AC Motors

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Chapter 19

• AC Motors

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Objectives

• After studying this chapter, you will be able to
• Describe the construction of various types of AC
  motors
• Explain the principle of operation of various
  types of three-phase motors and single-phase
  motors

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Objectives (contd.)

• Discuss the reasons for the difference in the
  values of starting and running currents in AC
  motors

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AC Motor Construction

• Induction motor is most common AC motor
• Two main components stator and rotor
• Stator has electromagnets secured to frame
• Rotor made of steel lamination in shape of
  cylinder
• Windings placed in slots on rotor surface
• Squirrel-cage rotor winding
• Heavy copper bars welded to end rings

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AC Motor Construction (contd.)

• Wound-rotor motor
• Copper wire wound into rotor slots
• Connected in wye
• Ends connected to slip rings on shaft

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Three-Phase Motor Theory

• Two-pole, three phase stator winding
• When energized from three-phase source,
  three-phase currents are 120 degrees apart and
  continuously changing in value and direction
• The effect of this variation in strength and
  direction produces a rotating field
• See Figure 19-4C

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Three-Phase Motor Theory (contd.)

• Speed of the rotating magnetic field
• Synchronous speed of two-pole induction motor is
  60 revolutions per second when supplied with 60
  Hz.
• Lower speeds obtained with more poles

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Three-Phase Motor Theory (contd.)

• Rotor speed
• Depends on synchronous speed and load
• Rotor speed lags behind synchronous speed one
  percent (no load) to five percent (full load)
• Direction of rotor rotation
• Depends on the phase sequence
• Revolves in same direction as stator field

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Three-Phase Motor Theory (contd.)

• Torque
• Varies with the strength of the stator and rotor
  fields
• Also affected by phase relationship between the
  two fields

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Three-Phase Motor Theory (contd.)

• Slip
• Difference between synchronous speed and rotor
  speed
• May be stated in rpm or a percentage

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Three-Phase Motor Starting and Running Current

• Starting current is limited to four to six times
  the full load current
• Loading a squirrel-cage motor
• For large motors, reduced voltage starters are
  used to limit the starting current to a lower
  value that will not cause line drops

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Three-Phase Motor Starting and Running Current
(contd.)

• Double squirrel-cage rotor
• Has two squirrel-cage windings
• Inner winding has low resistance and high
  inductance
• Outer winding has high resistance and low
  inductance
• Results in large current and high power factor in
  outer winding high starting torque

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Types of Three-Phase Motors

• Multispeed squirrel-cage motors
• Number of poles may be varied by changing
  external connections
• Wound-rotor induction motors
• Provides high starting torque at low current
  value
• Accelerates smoothly under heavy loads
• Poor speed regulation when operating with
  resistance in the rotor circuit

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Types of Three-Phase Motors (contd.)

• Adjustable-speed induction motor (brush-shifting
  motor)
• Provides a wide range of speeds, depending on
  position of the brushes
• High-frequency motors
• Operate at speeds greater than can be obtained
  from a 60Hz supply

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Types of Three-Phase Motors (contd.)

• Synchronous motors
• Operates at constant speed from no load to full
  load
• Power factor can be controlled by varying amount
  of current in the exciter winding
• Generally used for driving loads requiring
  constant speeds and infrequent starting and
  stopping

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Single-Phase Motors

• Disadvantages when compared to polyphase motors
• Torque developed is not as smooth
• Larger in physical size and less efficient

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Starting Single-Phase Motors

• Phase-splitting principle one method of making a
  single-phase motor self-starting
• Stator has two windings main and auxiliary
• Two currents out of phase by less than 90 degrees
• Resistance split-phase motor
• Resistance connected in series with auxiliary
  winding, resulting in better starting torque

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Starting Single-Phase Motors (contd.)

• Capacitor split-phase motors
• Three classes 1) low starting torque, 2)
  capacitor start, induction run, 3) capacitor
  start, capacitor run
• Reversing split-phase motors
• Reverses direction of rotor rotation by reversing
  the polarity of one of the fields

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Starting Single-Phase Motors (contd.)

• Shaded-pole motors
• Started by a low-resistance, short-circuited coil
  placed around one tip of each pole
• Have poor starting torque
• Repulsion motors
• Starts based on the principle that like poles
  repel

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Starting Single-Phase Motors (contd.)

• Series AC motors
• More complex structure and heavier per horsepower
• More expensive than DC motors of the same rating
• Operating characteristics very similar to DC
  motors

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Universal Motors

• Series motor that will operate on both AC and DC
• Manufactured in small sizes with fractional
  horsepower ratings
• Used for vacuum cleaners, portable electrical
  tools, and small household appliances

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Selection of Single-Phase Motors

• Shaded-pole motors adequate for small fans,
  measuring instruments and other applications
  requiring little starting torque
• Split-phase motors can be used for large machines
  that start without a load (small lathes, drills,
  grinders)
• Capacitor and repulsion motors for machines that
  start under load

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Torque Motors

• Have greatest torque output when stalled
• Applications opening and closing valves,
  dampers, doors, gates, windows, etc.
• Large torque motors operate on three-phase
• Small ones versions of universal motor
• Not intended for continuous duty

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Dual-Voltage Windings

• Motors that can operate on either of two voltages
• Accomplished by dividing each phase into two
  sections (either delta or wye)

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Multispeed Induction Motors

• Synchronous speed of induction motor depends on
  supply frequency and number of poles
• Changing speed by varying frequency requires use
  of variable frequency drive and means of
  adjusting motor current
• Solid-state controller can be used to accomplish
  this

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Summary

• Induction motor made up of stator and rotor
• Types of three-phase motors include multispeed
  squirrel-cage, wound-rotor induction, adjustable
  speed induction, high-frequency and synchronous
  motors
• Single-phase motors are larger in size and less
  efficient than polyphase motors

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Summary (contd.)

• Universal motors can operate on either AC or DC
• Dual voltage windings can operate on either of
  two voltages
• Multispeed induction motors offer varying speeds
  by changing frequency or number of poles