12/3/2002BAE 4353 1 Electric Motors Classification / types –DC Motors –AC Motors –Stepper...
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Transcript of 12/3/2002BAE 4353 1 Electric Motors Classification / types –DC Motors –AC Motors –Stepper...
12/3/2002BAE 4353
1
Electric Motors
• Classification / types– DC Motors– AC Motors– Stepper Motors– Linear motors
• Function– Power conversion - electrical into mechanical– Positional actuation – electrical signal to position
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DC Motors
– DC Motors• Fundamental characteristics
– Basic function
• Types and applications– Series– Shunt– Combination– Torque characteristics
• Modeling
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Fundamental characteristics of DC Motors
N
S
StatorCoils
N
SS
N
Rotor
Stator
S
N
S
N
N
S
End viewTime 0
N
S
StatorCoils
N
S NRotor
Stator
S
N
S
N
N
S
S
End viewTime 0+
Shifting magnetic field in rotor causes rotor to be forced to turn
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Nature of commutation
• Power is applied to armature windings– From V+– Through the +brush– Through the commutator
contacts– Through the armature (rotor)
winding– Through the – brush– To V-
• Rotation of the armature moves the commutator, switching the armature winding connections
• Stator may be permanent or electromagnet
Rotor
V-
V+Brush
Assembly
S
S
N
N
N
Stator
Stator
Comutator
V-
V+
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DC motor wiring topologiesP
erc
en
t o
f ra
ted
Sp
ee
d
Percent of Rated Torque
120
100
80
60
40
20
0
400300200100
0
Sh
un
t F
ield
Series Field
Sh
un
t F
ield
Series Field
Shunt
Series
Compound
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Series Wound DC motors
• Armature and field connected in a series circuit.
• Apply for high torque loads that do not require precise speed regulation. Useful for high breakaway torque loads.
– locomotives, hoists, cranes, automobile starters
• Starting torque – 300% to as high as 800% of full load torque.
• Load increase results in both armature and field current increase– Therefore torque increases by the square of a current increase.
• Speed regulation– Less precise than in shunt motors
» Diminished load reduces current in both armature and field resulting in a greater increase in speed than in shunt motors.
– No load results in a very high speed which may destroy the motor.» Small series motors usually have enough internal friction to prevent
high-speed breakdown, but larger motors require external safety apparatus.
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Shunt wound DC motors
• Field coil in parallel (shunt) with the armature.– Current through field coil is independant of the armature.
» Result = excellent speed control.
• Apply where starting loads are low– fans, blowers, centrifugal pumps, machine tools
• Starting torque– 125% to 200% full load torque (300 for short periods).
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Compound wound DC motors
• Performance is roughly between series-wound and shunt-wound
• Moderately high starting torque
• Moderate speed control
• Inherently controlled no-load speed– safer than a series motor where load may be disconnected
» e.g. cranes
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Permanent magnet DC motors
Permanentmagnetpoles
Pe
rce
nt
of
rate
d S
pe
ed
Percent of Rated Torque
40
20
02001000
120
100
80
60
400300
PermanentMagnet
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Permanent Magnet DC Motors
– Have permanent magnets rather than field windings but with conventional armatures. Power only to armature.
– Short response time– Linear Torque/Speed characteristics similar to shunt wound
motors. Field magnetic flux is constant• Current varies linearly with torque.
– Self-braking upon disconnection of electrical power• Need to short + to – supply, May need resistance to dissipate heat.
– Magnets lose strength over time and are sensitive to heating.• Lower than rated torque.
• Not suitable for continuous duty
• May have windings built into field magnets to re-magnetize.
– Best applications for high torque at low speed intermittent duty.• Servos, power seats, windows, and windshield wipers.
