direction, but the lines of force below the conductor are weakened because the two fields below the conductor are opposite in directions and hence tends to destroy each other.

FORCEFIG. (c)

N S

29


Magnetic lines like rubber bands have a tendency to strengthen out and, therefore, a force is experienced on the conductor in the downward direction, as shown in the fig.( C ).

FORCEFIG. (c)

N S

30


If the direction of current is reversed in the conductor , as shown in the Fig. (d), the direction of force experienced is reversed. In this case the lines of force above the conductor are weakened while those below the conductor are strengthened.

Fig. (d)

FORCE

N S

31


Hence it is observed that when a current carrying conductor is placed at right angle to the direction of magnetic field, a mechanical force is Experienced on the conductor in a direction perpendicular to both the direction of magnetic field and flow of current.

Fig. (d)

FORCE

N S

32


Magnetic Field in DC Motor

33


Current in DC Motor

34


Force in DC Motor

35


Torque in DC Motor

36


DC Motor Operation

37


COMMUTATOR ACTION IN A DC MOTORIn the case of dc motor, it is necessary that the current through the coils of armature winding be reversed as a particular coil leaves one pole (say, north pole), crosses the neutral line and comes under the influence next pole which is of opposite polarity (i. e. the south pole) The operation of commutator, that serves the above purpose, is given below:Consider a single turn coil, whose leads are soldered to cummutator segment (a) and (b), each carrying a brush as illustrated in fig shown in next slide Commutator action continues….

38


COMMUTATOR ACTION IN A DC MOTOR

Commutator action continues….

... ..

II

1

..

2+

_

a

b

..2

1

+_

b a

III

+

.

..

.2

1

_a b

I39



The positive side of the supply line is connected to left hand brush and negative side to the right side brush. In position I, the line current arrives at the commutator segment (a), flows through the bottom side 1 of the coil away from the reader and the upper side 2 of the coil towards the reader, reaches the commutator segment (b) and flows again into the line through the brush. The coil will tend to rotate in clock-wise direction, as determined by Fleming’s left hand rule.

Commutator action continues…. 40



In position II, the coil is on the magnetic neutral line; there is no contact between the commutator segments and the brushes, there is no flow of current through the coil. The coil crosses the neutral line by inertia. In case of multi-turn coil, the remaining turns of the coil will supply the necessary torque.

In position III, the two sides of the coil, 1 and 2, have changed poles, and the current flowing them has reversed. The commutator segments, however, have also changed contact with the brushes. Thus the coil will continue to rotate in the same direction as before, i.e. clock-wise. 41


BACK EMF IN A DC MOTOR AND ITS IMPORTANCE

When the armature of motor continues to rotate due to motoring action, the armature conductors cut the magnetic flux and therefore emfs are induced in them. The direction of this induced emf, known as back emf, is such that it opposes the applied voltage.

Back emf continues…..

42



Since the back emf is induced due to the generator action, the magnitude of it is, therefore, given by the expression, Back emf, Eb = ØZNP …..(1.1)

60 x A Where, Ø is flux per pole in webers, Z is the total number of armature conductors or coil sides on the armature, P is the number of poles, A is the number of parallel paths in the armature, N is the rotational speed of the armature in rpm

Back emf continues…..43



The equivalent circuit of a dc motor is shown in fig below. The armature circuit is equivalent to a source of emf, Eb in series with a resistance, Ra put across a dc supply mains of V volts. It is evident from fig that the applied voltage V must be large enough to balance both the voltage drop in armature resistance and the back emf at all times i.e. V = Eb + Ia Ra or Ia = V-Eb …….(1.2) Ra


V

Eb

V

IaRa

Eb

+

_

Ia

Ia

+

_

Equivalent circuit of a motor armature 44


Back emf continues….. V = Eb + Ia Ra or Ia = V-Eb …….(1.2) RaWhere, V is the applied voltage across the armatureEb is the induced voltage in the armatureIa is the armature currentRa is the armature resistance

It is obvious from the expressions (1.1) and (1.2) that the induced emf in the armature of a motor, Eb Back emf continues…..

