Chapter 6 AC Motors * * Fundamental Operation AC Motors

1
Chapter 6

• AC Motors

2
Fundamental Operation

• AC Motors convert AC energy into mechanical
  energy
• Typical components include
• Stator (field pole winding)
• Rotor

3
Stator/Rotor Action

• The diagram to the right demonstrates one
  complete AC cycle being applied to the windings
  of an AC motor

4
Stator Construction

• Two-phase operation is necessary to create the
  magnetic field conditions needed to generate
  torque in an AC motor
• Phase 1 is applied to the vertical stator
  windings
• Phase 2 is applied to the horizontal stator
  windings

5
Three-Phase

• Three-phase power is commonly used in industrial
  factories
• Three-phase power is ideal for powering rotating
  stator fields in motors

6
RotorConstruction

• AC motors use electromagnets for their rotors
• Two methods are used for energizing the rotor
• Connect an electrical current to the windings
• Use induction as the means of producing magnetic
  fields

7
Types of Rotors

• Squirrel Cage - cage is made of aluminum or
  brass, connected to end rings
• Wound Rotors - rotor is wound with coils of wire.
  The number of coils must be equal to the number
  of stator coils

8
Principles of Operation

• The stator winding, when energized, produces a
  rotating magnetic field that causes the motor to
  turn at synchronous speed
• However, the rotor will never catch up to the
  stator field
• Speed - determined by frequency of the applied
  voltage and number of stator poles per phase

9
Speed and Slip

• Speed of a motor is determined by the following
  formula
• N RPM
• P Number of pole pairs
• f Applied frequency
• 60 Formula constant based upon seconds/minute

10
AC Motor Synchronous Speeds
11
Single-Phase Induction Motors

• Single-phase commercial power is typically
  supplied to residential customers
• AC induction motors require two phases
• Single-phase AC motors are classified by the
  means of obtaining the two phases
• Resistance-start, induction-run
• Capacitor-start, induction-run
• Shaded-pole motor

12
Resistance-Start Induction-Run Motor

• Has two separate windings connected in parallel
• Main (run) winding
• Auxiliary (start) winding
• Start winding has high resistance

13
Capacitor-Start Induction-Run Motor

• Two windings, a start and a run winding
• Capacitor is placed in series with the start
  winding to obtain higher starting torque
• At 70-80 of full speed, capacitor is
  disconnected by a centrifugal switch

14
Shaded-Pole Motor

• Uses a squirrel cage rotor and a main winding
• Uses shaded poles to develop rotating field
• Shaded-pole motors are useful in light load
  applications

15
Troubleshooting Split-phase Motors

• The most frequent cause of failure in
  capacitor-start motors is a defective capacitor
• Resistance-start motors usually fail as a result
  of a defective centrifugal switch or open start
  winding

16
Universal Motors

• Usually classed as an AC motor, but can operate
  on DC voltages as well
• Similar to a DC series-wound motor
• Universal motors use a laminated iron core versus
  the solid iron cores of DC motors
• Output torque is less compared to a DC motor

17
Three-Phase Motors

• Most industrial motors operate on three-phase
  power
• Also referred to as polyphase motors
• Simpler in construction and more powerful than
  single-phase motors
• Three types
• Induction motor
• Wound-rotor motor
• Synchronous motor

18
Induction Motor

• Most industrial machines are powered by
  three-phase squirrel cage induction motors
• Small size, efficiency, relatively low-cost

19
Rotating Field
20
Wound-Rotor Motor

• Rotor consists of a set of three coils in place
  of conducting bars found in squirrel cage motors
• Wound-rotor motors are used where speed and
  torque need to be adjustable

21
Synchronous Motor

• Synchronous motors turn at the same speed as the
  stators magnetic field
• Synchronous motors perform two functions
• Convert electrical energy into mechanical energy
• Perform power factor correction

22
Synchronous Motor Construction and Operation

• The most common type of synchronous motor has two
  different rotor circuits
• Damper or amortisseur winding
• Squirrel cage bars
• Damper windings are locked on the outer periphery
  of the pole core on pole face
• The other circuit coils are wound on laminated
  core bodies called salient poles

23
Synchronous Operation

• Sometimes, pony motors are used to help get the
  synchronous motor up to speed
• Once up to speed, the pony motor becomes a DC
  generator to provide the excitation current for
  the salient poles
• The DC generator may be referred to as an exciter

24
Power Factor Correction

• Synchronous motors are often used for power
  factor correction in industrial settings
• Power factor correction is necessary because
• Current-carrying capabilities of power systems
  are reduced
• Power companies assess penalties to industrial
  users whose power factor is below a specified
  level