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Modeling DC motors
• A linear speed/torque curve can be used to model DC motors. This works well for PM and compound designs and can be used for control models for narrow ranges for the other configurations
• Model will assume!– Linearity
– Constant thermal characteristics
– No armature inductance
– No friction in motor
Pe
rce
nt
of
rate
d S
pe
ed
Percent of Rated Torque
120
100
80
60
40
20
0
4003002001000
nNo load speed
Stalled rotortorque
s
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DC Motor modeling
+
V R
Armature
E (back emf)
I
T,
]/[ ANmK t
bEIRV
eb KE
IKT t
Motor equations
From the circuit
et
KRK
TV
RKK
T
K
V
tet
Substituting the above:
R
VKT es
tn K
V
For stalled rotor torque
And no-load speed
TKK
R
ten
In terms of no-load speed torque/speed equation is:
2TKK
RTTP
ten
Power is:
Max power is:
R
VP
4
2
max
]/[ radVsKe Units:
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Application
• Use motor voltage and no-load speed to calculate Kt
• Kt = Ke in SI units
• Use stalled rotor torque, V, and Ke to find R– Note, R varies with speed and cannot be measured at rest
• See web download for explanation of Kt, Ke:
http://biosystems.okstate.edu/home/mstone/4353/downloads/
Development of Electromotive Force.pdf
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DC motor control – H-bridge
• Switches control direction– “A” switches closed for
clockwize– “B” switches for counter-
clockwise
• PWM for speed control– “A’s” duty cycle for clockwise
speed– “B’s” duty cycle for counter-
clockwise speed
• Can be configured to brake– Bottom “B” and “A” to brake
M
12V
A
A
B
B
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H-Bridge implementation
• Elements in box are available as single IC
InputLogic
PWM
Direction
Brake
Vsupply
M
GroundH-Bridge Circuit
DC Motor
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Brushless designs
• Commutation is done electronically– Encoder activated switching
– Hall effect activated switching
– Back EMF driven switching
• PM armature• Wound/switched fields• Application
– Few wearing parts (bearings)
– Capable of high speed
– Fractional HP• Servos• Low EMC
+V
Optical Encoder
Armature
+V
Field
Encoder activated switching
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AC Motors
– AC Motors• Fundamental characteristics
• Types– Fractional
» Shaded Pole» Capacitor Start Induction Run
– Integral» Service Factor» Insulation class
– Stepper
• Modeling
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AC motor model
Rs Ls Lr
Lm (Magnetizing Inductance) Rr
mm Lf
EI
2
E Iw
f
E wIkT
E - Magnetizing voltageIm - Magnetizing currentf - FrequencyT - TorqueIw - rotor current - Magnetic Flux, rotor
22wms III
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AC Motors
• Relationship between number of poles and motor speed
P
fN s
120
Poles Speed
(RPM)
2 3600
4 1800
6 1200
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Squirrel Cage Rotor
Seimens AG
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Torque/speed curve
% o
f F
ull-
Lo
ad
To
rqu
e
% of Synchronous Speed
Slip (Full load)
100806040200
250
200
150
100
50
0
Breakdown
Torque
Full-Load Torque
Pull-up Torque
Locked rotor
torque
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Typical starting current
% o
f F
ull-
Lo
ad
Cu
rre
nt
Time
500
400
300
200
100
0
Full-Load CurrentLocked Rotor
(Starting Current)
700
600
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NEMA Motor Characteristics
Design Locked Rotor
Torque
% FL
Pull-up Torque
% FL
Breakdown Torque
% FL
Locked Rotor
Current
% FL
Slip
%
Efficiency
A 70-275 65-190 175-300 NA 0.5-5 Med-High
B 70-275 65-190 175-300 600-700 0.5-5 Med-High
C 200-285 140-195 190-225 600-700 1-5 Med
D 275 NA 275 600-700 5-8 Low
E 74-190 60-140 160-200 800-1000 0.5-3 High
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PWM Variable Frequency Drives
L1
L3L2
Co
ntr
ol L
og
ic
M
Rectifier Filter Inverter
480V
650 V