V

Eb

V

Ia

Ra

Eb+_

Ia

Ia

+

_Equivalent circuit of a motor armature

45



Depends among other factors upon the armature speed and the armature current depends upon the back emf Eb for a constant applied voltage and armature resistance.If the armature speed is high, back emf will be large and therefore, armature current small. If the speed of the armature is low, then back emf will be less and armature current Ia will be more resulting in development of large torque.

46


POWER RELATIONSHIP IN A DC MOTOR

Voltage equation for a motor is: V = Eb + Ia RaMultiplying each term of the voltage equation by Ia we get VIa = Eb Ia + Ia2 Ra …….(1.3)

The equation (1.3) is known as power equation of the dc motorThe term VIa represents the power supplied to the motor armature, Ia2 Ra represents the power lost in the armature, Eb Ia represents developed in the armature causing rotation of the armature.

Power relationship continues……47


Power relationship continues……

The power developed ‘Eb Ia’ is not all available at the shaft since some it is used to overcome the mechanical power losses of the motor As mentioned above the mechanical power developed in the motor is given by P m = Eb Ia watt ……(1.4)

Power relationship continues……

48


TYPES OF DC MOTORS

Different types of dc motors are:

1. Permanent magnet dc motors 2. Separately excited dc motors 3. Series wound dc motors 4. Shunt wound dc motors 5. Compound wound dc motors

49


1. Permanent magnet dc motors: It consists of an armature and one or several

permanent magnets encircling the armature. Field coils are not usually required. However,

some of these motors do have coils wound on the poles. If they exist, these coils are Intended only for recharging the magnets in the event that they loose their strength.

ARMATURE

+

_

N S

Fig. Permanent Magnet Motor

50


2. Separately excited dc motors:

These motors have field coils similar to those of a shunt wound dc motor, but the field coils and armature are fed from different supply source.In a separately excited dc motor, Armature current, Ia = Line current, IL

Back emf developed, Eb = V – I Ra

Separately Excited dc motors continues…….

DC SUPPLY MAINS

DC SUPPLY MAINS

+

-

I

+

-

ARMATURE

FIE

LD

If

+

-

RHEOSTAT

Separately excited dc motor51


Separately Excited dc motors continues…….

Power drawn from the mains, P = VI Mechanical power developed, Pm = Power input to armature - power lost in armature = VI – I2 Ra = I (V – I Ra ) = Eb I watt

52


3. Series wound dc motors:

As the name implies, the field coils, consisting of a few turns of thick wires, are connected in series with the armature as shown in the fig. The cross-sectional of the wire for the coils has to be fairly to carry the armature current, but owing to large current, the number of turns of wire in them need not be large.

DC Series Motors continues…….

ARMATURE

+

_

Series field

DC SUPPLY MAINS

+

_

Series wound dc motors53


DC Series Motors continues…….

In a dc series motor, Armature current, Ia = series field current, Ise = Line current, IL = I (say)Armature current, Ia = series field current, Ise = Line current, IL = I (say)

Back emf developed, Eb = V – I (Ra + Rse ) Power drawn from the mains, P = VI Mechanical power developed, Pm = Power input to armature - power lost in armature = VI – I2 (Ra + Rse) = I [V – I ( Ra + Rse) ] = Eb I watts

54


4. Shunt wound dc motors:

The word “ shunt ’’ means “ parallel ’’ . In these motors , the field coils are connected in parallel with the armature. The field winding consists of a large number of turns of thin wire so as to provide large resistance. The field current is much less than the armature current, sometimes as low as 5%.

DC Shunt Motors continues…….

DC SUPPLY MAINS_

_

Shunt wound dc motors

Shun

t fi

eld

ARMATURE

IshIshIa

IL

++

55



The current supplied to the motor is divided into two paths, one from the field winding and second through the armature i.e. Input line current, IL = Ia + Ish

Where Ia is the armature current and Ish is the shunt field current and is given by where V is the supply voltage, Rsh is the shunt field resistance

Ish = V RshDC Shunt Motors continues……. 56


Back emf developed, Eb = V – I Ra

Power drawn from the mains, P = VIL

Mechanical power developed,

Pm = Power input to armature - power lost in armature

= VIL – V Ish - I2a Ra

= V( IL – Ish ) - I2a Ra

= V Ia- I2a Ra = Ia( V – Ia Ra )

= Eb Ia watt


57


5. Compound wound dc motors:

Compound wound motors are of two types namely cumulative compound motor wound and differential compound wound motor.

