dc machinesdc machines - university of florida mechanism is called a commutator, and dc machines are...
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DC MachinesDC Machines
Revised October 6, 2008
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DC Machines:
DC Motors are rapidly losing popularity.
Until recent advances in power electronics DC motors excelled inUntil recent advances in power electronics DC motors excelled in terms of speed control.
Today, induction motors with solid-state control are replacing DC motors.
DC motors are still popular in cars.
DC Generators are essentially non existentDC Generators are essentially non existent.
DC motors are the simplest motor to understand.
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DC Machines :
DC machines are like AC machines in that they have AC voltages and currents within them.
DC machines have a DC output only because they have a means of converting the AC into DC.
This mechanism is called a commutator, and DC machines are also known as commutating machinery.ow as commutating machine y.
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DC Machines :
Remember this picture? (Slide 66, Note Set 1)
B – Field
This is a simple (crude) DC machine.
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DC Machines :A better picture:A better picture:
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DC Machines :
Fundamental Rotor Construction: Windings lies in carved slots in a ferromagnetic core.
Note that for the DC generator, the rotor is the armature and the stator provides the “field” magnetic field.
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DC Motors:
Note the constant width air gap between rotor and stator. The magnetic flux takes the shortest path between rotor and stator and so is everywhere perpendicular between the two surfaces The magneticis everywhere perpendicular between the two surfaces. The magnetic flux density is constant everywhere under the pole faces.
N S
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DC Machines : Induced Voltage in a Rotating Loop – We've done this before at the beginning of the coursethis before at the beginning of the course
bcB
c
B
xxx
vv B
r
vvbd
B
xxx v
v BN S
ada x
inducede inducede v B
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induced
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DC Machines : Induced Voltage in a Rotating Loop
Segment ab:bc e v B
B
bae v B
vB
xxx
vv B
Segment cd:B
xxx v
v B
adx dce v B
vB
inducede
vB
0bc dae e
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DC Machines : Induced Voltage in a Rotating Loop – We've done this before at the beginning of the coursethis before at the beginning of the course
bc 0cbe
B
er
vv baedce
0N S
ad 0dae
total
ba cb cd da
ee e e e
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vB
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DC Machines : Induced Voltage in a Rotating Loop –
bWhen the loop rotates by 180o: b c
xx vv B
B
B
xxx
Bb
v a d
x
x vv B
N Sra c v
a d
inducede d
00
ab ab bae e e signse e e flip
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0cd cd dce e e flip
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DC Machines : Induced Voltage in a Rotating Loop
totale
2 B2v B
2 B
t
2v B
Clearly an AC voltage.
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DC Machines : Induced Voltage in a Rotating Loop
totale
t
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DC Motors: Induced Voltage in a Rotating Loop
Alternate expressions: Rotor Surface Area
22total ve B BB
r
2
2 2r BB r
2PA B
Rotor Surface Area-per Pole
PA r
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DC Machines : Induced Voltage in a Rotating Loop
Alternate expressions:
22
2
total PBe A
2
The voltage generated in the machine is equal to the product of the flux in the gap time the speed of rotation of the machineflux in the gap time the speed of rotation of the machine.
How do we get a DC voltage out of the loop?
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DC Machines : Getting DC voltage out of the loop –
Commutation
Commutator RingsS t
inducede or Segments
induced
Brushes
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DC Machines : Getting DC voltage out of the loop –
Commutation
I I
The current in the loop isThe current in the loop is AC and reverses direction every half cycle, but so do
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the contacts, so the current Iin the brushes is DC.
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DC Machines : Induced Torque in a Rotating Loop
Though not explicitly shown in this figure theshown in this figure, the loop is between circular magnetic poles.
e
0t
iWe did this back in Note Set 1,
inducedeR
0t except there the magnetic field was not radial. Let’s quickly review the process.
BV
review the process.
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DC Machines : Induced Torque in a Rotating Loop
F i B
B
r b
c
dN Sx abF
rcdF
a
dN S
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DC Machines : Induced Torque in a Rotating Loop
F B B
B
sin 90b b b
ab
r F rF
F i B i B
b
c
dN Sx abF
sin 90ab ab abr F rF
ri BC t l k i
rcdF
a
bdN S
Counterclockwise
F i B i B
sin 90cd cd cd
cd
r F rF
F i B i B
cd cd cd
ri BCounterclockwise
&
0
Segments bc da
B F F
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Counterclockwise0bc daB F F
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DC Motors: Induced Torque in a Rotating Loop
22 2
induced ab cd ri B
2 2PiBr BAi
2 i
2 2induced i
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Commutation in a 4-Pole DC Machine
Commutation is the process of converting the AC voltages and currents produced by a machine into DC voltages and currents at the terminal (and vice versa)terminal (and vice versa).
Commutation is the most critical part in the design and operation of a DC machine.
