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AC Machine Stator

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RMF(Rotating Magnetic Field) LP12. 5. Induction Motor ... are rotating with the rotor and carbon brushes that are static. LP12. 8 ... – PowerPoint PPT presentation

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Title: AC Machine Stator


1
AC Machine Stator
2
Currents in different phases of AC Machine
3
MMF Due to a phase current
4
RMF(Rotating Magnetic Field)
5
Induction Motor
  • Most popular motor today in the low and medium
    horsepower range
  • Very robust in construction
  • Speed easily controllable using V/f or Field
    Oriented Controllers
  • Have replaced DC Motors in areas where
    traditional DC Motors
  • cannot be used such as mining or explosive
    environments
  • Of two types depending on motor construction
    Squirrel Cage
  • or Slip Ring
  • Only Disadvantage Most of them run with a
    lagging power factor

6
Squirrel Cage Rotor
7
Slip Ring Rotor
  • The rotor contains windings similar to stator.
  • The connections from rotor are brought out using
    slip rings that
  • are rotating with the rotor and carbon brushes
    that are static.

8
Torque Production in an Induction Motor
  • In a conventional DC machine field is stationary
    and the current carrying conductors rotate.
  • We can obtain similar results if we make field
    structure rotating and current carrying conductor
    stationary.
  • In an induction motor the conventional 3-phase
    winding sets up the rotating magnetic field(RMF)
    and the rotor carries the current carrying
    conductors.
  • An EMF and hence current is induced in the rotor
    due to the speed difference between the RMF and
    the rotor, similar to that in a DC motor.
  • This current produces a torque such that the
    speed difference between the RMF and rotor is
    reduced.

9
Slip in Induction Motor
  • However, this speed difference cannot become zero
    because that would stop generation of the torque
    producing current itself.
  • The parameter slip s is a measure of this
    relative speed difference
  • where ns,?s are the speeds of the RMF in RPM and
    rad/sec respectively
  • nm,?m are the speeds of the motor in RPM and
    rad/sec respectively
  • The angular slip frequency and the slip frequency
    at which voltage is induced in the rotor is given
    by??

10
Induction Motor Example
A 100 hp, 8 pole, 60 Hz, 3 phase induction motor
runs at 891 rpm under full load. Determine the
synchronous speed in rpm, slip, slip frequency at
full load. Also estimate speed if load torque
becomes half of full load torque, given the fact
that torque is proportional to slip in the region
between breakdown torque or zero torque.
Solution on Greenboard
11
Rotor Equivalent Circuit
12
Induction Motor Equivalent Circuit
13
Synchronous Machine
  • The stator is similar in construction that of a
    induction motor
  • The rotor can be Salient or Non-Salient
  • Field excitation is provided on the rotor by
    either permanent or
  • electromagnets with number of poles equal to the
    poles of the
  • RMF caused by stator
  • Non-excited rotors are also possible as in case
    of reluctance motors

14
Synchronous Machine (2)
  • The rotor gets locked to the RMF and rotates
    unlike induction motor at synchronous speed under
    all load condition
  • All conventional power plants use synchronous
    generators for converting power to electrical
    form
  • They operate at a better power factor and higher
    efficiency thanequivalent induction machines

15
Synchronous Machine Construction
16
Synchronous Machine Equivalent Circuit
17
Synchronous Machine Phasor Diagram
18
Excitation and Stator induced voltage
19
Effect of Load Change (Field constant)
20
Effect of Field Change (Load constant)
21
V curves
22
Torque versus Load Angle
23
Torque versus Speed
24
Example 1
A six-pole 60 Hz synchronous motor is operating
with a developed power of 5 hp and a torque angle
of 5o. Find the speed and developed torque.
Suppose that the load increases such that the
developed torque doubles. Find the new torque
angle. Find the pull-out torque and maximum
developed power for this machine.
25
Example 2
An eight-pole, 240 V-rms, 60 Hz, delta connected
synchronous motor operates with a constant
developed power of 50 hp and a torque angle of
15o at unity power factor. Suppose the field
current is increased by 20. Find the new
torque angle and power factor. Is the new power
factor lagging or leading? Assume linear
magnetic characteristics.
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