1. Electric Motors Classification / types Function DC Motors AC Motors
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

2. DC Motors DC Motors Fundamental characteristics Types and applications
BAE 4353
12/3/2002
DC Motors
DC Motors
Fundamental characteristics
Basic function
Types and applications
Series
Shunt
Combination
Torque characteristics
Modelling

3. Fundamental characteristics of DC Motors
BAE 4353
12/3/2002
Fundamental characteristics of DC Motors
End view
Time 0
End view
Time 0+
Shifting magnetic field in rotor causes rotor to be forced to turn

4. Nature of commutation Power is applied to armature windings
BAE 4353
12/3/2002
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

5. DC motor wiring topologies
BAE 4353
12/3/2002
DC motor wiring topologies

6. 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.

7. 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).

8. Compound wound DC motors
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

9. Permanent magnet DC motors
BAE 4353
12/3/2002
Permanent magnet DC motors

10. Permanent Magnet DC Motors
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.

11. BAE 4353
12/3/2002
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

12. DC Motor modeling From the circuit Motor equations Power is:
BAE 4353
12/3/2002
DC Motor modeling
From the circuit
Motor equations
Power is:
Substituting the above:
And no-load speed
Max power is:
In terms of no-load speed
torque/speed equation is:
For stalled rotor torque
Units:

13. http://biosystems.okstate.edu/home/mstone/4353/downloads/ Application
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:
Development of Electromotive Force.pdf

14. DC motor control – H-bridge
BAE 4353
12/3/2002
DC motor control – H-bridge
Switches control direction
“A” switches closed for clockwise
“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

15. H-Bridge implementation
BAE 4353
12/3/2002
H-Bridge implementation
Elements in box are available as single IC

16. Brushless designs Commutation is done electronically PM armature
BAE 4353
12/3/2002
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

17. Stepper Motors Description Two general types of windings
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

18. Winding configurations
BAE 4353
12/3/2002
Winding configurations
Bi-polar design
6 wire
Unipolar design
4 wire

19. AC Motors AC Motors Fundamental characteristics Types
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

20. Fractional horsepower designs
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
Universal? (intermittent use, brushes!)
DC motor with inductance managed to allow AC operation
Synchronous (clocks, synchronization)
Permanent magnet rotor always in phase with AC

21. AC motor model See Siemens AC motor info for modeling info. BAE 4353
12/3/2002
AC motor model
See Siemens AC motor info for modeling info.

22. BAE 4353
12/3/2002
AC Motors
Relationship between number of poles and motor synchronous speed
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.
Poles
Synchronous
Speed
(RPM) 2
3600 4
1800 6
1200

23. BAE 4353
12/3/2002
Squirrel Cage Rotor
Seimens AG, 2002

24. Inducing magnetism in the rotor
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.
This current in turn magnetizes the rotor

25. BAE 4353
12/3/2002
Torque/speed curve

26. Typical starting current
BAE 4353
12/3/2002
Typical starting current

27. Motor characteristics
BAE 4353
12/3/2002
Motor characteristics
Enclosure / frame
Voltage / frequency
3 or 1 phase
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
60 Hz
50 Hz
115
380
200
400
230
425
460
220/380
575

28. NEMA Torque characteristics summarized
BAE 4353
12/3/2002
NEMA Torque characteristics summarized
NEMA DESIGN
STARTING TORQUE
STARTING CURRENT
BREAK- DOWN TORQUE
FULL LOAD SLIP
TYPICAL APPLICATIONS A
Normal
High
Low
Mach. Tools, Fans B
Same as Design "A" C
Loaded compressor Loaded conveyor D
Very high
High Punch Press

29. NEMA Motor Characteristics
BAE 4353
12/3/2002
NEMA Motor Characteristics
Design
Locked Rotor Torque
% FL
Pull-up Torque
Breakdown Torque
Locked Rotor Current
Slip %
Efficiency A
70-275
65-190 NA
0.5-5
Med-High B
(most common) C
1-5
Med D
275
5-8
Low E
74-190
60-140
0.5-3
High

30. PWM Variable Frequency Drives
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