Practical commutation methods have problem that we will need to ac ca co u a o e ods ave p ob e a we w eed ounderstand and deal with.
First let us look at a somewhat more sophisticated (and moreFirst let us look at a somewhat more sophisticated (and more realistic) commutation scheme than what we have considered thus far.
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Commutation in a 4-Pole DC MachineRotor Winding 1:Rotor Winding 1:
11 11
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Commutation in a 4-Pole DC MachineRotor Winding 1:Rotor Winding 1:Rotor Winding 2:
11
2
11
2
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Commutation in a 4-Pole DC MachineRotor Winding 1:Rotor Winding 1:Rotor Winding 2:Rotor Winding 3:
11
2
33 11
2
33
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Commutation in a 4-Pole DC MachineRotor Winding 1:Rotor Winding 1:Rotor Winding 2:Rotor Winding 3:Rotor Winding 4:Rotor Winding 4:
11
2
334
112
33
4
Note that there are two parallel paths for current. This is a feature present f ll i h
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for all commutation schemes.
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Commutation in a 4-Pole DC Machine The winding loops are buried in slots in the rotor steel core.buried in slots in the rotor steel core.
Note the unprimed ends are on the outside, while the primed ends are on the inside of the slot on the inside of the slot.
2 32
1
3
4 bCommutator
Sections
N S23
a c
dB
2
1
3
4d
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Commutation in a 4-Pole DC Machine At the instant shown (call this t = 0), the ends 1, 2, 3’, and 4’ of the loop are under the north
e v B
this t 0), the ends 1, 2, 3 , and 4 of the loop are under the north pole face while ends 1’, 2’, 3, and 4 are under the south pole face. The voltages at the north end will be positive with respect to those at the south end as determined from as usual
2 3
e v B the south end as determined from as usual. Equivalently, we have ...
2
1
3
4 b
N S23
a c
dB
2
1
3
4d
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21Back side of windings
b
21
b
cE 21
34a cE
x y
d
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34
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21
4 4E e v B Clearly,
b
4 4 ,0
E e v Bt
21
b
cE 21
34a cE
x y
d
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As the machine continues to rotate, consider now the case for
45t
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Commutation in a 4-Pole DC Machine For t = 45o, clearly no voltage is induced in loops 1 and 3. At this point, the commutatorvoltage is induced in loops 1 and 3. At this point, the commutator brushed are shorting out commutator segments ab and cd. Our equivalent model is...
1 3
N S2 2b c
N S44 a d
B
1 3
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2 22 45E e t Now,
+ e - + e -2
12
3
2 , 45E e t
+ +
bc
E
+0-
+0-
a d
E x y
+0
+0
1 3
a d0-
0-
1 3
44+ e - + e -
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As the machine continues to rotate, consider now the case for
90t
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Commutation in a 4-Pole DC Machine For t = 90o, now loop ends 1’, 2, 3, and 4’ are positive with respect to 1, 2’, 3’, and 4,ends 1 , 2, 3, and 4 are positive with respect to 1, 2 , 3 , and 4, respectively
3
N S1
24
b
c
dN S2 34 a
b d
B
1
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2 3
4 90E t Now,
c
4 90E e t
2 3b
c
dE 2
134
b dE x y
a
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1 4
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Comparing t = 0 with t = 90o, we see that the polarity across loops 1 and 3 has reversed, but so have their connections, so the p , ,terminal polarity remains the same. This is the essence of commutation.
2 321
2 3b
c
dE 21 a
b
cE 2
134
a
b dEx y34
d
x y
1 434
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90t 0t
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Output Voltage: Note that this is a better approximation to DC than is a single loop. As the number of loops on the rotor increases, the g p p ,approximation to a DC voltage gets better and better.
totaletotal
4e2
t2e
0 45 90 135 180 225 270 315 360
Real DC machines have a similar construction to the four-pole machine considered except they have more loops and more
0 45 90 135 180 225 270 315 360
machine considered, except they have more loops and more commutation segments.
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Practical Issues Associated with Commutation
1. Armature reaction – Current flowing in the rotor armature winding produces its own magnetic field that interacts with the pole fieldproduces its own magnetic field that interacts with the pole field due to the permanent magnet field causing a distortion in the uniform radial field desired.
2. Ldi/dt Voltages – The commutator segments shorted by the brushes cause what is known as inductive kickback. b us es cause w a s ow as inductive kickback.
Each of these issues (problems) and ways to fix them are now discusseddiscussed.
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Armature Reaction
Armature Field Pole Field
AddingSubtracting
N S
Adding SubtractingAdding Subtracting
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Armature Reaction
Wh N SWhat wewant:
N SWhat we N Sget:
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Armature Reaction
The Magnetic Neutral Plane is the plane in the machine where the velocity of the rotor wires is parallel to the magnetic flux lines so that the induced voltage in the plane is zerothe induced voltage in the plane is zero.