58


Cumulative compound wound motor:

in this motor field winding are connected in such a way that the direction of flow of current is same in both the field windings as shown in fig. In the motor of this type the flux due tothe series field windingstrengthens the field due to theshunt field winding.

SERIES FIELD

_DC SUPPLY MAINS

+

IL IshIsh

Shun

t fi

eld

Ia+

_ARMATURE

Cumulative compound wound motor59


Differential compound wound motor:

In this motor field winding are connected in such a way that the direction of flow of current is opposite to each other in both the field windings as shown infig. In the motor ofthis type the flux due to the seriesfield winding weakens the fielddue to the shunt field winding.

Differential compound wound motor

DC SUPPLY MAINS

+

SERIES FIELD

Shun

t fi

eld

Ish

IshIa

IL

IL

+

__

ARMATURE

60


SPEED EQUATION OF DC MOTORAs already in the expression for back emf developed in the armature of a dc motor is given by the expressions Eb = Ø Z N P volts ……(1)

60xA

and Eb = V – Ia Ra volts ……(2)

comparing expressions (1) and (2) , we get Ø Z N P = V – Ia Ra 60xA

or N = V – Ia Ra x 60A Ø Z P

Speed equation of dc motor continues……61


Speed equation of dc motor continues……

or N α V – Ia Ra Ø

Since Z, A and P are constant for a particular machine. Now in the above expression for speed, voltage V is constant and the armature drop Ia Ra is negligible compared to supply voltage V . Therefore, the speed of the motor is inversely proportional to the flux per pole i.e. the speed of the motor increases with the fall in flux.

62


TORQUE

The measure of causing rotation of the wheel or the turning or twisting moment of a force about an axis is called the torque.

Torque is measured by the product of force and the radius at which this force acts.

Torque continues…..F

r

ROTATION

64


Torque continues…..

Consider a wheel of radius r meters acted by circumferential force F newtons, as shown in the fig. Below. Let this force cause the wheel to rotate at n rps Torque, T = F x r newton metres Work done per revolution = Force x distance moved = F x 2 Π r joules Work done per second = F x 2 Π r x n

= F x r x 2 Π n = T x 2 Π n joules 65


ARMATURE TORQUE OF A DC MOTOR

Let Ta be the torque developed in newton metres by the motor armature running at n rps Power developed = Work done per second = Ta x 2 Π n watts …..(1)Electrical equivalent of mechanical power developed by the armature also = Power input to armature – armature resistance loss

= V Ia- I2a Ra

= Ia( V – Ia Ra ) = Eb .Ia watt ….(2)

Armature torque continues…….66


Armature torque continues……. Comparing expressions (1) and (2), we get Ta x 2 Π n = Eb Ia = Ø Z N P x Ia 60xA = Ø Z n x P x Ia A ( since Ø Z N x P = Ø Z n x P 60 A A ( as n = N 60

Armature torque continues…….

67


or Ta = 1 x Ø Z x P x Ia N-m 2 Π A = 0.159 Ø Z P x Ia N-m Asince Z, P and A are constant for a particular machine therefore Ta α Ø Ia

i.e. torque developed by armature is proportional to the product of flux per pole and armature current.



68


In case of series wound motor Ø is proportional to Ia ( before saturation) because current in field winding and armature winding is same and

therefore Ta α Ia2

In case of shunt wound motor Ø is practically constant and

therefore Ta α Ia


69


OPERATING CHARACTERISTICS OF DC MOTORS

The performance and, therefore, suitability of a dc motor is determined from its characteristics known as performance characteristics. The important characteristics of dc motors are:

1. Torque – Armature Current Characteristic. This

characteristic curve gives relation between torque developed in armature, T and armature current, Ia. This is also known as electrical characteristic.

