Chapter_2-1_dc_motors.pdf

ELECTRICAL MACHINES (DC Motors)

Contents

• Introduction to Electrical & Direct Current Machines• Constructional Details of DC Machines• Principle of Operation of Dc Machines• Types of DC Motor• Power Flow Diagram• Characteristics of DC Motors• Armature Reaction• Speed Control of DC Motors• Applications

1

Chapter 2-1 (201305)

Learning Outcomes

Upon completion of chapter-2 you should be able to:

•State the principle by which machines convertmechanical energy to electrical energy and vice versa.

•Discuss the operating differences between differenttypes of dc machines.

•Explain the characteristics of dc machines.

2

3

IntroductionEnergy Conversion•Energy exists in many forms.•One form of energy can be obtained from the otherform with the help of converters.•Light bulbs and heaters require energy in electricalform.

Electrical Machines:•Converters that translate an electrical input to amechanical output or vice versa are called the electricmachines.•This process of translation is electromechanical energyconversion.•The magnetic system acts as the link between theelectrical and mechanical systems

4

Introduction . . .

Electromagnetic phenomena:Electrical machines use the following electromagneticphenomena for their electromechanical energyconversion:•Whenever the field in the vicinity of a conductor changes(or) flux linking a conductor changes, an emf is inducedinto that conductor (Faraday’s Law).•Whenever a current-carrying conductor is placed in amagnetic field, the conductor experiences a mechanicalforce.

Introduction . . . .

55

Energy flow diagram

Gen Transformer(step-up)

TransmissionLine

Transformer(step-down)

Distribution /Utilization: Loads could beMotors,Lighting,

Heaters, Coolers, …

66

Over view of DC Machines

Direct-current (DC) machines are divided into dcgenerators and dc motors.

DC Generator•A dc generator is a machine that converts mechanicalenergy into electrical energy (dc voltage & current) byusing the principle of magnetic induction.

•DC generators are not as common as they used to be,because dc, when required, is mainly produced byelectronic rectifiers.

77

Over view of DC Machines . . .

•A dc motor is a machine that converts electrical energyinto mechanical energy by supplying a dc power(voltage and current).

•DC motors are widely used in many applications.

•DC motors are everywhere! In a house, almost everymechanical movement that you see around you iscaused by an DC (direct current) motor.

DC Motor

88

Over view of DC Machines . . .

99

CONSTRUCTIONAL DETAILS OF DC MACHINES

• Stator: Field Poles (minimum 2) - magnetic flux• Rotor : Armature - carries armature winding• Air gap (between poles and armature)• Commutator (ac to dc)• Carbon brushes (collects and carry current from

the commutator)

1111

Rotor of a DC machine

Tooth

Slot

12

Constructional details of dc machines . . .

16

•The field poles which produce the magnetic flux, aremounted on the yoke made up of cast iron and carry fieldwindings.•The pole cores and pole faces are laminated to reduceeddy current loss.•The armature core, which carries armature windings ison the rotor and is made of sheet-steel laminations.•The slots are cut on the surface of armature core toplace armature windings.

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•The ends of the windings are connected to thecommutator segments (built in copper and are very goodconductors).•Carbon brushes are placed overcommutator segments and serve asleads for the electric connection.

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18

ARMATURE (Rotor)


•The entire assembly of iron core, commutator, andwindings is called the armature.

•The commutator is connected to the slotted iron core.

•The windings of armatures are connected in differentways depending on the requirements of the machine.

•More turns of conductor = higher rectified voltage 18

19

There are two types of armature winding: Lap windingand Wave winding.

Lap Wound Armatures•are used in machines designed for low voltage and highcurrent•armatures are constructed with large wire because ofhigh current•Number of parallel paths = number of poles

ARMATURE . . .


19

20

Wave Wound Armatures•are used in machines designed for high voltage and lowcurrent•are used is in the small generator•No of parallel paths = 2

ARMATURE . . .


