Electric Motors

1. BAE 4353 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. BAE 4353 DC Motors • DC Motors • Fundamental characteristics • Basic function • Types and applications • Series • Shunt • Combination • Torque characteristics • Modelling

3. BAE 4353 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. BAE 4353 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. BAE 4353 DC motor wiring topologies

6. BAE 4353 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 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. BAE 4353 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. BAE 4353 Permanent magnet DC motors

10. BAE 4353 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 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. BAE 4353 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. BAE 4353 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

14. BAE 4353 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. BAE 4353 H-Bridge implementation • Elements in box are available as single IC

16. BAE 4353 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. BAE 4353 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. BAE 4353 Winding configurations • Bi-polar design • 6 wire • Unipolar design • 4 wire

19. BAE 4353 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. BAE 4353 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

21. BAE 4353 AC motor model • See Siemens AC motor info for modeling info.

22. BAE 4353 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.

23. BAE 4353 Squirrel Cage Rotor Seimens AG, 2002

24. BAE 4353 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 Torque/speed curve

26. BAE 4353 Typical starting current

27. BAE 4353 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

28. BAE 4353 NEMA Torque characteristics summarized

29. BAE 4353 NEMA Motor Characteristics

30. BAE 4353 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