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Electromechanical Electromechanical Systems & ActuatorsSystems & Actuators

DC MACHINESDC MACHINES

Dr. Adel Gastli

These slides are the contributions of: Dr. A. Gastli, Dr. A. Al-Badi, and Dr. Amer Al-Hinai

Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 2

DC Machines

LEARNING GOALS IntroductionApplication of DC MachineAdvantages & Disadvantages of DC Machine

Construction of DC MachineField SystemArmatureCommentatorBrush

Principle of OperationFaraday’s LawArmature Voltage & Developed Torque

Classification of DC MachinePermanent MagnetSelf-ExcitedSeparately-Excited

DC Machine Representation

Magnetization Curve (Saturation)

DC Motor & Generator Equations

Power Flow & Efficiency

Torque-Speed Characteristics

Starting of DC Machine


Introduction

Most of the electrical machine in service are AC type.

DC machine are of considerable industrial importance.

DC machine mainly used as DC motors and the DC

generators are rarely used.

DC motors provides a fine control of the speed which

can not be attained by AC motors.

DC motors can developed rated torque at all speeds

from standstill to rated speed.

Developed torque at standstill is many times greater

than the torque developed by an AC motor of equal

power and speed rating.


The d.c. machine can operate as either a motor or a

generator, at present its use as a generator is limited because

of the widespread use of ac power.

Large d.c. motors are used in machine tools, printing

presses, fans, pumps, cranes, paper mill, traction, textile mills

and so forth.

Small d.c. machines (fractional horsepower rating) are

used primarily as control device-such as tachogenerators for

speed sensing and servomotors for position and tracking.

Application of DC Machines


DC Motor

Paper Mills

Oil Rigs

MiningMachine Tools Petrochemical

RobotsSteel Mills

Application of DC Machines


Advantages

• High starting torque

• Rapid acceleration and deceleration.

• Speed can be easily controlled over wide speed range.

• Used in tough gobs (traction motors, electric trains,

electric cars,….)

• Built in wide range of sizes.

Disadvantages

• Needs regular maintenance

• Cannot be used in explosive area

• High cost

Advantages & Disadvantages Of D.C. Motors


GeneratorMechanical

InputElectricalOutput

MotorElectricalInput

MechanicalOutput

Energy FlowMotor

Generator

Electromechanical Energy Conversion

T Mechanical systemElectrical system v

ωi

+

_

Ideal Electric Machine

Introduction

Electric Machine

v i=T ω


Shaft

Field yoke

Trailing pole tip

Leading pole tip

Pole core

Field coil

Armature winding

Pole face

Pole axisRotation

Armature coreParts of a DC MachineParts of a DC Machine

Construction of DC Machine

Dr. Adel Gastli MCTE3210: Electromechanical Systems & Actuators 92 Pole DC Machine

Stator

Armature

CommutatorCommutator

Shaft

Field coil

Construction of DC Machine


2000HP DC Motor field System

The field system is to produce uniform magnetic field within which the armature rotates. This consists of Yoke or frame: Acts as a mechanical support of the machine

Construction of DC Machine: Field System


Cooling ducts for air circulation

TeethSlots

Slots for wedges

Portion of an armature lamination of a dc machine showing slots and teeth

The rotor or the armature core, which carries the rotor or armature winding, is made of sheet-steel laminations. The laminations are stacked together to form a cylindrical structure

The armature coils that make the armature winding are located in the slots

Non-conducting slot liners are wedged in between the coil and the slot walls for protection from abrasion, electrical insulation and mechanical support

Construction of DC Machine: Armature


Construction of DC Machine: Armature

Armature of a DC Machine


Commutator

Construction of DC Machine: Commutator

Commutator: is a mechanical rectifier, which converts the alternating voltage generated in the armature winding into direct voltage across the brush. It is made of copper segments insulated from each other by mica and mounted on the shaft of the machine. The armature windings are connected to the commutator segments.


Commutator and Brushes

Construction of DC Machine: Brush

The purpose of the brush is to ensure electrical connections between the rotating commutator and stationary external load circuit. It is made of carbon and rest on the commutator.


1 2 3

Brush

Bottomcoil sides

Topcoil sides

Commutator 1 2

Brush

Topcoil sides

Elements of Lap Winding Elements of Wave Winding

Construction of DC Machine: Armature Winding


Conductors

Turn Coil

End connection

Winding



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

a b c d e

f g h

19 20 21

a b c d e

f g h

+ - + -

+-+

+

+

---

+ +

--

Ia

Icoil

+

-

N SSN

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

pba ==// paths brushes poles

Lap WindingLap Winding



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

a b c d e

f g h

19 20 21

a b c d e

fgh

- + - +

-+

++ +

---

+

-

Ia

Icoil

+

-

N SSN

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

i j k1817

i j k

2=a

Wave WindingWave Winding

Nb. of // paths



The Faraday Disk and FaradayThe Faraday Disk and Faraday’’s Laws Law

S

N

ω

φ

+

_

V

Copper disk

Magnet

Conducting shaft

Brush

An emf is induced in a circuit placed in a magnetic field if either:• the magnetic flux linking the circuit is time varying• or there is a relative motion between the circuit and the magnetic field such that the conductors comprising the circuit cut a cross the magnetic flux lines.