20

Field winding


Most DC machines use electromagnets to provide themagnetic field.

Two types of field windings are used :–series field winding–shunt field winding

Series field windings–are so named because they are connected in series withthe armature–are made with relatively few turns of very large wire(sufficiently large to carry the current) and have a verylow resistance.

21


Shunt field windings–Have relatively many turns of small wire, thus, it has amuch higher resistance than the series field.–is intended to be connected in parallel with, or shunt,the armature.–high resistance is used to limit current flow through thefield.

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When a DC machine uses both series and shunt fields,each pole piece will contain both windings.

The windings are wound on the pole pieces in such amanner that when current flows through the winding itwill produce alternate magnetic polarities.

Factors affecting the machine output•Speed•Field strength•No. of turns in the windings

DC motor principles•The same dc machine can act as a generator or amotor.•The motor converts electric energy to mechanicalenergy.•DC motors are used in industrial applications thatdemand a high degree of flexibility in the control ofspeed and torque (the torque-speed characteristics ofdc motors can be varied over a wide range whileretaining high efficiency).•They drive devices such as hoists, fans, pumps,calendars, punch-presses, cars, steel mills, mines,electric trains etc.

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DC motor principles . . .The Advantages

•The greatest advantage of DC motors may be speedcontrol.

•Today, adjustable frequency drives can provideprecise speed control for AC motors, but they do so atthe expense of power quality, as the solid-stateswitching devices in the drives produce a richharmonic spectrum. The DC motor has no adverseeffects on power quality.

26

• Power supply, initial cost, and maintenancerequirements are the negatives associated with DCmotors

• Rectification must be provided for any DC motorssupplied from the grid. It can also cause power qualityproblems.

• The construction of a DC motor is considerably morecomplicated and expensive than that of an AC motor,primarily due to the commutator, brushes, andarmature windings.

The drawbacksDC motor principles . . .

DC motor – principle of operation• DC motors consist of rotating armature windings and

stationary field windings. The current must be conductedto the armature windings by passing current throughcarbon brushes that slide over a set of copper surfacescalled a commutator, which is mounted on the rotor.

• The commutator bars are soldered to armature coils.• The brush/commutator combination makes a sliding

switch that energizes particular portions of the armature,based on the position of the rotor.

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• This process creates north andsouth poles on the rotor that areattracted to or repelled by northand south poles on the stator,which are formed by passingdirect current through the fieldwindings. It's this magneticinteraction that causes the rotor torotate.

DC motor – principle of operation . . .

• All motors rely upon the force exerted by a magnetic fieldon a current-carrying conductor.

• If a straight current carrying conductor is placed at rightangles to the uniform magnetic field existing between theNorth and south poles of a permanent magnet, the resultis shown in Fig.a.

• Two fields are present: the uniform field due to themagnet with lines of force that are straight and parallel,and the circular field around the current-carryingconductor, shown dotted.

28

DC motor – principle of operation . . .

29

DC motor – principle of operation . . .• As the lines of force above the conductor in Fig.a point in

the same direction, they add together, and as the lines offorce below the conductor oppose each other, theysubtract. The resultant magnetic field is shown in fig. b.

• Because the field is strong above the conductor andweak below the conductor, the distorted lines of forcetend to straighten like stretched elastic bands.

• A force is thus exerted on the conductor, tending tomove it down, as indicated by the arrow.

• If it were free to move, the conductor would leave themagnetic field.

• If the current is reversed through the conductor, thecircular field around the conductor will also reverse.Hence, the conductor will tend to move in the oppositedirection, i.e., upwards.

• Similarly, if the polarity of the main magnetic field isreversed, the direction of conductor motion will change. 30

31

N

S

TorqueIt is the turning or twisting force about an axis.

32

sin

Forc

e on

the

cond

ucto

r

33

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Counter emf (Back emf) in dc motors

0 60E Zn P A

• When a dc supply is connected to the dc motor, alarge current will flow through the armature conductorsbecause its resistance is very low.

