Electric Motors

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# Electric Motors - PowerPoint PPT Presentation

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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Presentation Transcript
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
• AC
• Induction motors
• 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
• 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

stator & rotor – a mechanical distinction

field & armature – an electrical distinction

EMC_Intro_to_Electric_Motors Roger Enns

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
• 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.

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
• 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 Motors

EMC_Intro_to_Electric_Motors Roger Enns

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
• 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
• 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
• Field windings in parallel with armature
• High field resistance.