Magnetic Neutral Plane
v
N Sv
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Armature Reaction Magnetic Neutral Plane
The greater the current that flows in the rotor, the greater
N Se o o , e g e e
the armature field, and the greater the shift in the neutral
Shif d i l l
shift in the neutral plane.
N S
Shifted Magnetic Neutral Plane(for a generator)
N S
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Armature Reaction Magnetic Neutral Plane
N S
Shif d i l l
N S
Shifted Magnetic Neutral Plane(for a motor)
N S
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Armature Reaction
Why is armature reaction a problem?
The brushes must short out the commutator segments at exactly theThe brushes must short out the commutator segments at exactly the point where the voltage across them is zero.
If the brushes were located to short out the conductors for the case of the vertical neutral plane, then when the machine is loaded the brushes will short out segments with a finite voltage across them.b us es w s o ou seg e s w a e vo age ac oss e .
This is a serious problem.
If we placed the brushes at their full-load location, it would then spark at no-load.
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Armature Reaction
Another problem caused by armature reaction is called flux weakening.
Recall that to achieve maximum flux most machines operate near the saturation point of the magnetic material of the armature.
YEEL 3211 (© 2008, H. Zmuda) 7. DC Machines 46
Y
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Armature Reaction Flux Weakening
In places where the rotor mmf adds to the
p
pole mmf only a small increase in flux occurs as compared to places where the two mmf’s subtract.
This means that the total average flux under the entire pole face is decreased.
Y
Pole mmf – Armature mmf
Pole mmf
Pole mmf + Armature mmf
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Armature Reaction Flux Weakening
In a generator, a reduced flux reduces the generated voltage.
For a motor the situation is much more serious Consider our simpleFor a motor the situation is much more serious. Consider our simple motor form Slide 18...
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Armature Reaction Flux Weakening
.Radial B field
e
0t
i
inducedeR
0t
BV
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Armature Reaction Flux Weakening
First, assume that there is no load on the shaft. When the switch closes at t = 0, an current will flow in the loop,
VB inducedV eiR
But since the loop is at first stationary,
0 Binduced
Ve iR
This current produces a torque,
induced R
2 i
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Armature Reaction Flux Weakening
This torque causes the rotor to accelerate.
This acceleration clearly cannot continue forever What causes theThis acceleration clearly cannot continue forever. What causes the acceleration to decrease and a steady-state rotation to exist?
As the rotor accelerates, the moving loop now produces an induced voltage (generally termed the counter electromotive force) given by,
2inducede
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Armature Reaction Flux Weakening
The induced voltage causes the current to drop since
B inducedV ei
As the current decreases, the induced torque decreases since,
B inducediR
2 i
we eventually reach the steady-state operating conditions of,
0, ,induced Be V ,
. ., 0induced loadi e 0,
a constanti
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What is ss ? a constantss
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Armature Reaction Flux Weakening
Since,0 0
2i
si 2nce
d
i
Since i = 0, einduced = VB, and since
0and
S ce i 0, einduced VB, a d s ce
2induced
Bss
Ve 2induced ss
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Armature Reaction Flux Weakening
induced Be V 2 2
For a fixed voltage, as the flux decreases the speed increases.
But increasing the speed increases the load resulting in more flux u c eas g e speed c eases e oad esu g o e uweakening.
This can cause a run away situation to occurThis can cause a run-away situation to occur.
This too poses a serious problem.
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Armature Reaction Flux Weakening
Examine the mmf and the fluxExamine the mmf and the flux.
ll ˆ gapB A B AY e e
Recall: ˆ gapgap gap
o
B A B nAA
Y e e
n̂
N S
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Armature Reaction Flux Weakening
Pole Fieldn̂
N S gapB AA
Y
o A
Y
t
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Armature Reaction Flux Weakening
Armature Field
N S gapB AA
Y
o A
Yt
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Armature Reaction Flux Weakening
Add these to get the net mmf:
Y
t
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Armature Reaction Flux Weakening
Y
Flux SaturationPole Only NeutralPlaneY Plane
t
t
New NeutralPlane
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Armature Reaction Flux Weakening
W. H. Yeadon & A. W. Yeadon, HANDBOOK OF SMALL ELECTRIC MOTORS, McGraw-Hill 2001, Chapter 4, Direct Current Motors
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Ldi/dt Voltages
The Ldi/dt voltage occurs in commutator segments being shorted out by the brushes.
Consider the situation on the next three slides and notice that when a commutator segment is shorted out, the current flow through that
t t t tcommutator segment must reverse.
The question is, “How fast does this reversal in direction occur?”
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Ldi/dt Voltages i i
0t b
2i2ii i2i2ia c2i2ii
i
i
i
2i2i
d
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i i
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Ldi/dt Voltages
i i45t
?
i i
i?
i
bc
2i 2i
i i
2i 2i
a d
i i
i i
? ?