• 1st form of the law is the basis of transformers.• 2nd form is the basic principle of operation of electric generators.

Principle of Operation


B

u

V

Voltmeter

Conductorrails

Movingconductor

le

emf, e

Flux density, B

Velocity, u

The rightThe right--hand rule and generator actionhand rule and generator action

e=BluFaraday’s law or flux cutting rule

s.l.B

A.B

=Φ=Φ


dt

s.l.dB

dt

de =

Φ=

dt

dsu,

dt

ds.l.Be ==


t

v

External circuit

NS

ω

N-turn coil

brushes

Slip rings

v

φ

Field pole

l1


Without CommutatorWithout Commutator


t

v

NS

ab

ω

coil

brushesCommutator

segments

v


With CommutatorWith Commutator


Single-Phase Full wave Rectifier


p

oo md360ed180pitch pole One ==

Multi-Pole Machines

If p is the number of poles, then p/2 cycles of variation of the flux are encountered every complete mechanical rotation.

N

N

SS

θ

mded

p θθ2

=

θed : electrical degrees or angular measure in cyclesθmd : mechanical degrees or angular measure in space

N N

S S

θedθ3π2ππ 4π

Pole pitchB(θ)

2ππ

θmd


Principle of Operation: Armature Voltage

( ) 60

..

/60

.

.Re/

.Re/ m

mconductor

Np

N

p

vtime

vFluxEmf

Φ=

Φ==

pathconductorofNumber

EmfEmf conductor

Total /=

where

p = number of poles

Z = total number of armature conductors

a = number of parallel paths, 2 for wave and p for lab.

Φ = flux per pole (Weber)

Nm = speed of the motor in the revolutions per minute (rpm)

time of 1 revolution = 60/Nm (sec)

a

NZp

a

ZNpEmf mm

Total 60

.../

60

.. Φ=⎟

⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛ Φ

=


Principle of Operation: Armature Voltage

Let π

ωπω.2

60.

60

..2 mm

mm N

N=⇒=

a

Zp

a

ZpEmf mm

Total ..2

...

.2

60..

60

..

πω

πω Φ

=Φ

=

ωm= speed of the motor in radians per second

maTotal KEmf ω..Φ= Ka: armature constanta

ZpK a ..2

.

π=

Generated voltage : generator operationBack emf : motor operation


Developed (or Electromagnetic) Torque

Consider the turn shown in the following Figure.

Area per pole A =p

rlπ2

The force on a conductor is a

IlBf a

c =

The torque developed by a conductor is rfT cc = a

Ipr

a

IlB aa

π2

Φ==

The total torque developed is

m

aaaa

ae

IEIK

a

IZpT

ωπ=Φ=

Φ=

2

lr

p

AB

π2

Φ=

Φ=Flux density

Current / conductor isa

II a

c =


Production of Unidirectional Torque and Operation of an Elementary

B

I

F

Left-hand rule

With this configuration the torque is unidirectional and independent of conductor position

Position of conductor a under N-pole

+a b

a

b

ω

I1 2

N S

F

F

Position of conductor a under S-pole

+a b

ba

ω

I1 2

N S

F

F


DC Machine

DifferentialCumulative

LongShunt

ShortShunt

Classification of DC Machine

Self-excited

Separately excited

Permanentmagnet

Shunt

Series

Compound


Field Armature

Separately excited

Field

Armature

Shunt

Field

Armature

Series

Short-shunt

φfφs

S1 S2F2F1

A2

A1

Long-shunt

φfφs

S1 S2F2F1

A2

A1


Motor operationGenerator operation


Cumulative compound

φfφs

S1 S2F2F1

A2

A1

Differential compound

φfφs

S1 S2F2F1

A2

A1


Motor operationGenerator operation


DC Machine Representation

Field

Armature

q-axis

d-axis

The mmf’s produced by the field circuit and the armature circuit are in quadrature.

Field mmf

d-axis

Arm

ature m

mf

q-a

xis

φf

φaArmature mmf

Field mmf

Φ

Fp

Flux-mmf relation in a dc machine

Saturation

Linear


Magnetization (or Saturation) Curve of a DC Machine

Ea

If

Magnetization curve

Speed ωm

0.5 ωm

The magnetizing curve is obtained experimentally by rotating the the dc machine at a given speed and measuring the open-circuit armature terminal voltage as the current in the field winding is changed.

MagnetizationCurve

Represents the saturation level in the magnetic system of the dcmachine for various values of excitation mmf .

If Nf

Φ

Flux-mmf relation in a dc machine

Saturation

Linear


Separately Excited DC MotorSeparately Excited DC Motor

Rfw

Rfc

If Vf+ −

Ra

Ia

It

Vt

+

−

ωm

aae

maa

aata

fff

IKT

KE

RIVE

IRV

Φ=Φ=−=

=

ω

Rfw: resistance of field winding.

