Induction Motor Protection System

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Induction Motor Protection System
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Induction Motor Protection System

• Introduction
• Three-phase induction motors are the most common
  and frequently encountered machines in the
  industry.
• An induction motor works on transforming action.
  The stator works as the primary, while the rotor
  works as the secondary. It is also called
  asynchronous motor.
• It is simple in design, rugged, low-price and has
  easy maintenance.
• It runs essentially as constant speed from
  no-load to full load.

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Induction Motor Protection System

• Its speed depends on the frequency of the power
  source.
• It is not easy to have variable speed control and
  requires a variable-frequency power electronic
  drive for optimal speed control.

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Induction Motor Protection System

• Description
•  
• To provide safety to the industrial
  motor/pump/lift Motor etc. If any of the phases,
  out of the 3 phases is missing or if the
  temperature of the motor during operation exceeds
  the threshold value, motor stops immediately.
• The system uses a 3-Phase power supply where
  three single phase transformers are connected to
  it.
• If any of the phases is not available the
  corresponding transformer stops supplying power
  to the circuit. This leads to one of the four
  relays getting switched OFF.
• The main relay which is powered through a set of
  four relays gets disconnected because of one
  relay not being powered.

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Induction Motor Protection System

• Thus the motor gets disconnected. A thermistor is
  connected to the motor body to sense the
  temperature. If the temperature increases then
  supply to the fourth relay is disconnected.
•  

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Induction Motor Protection System

• An induction motor has two main parts
• Stator - It is the stationary part of the motor.
• Rotor - It is the rotating part of the motor.

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Induction Motor Protection System
Stator has three main parts Outer Frame It
is the outer body of the motor. It protects the
inner part of the machine. Stator Core Built
up of high grade silicon steel. Carries the
alternating magnetic field. Stator winding Has
a three phase winding.
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Induction Motor Protection System

• There Are Two Types Of Rotors Which Are Employed
  In 3 Phase Induction Motor
• Squirrel
• Phase Cage Rotor or wound/ Slip Ring Rotor.
•  

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Induction Motor Protection System

• Squirrel
• It consists of a laminated cylindrical core
  having semi closed circular slots at the outer
  periphery.
• Copper or aluminum bar conductors are placed in
  these slots and short circuited at each end by
  copper or aluminum rings called short circuiting
  rings.
• The rotor winding is permanently short circuited
  and it is not possible to add any external
  resistance.
• The rotor slots are not parallel to the shaft but
  skewed to reduce humming, provide smoother torque
  for different positions of rotor and reduce
  magnetic locking of stator and rotor.

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Induction Motor Protection System

• Phase Wound Rotor
• It is also called SLIP RING ROTOR
• Consists of a laminated core having semi closed
  slots at the outer periphery and carries a
  3-phase insulated winding.
• The rotor is wound for the same number of poles
  as that of stator.
• The three finish terminals are connected together
  forming a star point and the three star terminals
  are connected to three slip rings fixed on the
  shaft.

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Induction Motor Protection System

• Slip
• Induction motor rotor always rotate at a speed
  less than synchronous speed.
• The difference between the flux (Ns) and the
  rotor speed (N) is called slip.
• Slip (Ns N) N
• Slip speed Ns N 100

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Induction Motor Protection System

• The Working of an Induction Motor
• A 3 phase AC current passing through a stator
  winding produces a rotating magnetic field. So as
  in the previous case, current will be induced in
  the bars of the squirrel cage and it will start
  to rotate.
• This is due to the rate of change of magnetic
  flux in one squirrel bar pair which is different
  from another, due to its different orientation.
• This variation of current in the bar will change
  over time. That's why the name induction motor
  used in electricity is induced in rotor by
  magnetic induction rather than direct electric
  connection.
• Such small slices of iron layers make sure that
  eddy current losses are at a minimum.

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Induction Motor Protection System

• The bars of a squirrel cage are inclined to the
  axis of rotation, or it has got a skew. This is
  to prevent torque fluctuation.
• If the bars were straight there would have been a
  small time gap for the torque in the rotor bar
  pair to get transferred to the next pair.
• This will cause torque fluctuation and vibration
  in the rotor. By providing a skew in the rotor
  bars, before the torque in one bar pair dies out,
  the next pair comes into action. Thus it avoids
  torque fluctuation.
•  

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Induction Motor Protection System

• Principle of Operation
• A rotating field is set up in the stator when a
  3- Phase supply is given.
• The stationary rotor cut the revolving field and
  due to electromagnetic induction an e.m.f. is
  induced in the rotor conductor.
• As the rotor conductor is short circuited current
  flows through them.
• It becomes a current carrying conductor in
  magnetic field and start rotating.

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Induction Motor Protection System

• Future Scope
• Single phasing and high temperature protections
  are easily achieved by this simple circuit using
  comparator and relay circuits.
• Further it can be enhanced by using current
  sensors for over load protection and phase
  sequence sensor for protecting the motor from
  applying wrong phase sequence.
• It can also modify as once the temperature comes
  to the normal position it will automatically come
  to normal operation of the motor.

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Induction Motor Protection System

•  
• Conclusion
• Induction Motor Protection System is against the
  single phasing and the high temperature is
  achieved by simply interrupting the power supply
  to the load effectively by the use of comparator
  - transistor driven relays.
• Without using microcontroller and other
  programmable devices makes the protection scheme
  simple and less complex.

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