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Ldi/dt Voltages i i
90t c
2i2ii i2i2ib d2i2ii
i
i
i
2i2i
aNote that the direction of the currents in theof the currents in the blue and greensegments has flipped.S t lid
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i iSee next slide...
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Ldi/dt Voltages
90t 45t 0t a b
cb
i
b d
a
b c
d
a c
ad
ib a
Let’s ‘unroll’ this...
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Ldi/dt Voltages
b
I I
a
b
c
Let’s ‘unroll’ this...2I
dI I
0t
Direction of Motion (Brush stays still)2I
a b c d a b c dd a b c d
I I I I
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Direction of Motion (Brush stays still)2I 0t
a b c d a b c dd a b c d
I I
Direction of Motion (Brush stays still)2I 45t
a b c d a b c dd a b c d
( y )
I I
Direction of Motion (Brush stays still)2I 90t
a b c d a b c da b c d
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Direction of Motion (Brush stays still)2I 0t
c d a b c dd a b c a bd
I I
Direction of Motion (Brush stays still)2I 45t
a b c d a b c dd a b c d
( y )
I I
Direction of Motion (Brush stays still)2I 90t
d a b c da b c d a b c
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Ldi/dt Voltages 2I2I
a bd
I I
a b c
IBrush reachesend of a
Brush reachesbeginning of
“Real”Transition
men
t
I II I
t
end of a commutatorsegment
beginning ofnext commutator segment
in S
egm
t
Note how di/dtcan get quite large here
“Ideal”Transition
Cur
rent
i
tI
can get quite large hereC
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Commutator Shorting Region
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Ldi/dt Voltages
How much time elapsed while the current to changed directions?
Suppose the motor is turning at n rpm and suppose that there are NSuppose the motor is turning at nm rpm, and suppose that there are Ncommutator segments. Then,
60rev rev 60 secrpmmin 60 sec rev
mm
m
nnn
1 rev 60 sec 60 secondssegments revm m
tN n Nn
g m m
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Ldi/dt Voltages
What voltage is induced? Take some reasonable numbers.
1000 rpmmn p50
100 amps
m
NI100 ampsI
2 volts166,66760 henrym
dI I I Nndt t
Even with a small inductance, large induced voltages th b h d thi i bl
dILdt
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on the brushes can occur, and this is a problem. dt
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Solutions to the Commutation Problem
There are two common techniques to fix (partially or completely) the problems associated with armature reaction and Ldi/dt voltages. These areThese are,
1. Commutating Poles or Interpoles
2. Compensating Windings
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Solutions to the Commutation Problem
Commutating Poles – Basic idea: If the voltage in the wires undergoing commutation can be made zero, no sparking will occuroccur.
To accomplish this, small poles are placed midway between the main l th t lidpoles as seen on the next slide...
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Solutions to the Commutation ProblemAI
Interpoles:
N S
TV
AI
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Solutions to the Commutation Problem
Q: What caused the problem? A: The armature field.
Pole Field
AddingSubtracting Armature Field
N SN S
Adding Subtracting
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Solutions to the Commutation ProblemAI
Interpoles:
N S
TV
AI
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Solutions to the Commutation ProblemAI
Interpoles:
N S
TV
AI
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Solutions to the Commutation Problem
Commutating Poles – The commutating poles are placed directly over the conductors being commutated. These poles provide a flux that (ideally) exactly cancels the effect of that portion of thethat (ideally) exactly cancels the effect of that portion of the armature reaction which caused problems.
Th t ti l ll th t th ff t l th fThe commutating poles are so small that they effect only the few conductors about to undergo commutation.
Note that the armature reaction under the main pole faces are unaffected since the effects of the commutating poles do not extend that farextend that far.
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Solutions to the Commutation Problem
Commutating Poles – Note that the commutating poles or interpoles are connected in series with the rotor winding.
I AI
N STV
AI
EEL 3211 (© 2008, H. Zmuda) 7. DC Machines 79
AI
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Solutions to the Commutation Problem
Commutating Poles – As the load increases and the rotor current increases, the amount of shift in the neutral plane and the magnitude of the Ldi/dt voltage increases as well However themagnitude of the Ldi/dt voltage increases as well. However, the interpole flux also increases, producing a proportional voltage in the conductors that opposes the voltage causing the neutral plane hiftshift.
The net result is a cancellation that is effective over a broad range of loads.
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Solutions to the Commutation Problem
Compensating Windings: Can completely cancel armature reaction and eliminate neutral plane shift and flux weakening.
Compensating windings are windings placed in slots carved in the faces of the poles parallel to the rotor conductors.
These windings are connected is series with the rotor windings.
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Solutions to the Commutation Problem
Q: What caused the problem? A: The armature field.
