1
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Describe the meaning of synchronous.
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Synchronous
- The term ‘syn’ means to ‘with’ and
‘chronous’ means to ‘in time’. Therefore the synchronous motor operates ‘in
time with’ the power supply.
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2
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At what principle the synchronous motor operates?
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Principle of synchronous motor
- When the three phase supply is given to the stator winding, a rotating
magnetic field of constant magnitude is produced in it.
- When the DC excitation is given to the field winding, rotor interlock
magnetically through the air gap and revolves synchronism with the rotating
magnetic poles of the stator.
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3
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Describe the function of the exciter.
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Function of exciter
- The function of the exciter is to supply magnetizing current to the
field winding of the synchronous motor.
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4
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Describe the difference between Synchronous motor
and Induction motor.
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Difference between Synchronous motor and Induction
motor
- The synchronous motor always runs as synchronous speed whereas the
speed of the induction motor always less than the synchronous speed.
- The power factor of the induction motor is always lagging whereas the
synchronous motor runs at lagging, leading as well as unity power factor.
- The DC supply is required in the synchronous motor not in the
induction motor.
- The cost of the synchronous motor is much higher than of the induction
motor of the same rating.
- The breakdown torque in the induction motor varies as the square of
the supply voltage whereas its value directly proportional to the supply
voltage.
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5
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Explain the terms : Normal excitation, Over
excitation and Under excitation
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Normal excitation
- If the DC excitation in the synchronous motor is such that the back
emf Eb is equal to supply voltage V, it is called as normal
excitation.
Over excitation
- If the DC excitation in the synchronous motor is such that the back
emf Eb is greater than the supply voltage V, it is called as over
excitation.
Under excitation
- If the DC excitation in the synchronous motor is such that the back
emf Eb is less than the supply voltage V, it is called as under
excitation.
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6
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Explain the term : Load angle or Coupling angle
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Load angle or Coupling angle
- The angle between stator pole and rotor pole of opposite polarity is
called as load angle.
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7
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Which parameters greatly affect the value of back
emf in the DC motor and synchronous motor?
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- The value of the back emf in the synchronous motor depends upon rotor
excitation only whereas its value depends upon flux per pole and speed of the
motor in the DC motor.
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8
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Which parameter is responsible for copper losses in
the synchronous motor?
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The copper loss in the synchronous motor depends upon rotor excitation
or direct voltage supplied to the field winding.
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9
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Explain the terms : Starting torque, Running torque
or Synchronous torque, Pull - in
torque and Pull - out torque
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Starting torque
- It is the torque developed by the synchronous motor when voltage is
applied to the stator terminal as per name plate rating ( or at the instant
of starting at zero speed ).
Running torque ( synchronous torque )
- It is the torque developed by the synchronous motor under running
conditions.
Pull – in torque
- The synchronous motor starts as induction motor and runs 4 – 5% below
the synchronous speed.
- When the excitation is switched on, the synchronous
motor rotor pulls into step with the synchronous rotating stator field.
- The
amount of torque at which synchronous motor pulls into steps is called as
pull – in torque.
Pull – out torque
- The maximum torque which can be developed by synchronous motor without
losing synchronism is called as pull – out torque.
OR
- It is defined as maximum steady state torque developed by the
synchronous motor, for one minute, before it pulls out of step due to
overload.
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10
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What is meaning of the slipping poles?
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Slipping
poles
- When the
angular displacement of the rotor exceeds the stability limit, the rotor
slips a pole pitch or rotor flux slips with respect to the stator flux.
- This
condition is called pole slipping.
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11
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What is condition for maximum power developed in the
synchronous motor?
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The condition for the maximum power developed in the synchronous motor
is internal angle θ is equal to load angle α.
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12
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Describe the effect on armature current of the field
current in the synchronous motor?
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The armature current increasing for both lagging and leading power
factor and its value becomes minimum at unity power factor.
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