• Each current carrying conductor experiences a force(because they are immersed in the magnetic field).

• These forces add up to produce a powerful torque,causing the armature to rotate.

• As soon as the armature begins to turn, a 2nd

phenomenon takes place: the generator effect. Withthe armature rotating in the magnetic field, thearmature conductors generate an emf.

• This generated (induced) emf is proportional to thespeed of rotation of the motor and the flux per pole, and is as follows:

35

0SI E E R

Counter emf (Back emf) in dc motors . . .Where, Z = total number of armature conductors

= effective flux per pole (Wb)n = speed of rotation (rpm)P = no. of polesA = no. of parallel paths

The generated voltage opposes the supply voltage, thuslimiting the armature current.

In case of a motor, the induced voltage, E0 is calledcounter emf because it opposes the source voltage.

The armature current is given by

where ES = line supply voltage & E0 = counter(generated) emf.

36

Counter emf (Back emf) in dc motors . . .

When the motor is at rest, the counter emf (cemf) is zeroand so the starting current is given by: I = (ES-0) / R.

As the speed increases, the cemf increases, with theresult that the value of armature current diminishes.

When a motor runs at no-load, the counter-emf must beslightly less than ES, so as to enable a small current toflow, sufficient to produce the required torque.

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Mechanical power and torque

The electrical power supplied to the armature, which isconverted to mechanical power (mechanical powerdeveloped), is

where, P =mechanical power developed by the motor (W)E0 = induced voltage in the armature (cemf) (V)I = total current supplied to the armature (A)

The mechanical power P is also given by the expression,where n is the speed of rotation.

Combining the above two equations for P,

0P E I

9.55P T nT

09.55 6.28PZ InT E I TA

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where, T = torque developed (N-m)Z = total number of armature conductorsF = effective flux per pole (Wb)I = armature current (A)6.28 = constant, to take care of units (=2)

Mechanical power and torque . . .

09.55 6.28PZ InT E I TA

Speed of rotation

0 60E Zn P A

0, 60 ( )Speed n E A Z P

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Classification of dc motorsThe magnetic flux in DC machine is produced by fieldcoils carrying current.

The production of magnetic flux in the machine bycirculating current in the field winding is called excitation.

There are two methods of excitation: separate excitationand self excitation.

In separate excitation the field winding is energized by aseparate DC source.

Fig. Separately excited DC motor

4040

Classification of dc motors . . .

In self excitation the current flowing through the fieldwinding is supplied by the machine itself.

DC machines are named according to the connection ofthe field winding with the armature.

The principle types of DC machine are:(i) separately excited DC machine(ii) self excited DC machine: 3 types

Fig. Self excited DC motor

•Shunt machine.•Series machine.•Compound machine.

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Series motorThe field is connected in series with the armature andmust, therefore, carry the full armature current.

V= E+Ia(Ra+Rse)

IL = Ia

LI

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P

PoutPin= VTiL

Pca=ia2Ra

Pcf=ia2Rf

Pm

Power Flow Diagram

P is normally given

Pin = Pout + total losses

Where,Pca =armature copper lossPcf =field copper lossP=stray, mech etc

in

out

mm

oo

P

PEfficiency

N

Ptorquemechanicalfor

N

Ptorqueloadoutputfor

N

P

,

2

60,

2

60,/

2

60

Pm= Ea ia

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Shunt motorThe field is connected in parallel with the armature.

V= E+IaRa

IL = Ia+Ish

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Power Flow DiagramP

PoutPin=VTiL

Pca=ia2Ra

Pcf=if2Rf

Pm

P is normally given



Pm= Ea ia

in

out

mm

oo

P

PEfficiency

N


N


N

P

,

2

60,

2

60,/

2

60

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Compound motorA compound motor carries both a series field and a shuntfield.

The shunt field is always greater than the series field.