70


2. Speed - Armature Current Characteristic. This characteristic curve gives relation between speed, N and armature current, Ia. This is also known as speed characteristic.

3. Speed - Torque Characteristic. This characteristic curve gives relation between speed, N and torque, T. This is also known as mechanical characteristic

The important relations to be kept in mind while discussing motor characteristics are:

(i) Ia = V – Eb (ii) N α Eb Ra Ø

and (iii) T α Ia 71


CHARACTERISTICS OF DC SREIES MOTORS

1. Magnetic characteristic.

In case of dc series motors the flux Ø varies with the variation in armature current as the field is in series witharmature. The flux first increases following in linearlaw with the increase in load current, becomesmaximum at saturation point and finally becomes constant.

FL

UX

IN

W

b

ARMATURE CURRENT IN AMPS

Magnetic characteristic DC series motor

FLUXsaturation point

72


2. Speed – Current Characteristic of DC Series Motor.

From the expression for speed, N α V – Ia Ra Ø

it is obvious that speed is directly proportional to applied voltage and inversely proportional to flux per pole. If the applied voltage remains constant, speed is inversely proportional to flux. So if a curve is drawn between reciprocal of the flux and current I, the speed – current characteristic is obtained, which is a rectangular hyperbola in shape as shown in fig. in next slide.

Speed – Current characteristic DC series motor continues……73


Speed – Current characteristic DC series motor continues……

Since on no load the speed is dangerously high,As obvious from speed current characteristic curve, which will result in heavy centrifugalForce which in turn will ]damage the motor.That is why, series motors are never started on No-load

Speed – Current characteristic DC series motor


SPE

ED

IN

RP

M

74


3. Torque – Current Characteristic of dc series motor:From the expression of mechanical torque T α Ø Ia , it is obvious that torque is directly proportional to the product of flux per pole Ø and armature current Ia. up to saturation point flux is proportional to field current and hence to the armature current, because Ia = If . Therefore on light load mechanical torque T is proportional to the square of the armature currenti.e. Ta α Ia2 and


TO

RQ

UE

IN

N -

m

Torque – Current Characteristic of dc series motor continues….75


Torque – Current Characteristic of dc series motor continues….

and hence curve drawn between Torque and armature current is a parabola as shown in the fig. After saturation point flux is almost independent of excitation current and so torque is directly proportional to the armature current i.e. T α Ia . Hence the characteristic becomes a straight line.

76


4. Speed – Torque Characteristic of dc series Motor:

Fig below shows the speed – torque characteristic of a dc series motor. It is obvious from the fig below that as the torque increases speed decreases.Hence series motors are best suited for the services where the motor is directly coupled to the loadsuch as fans whosespeed falls with increase in torque.

Fig. Speed - torque Characteristic of dc series motor

TORQUE IN N - m

SPE

ED

IN

RP

M

77


CHARACTERISTICS OF DC SHUNT MOTORS

1. Speed – Current characteristic of dc Shunt motor:

If the voltage V is kept constant the field current will remain constant hence flux will have maximum value on no-load but will decrease slightly due to armature reaction as the load increases but for most purpose the flux is considered to be constant, neglecting the effect of armature reaction.

Speed – Current characteristic of dc Shunt motor continues……


SP

EE

D I

N R

PM

78


Speed – Current characteristic of dc Shunt motor continues……

From the expression of speed for a dc motor, N α V – Ia Ra or Eb Ø Øspeed N is directly proportional to back emf, Eb or( V – Ia Ra ) and inversely proportional. Since flux is considered to be constant as mentioned above, so with the increase in load current the speed slightly falls due to increase in voltage drop in armature. Since voltage drop in armature at full load is very small as compared to applied voltage so drop in speed from no-load to full load is very small and for all practical purposes the shunt motor is taken as a constant speed motor 79


2. Torque – Current Characteristic of a dc Shunt Motor :From the expression for the torque of a dc motor, T α Ø Ia , the torque is directly proportional to the product of flux per pole and the armaturecurrent. Since in case of shunt motor the flux is considered to be constant, therefore, torque increaseswith the increase in loadcurrent following linearlaw i.e. torque -armature current characteristic is a straight line passing through origin as shown in fig. Speed - Current Characteristic of

dc shunt motor


TO

RQ

UE

IN

N

- m

80


3. Speed - Torque Characteristic of a dc Shunt Motor:

This characteristic curve can be drawn from the two other curves i.e. speed – current curveand torque – current curve and is shown in the fig.

