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.

 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.

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.

 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.

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.

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.

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

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.

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.

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

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.

 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.

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.

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.

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.

Induction Motor Protection System

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