If the magnetic flux produced by the series winding aidsthe flux produced by the shunt field winding, the machineis said to be cumulatively compounded.

If the series field winding opposes the shunt field flux, themachine is said to be differentially compounded.

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Compound motor . . .Either type may be long shunt or short shunt type.

4747

Compound motor . . .

Short shunt

V =E+ IaRa + ILRse

IL = Ia+Ish

Long shunt

V= E+Ia(Ra+Rse)

IL = Ia+Ish

48

Power Flow DiagramP

PoutPin=VTiL

Pca=ia2Ra

Pcf1=if2Rf1

Pm

Pcf2=ia2Rf2

P is normally given



Pm= Ea ia

in

out

mm

oo

P

PEfficiency

N


N


N

P

,

2

60,

2

60,/

2

60

49

Characteristics of dc motors: Load characteristics

Shunt motorConsider a dc shunt motor running at no-load. If amechanical load is suddenly applied to the shaft, themotor begins to slowdown.

This causes the counter emf to decrease, resulting in ahigher armature current and a corresponding highertorque.

The speed of a shunt motor stays relatively constant(changes by 5 to 15 percent) from no-load to full-load.

As load current increases the torque developed alsoincreases linearly as the field flux remains almostconstant.

ΦE

n aIΦT

50

Characteristics of dc motors: Load characteristics …

51

Characteristics of dc motors: Load characteristics …

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Characteristics of dc motors: Load characteristics . . .

Series motorWhen a series motor starts up (Eb is zero), the armaturecurrent is higher than the normal, with the result the fluxalso greater than normal.

Therefore the starting torque of the series motor isconsiderably greater than that of a shunt motor.

At light loads the armature current and the flux are small.The weaker field causes the motor to run faster than thenormal speed

At no-load the speed may rise to a dangerously highvalue. For this reason we never operate a series motor atno-load

aIΦ

aIΦT 2aIT

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Characteristics of dc motors: Load characteristics . . .Compound motorIn a cumulative compound motor, the mmf of series fieldadds to the mmf of shunt field.

Hence, as the load increases, the total mmf increaseswhich in turn decreases the speed.

The speed drop from no-load to full-load is generallybetween 10 and 30 percent.

In a differential compound motor, the mmf of the seriesfield opposes the mmf of the shunt field.

In such a motor, the total mmf decreases with increasingload which in turn increases the speed of the motor.

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Speed control of dc motorsShunt motorThere are two methods of speed control, namelyarmature control and field control

Armature controlAccording to eqn.

if the flux per pole is kept constant, the speed dependsonly on the counter emf or mainly on the supply voltage(Armature resistance being very small).

The field excitation of the motor is kept constant, but thearmature voltage is varied from zero to maximum to varythe speed of the motor.

0 60E Zn P A

5555

Speed control of dc motors . . .Armature control. . .One way to control the speed of a dc motor by armaturecontrol is to place a resistance in series with thearmature. The voltage drop in the resistance reduces thearmature voltage.

This method enables us to reduce the speed below itsnominal speed.

This is only recommended forsmaller machines because alot of power is wasted in theresistance.

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Speed control of dc motors . . .Field controlWe can also vary the speed of a dc motor by varying thefield flux.

If the line supply voltage is maintained constant then thespeed is inversely proportional to the flux, F.

This method of speed control is frequently used when themotor has to run above the normal speed.

To control the flux we vary the field current by connectinga rheostat in series with the field.

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Speed control of dc motors . . .Field control. . .If we suddenly increasethe resistance of therheostat, both theexciting current Ix andthe flux will decrease.

This in turn reduces thecemf, causing thearmature current to jumpto a much higher value.

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Speed control of dc motors . . .Field control. . .

The motor develops a greater torque than before andhence runs faster.

This method of speed control enables high-speed/base-speed ratios as high as 3:1.

