Download - Chapter2-2 Types of Dc Motor
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DC Motor - 2
Muhamad Zahim
Ext : 2312
BEE2123
ELECTRICAL MACHINES
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Contents
Overview of Direct Current Machines
Construction Principle of Operation
Types of DC Motor
Power Flow Diagram
Speed Control
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DC motor principles
DC motors consist of rotor-mountedwindings (armature) and stationarywindings (field poles). In all DC motors,
except permanent magnet motors,current must be conducted to thearmature windings by passing currentthrough carbon brushes that slide overa set of copper surfaces called acommutator, which is mounted on the
rotor. Parts of an electric motor
The commutator bars are soldered to armature coils. The brush/commutatorcombination makes a sliding switch that energizes particular portions of thearmature, based on the position of the rotor. This process creates north andsouth magnetic poles on the rotor that areattracted to or repelled by northand south poles on the stator, which are formed by passing direct current
through the field windings. It's this magnetic attraction and repulsion thatcauses the rotor to rotate.
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The Advantages
The greatest advantage of DC motors may be speed
control. Since speed is directly proportional to armature
voltage and inversely proportional to the magnetic flux
produced by the poles, adjusting the armature voltage
and/or the field current will change the rotor speed.
Today, adjustable frequency drives can provide precise
speed control for AC motors, but they do so at the
expense of power quality, as the solid-state switching
devices in the drives produce a rich harmonic spectrum.
The DC motor has no adverse effects on power quality.
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The drawbacks
Power supply, initial cost, and maintenancerequirements are the negatives associated with DC
motors Rectification must be provided for any DC motors
supplied 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, and armaturewindings. An induction motor requires no commutator orbrushes, and most use cast squirrel-cage rotor bars
instead of true windings two huge simplifications.
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Major types of dc motors
Self excited dc motor Series dc motor
Shunt dc motor
Compound dc motor
Separately excited dc motor
Permanent magnet dc motor
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Series motors
Series motors connect
the field windings in
series with thearmature.
Series motors lack good
speed regulation, but
are well-suited for high-
torque loads like powertools and automobile
starters because of their
high torque production
and compact size.
Ea
Rf
M VT (dc
supply)
Raia
)( faaaT RRiEV
La iinote :
aa IKKE 21
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Series Motor Power Flow Diagram
P
Pout
Pin= VTiL
Pca=ia2Ra
Pcf=ia2Rf
Pm
P is normally given
Pin= Pout+ total losses
Where,
Pca =armature copper loss
Pcf =field copper loss
P=stray, mech etc
in
out
mm
oo
P
PEf f iciency
N
Ptorquemechanicalfor
N
Ptorqueloadoutputfor
N
P
,
2
60,
2
60,/
2
60
Pm= Eaia
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Series Motor (cont)
Example 1:
A dc machine in Figure 1 is
consumed a 6.5kW when the12.5 A of armature current ispassing thru the armature andfield resistance of 3.3and 2.0respectively. Assume straylosses of 1.2kW. Calculate
a) terminal voltage, VTb) back emf, Ea
c) net torque if the speed is at3560rpm
d) efficiency of the machine
[520V, 453.75V, 12N-m, 68.8%]
Ea
Rf
M VT (dc
supply)
Raia
Figure 1
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Series Motor (cont)
Example 2:
A 600V 150-hp dc machine inFigure 2 operates at its full rated
load at 600rpm. The armature andfield resistance are 0.12and0.04respectively. The machinedraws 200A at full load. Assumestray losses 1700W. Determine
a) the armature back emf at full load,
Eab) developed/mechanical power anddeveloped/mechanical torque
c) assume that a change in loadresults in the line current droppingto 150A. Find the new speed inrpm and new developed torque.
{Hint: Ea=K1K2ia}
Ea
Rf
M VT (dc
supply)
Raia
Figure 2
[568V, 113.6kW, 1808Nm, 811.27rpm, 1017Nm]
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Shunt motors
Shunt motors use high-resistance field windingsconnected in parallel with the
armature. Varying the field resistance
changes the motor speed.
Shunt motors are prone toarmature reaction, a distortionand weakening of the fluxgenerated by the poles that
results in commutationproblems evidenced by sparkingat the brushes.
Installing additional poles, calledinterpoles, on the statorbetween the main poles wired inseries with the armature
reduces armature reaction.
