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AC Machines

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Power losses in the brushes and slip rings is relatively small. Brushed and slip rings are in the field circuit. Field current is low. ... – PowerPoint PPT presentation

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Title: 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
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