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1/23/2004 ME 269 DC Machine1
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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Generated Voltage in a Loop (a coil of one turn)
• 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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DC Motor Fundamentals
Example: Find the force for a conductor with a length of 1 m.
B = 0.35 T
i = 2 A
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DC Motor Fundamentals
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.