Ea VT (dc
supply)
Raia
if
RfM
iL
)( aaaT RiEV
ffT
faL
RiV
iiinote
:
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Shunt Motor (power flow diagram)
PPout
Pin=VTiL
Pca=ia2Ra
Pcf=if2Rf
Pm
P is normally given
Pin= Pout+ total losses
Where,
Pca =armature copper loss
Pcf =field copper loss
P=stray, mech etc
in
out
mm
oo
P
PEf f iciency
N
Ptorquemechanicalfor
NPtorqueloadoutputfor
N
P
,
2
60,
260,/
2
60
Pm= Eaia
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Shunt Motor
Example :
A voltage of 230V is applied to armature of amachines results in a full load armature currents
of 205A. Assume that armature resistance is
0.2. Find the back emf, net power and torque by
assuming the rotational losses are 1445W at full
load speed of 1750rpm.
[189V, 37.3kW, 203.5Nm]
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Compound motors
the concept of the
series and shunt
designs arecombined.
Ea VT (dc
supply)
Raia
if
Rf1M
iLRf2
)(2faaaT
RRiEV
1
:
ffT
faL
RiV
iiinote
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Compound motor (power flow diagram)
P is normally given
Pin= Pout+ total losses
Where,
Pca =armature copper loss
Pcf =field copper loss
P=stray, mech etc
in
out
mm
oo
P
PEf f iciency
N
Ptorquemechanicalfor
NPtorqueloadoutputfor
N
P
,
2
60,
260,/
2
60
Pm= Eaia
PPoutPin=VTiL
Pca=ia2Ra
Pcf1=if2Rf1
Pm
Pcf2=ia2Rf2
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Separately Excited Motor
There is no direct connection between the
armature and field winding resistance
DC field current is supplied by an
independent source
(such as battery or another generator or prime
movercalled an exciter)
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Separately Excited Motor (Cont)
nKniKC
pnZ
E fffa 60
2
fff RiV
aaaT RiEV
Where p= no of pole pair
n= speed (rpm)
Z=no of conductor
=Flux per pole (Wb)C= no of current/parallel path
=2p (lap winding)
=2 (wave winding)
KVL:
Circuit analysis:
La iinote :
Ea
Ra Laia
M
Rf
VTVfLf
If
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Permanent Magnet motors
PMDC is a dc motor whose poles are made of
permanent magnets.
Do not require external field circuit, no copper losses
No field winding, size smaller than other types dc
motors
Disadvantage: cannot produce high flux density,
lower induce voltage
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Speed Control for shunt motor and
separately excited dc motor
Torquespeed characteristic for shunt and separately
excited dc motor
a
ff
a
ff
a
a
a
aa
aa
R
nIK
R
IVK
excitedseparatelyassame
nE
REV
n
IE
IEtorqueDeveloped
22
,
2
2
,
22
a
ff
R
IVKc
2
Starting
torque
a
ff
R
nIK
slope 2
22
nNLnn
m
=0
n=0
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Speed Control for shunt motor and
separately excited dc motor
By referring to the Torquespeed characteristic for shunt andseparately excited dc motor
note that, there are three variables that can influence the speed of
the motor, VIf
Ra Thus, there are three methods of controlling the speed of the
shunt and separately excited dc motor,
i. Armature terminalvoltage speed control
ii. Field speed control
iii. Armature resistance speed control
a
ff
a
ff
RnIK
RIVK
22
22
Variables
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Speed Control for shunt motor and
separately excited dc motor
i. Armature resistance speed control
- Speed may be controlled by changing Ra
- The total resistance of armature may be varied by means of a
rheostat in series with the armature
- The armature speed control rheostat also serves as a starting
resistor.
- From -n characteristic,
a
ff
a
ff
start
R
nIKslope
R
IVKc
2
2
22 Will be changed
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Speed Control for shunt motor and
separately excited dc motor
Torquespeed characteristic
Ra1
nNL nn1
m
Ra2
Ra3
Ra1 < Ra2< Ra3
n2n3
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Speed Control for shunt motor and
separately excited dc motor
Advantages armature resistance speed control:
i. Starting and speed control functions may be combined in one
rheostat
ii. The speed range begins at zero speed
iii. The cost is much less than other system that permit control
down to zero speed
iv. Simple method
Disadvantages armature resistance speed control :
i. Introduce more power loss in rheostat
ii. Speed regulation is poor (S.R difference nLoaded& nno loaded)
iii. Low efficiency due to rheostat
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Speed Control for shunt motor and
separately excited dc motor
ii. Field Speed Control
- Rheostat in series with field winding (shunt or separately ect.)