Pole Field
AddingSubtracting Armature Field
N SN S
Adding Subtracting
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Solutions to the Commutation Problem
Compensating Windings:
N S
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Solutions to the Commutation Problem
Compensating Windings:
N S
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Solutions to the Commutation Problem
For both cases:
Armature Reaction
Y
t
Compensating Winding
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Solutions to the Commutation Problem
Compensating Windings: Motors with compensating winding are more expensive to make.
Commutating poles or interpoles must still be used since compensating windings do not cancel di/dt effects though the i t l b ll th b f i th l d d tinterpoles can be smaller than before since they are only needed to cancel di/dt effects and not neutral plane shifts.
Because of the expense and complexity, dc motors that use compensating windings are used only in large motors when extreme load variations occurload variations occur.
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Types of DC Motors
There are five major types:
1 Separately excited DC motor1. Separately excited DC motor.2. Shunt DC motor3. Permanent magnet DC motor4 S i DC t4. Series DC motor5. Compounded DC motor
DC motors are assumed to be driven from a DC source whose voltage is assumed constant.
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DC Motor Equivalent Circuit(often neglected)
ARb hV AIadjR
ARbrushV AI
Equivalent total armatureField
FR AE
Equivalent total armature circuit model including rotor coils, interpoles, and compensating windings (if
FieldCircuitModel
FL
compensating windings (if present)
F
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DC Motor Equivalent Circuit
Recall from Slides 15 and 21 that:
E KA
induced A
E KK I
Where K is a constant that depends on the machine construction. (For the case derived earlier, K = /2).
These equations, the model, and the motor’s magnetization curves are all that is needed to analyze a DC motorare all that is needed to analyze a DC motor.
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DC Motor Equivalent Circuit
Fl
ux
The field current IF produces the mmf via:
F FN IY
produces the mmf via:
YMagnetomotive Force
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Y
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DC Motor Equivalent Circuit
Typical Curves:
AE AE K
gere
Vol
tag
Constant Speed o
FF
VIR
YArm
atur
FR
Field Current
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DC Motor Equivalent Circuit
Separately Excited DC Motor – one whose field current is supplied from a separate constant-voltage power supply.
I I
R
A LI IFI
adjRAR
FR AEFV TV
FL
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DC Motor Equivalent Circuit
Shunt DC Motor – one whose field current is supplied from the armature terminals.
AR LIAI
R
A
R
adjR
V
FI
FR
L
AE TV
FL
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DC Motor Equivalent Circuit
There is no practical difference between a separately excited and a shunt DC motor.
How does a shunt DC motor respond to a load?
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Shunt DC Motor
Suppose the load on the shaft of a shunt motor is increased.
Then the load torque will exceed the induced torqueThen the load torque will exceed the induced torque.
The motor will then start to slow down.
When it slows down, the internally generated voltage drops,
and the armature current increases since
AE K
and the armature current increases, since
T AA
V EIR
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AR
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Shunt DC Motor
As the armature current increases, the induced torque increases,
i d d AK I
Finally, the induced torque will equal the load torque at a reduced d f t ti
induced AK I
speed of rotation.
The output characteristic (torque and speed) of a motor can be expressed as follows:
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Shunt DC Motor
From the model, using AE K
T A A A
A A
V E I RK I R
A AK I R
inducedK I I butinduced
induced A AK I IK
thus, inducedT AV K R
K
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Shunt DC Motor
Solving for the motor’s speed ,
V R2 2
T Am induced
V RK K
mTV With armature reaction
(flux weakening)
l ARK
slope: 2 2A
K
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Shunt DC Motor Speed Control
The speed of a shunt DC motor is controlled in one of two ways:
1 – Adjusting the flux by adjusting the field resistance1. Adjusting the flux by adjusting the field resistance
2. – Adjusting the terminal voltage applied to the armature.
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Shunt DC Motor Speed Control
Suppose the field resistance RF is increased.
The field current decreases and so does the flux The field current decreases and so does the flux .
A decrease in flux causes a decrease in the internally generated lt E ivoltage EA, since
This causes a (large*) increase in the machine’s armature current,AE K
T AA
A
V EIR
*Recall that since we are assuming that we are operating near saturation, a small change in flux can cause a large change in
t
A
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current.
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Shunt DC Motor Speed Control
Since
if IA increases and decreases what does he induced torque do?
induced AK I
if IA increases and decreases, what does he induced torque do?
Look at some typical model parameters:
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Shunt DC Motor Speed Control
5 V0.25AR
5 V 20 A0.25AI
R
5 V+
FR245 VAE 250 VTV
FL
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Shunt DC Motor Speed Control
Now suppose that 1% decrease in flux. SinceEA will also decrease by 1%.
AE K
Th t t i
0.99 245 242.55AE
The new armature current is
250 242 55
A 1% decrease in flux produced a 49% increase in armature current
250 242.55 29.80.25AI
A 1% decrease in flux produced a 49% increase in armature current.
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Shunt DC Motor Speed Control
Sinceinduced AK I
if IA increases and decreases, we see that the torque increases, and so the motor speed up.