Speed - Current Characteristic of dc shunt motor

TORQUE IN N - m

SP

EE

D I

N R

PM

81


CHARACTERISTICS OF DC COMPOUND WOUND MOTORS

Speed Characteristic of a dc Cumulative Compound Wound Motor:

The characteristics of a the cumulative compound wound motor are the combination of shunt and series characteristics. As the load is increased, flux due to series field winding increases, and cause the torque greater than it would have with shunt field winding alone for a given machine and for a given current. The increase in flux due to series field winding on account of increase in load causes the speed to fall more rapidly than it would have done in shunt motor. The characteristics are shown in the fig given below82


Characteristic of a dc Cumulative Compound Wound Motor:

Characteristics of Cumulative Compound Wound Motors


TO

RQ

UE

IN

N -

m

CUMULATIVE

SHUNT


SP

EE

D I

N R

PM

CUMULATIVE

SHUNT

83


Characteristic of a dc Differential Compound Wound Motor:

In differential compound wound motors, the series field winding is connected in such away that the series field opposes the shunt field while in cumulative compound wound motor series field helps the shunt field. Since the flux decreases with the increase in load so the speed remains nearly the constant as the load is increased and in some cases the speed will increase even.

Characteristic of a dc Differential Compound Wound Motor continues……

84


Characteristic of a dc Differential Compound Wound Motor continues……..

The decrease in flux with the increase in load causes the torque to be less than that of shunt motor. The characteristics are similar to those of a shunt motor. Since the shunt motor develops a good torque and almost constant speed, therefore, differential motor is seldom used.

85


Characteristic of a dc Differential Compound Wound Motor:

Characteristics of Differential Compound Wound Motors


TO

RQ

UE

IN

N -

m

SHUNT

DIFFERENRIAL


SP

EE

D I

N R

PM

SHUNT

DIFFERENRIAL

86


SPEED CONTROL OF DC MOTOTRS

MEANING OF SPEED CONTROL:

Speed control means intentional change of the drive speed to a value required for performing the specific work process. The concept of speed control or adjustment should not be taken to include the natural change in speed which occurs due to change in load on the drive shaft. Speed can be controlled manually by operator or by some automatic control device.

87


SPEED CONTROL BY MECHANICALS :

Speed can be adjusted mechanically by means of stepped pulleys, sets of change gears, variable speed friction clutch mechanism and other mechanical devices. But the electrical speed control has many economical as well as engineering advantages over mechanical speed control.

88


SPEED CONTROL BY ELECTRICAL METHOD:

Expression of speed for a dc motor, N = k V – Ia ( R + Ra )

ØThe above expression reveals that the speed can be controlled by adjusting any one of the three factors appearing on the right hand side of the expression:

Applied voltage to the armature terminals, V(ii) External resistance in the armature circuit, R(iii) Flux per pole, Ø

89


SPEED CONTROL OF DC SHUNT MOTORS

1.Field Control Method For DC Shunt Motor: In this method speed variation is accomplished by

means of a variable resistance inserted in the series with the shunt field. The power wasted in the controlling resistance

is very small as the field current is very small.