- If field current, If is varied, hence flux is also varied
- Not suitable for series field
- Refer to -n characteristic,
- Slope and nNLwill be changed
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Speed Control for shunt motor and
separately excited dc motor
Torquespeed characteristic
If1 < If2< If3
1< 2< 3
nNL3 nn1
m
n2 n3 nNL2
nNL1Base speed
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Speed Control for shunt motor and
separately excited dc motor
Advantages field speed control:
i. Allows for controlling at or above the base speed
ii. The cost of the rheostat is cheaper because Ifis small value
Disadvantages field speed control :
i. Speed regulation is poor (S.R difference nLoaded& nno loaded)
ii. At high speed, flux is small, thus causes the speed of themachines becomes unstable
iii. At high speed also, the machines is unstable mechanically,
thus there is an upper speed limit
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Speed Control for shunt motor and
separately excited dc motor
iii. Armature terminal voltage speed control
- Use power electronics controller
- AC supplyrectifier
- DC supplychopper
- Supply voltage to the armature is controlled
- Constant speed regulation
- From -n characteristic,- C and nNLwill be change
- Slope constant
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Speed Control for shunt motor and
separately excited dc motor
Torquespeed characteristic
nNL1nn1
m
V3< V2< V1
n2n3 nNL2
nNL3
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Speed Control for shunt motor and
separately excited dc motor
Advantages armature terminal voltage speed control:
i. Does not change the speed regulation
ii. Speed is easily controlled from zero to maximum safe speed
Disadvantages armature terminal voltage speed
control :
i. Cost is higher because of using power electronic controller
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FACTORS AFFECTING THE
PERFORMANCE OF DC MACHINE
There are two factors affecting the
performance of dc machine
1. Armature reaction
2. Armature inductance
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Armature Reaction
Definition of armature reaction:1. It is the term used to describe the effects of the armature
mmf on the operation of a dc machineas a "generator" nomatter whether it is a generator or motor.
2. It effects both the flux distribution and the flux magnitude inthe machine.
3. The distortion of the flux in a machineis called armaturereaction
Two effects of armature reaction:1. Neutral Plane Shift
2. Flux Weakening
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Armature Reaction
Effect on flux distribution:Neutral plane shift
When currentis flowing inthe field winding, hence aflux is producedacross themachine which flows from
the North pole to the Southpole.
Initially the pole fluxisuniformly distributed andthe magnetic neutral planeis vertical
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Armature Reaction
Effect on flux distribution:
Neutral plane shift
effect by the air gapon the
flux fieldcauses the
distribution of flux is no longer
uniform across the rotor.
There are two points on the
periphery of the rotor where
B= 0.
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Armature Reaction
Effect on flux distribution:Neutralplane shift
when a load connected to themachines a resulting magneticfield produced in the armature
If the armatureis rotated at aspeed by an external torqueeach armature coil experiences
a change in flux t as itrotates.
A voltage is generatedacrossthe terminals of each windingaccording to the equation e =t
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Armature Reaction
Effect on flux distribution:Neutral plane shift
Both rotor and pole fluxes (fluxproduced by the field windingand the flux produced by thearmature winding) are addedand subtracted togetheraccordingly
The fields interact to produce adifferent flux distribution in therotor.
Thus, the flux on the middleline, between the two field
poles, is no longer zero.
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Armature Reaction
Effect on flux distribution:Neutral planeshift
The combined flux in the machinehasthe effect of strengthening or weakeningthe flux in the pole.Neutral axis istherefore shifted in the direction ofmotion.
The result is current flow circulatingbetween the shorted segments and largesparks at the brushes. The ending resultis arcing and sparking at the brushes.
Solution to this problem:
placing an additional poles on theneutral axis or mid-point that willproduce flux density component,which counter-acts that produced bythe armature.
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Armature Reaction
Effect on flux magnitude:FluxWeakening
Most machine operate at saturationpoint
When the armature reaction happen,at location pole surface:
The add of rotor mmf to pole mmfonly make a small increase in flux
The subtract of rotor mmf frompole mmf make a large decreasein flux.
The result is the total average fluxunder entire pole face isdecreased.
This is called Flux Weakening
dflux decrease under subtracting section of poles
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Armature Inductance
When rotor turns, thus we have inductance value, e1= L(di/dt). Let say current ia1.
That means, we have ability to store energy If the machine is turn off, thus, e1will decreased.
This will affect the current as well. Say ia2. When the machine is turn on again, it will produce
e2while e1is still inside. The current now is reverseddirection from previous (decreasing) current.
Thus, it will cause sparking resulting the sameaching problem caused by neutral plane shift.