As the motor speed up the internally generated voltage EA rises causing IA to decrease.A
As IA decreases, so does the induced torque.
This continues until induced = load but at a higher steady-state speed.
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Shunt DC Motor Speed Control
m
2FR 2 2T A
m inducedF F
V RK R K R
ns
1FR The no-load torque (the intercept) i i l h i l fC
ondi
tion
R R
is proportional to the reciprocal of the flux, while the slope is proportional to the reciprocal of the flux squared Thus decrease inN
o-Lo
ad
2 1F FR R the flux squared. Thus decrease in flux results in a steeper slope.
N
inducedfull load
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Shunt DC Motor Speed Control
RVm
2FR
ns
2 2A
inducedF
T
Fm
RK R
VK R
1FR
Con
ditio
n For small flux, the second term is dominant. For larger flux, the first termhas more of an effect.
R RNo-
Load
induced2 1F FR RN
Very Low Speeds
full load Stall Conditions
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Shunt DC Motor Speed Control
At very slow speeds, an increase in the field resistance can actually decrease the speed of the motor.
This is because at very low speeds the increase in armature current caused by the decrease in EA is no longer large enough to
t f th d i fl i th i d d tcompensate for the decrease in flux in the induced torque equation.
When the flux increase is larger than the armature current increase, the induced torque decreases, and the motor slows down.
If very low speeds are anticipated, this type of speed control should not be used. Instead, the armature voltage control method should b d
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be used.
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Shunt DC Motor Speed Control - Armature Voltage Control –Controls the motor speed by changing the armature voltage p y g g gwithout changing the voltage applied to the field circuit. In effect, this is a separately excited motor.
AR
AIVariableVoltage
Controller
LI
FR
ControllerFI
AE AV
F
TV
FL
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Shunt DC Motor Speed Control - Armature Voltage Control –
V EAs VA is increased, so is IA, A A
AA
V EIR
As IA increases so does the induced torque,
i th t t l t A it d d th i t ll
induced AK I
causing the motor to accelerate. As it does, so does the internally induced voltage EA, AE K
causing the armature current to decrease, lowering the torque, and causing
at a higher rotational speed .
induced load
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Shunt DC Motor Speed Control
m
2AV
1AV
2AVNote that the slopes remain the same.
1AV
2 1A AV V
induced
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Shunt DC Motor Speed Control
In field resistance control, the lower field current in a shunt or separately excited DC motor, the faster it turns, and the higher the field current the slower it turnsfield current the slower it turns.
The minimum achievable speed occurs when the field circuit has the i ibl t fl i th h itmaximum possible current flowing through it.
A motor operating at rated terminal voltage, power, and field current will be running at rated speed. This is known as the base speed.
Field resistance control circuits can control the speed of the motor forField resistance control circuits can control the speed of the motor for speeds above the base speed but not below, since speed below the base speed often result in excessive field currents.
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Shunt DC Motor Speed Control
For armature voltage control, the lower the armature voltage the slower the motor turns, and the higher the armature voltage the faster it turnsfaster it turns.
Since an increase in armature voltage causes an increase in speed, th i i hi bl d hi h h ththere is a maximum achievable speed which occurs when the armature voltage reaches its maximum permissible level.
Armature control can be used to control the speed of a motor below the base speed but not above, since faster speeds could result in excessive armature voltageexcessive armature voltage.
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Shunt DC Motor Speed Control
Clearly these two forms of speed control are complementary.
Motors which combine these two methods of speed control can haveMotors which combine these two methods of speed control can have a very wide range of speed variation.
Th li iti f t h i l th h ti t th tThe limiting factor, however, is always the heating to the armature conductors, which places a limit on the magnitude of the armature current IA.
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Shunt DC Motor Speed Control
Note that for armature voltage speed control, the flux in the motor is constant, so the maximum torque of the motor is:
Th i i dl f h d f h
maxmax AK I
The maximum torque is constant regardless of the speed of the motor.
But, sinceP
the maximum power out of the motor is directly proportional to its operating speed under armature voltage control
max maxP
operating speed under armature voltage control.
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Shunt DC Motor Speed Control
Note that for field resistance speed control, the flux in the motor changes. An increase in speed is caused by a decrease in flux. For the armature current limit not to be exceeded the induced torquethe armature current limit not to be exceeded, the induced torque limit must decrease as the speed of the motor increases. Since
P and the torque limit decreases as the speed of the motor increases, the
maximum power out of a DC motor under field current control is constant, while the maximum torque varies as the reciprocal of the motor speed.