Fig. Field Control Method For DC Shunt Motor

+_ ARMATURE SHUNT

FIELD

FIELD RHEOSTATE

IshIa +

_

V

IL

90

DIRECT CURRENT MOTORField Control Method For DC Shunt Motor continues………

Since in this method of speed control the flux can only be reduced (not increased) so the speed only above the normal one can be obtained. The speed is minimum at the maximum value of flux and the speed is maximum at the minimum value of flux. The high speed limitis also restricted due to mechanical considerationas the centrifugal forces are set up at high speed

TORQUE

SP

EE

DNORMAL FIELD

WEAK FIELD-1

WEAK FIELD-2

Fig. Speed – Torque Characteristics of Shunt Motor With Field Control91

DIRECT CURRENT MOTORSPEED CONTROL OF DC MOTOTRS

2. Armature Control Method For DC Shunt Motors :

This method consists of a variable resistance connected in series with the armature as shown in the fig in next slide. The speed at thefull load may be reduced any desired value depending on the amount of resistance. With this method the voltage across the armature is lower than the line voltage.

Armature Control Method For DC Shunt Motors continues…… 92


Armature Control Method For DC Shunt Motors continues……

WITH RESISTANCE

CONTROL RESISTANCE

SHUNT FIELD

ARMATURE

+

_

+

_

ILIa

Ish

V

TORQUESP

EE

D

NO RESISTANCE

Fig. Armature Control Method For DC Shunt Motors

93


SPEED CONTROL OF DC SERIES MOTORS:Speed of dc series motor may be obtained through either armature or field control.

1. ARMATURE CONTROL METHODS FOR DC SERIES MOTORS:

(i) Armature resistance control For DC Series Motors:

Fig. Armature Resistance Control For DC Series Motors

SERIES FIELD

CONTROL RESISTANCE

_

+

ARMATUREV+_

94


2. FIELD CONTROL METHOD FOR DC SERIES MOTORS:

(i) Field diverter method of Speed Control For dc Series Motors:

Fig. Field diverter method of Speed Control For dc Series Motors

ARMATURE

+_

SERIES FIELD

DIVERTOR

V

_

+

I

I div

Ise

96


(ii) Tapped Field Control Method For Speed Control of DC Series Motors:

Fig. Tapped Field Control Method For Speed Control of DC Series Motors

TAPPED SERIES FIELD

ARMATURE

+

_

V

+

_

I

I

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POWER LOSSES IN A DC MOTOR

The basic function of dc motor is to convert electrical energy into mechanical energy. Whole of the input energy is not converted into useful output energy but a part of the input energy is converted into heat and the same is lost. The basic power equation for a motor is

P input = P output + P losses

Power losses of dc motor continues……..98



Power losses of dc motor continues……..

The power losses in a dc motor consist of input power that is converted into heat. These losses are divided into

(i) Copper losses or electrical losses (ii) Iron or magnetic losses(iii) Mechanical losses


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POWER LOSSES IN A DC MOTORPower losses of dc motor continues……..

1. Copper or Electrical Losses:

Copper or electrical losses include power wasted in armature winding, series field,shunt field interpole field brush contacts

Armature copper losses = I2aRa

Shunt Field Copper losses = I2sh Rsh

Series Field Copper Losses = I2se Rse

Power losses of dc motor continues……..100

DIRECT CURRENT MOTORPower losses of dc motor continues……..

2. Iron or Magnetic Losses:

These losses are also called core losses and include the hysterisis and eddy current losses

(a) Hysterisis Losses P h = ŋ ( B max ) 1.6 .f .V watts

(b) Eddy Current losses P e = K e . B max. f 2 V t2 watts

where ŋ = Steinmetz hysterisis coefficient, V =Volume of core in cubic meters , f = Frequency of the magnetic cycles

per second t = Thickness of core steel laminations B max. = Maximum flux density 101



Value of Steinmetz hysterisis coefficient, ŋ for:

Good dynamo sheet steel = 502 J/m3 ,

Silicon steel = 191 J/m3,

Hard cast steel = 7040 J/m3,

Cast steel = 750 – 3000 J/m3,

Cast iron = 2700 - 4000 J/m3

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3. Mechanical Losses in a DC Motor:

These losses consist of power loss due to frictionof bearings, air friction or windage and are caused by the motion of the moving parts.


