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BAE 4353 12/3/2002
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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BAE 4353 12/3/2002
DC Motors
– DC Motors
• Fundamental characteristics
– Basic function
• Types and applications
– Series
– Shunt
– Combination
– Torque characteristics
• Modelling
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BAE 4353 12/3/2002
Fundamental characteristics of DC Motors
Stator Stator
S Stator S Stator
N Coils Coils
N S
N N N N
S S
Rotor S Rotor N
SN N
S S S
N S
S
N N
End view End view
Time 0 Time 0+
Shifting magnetic field in rotor causes rotor to be forced to turn
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BAE 4353 12/3/2002
Nature of commutation
• Power is applied to armature V+
windings Stator
V+
– From V+ Brush
N Assembly
– Through the +brush
S
– Through the commutator Rotor
contacts N
– Through the armature (rotor) S
winding N Comutator
V-
– Through the – brush Stator
– To V- V-
• Rotation of the armature
moves the commutator,
switching the armature winding
connections
• Stator may be permanent or
electromagnet
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BAE 4353 12/3/2002
DC motor wiring topologies
Shunt Field
Shunt
120
100 Sh u n Series Field
Percent of rated Speed
t
80
Series
60 Co
Se m
rie po
s un
40 d
Series Field
20
Shunt Field
0 Compound
100 200 300 400
0
Percent of Rated Torque
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BAE 4353 12/3/2002
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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BAE 4353 12/3/2002
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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BAE 4353 12/3/2002
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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BAE 4353 12/3/2002
Permanent magnet DC motors
120
100 Pe Permanent
Percent of rated Speed
rm Permanent
an magnet
en Magnet
80 tM poles
ag
ne
t
60
40
20
0
0 100 200 300 400
Percent of Rated Torque
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BAE 4353 12/3/2002
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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BAE 4353 12/3/2002
Modeling DC motors
• A linear speed/torque curve
can be used to model DC
motors. This works well for n No load speed
PM and compound designs 120
and can be used for control
100
models for narrow ranges for
Percent of rated Speed
Id
e
the other configurations 80 al
lin
ea
• Model will assume! 60
rm
od
el
– Linearity
40
– Constant thermal
Stalled rotor
characteristics 20
torque
s
– No armature inductance 0
– No friction in motor 0 100 200 300 400
Percent of Rated Torque
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BAE 4353 12/3/2002
DC Motor modeling
T,
From the circuit
Armature
V IR Eb
+
V R E (back emf)
Motor equations
Eb K e I
Power is:
T Kt I
R 2
Substituting the above: And no-load speed K K T
P T T n
T e t
V R K e V
Kt n
Kt Max power is:
V T
R In terms of no-load speed
Kt Ke Kt V2
torque/speed equation is: Pm ax
4R
For stalled rotor torque
R Units:
KV
Ts e K K T
n
e t K e [Vs / rad ]
R
K t [ Nm / A]
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BAE 4353 12/3/2002
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
13
BAE 4353 12/3/2002
DC motor control – H-bridge
• Switches control direction 12V
– “A” switches closed for
clockwise
A B
– “B” switches for counter-
clockwise M
• PWM for speed control A
B
– “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
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BAE 4353 12/3/2002
H-Bridge implementation
• Elements in box are
available as single IC
M
DC Motor
Vsupply
PWM
Input
Direction
Logic
Brake
H-Bridge Circuit
Ground
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BAE 4353 12/3/2002
Brushless designs
• Commutation is done
electronically
+V +V
– Encoder activated switching
– Hall effect activated switching
– Back EMF driven switching
• PM armature
• Wound/switched fields Field
• Application
– Few wearing parts (bearings)
Armature
– Capable of high speed
– Fractional HP
• Servos Optical Encoder
• Low EMC
Encoder activated switching
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BAE 4353 12/3/2002
Stepper Motors
• Description
– Generally a two phase motor
– permanent magnet rotor and wound fields
– Rotor normally has many poles
• 200 poles = 1.8 degrees per step
– Used primarily for position or velocity control
– Typically no position feedback
• Torques are managed so that an intended step is always achieved
– Accelerations, decelerations and loads must be managed intelligently
• Two general types of windings
– Unipolar
– Bi-polar
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BAE 4353 12/3/2002
Winding configurations
• Bi-polar design
12V
– 6 wire
N
12V
S
12V
H-Bridge
• Unipolar design N
H-Bridge
– 4 wire
12V
S
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BAE 4353 12/3/2002
AC Motors
• AC Motors
– Fundamental characteristics
– Types
• Fractional horsepower (single phase)
• Integral
– Single phase (Cap start Induction run)
– Three phase
– NEMA Torque characteristics
– Modelling
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BAE 4353 12/3/2002
Fractional horsepower designs
– Shaded Pole (low starting torque, simple, cheap)
• uses a short circuited coil embedded in face of field to cause one
side of field to be magnetized before the other
– Split phase (low starting torque)
• Two windings (2-phase), one with high resistance hence different
RL and phase
• Centrifugal switch on starting winding
– Capacitor Start Induction Run (medium starting torque)
• Two windings (2-phases)
• Capacitor used on second winding to create leading phase
• Centrifugal switch on starting winding
– Universal? (intermittent use, brushes!)
