AC Machine Stator

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AC Machine Stator
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Currents in different phases of AC Machine
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MMF Due to a phase current
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RMF(Rotating Magnetic Field)
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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

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Squirrel Cage Rotor
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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.

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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.

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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??

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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
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Rotor Equivalent Circuit
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Induction Motor Equivalent Circuit
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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

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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

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Synchronous Machine Construction
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Synchronous Machine Equivalent Circuit
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Synchronous Machine Phasor Diagram
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Excitation and Stator induced voltage
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Effect of Load Change (Field constant)
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Effect of Field Change (Load constant)
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V curves
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Torque versus Load Angle
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Torque versus Speed
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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.
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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.