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SUMMERY OF POWER LOSSES IN A DC MOTOR

Useful Output

Total losses

Copper losses

Iron losses

Mechanical losses

Armature copper loss

Field copper losses

Hysterisis loss

Eddy current loss

Friction loss

Windage loss

Input

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EFFICIENCY OF A DC MOTOR

The ratio of useful output to the total input is called the efficiency of the machine is expressed as ŋ = Output = Input - total losses Input Input

= Output Output + total losses

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APPLICATION OF DC MOTORS

1. Application of DC Series Motors:

As observed from the different characteristics for dcseries motor, these motors are suitable where highstarting torque is required such as , electric traction,Hoists, trolleys, cranes, gears drives.

DC Series motors have dangerously high speed at no-loads. So these motors are not suitable for the services where the load may be entirely removed and also these motors are not suitable for belts drives.

Applications of dc motors continues…….. 112

DIRECT CURRENT MOTORAPPLICATION OF DC MOTORS

Applications of dc motors continues……..

2. Application of DC Shunt Motors:As observed from the different characteristics for dc shunt motors, these motors have almost constant speed and due to this feature these motors are suitable where constant speed is required in wide range of load such as, lathe machines, milling machines, conveyors line shafts etc. These are also useful where medium starting torque is required such as boring machines, blowers, fans, conveyors centrifugal pumps, shapers, spinning and weaving machines, machine tools, printing presses DC shunt motors are not suitable for use with flywheel or with fluctuating loads services.

Applications of dc motors continues…….. 113

DIRECT CURRENT MOTORApplications of dc motors continues……..

3. Application of DC cumulative Compound Motors:As observed from the different characteristics for dccumulative compound motors, these motors are used in driving machines which are subjected to sudden applications of heavy loads such as occur in rolling mills, punching and shearing machines, lifts, mine-hoist. These motors are also used where high starting torque is required but series motors cannot be employed conveniently such as cranes and elevators.These motors are not suitable for applications requiring adjustable speed for field control

Applications of dc motors continues…….114


APPLICATION OF DC MOTORSApplications of dc motors continues……..

4. Application of DC differential Compound Wound Motors:

In differential compound wound motors, since series field opposes the shunt field, the resultant flux decreases with the increase in load; thus the machine runs at a higher speed than it would do as a shunt motor. The decrease in flux with the increase in load causes the torque to be less than that of a shunt motor. So such motors are rarely used in practice as the differential arrangement causes difficulties during overloads and starting. 115


SPECIFICATIONS OF DC MOTORS

Specifications of dc motors are shown on the specification plate of the motors. The meaningof different specifications of Kirloskar make dc motor as:

Specification of DC Motors continues….

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KW / HP - Maximum power output of the motor

VOLTS – Rated Armature Voltage

AMPS - Rated motor output current

DUTY – S1 for continuous operation S4 for intermittent operation

XXX / XXX

Maximum Speed

Base Speed

RPM -

Specification of DC Motors continues….117


Specification of DC Motors continues…. Frame:

SX / MX / LX (Core length in mm)

K X B DC XXX - X

100…250 ( Centre height in mm)

DC Motor

B for flange mountingNone for foot mounting

L for IC 01, 06, 17, 37S for IC 0041H for IC 0666W for IC W37, A86Make - Kirloskar

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WDG – Connections Scheme of Winding of motor

EXC. V - Field Excitation Voltage

EXC. A – Field Excitation Current

INS. CL. – Insulation class used in winding

FWR – Field Winding Resistance

AMB – Ambient or room temperature

GD2 - Moment of inertia

Specification of DC Motors continues…. 119



IP – Type of Enclosure, IP21, IP22, IP23, IP44 (i) IP21: Screen protected

(ii) IP22: Screen protected drip proof (iii) IP23: Screen protected splash proof (iv) IP44: Totally enclosed duct ventilated

IC – Type of Cooling , IC06, IC17, IC37 (i) IC06: Motor mounted blower and free

outlet (ii) IC17: Cooling air inlet via duct, free

outlet (iii) IC37: Cooling air inlet and outlet via

ducts 120



BRG: CE – Bearing at commutator end NCE – Bearing at non commutator end

Air Flow – Amount of Air required for the cooling of Motor

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THANKS…..

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working principle of dc motor

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