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Electric Motors. Physics Review Motors convert electrical energy (P in ) to mechanical energy (P out ) Mechanical power = Torque * Angular Velocity = Force * Linear Velocity Theoretically, P out = P in Practically, P out < P in Units: Metric: watts, kW Imperial: horsepower

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electric motors
Electric Motors
  • Physics Review
    • Motors convert electrical energy (Pin) to mechanical energy (Pout)
    • Mechanical power = Torque * Angular Velocity = Force * Linear Velocity
    • Theoretically, Pout = Pin
    • Practically, Pout < Pin
  • Units:
    • Metric: watts, kW
    • Imperial: horsepower
  • Example:
    • A 1740 rpm motor raise a 200 lb load at a vertical speed of 2 inch/sec
    • Find: Power required, theoretical motor torque.

EMC_Intro_to_Electric_Motors Roger Enns

electric motor types
Electric Motor Types
  • AC
    • Induction motors
    • Shaded pole
    • Capacitor start
    • Gearmotors
    • Synchronous
  • DC
    • Shunt-wound
    • Series-wound
    • Gearmotors
    • Brushless

About 2/3 of the electrical energy generated in the US is consumed by motors, over ½ of this by induction motors.

EMC_Intro_to_Electric_Motors Roger Enns

nema national electrical manufacturers association
NEMA – National Electrical Manufacturers Association
  • Sets North American standards for electric motors
    • Motor Ratings (power output, typically in hp)
      • Ratings depend on motor classification, overload allowance
      • Actual motor power capability > rating
    • Frame size
      • Physical size of motor, interface, mounting, shaft size, etc.
    • Housing protection/classification
      • Selected based on environment, code requirements, etc.

EMC_Intro_to_Electric_Motors Roger Enns

motor terminology
Motor Terminology

stator & rotor – a mechanical distinction

field & armature – an electrical distinction

EMC_Intro_to_Electric_Motors Roger Enns

ac alternators
AC Alternators
  • Basic Principle
    • As a conductor (wire) moves across a magnetic field, a voltage is produced causing current to flow in the wire.

EMC_Intro_to_Electric_Motors Roger Enns

dc generators
DC Generators
  • Basic Principle
    • As a conductor (wire) moves across a magnetic field, a voltage is produced causing current to flow in the wire.

EMC_Intro_to_Electric_Motors Roger Enns

dc generators cont d
DC Generators cont’d.

If a single conductor were rotated through a magnetic field, AC voltage would be produced

Commutator: Segmented conductor that effectively reverses the polarity of the rotor every 180 degrees. Results in ‘lumpy’ DC voltage produced.

Multiple rotor coils used to smooth the output.

EMC_Intro_to_Electric_Motors Roger Enns

dc motors
DC Motors
  • Reverse the operation of the generator by:
    • Applying DC voltage to the brushes.
    • Magnet field created is by current in rotor windings.
    • This magnetic field is attracted/repelled by the corresponding stator field.
    • Stator field may be created by permanent or electo-magnets.
    • As motor turns, commutator switches between active rotor windings to maximize motor torque.

EMC_Intro_to_Electric_Motors Roger Enns

dc motors1
DC Motors

EMC_Intro_to_Electric_Motors Roger Enns

counter emf
Counter EMF
  • As the DC motor turns, a voltage is induced within the coil windings. This induced voltage opposed the applied voltage to the machine.
  • Induced voltage is known as “Counter EMF”, typically shown as E0

Z = # of conductors in winding

n = RPM of machine

Φ = Flux per pole

EMC_Intro_to_Electric_Motors Roger Enns

counter emf cont d
Counter EMF Cont’d
  • At startup, full applied voltage is present across motor. As motor starts to turn, it immediately acts as a generator, whose counter EMF opposes the applied voltage.
  • When counter EMF generated = Applied Voltage, maximum theoretical motor speed is attained.

Example: Motor has a winding made up of 50 coils with 10 conductors per coil. Assuming flux per pole = 0.02, find motor speed as a function of applied voltage.

EMC_Intro_to_Electric_Motors Roger Enns

electrical analysis of dc motors
Electrical Analysis of DC Motors
  • Armature Current (IA) = (Es-E0)/R
  • At startup, E0 = 0, therefore very high initial startup current – may be 20 to 30 times normal operating current
  • DC motor starters often use current limiting techniques.

EMC_Intro_to_Electric_Motors Roger Enns

shunt wound dc motor
Shunt-Wound DC Motor
  • Field windings in parallel with armature
  • High field resistance.
  • Nearly constant speed operation from no-load to full-load

EMC_Intro_to_Electric_Motors Roger Enns

series wound dc motor
Series Wound DC Motor
  • Field windings in series with armature
  • Low field resistance – must conduct armature current.
  • Low-speed under high load. High-speed under low load.
  • DO NOT OPERATE UN-LOADED!!

EMC_Intro_to_Electric_Motors Roger Enns