Under certain abnormal conditions, the flux may drop todangerously low values which in turn may cause themotor to rotate at a dangerously high speed. Safetydevices are introduced to prevent such runawayconditions.

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Speed control of dc motors . . .Series motor

The speed of a series motor can be increased by placinga low resistance in parallel with the series field.

The field current is then smaller than before, whichproduces a drop in flux and an increase in speed

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Starting of dc motors

When a motor is at rest, the cemf is zero and so thestarting current is given by I = ES / R. The starting currentmay be 20–30 times greater than the nominal full-loadcurrent.

All dc motors must, therefore, be provided with a means tolimit the starting current to reasonable value.

One method is to place a rheostat in series with thearmature.

The resistance is gradually reduced as the motoraccelerates and is eventually eliminated when themachine has attained full speed.

6161

Reversing the direction of rotationThe direction of the rotation may be reversed by reversingeither the armature connection or the field connection (butnot both).

When there is no load on thearmature, the armaturecurrent is almost zero andhence the flux is also zeroand there is no armaturem.m.f.

Armature reactionArmature reaction is the effect of the magneticinteraction between the two fields set up by the rotor andthe stator on the distribution of main field flux.

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Armature reaction . . .When a d.c. machine is loaded,current flows in the armaturewinding. This armature currentsets up an armature flux.

This armature reactionproduces 2 undesirableeffects on the main field flux.

Net reduction in the field fluxper pole. (demagnetizes /weakens the main flux).

Distortion of main field fluxwave along the air-gap.(cross-magnetizes / distorts).

63

Armature reaction . . .Interpoles: Probably the most common method forcompensating the armature reaction is the use ofinterpoles. These are smaller than the main field polesand are connected in series with the armature. Thepolarity of an interpole is that of the main pole left behindwith respect to the direction of rotation for motoringmode and that of the main pole ahead in the direction ofarmature rotation for the generating mode.

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Losses and efficiencyThe losses of dc machines may be classified as follows:Copper losses. The ohmic losses (I2R losses) in thearmature and field windings.

Iron losses. The power absorbed by the core is called ironlosses and is due to eddy currents and hysteresis.

Rotational losses. These are bearing friction, friction of thebrushes & windage losses.

Iron losses and other rotational losses (Mechanicallosses) are made very small by design.

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Eddy current loss

As an armature rotates in a magnetic field, the armaturecore also cuts the magnetic flux and an emf is induced init causing a current flow (called an eddy current) in thecore.

Owing to this eddy current and resistance, a certainamount of power will be absorbed, producing heat.

Although eddy current losses are effectively reduced bylaminating the core, they are never eliminated.

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Hysteresis loss

The alternating flux causes changes in the alignment ofthe molecules in the magnetic core.

This change is energy consuming and heat is producedwithin the core.

The energy loss is referred to as hysteresis loss, whichdepends on the nature of the material used for thelaminations. Silicon steel has low hysteresis losses

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EfficiencyEfficiency is the ratio of the output power to the inputpower

100Input power losses

Input power

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Applications of dc motorsShunt motors: Shunt motors are prone to armaturereaction, a distortion and weakening of the fluxgenerated by the poles that results in commutationproblems evidenced by sparking at the brushes.

Installing additional poles, called interpoles, on the statorbetween the main poles wired in series with the armaturereduces armature reaction.

They are suitable for applications requiring constantspeed and where accurate control of speed or position ofthe load is required.• specific applications: fans, conveyor drives,

machine tools, packaging etc.

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Applications of dc motorsSeries motors: Series motors lack good speedregulation, but are well-suited for high-torque loads likepower tools and automobile starters because of theirhigh torque production and compact size. They are usedon equipments requiring a high starting torque.• suitable to use in traction, electric cranes & hoists.

Compound motors: The concept of the series and shuntdesigns are combined. They are used when high startingtorque compared to shunt motors are required.• suitable for lifts or hoists.

Chapter_2-1_dc_motors.pdf

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