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Lecture NotesME 269

Ayman El-Hag

DC Machine

Part no. 1

Basic principles and physical construction

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DIRECT CURRENT (DC) MACHINES Fundamentals

• Generator action: An emf (voltage) is induced in a conductor if it moves through a magnetic field.

• Motor action: A force is induced in a conductor that has a current going through it and placed in a magnetic field

• Any DC machine can act either as a generator or as a motor.

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DC Generator Fundamentals( )e

e B v= × ⋅=v B l

l sin cosα β

e = induced voltage, v = velocity of the conductor, B = flux density and l is the length of the conductor

" - angle between the direction in which the conductor is moving and the flux is acting.

$ - smallest possible angle the conductor makes with the direction of, the vector product, ( v × B)and for maximum induction, $ = 0. Hence, e =Blv for most cases.

( v × B) indicates the direction of the current flow in the conductor, or the polarity of the emf.

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DC Generator Fundamentals

Example: Find the induced voltage for a road with a length of 1 m.

B = 3 T

V = 1 m/s

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Generated Voltage in a Loop (a coil of one turn)

• For emf to be induced, the conductors must cut the flux lines as they move. Otherwise, ( v × B) = 0.

eloop = eab + ebc + ecd + eda

eloop = Blv + 0 + Blv + 0

eloop = 2 B l v

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• Note: Induced voltages are always AC.

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DC MACHINESReal DC machine Construction

• Stator: Stationary part of the machine. The stator carries a field winding that is used to produce the required magnetic field by DC excitation. Often know as the field.

• Rotor: The rotor is the rotating part of the machine. The rotor carries a distributed winding, and is the winding where the emf is induced. Also known as the armature.

N S

Rotor

Field

Stator with with polesBrush

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DIRECT CURRENT MACHINES

DC machine Construction

• The picture shows the stator of a large DC machine with several poles.

• The iron core is supported by a cast iron frame.

Dc motor stator construction

Field poles

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DIRECT CURRENT MACHINES

DC machine Construction

• The rotor iron core is mounted on the shaft.

• Coils are placed in the slots.

• The end of the coils are bent and tied together to assure mechanical strength.

• Note the commutator mounted on the shaft. It consists of several copper segments, separated by insulation.

DC motor rotor construction

Poles

Rotor winding

Fan

Bearing

Brushes

Commutator

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DIRECT CURRENT MACHINESDC Machine Construction

• The adjoining picture shows thecommutator of a large DC machine.

• The segments are made out of copper and mica insulation is placed between the segments.

• The end of each segment has a flag attached. The coil endings are welded to these flags.

• An insulated ring is placed on the coil ends to assure proper mechanical strength.

Insulator

Copper

FlagRing

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DC MACHINESGenerated EMF in a Real DC Machine

WhereZ = total number of conductors, P = total number of polesa = P for lap winding, a = 2 for wave winding, φ = flux, ω = speed in rad/s and n = speed in rpm.

φωφωπ

φφ mgG ka

ZPnkna

ZPE ====260

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DC Motor Fundamentals

( )F B i= ×l F = induced force, B = flux density, I is the current passing in the conductor and l is the length of the conductor

R is a vector in the direction of the flow of the current.

(R×B) direction indicates the direction of force.

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Example: Find the force for a conductor with a length of 1 m.

B = 0.35 T

i = 2 A

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Counter EMF:• When the motor is running, internally

generated emf, (EG = EC) opposes the applied voltage, thus:

IV ERA

T C

A=

− Where: VT = terminal voltage, Ec = counter EMF, RA is the armature resistance and IA is the armature current

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The relationship between the induced EMF and torque

{ {

E B vT B irthereforeET

B vB ir

virv r

ET iEi T

conductor

conductor

electricpower

mechanicalpower

==

= = =

=

=

l

l

l

l

,

, ω

ω

ω

TEI ZP

aI

T k I

A

m A

= =

=ω π

φ

φ2

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Example A six-pole DC machine has a flux per pole of 30 mWb. The armature has 536 conductors connected as a lap winding. The DC machine runs at 1050 rpm and it delivers a rated armature current of 225 A to a load connected to its terminals, calculate:

A) Machine constant, KmB) Generated voltage, EGC) Conductor currentD) Electromagnetic torque.E) Power delivered by the machine.

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