AC Machines

1
Chapter 16

• AC Machines

2
Objectives

• Describe the theory of operation of the
  three-phase alternator, and calculate an
  alternators input power, output power, losses,
  output frequency, and efficiency.
• Discuss the process to connect an alternator to a
  power grid and remove an alternator from a power
  grid.
• Describe the principles of operation of the
  three-phase synchronous motor.
• State the parts of a three-phase induction motor
  and describe its principles of operation.
  Perform calculations of three-phase induction
  motor synchronous speed, rotor speed, slip, and
  efficiency.

3
Objectives

• Describe the operating principles of the
  single-phase induction motor, define the
  operating characteristics of various types, and
  perform calculation of speed, slip, and
  efficiency.
• State the operating principles of the reluctance
  motor.
• Describe a universal motor and state the
  differences and similarities between the
  universal motor and the series dc motor.

4
16-1 Introduction
5
16-2 AC Generator (Alternator)

• Output Voltage
• Important Characteristics
• Output Frequency
• Taking an Alternator Online
• Taking an Alternator Offline
• Efficiency
• Synchronous Motor

6
Output Voltage

• Faradays Law e N dF/dt
• e the induced voltage in volts (V)
• N the number of series-connected turns of wire
  in turns (t)
• dF/dt the rate of change in flux in
  Webers/second (Wb/s)

7
Fundamental ac generator.
8
AC generator with rotating field and stationary
armature.
9
Simple three-phase ac alternator and the
resulting output waveform.
10
Important Characteristics of Alternators

• The output frequency of an alternator can be
  varied only by changing the rotor speed.
• The output voltage of an alternator can be varied
  by changing either the field current or the rotor
  speed.
• The power input to the field (Vf x If) is
  dissipated entirely inside the alternator in the
  form of heat.
• None of the field power is converted to output
  power.

11
Important Characteristics of Alternators

• Power losses in the brushes and slip rings is
  relatively small.
• Brushed and slip rings are in the field circuit.
• Field current is low.
• The output current capacity of the alternator can
  be increased simply by increasing the size of the
  stator wire.
• Higher output current will cause a proportional
  increase in the input shaft torque.
• This will increase in the mechanical stress on
  the field windings.
• The rotating field causes an alternating field in
  the stator core.
• The core will suffer hysteresis and eddy current
  losses.
• The stator core must be laminated to minimize
  eddy currents.
• The magnetic field in the rotor core is constant,
  so the rotor core does not need to be laminated.

12
Two-pole three-phase alternator armature.
13
Two-pole three-phase alternator and output
voltage waveforms.
14
Output Frequency

• f (P nr) / 120
• f the frequency in Hertz (Hz)
• P the number of poles per output phase
  (dimensionless)
• nr the rotor speed in revolutions per minute
  (rpm)

15
Taking an Alternator Online

• With the alternator offline and field excitation
  set to a typical value, increase the speed of the
  alternator and prime mover until the output
  frequency of the alternator is the same as the
  power grid frequency.
• Adjust the field excitation until the output
  voltage of the alternator is the same as the
  voltage of the power grid.
• Fine-tune the rotor speed until the output phase
  of the alternator is the same as the phase of the
  power grid.
• Connect the alternator to the grid.

16
Taking an Alternator Offline

• Reduce the power output of the prime mover until
  the electrical power being transferred from the
  alternator to the grid is near zero (this is
  normally monitored using wattmeters).
• Open the circuit connecting the alternator to the
  grid.
• Switch off the prime mover and allow the
  alternator to come to a stop.

17
A 12-V dc automotive alternator system.
18
Efficiency

• ? Pout / Pin
• Pout / (Pshaft Pfield)
• (3 Vphase Iphase cos ?) / (T nr / 7.04 Vf If)
• Ploss Pin Pout
• Pshaft Pfield Pout
• (T nr / 7.04) Vf If 3 Vphase Iphase cos ?
• Losses in Alternator
• Field Loss
• Stator Copper Loss
• Stator Core Loss
• Friction and Windage Loss
• Stray Loss

19
16-3 Three-Phase Induction Motor

• Synchronous Motor Speed
• ns 120 f / P
• Slip Speed
• n ns nr
• Slip
• s n / ns (ns nr) / ns

20
Squirrel-cage rotors.
21
16-4 Single Phase Induction Motor

• Resistance-Start Split-Phase Induction Motor
• Capacitor-Start Split-Phase Induction Motor
• Permanent Split Capacitor (PSC) Motor
• Two-Value Capacitor, Capacitor-Start-Capacitor-Run
  (CSCR) Motor
• Shaded-Pole Induction Motor

22
Stator of a two-pole single-phase induction motor.
23
Single-phase motor (a) main and quadrature flux,
and (b) rotor currents.
24
Main and quadrature flux waveforms and the
resulting sum.
25
Single-phase motor with auxiliary winding.
26
Shaded-pole induction motor.
27
Shaded-pole motor, (a) main pole flux followed by
(b) shaded-pole flux.
28
Shaded-pole motor.
29
Practical Aspects of Single-Phase Induction Motors

• With the exception of the PSC and shaded-pole
  motor, all single-phase induction motors use a
  centrifugal switch connected in series with the
  auxiliary winding.
• The capacitor-start, PSC, and CSCR motors contain
  one or two capacitors.

30
16-5 Reluctance Motor
31
(a) Reluctance motor rotor construction and (b)
low-reluctance paths.
32
16-6 Universal Motor

• Series-connected motor designed for ac
  applications.
• Same operating characteristics as series dc
  motor.
• High Starting Torque
• Poor Speed Regulation
• Very High Speed
• Portable power tools and small kitchen appliances