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Shunt DC Motor Speed Control
P Power Torque
max
constant
max
constantP max
constantPmax
constant
constant constantArmatureVoltageControl
ArmatureVoltageControl
FieldControl
FieldControl
mn mnbasen basenControl ControlControl Control
max maxP
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Permanent Magnet DC Motors (PMDC)
Advantages:• No external field circuit required• No field circuit copper lossesNo field circuit copper losses• Can be smaller than motors with field windings• Often the least expensiveDi d tDisadvantages:• Permanent magnets (PM) cannot provide as high a flux density as
an externally applied shunt field (though this is changing), so PMDC motors have a lower induced torque per amp of armature current compared with a shunt motor of the same size and constructionconstruction
• A PM can become demagnetized. This can happen through heating, bumping, or, most of all, armature reaction. Again, new d l d t i l h l i i i thi ff t
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developed materials help minimize this effect.
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Permanent Magnet DC Motors (PMDC)
A PMDC motor is essentially the same as shunt DC motor expect that the flux of a PMDC motor is fixed. Consequently it is not possible to control the speed using the field control methodpossible to control the speed using the field control method.
The only way to control the speed of a PMDC motor is to vary the t lt t i tarmature voltage or armature resistance.
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Series DC Motors
A series DC motor is one whose field winding consists of a relatively few turns connected in series with the armature circuit.
SR SLAR L S AI I I
The armature, line, and field currents are the same.
AE TV
T A A A SV E I R R
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Series DC Motors
Since the flux is directly proportional to the armature current (until saturation is reached).
This makes the behavior of a series DC motor quite different than what we have considered thus far.
As the load on the motor increases, its flux increases too. But recall that an increase in flux causes a decrease in speed.
Consequently the series DC motor has a sharply decreasing torque-speed characteristicspeed characteristic.
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Series DC Motors – Terminal Characteristics
Recall again that the induced torque is given by,
K I
but the flux is proportional to IA,
induced AK I
A AI cI
thus,2
induced AcKI
The torque is proportional to the square of its armature current.
induced A
As a consequence, the series DC motor gives more torque per ampere than any other DC motor.
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Series DC Motors – Terminal Characteristics
Neglecting saturation, let
Now
AcI T A A A SV E I R R Now
and since
T A A A SV
2 inducedinduced A AcKI I
cK
cK
i d dV E R R inducedT A A S
A
V E R RcK
E K
inducedT A SV K R R
cK
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Series DC Motors – Terminal Characteristics
Now express
along with
AIc
2d d cKI along with
th
induced AcKI2
2 2KKI K thus 2 2
induced AcKI cKc c
andinduced
cK
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Series DC Motors – Terminal Characteristics
Substituting: inducedT A SV K R R
cK
2induced
Kc
inducedT induced A S
cV K R RK cK
1R RV , 1A ST
induced
R RV orcK Kc
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Series DC Motors – Terminal Characteristics
m0
Clearly a series DC motor can NEVERoperate without a load!
02
0 induced start
TVK
T
startA S
cKR R
inducedstart
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Series DC Motors – Speed Control
Clearly the only way to vary the speed of a series DC motor is to change VT, since
A ST
induced
R RVcKcK
As VT is increased so is the speed for any given torque.
induced
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Compounded DC Motor
A DC motor with both a shunt and a series field.
The compounded DC motor combines the best features of the shuntThe compounded DC motor combines the best features of the shunt and series motors.
Th d d DC t h hi h t ti t th h tThe compounded DC motor has a higher starting torque than a shunt motor (whose flux is constant) but a lower starting torque than a series motor (whose flux is proportional to armature current).
Like a series motor, it has extra torque for starting but does not overspeed under no-load conditionsoverspeed under no load conditions.
Needs two field windings.
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Compounded DC Motors – Needs two field windings
Rotor
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Compounded DC Motors
S i Fi ld Sh t Fi ldSeries Field Shunt Field
Rotor
Short-Shunt Connection
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Compounded DC Motors
S i Fi ld Sh t Fi ldSeries Field Shunt Field
Rotor
Long-Shunt Connection
EEL 3211 (© 2008, H. Zmuda) 7. DC Machines 130
g
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Compounded DC Motors Series FieldWinding
Long-Shunt ConnectionShunt Fieldi iWinding
SeriesFieldField
Winding
Shunt Field
Winding
Short-Shunt Connection
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DC Motors 4 possibilities Series Field
Fi ld
Series
1 2
Field
Separately Excited
Excited
4-b
ShuntExcited
C d d
Series Field3 4 4-a
CompoundedExcited
Shunt Field Shunt Field
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Shunt Field Shunt Field
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Compounded DC Motor
Model:
AR LISR SL
adjR
FIAI
FR AE TVShort-Shunt Connection
FL
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Compounded DC Motor
Model:
AR LISR SL
adjR
FIAI
FR AE
F
TVLong-Shunt Connection
FL
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Compounded DC Motors – These are coupled!
Rotor
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Compounded DC Motors – These are coupled!
RotorFlux
Cumulatively Compounded
EEL 3211 (© 2008, H. Zmuda) 7. DC Machines 136
y p
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Compounded DC Motors – These are coupled!