Rfc: resistance of control rheostat used in field circuit.

Rf=Rfw+Rfc: total field resistance

Ra: resistance of armature circuit, including the effect of brushes. Sometimes Ra is shown as the resistance of armature winding alone; the brush-contact voltage drop is considered separately and is usually assumed to be about 2V.

Dc Motors Equations


Shunt or SelfShunt or Self--Excited DC MotorExcited DC Motor

ftaLtt

aaemaa

aata

tfff

IIIRIV

IKTKE

RIVE

VIRV

−==Φ=Φ=

−=

==

,

,ω

Rfw

Rfc

If

+

−

Ra

Ia

It

Vt

+

−

ωm

Dc Motors Equations


Separately Excited DC GeneratorSeparately Excited DC Generator

La

LLt

maa

aata

ffffcfwf

II

RIV

KE

rIVE

IRIRRV

==

Φ=+=

=+=

ω

)(

Rfc

ra

RL

Ia IL

Vt

+

−

ωm

Rfw

If Vf+ −

Ea

+

−

Dc Generator Equations


SelfSelf--Excited DC GeneratorsExcited DC Generators

1. Shunt generator 1. Shunt generator

Rfc

RLRfw

If

+

−

ra

Ia

IL

Vt

+

−

ωm

−

Ea

fLa

LLt

maa

aata

tfff

III

RIV

KE

rIVE

VIRV

+==

Φ=+=

==

ω



2. Series Generator2. Series Generator

IL

RL+

−

ra

Rs

Ia

Vt

+

−

Ea

msaa

faL

saaat

KE

III

RrIEV

ωΦ=

==+−= )(



3. Compound3. Compound DC Generator DC Generator

Rfc

If

Rfw

+

−

Ra

Rs

Ia

IL

Vt

+

−

Ea

msshaa

fcfw

aaaf

faL

sLaaat

KE

RR

RIEI

III

RIRIEV

ω)( Φ±Φ=

+−

=

−=−−= ( )

msshaa

fcfw

tf

faL

saaat

KE

RR

VI

III

RRIEV

ω)( Φ±Φ=

+=

−=+−=

Rfw

Rfc

+

−

Ra

Rs

Ia

IL

Vt

+

−

Ea

If

Long ShuntShort Shunt

( ) msshaa KE ωΦ±Φ=

+ −

Cumulative Differential



Power Flow and Efficiency

Rfw

Rfc

If

+

−

Ra

Rs

Ia

IL

Vt

+

−

Ea

Poutput= Pelectrical

Pinput= Pmech = Pshaft

Rotational losses

a2a RI f

2f RI

aa IELaIVaa IV

sL RI 2

Lt IV

LossesRotationalIE

IV

LossesRotationalRIIV

IV

LossesP

P

P

P

aa

Lt

Lt

Lt

output

output

input

output

+=

++=

+==

∑

η

η

η

2

DC Generators


Power Flow and Efficiency

DC Motors

Rfw

Rfc

If

+

−

Ra

Rs

Ia

IL

Vt

+

−

Ea

Pinput = Pelectrical

Rotational lossesa

2a RIf

2f RI

aaIELaIV aaIV

sLRI 2

Poutput= Pmech= PshaftLtIV

Lt

aa

Lt

Lt

input

input

input

output

IV

LossesRotationalIE

IV

LossesRotationalRIIV

P

LossesP

P

P

−=

−−=

−==

∑

η

η

η

2


Ia


Therefore , TK

r

K

V

a

a

a

tm 2)( Φ

−Φ

=ω

Φ−

=a

aatm K

rIVω

aa IKT Φ=

aaat rIEV +=maa KE ωΦ=

Separately excited & Shunt motors

(φ is independent of the load torque )

ωm

T

Slope 2)( Φa

a

K

rΦa

t

K

V



Series motors

maa

saata

KE

RRIVE

ωφ=+−= )(

s

sa

s

tm

asaaaa

s

sa

as

tm

K

RR

TK

V

IKIKKIKTBut

K

RR

IK

V

+−=∴

===

+−=

ω

φ

ω

22

1

af IKIK 11 ==φNeglecting saturation

masmaaa IKIKKE ωω == 1



Compound motors

seriesshuntt φφφ ±=Differential Compound

Cumulative Compound

seriesshuntt ATATAT ±=

TK

r

K

V

ta

a

ta

tm 2)( φφ

ω −=

Shunt motor


Starting of DC Machine

If a d.c. motor is directly connected to a d.c. power supply, the starting current will be dangerously high.

a

ata r

EVI

−= at starting 0E0 a =→=ω

a

tStartinga r

VI =∴

Since ra is small, the starting current is very large.The starting current can be limited by the following methods:1- Use a variable-voltage supply.2- Insert an external resistance at start, as shown in the Figure.