• Protection of 3 Φ Induction Motor:
• Abnormal operating conditions:
• 3 Φ IM are most widely used for industrial applications. Different abnormal
conditions expected are
• A. Over loading: it is caused by mechanical load on the motor. Over loading
results in temperature rise of the winding and deterioration of insulation, which
may results in winding short circuits. Similar situation arises during prolonged
starting.
• Stator wing faults: mainly caused by insulation failure . Faults can be a. phase
to earth faults b. phase to phase faults c. internal faults.
• Rotor faults: these are likely to occur in wound rotor motors due to insulation
failure.
• Single phasing: it occurs when one of the supply lines gets disconnected due to
rupturing of a fuse or open circuit in one of the three supply connections. The
motor then continues to run with the power supplied through two windings,
which may over heat them.
• Unbalanced supply voltage: this causes heating up of rotor due to negative
sequence current in the stator winding.
• Supply under voltage: this causes increase in the motor current, for the same load.
• Stalling: if the motor does not start due to excessive load it draws heavy current.
• Phase reversal: if the phase sequence of the supply is reversed , the motor runs in
reverse (wrong) direction.
• Protection schemes: motor protection should be simple and economical. It should
not operate during starting and permissible overloads. The choice of motor
protection scheme depends upon
• Size of the motor, rated voltage
• Type of the motor
• Type of starter
• Cost of motor
• Type of load, starting current etc.
• Normally contactor started with H R fuse & thermal over load current
relays are for motors up to about 150 HP
• For large motors circuit breakers and associated protective relays are used.
• Table shows to give the protective schemes for different types of
abnormalities.
• Sl no Abnormal conditions Alternate forms of protections
1 Over loads a. Over load relay(associate with starter)
b. Thermal over load relay
c. Inverse over current relays
2. Stater phase and earth fault a. H.R.C fuse
b. High set instantaneous o.c relay
c. Differential protection
3 Rotor faults Instantaneous O.C.relay(for slip ring motor)
4 Single phasing a. Thermal over load relay
b. Single phase preventer
5. Unbalanced voltage Negative phase sequence relays
6 Supply under voltage a. Under voltage relay
Sl no Abnormal conditions Alternate forms of protections
7 stalling a. Thermal relays
b. Instantaneous over current relay
8 Phase reversal Phase reversal relay
• Over load protection of Induction Motor:
• It can provided by :devices which respond to motor current: eg bimetal relay,
over current relay etc.
• Devices which respond to winding temperature eg: thermistors, thermostats
embedded in slots.
• Thermal over load Protection:
• The purpose of thermal overload protection is to protect the motor insulation
from excessive thermal stresses during overloading, the temperature of the
motor winding exceeds the safe limit and the life of the insulation is reduced.
• The time take to reach the limit of temperature rise depends on the load on the
machine.
• Replica type thermal relay i.e a relay whose characteristics is an appropriate
replica of motor heating is used before the protection.
• In fig shows curve A indicates the motor heating characteristics i.e time taken to
reach permissible temperature and curve B gives the characteristics of the
replica relay.
From the characteristics it can be seen that in the event of
overloading, replica relay operates and disconnects the motor well
before the limit of safe temperature is reached.
• Protection against unbalanced supply:
• The unbalanced supply voltage results in unbalanced currents. The negative
sequence currents produce rotating magnetic field revolving at synchronous
speed in the opposite direction to that of the rotor.
• Hence double frequency currents are induced in the rotor body & conductors.
The rotor thus gets overheated and the temp may reach above the safe limit.
• The unbalanced protection provided to a motor should prevent prolonged
unbalanced condition, but should not disconnect the motor for permissible
unbalance of short duration.
• For smaller motors; normally separate phase unbalance relay are not provided.
The unbalance protection is given by bimetallic relays or thermal relays.
• For large motors: additional relay is used to protect the motor against
unbalanced supply . The secondary currents of CTs are fed to negative phase
sequence filter.
• The o/p of the negative sequence filter is given to an O.C relay ,whose setting is
based on the permissible time for percent unbalance.
• Protection against single phasing:
• During single phasing motor continues to run drawing excessive current from
the two healthy lines, which can over heat the motor windings also, since the
current are unbalanced, negative sequence component of the current can over
heat the rotor.
• In smaller motor, thermal over load relays sense the over load current in
healthy phase due to single phasing & provide the required protection
• In case of large motors, separate protection against single phasing in the form
of single phase preventer is provided.
• Fig shows the connection of a single phasing preventer: Single phasing
preventers are connected to secondaies of line CTs .
• These contain a negative sequence filter is fed to a level detector or a relay.
When the negative sequence current exceeds a pre set limit , it sends tripping
command to the starter or the circuit breaker.
• Earth fault protection:
• These faults are relatively frequent and hence protection is required against these
which is provided with the help of Earth leakage CB.
• The ground fault protection is achieved using earth leakage circuit breaker
(ELCB). When the fault current or leakage current flows through earth return
path then it form the earth fault.
• As shown in the Fig. ELCB consists of a small current transformer
surrounding live and neutral wire.
• The secondary winding of current transformer is connected to relay circuit
which can trip the CB which is connected in the circuit.
• Under normal conditions, the current in line and neutral conductor is same so
that net current (IL -IN) flowing through the core is zero. Eventually there will
not be any production of flux in the core and no induced emf. So the breaker
does not trip.
• ELCB which will typically trip in around 25 ms if current exceeds its preset
value.
• If there is a fault due to leakage from live wire to earth or a person by mistake
touching to the live terminal then the net current through the core will no
longer remain as zero but equal to IL -IN or If which will set up flux and emf in
C.T.
• As per the preset value the unbalance in current is detected by C.T. and relay
coil is energized which will give tripping signal for the circuit breaker. As C.T.
operates with low value of current, the core must be very permeable at low flux
densities.
• In case of three phase circuits, single ring shaped core of magnetic material,
encircles the conductor of all three phases as shown in the Fig. . A secondary is
connected to relay circuit. Under normal condition, the component of fluxes
due to field of three conductors are balanced and secondary carries negligible
current.
• During faulty condition, the balance is distributed and current is induced in the
secondary to trip the circuit breaker through relay.
• This method to provide earth fault protection is called core balance type
protection or zero sequence current transformer (ZSCT) protection. In case of
earth faults, to avoid burning of coils and stampings the motor must be
disconnected as quickly as possible from the supply.
• Phase reversal protection:
• The direction of induction motor depends on the direction of rotating magnetic
field produced by the stator windings.
• For a particular phase sequence RYB the motor rotates in a particular direction
due to corresponding direction of rotating magnetic field. But if any two lines are
interchanged after repairs the phase sequence reverses such as YRB.
• Then the direction of rotating magnetic field also reverses and induction motor
starts rotating in opposite direction.
• Such a change of direction is dangerous if the induction motor is used for
cranes, lifts or in threading mills etc.
• Thus to disconnect induction motor from supply, phase reversal protection is
provided.
• This protection is provided using motor driven disc working on electromagnetic
principle.
• The secondaries of two current transformers connected in two lines drive the
motor to operate the disc.
• The arrangement in such that for a normal direction of motor, disc rotates in a
particular direction which keeps the auxiliary contacts closed.
• But if there phase reversal then the torque produced reverses to rotate the disc in
opposite direction. Due to this auxiliary contacts get opened.
• This in turn either operates the circuit breaker or de-energises starter coil to
disconnect the motor from the supply.
• Thus phase reversal protection for the induction motor is achieved. Now a days
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