RotorFlux
Differentially CompoundedEEL 3211 (© 2008, H. Zmuda) 7. DC Machines 137
Differentially Compounded
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Compounded DC MotorCoupled
Model:p
Coils
AR LISR SL
adjR
FIAI
Short-Shunt Connection
FRAE TV Cumulatively Compounded
Differentially Compounded
FL
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Compounded DC MotorCoupled
Model: Coils
AR LISR SL
adjR
I
AI
FR AE
FI
TVLong-Shunt Connection
FL
Cumulatively Compounded Differentially Compounded
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Compounded DC MotorAR LI
SR SL
adjR
A
AI
S
FR
adj
E
FI
V
T A A A S
A L F
V E I R RI I I
F
FL
AE TVA L F
TF
F
VIR
FL
,net F S ArmatureReaction
Cumulative CompoundingDifferential Compounding
Y Y Y Y
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Cumulatively Compounded DC Motor Torque-Speed Characteristicsq p
In a compounded DC motor there is a component of flux that is constant and a component that is proportional to the armatureconstant and a component that is proportional to the armature current and thus to the load.
At li ht l d th i fi ld h ll t d hAt light loads, the series field has a small current and hence very little effect, and the motor behaves as a shunt DC motor.
As the load gets very large, the series current/flux becomes significant and the motor behaves more like a series DC motor.
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Cumulatively Compounded DC Motor Torque-Speed Characteristics
m
q p
m
SeriesCumulativelyCompounded
Shunt
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Differentially Compounded DC Motor Torque-Speed Characteristicsq p
For a differentially compounded DC motor, the shunt and series magnetomotive forces subtract from each other sincemagnetomotive forces subtract from each other since
net shunt series shunt F series AN I N I Y Y Y
As the load increases, IA increases, the motor flux actually decreases because of the differencing. But as the flux decreases the speed increases which causes a further increase in the load, further increasing IA, further decreasing the flux, and increasing the speed again This is a run-away conditionagain. This is a run away condition.
Differentially compounded DC motors are extremely unstable and d
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Compounded DC Motor
A SR R R
SL L F AI I I AE Basic Motor
AE FIAI
TV
AI Equations:
FR
FLA T
Cumulative Compounding (upper sign)Differential Compounding (lower sign)
T A A
A L F
V E RII I I
F F F A A Ak N I k N I Circuit
Equations: Flux:
e e Co pou d g ( owe s g )
TF
F
VIR
Solve for the speed as a function of torque algebra
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Solve for the speed as a function of torque... algebra...
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Compounded DC Motor
V E RI k N I k N I
Cumulative Compounding
T A A F F F A A AA
T TF F F A A AA
V E RI k N I k N IE
V VI k N k N IIR R
F F
T A A
R RV E RI
V
Solve for
T A
TF F A A A A
F
Vk N k N I RIR
V RI
T A
TF F A A A
F
V RIVk N k N IR
Solve for AT
A F F A A A AF
VI k IR
IN k N I
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Compounded DC Motor Cumulative Compounding
Solve for T
F F A A AA AF
IVk N k N I IR
20 TF F A A A A
F
Vk N I k N IR
2 2 2
2 2 2 42 4
F F T F F TA A A
A A F A A F
k N V k N VI k Nk N R k N R
Substitute this into the expression for , using the upper sign tok iti (f lI
ti di )keep positive (for cumulaAI tive compounding) as
shown in the figure.
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Compounded DC Motor Cumulative Compounding
T A
TF F A A A
V RIVk N k N IR
2 2 2
2 2 2 42 4
F
F F T F F TA A A
R
k N V k N VI k Nk N R k N R
2 2 2
2 4A A F A A Fk N R k N R
k N V k N V 2 2 2
2 2 2
2 2 2
42 4
F F T F F TT A A
A A F A A F
k N V k N VV R k Nk N R k N R
2 2 2
2 2 2 42 4
T F F T F F TF F A A A A
F A A F A A F
V k N V k N Vk N k N k NR k N R k N R
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Compounded DC Motor Cumulative Compounding:
2 2 2
2 2 2 42 4
F F T F F TT A A
A A F A A F
k N V k N VV R k Nk N R k N R
2 2 2
2 2 2 42 4
T F F T F F TF F A A A A
F A A F A A F
V k N V k N Vk N k N k NR k N R k N R
Differential Compounding (change the sign of kA:
2 2 2
2 2 2 42 4
F F T F F TT A A
A A F A A F
k N V k N VV R k Nk N R k N R
2 2 2
2 2 2
2 4
42 4
A A F A A F
T F F T F F TF F A A A A
k N R k N R
V k N V k N Vk N k N k NR k N R k N R
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2 4F A A F A A FR k N R k N R
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Compounded DC Motor
Differential Compounding(inherently unstable)
EE
DSP
E
Cumulative Compounding
TORQUE
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