• DC motor with inductance managed to allow AC operation
– Synchronous (clocks, synchronization)
• Permanent magnet rotor always in phase with AC
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BAE 4353 12/3/2002
AC motor model
• See Siemens AC motor Rs Ls Lr
info for modeling info.
Lm (Magnetizing Inductance) E Iw Rr
E E - Magnetizing voltage
Im
E
T kI w Im - Magnetizing current
2 f Lm f f - Frequency
T - Torque
Is Im Iw
2 2
Iw - rotor current
- Magnetic Flux, rotor
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BAE 4353 12/3/2002
AC Motors
• Relationship between number of poles and motor synchronous
speed
Poles Synchronous
120 f Speed
Ns
P (RPM)
2 3600
4 1800
6 1200
• Squirrel cage motors must operate with some slip .5 to 8% to allow
the rotor to be magnetized.
– Actual speed is synchronous speed reduced by the slip.
(100 %slip)
N Ns
100
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BAE 4353 12/3/2002
Squirrel Cage Rotor
Seimens AG, 2002
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BAE 4353 12/3/2002
Inducing magnetism in the rotor
• Difference between
angular velocity of rotor
and angular velocity of
the field magnetism
causes squirrel cage
bars to cut the field
magnetic field inducing
current into squirrel cage
bars. Rotor
• This current in turn
magnetizes the rotor
N S
Difference in
rotation of field
magnetism and
rotor rotation
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BAE 4353 12/3/2002
Torque/speed curve
% of Full-Load Torque
250 Pull-up Torque Breakdown
Locked rotor Torque
torque
200
150 Full-Load Torque
100
Slip (Full load)
50
0
0 20 40 60 80 100
% of Synchronous Speed
25
BAE 4353 12/3/2002
Typical starting current
700
600
% of Full-Load Current
500
Locked Rotor
Full-Load Current
(Starting Current)
400
300
200
100
0
Time
26
BAE 4353 12/3/2002
Motor characteristics
• Enclosure / frame 60 Hz 50 Hz
115 380
• Voltage / frequency 200 400
230 425
• 3 or 1 phase 460 220/380
575
• Poles / speed
• Service factor
– Fraction of rated HP that motor can be operated at
• Insulation class/ Temp rise
– (operating temperature compatible)
• NEMA Design A,B,C,D, etc. (Torque curve type)
– See next page
• Efficiency
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BAE 4353 12/3/2002
NEMA Torque characteristics summarized
BREAK- FULL
NEMA STARTING STARTING DOWN LOAD TYPICAL
DESIGN TORQUE CURRENT TORQUE SLIP APPLICATIONS
A Normal High High Low Mach. Tools, Fans
B Normal Normal Normal Normal Same as Design "A"
Loaded compressor
C High Normal Low Normal
Loaded conveyor
D Very high Low ------- High High Punch Press
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BAE 4353 12/3/2002
NEMA Motor Characteristics
Design Locked Pull-up Breakdown Locked Slip Efficiency
Rotor Torque Torque Rotor %
Torque % FL % FL Current
% FL % FL
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
(most
common)
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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BAE 4353 12/3/2002
PWM Variable Frequency Drives
• Variable frequency drives use AC to DC converter then a
DC to AC converter (inverter)
– Inverter frequency and voltage output can be varied to allow
motor speed to be varied.
– Very efficient and cost effective variable speed for 1 HP and up
650 V
Control Logic
L1
480V L2 M
L3
Rectifier Filter Inverter
30
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