Electromagnetic relay

Electromagnetic relays
CONTENT
 Electromagnetic attraction relays
 Induction Relays
 Important Terms
Prepared By
Mr.K.Jawahar, M.E.,
Assistant Professor
Department of EEE
Electromagnetic Relays
Kongunadunadu College of Engineering and Technology Depar tment of EEE

Introduction
Role of protective relay
• Energy provides the power to progress.
• In a power system consisting of generators, transformers,
transmission and distribution circuits, it is usual that some
failure may occur somewhere in the system.
Principles reasons for clear the fault
• If the fault is not cleared quickly it may cause
unnecessaryinterruption of service to customers.
• Quick recovery from fault limits the amount of damage to
the equipment and prevents the effects of fault from
spreadinginto the system.

Basic Relay
• Relays used in power system operate by virtue of the current
and voltage supplied by current and voltage transformers
connected in various combinations to the system element that is
to be protected.
• Most of the relay in service on electric power system today are
of electro-mechanical type. they work on the following two
main operating principles:
(i) Electromagnetic attraction
(ii) Electromagnetic induction

Protective relaying
• The protective relaying is the monitoring process which
senses the abnormal conditions in a part of the power
system
and gives an alarm or isolates that part from the healthy
system.
• The relay detect the fault and supply information to the
C.B which performs the function of circuit interruption.

Electromagnetic attraction relays
• Electromagnetic attraction relays operate by virtue of an
armaturebeing attracted to the poles of an electromagnet.
Such relays may be D.C. or A.C. quantities.
• The important types of electromagnetic attraction relays are
(i) Attractedarmature type relay:

• Under normal operating conditions, the current through the coil C is
such that counterweight holds the armature in the position shown.
• When a S.C occurs, the current through the relay coil increases
sufficiently and the relay armature is attracted upwards.
• The minimum current at which the relay armature is attracted to
close the trip circuit is called pick up current.
(ii) Solenoid type relay:
• It consists of a solenoid and movable iron plunger.
• Under normal condition, the current through coil holds the plunger
by gravity or spring in the position.
• If fault occurs, the current through the relay coil becomes more than
the pickup value, causing the plunger to be attracted to the solenoid.
•The upward movement of the plunger closes the trip circuit, thus
operating the C.B. and disconnecting the faulty circuit.

(iii) Balanced beam type relay:
• It consists of an iron armature fastened to a balance beam.
• Under normal operating conditions, the current through the
relay coil is such that the beam is held in the horizontal position
by the spring.
• When a fault occurs, the current through the re-lay coil
becomes greater than the pickup value and the beam is attracted
to close the trip circuit.
• This causes the opening of the circuit breaker to isolate the
faulty circuit.

Induction Relays
• Electromagnetic induction relays operate on the principle of
induction motor and are widely used for protective relaying
purposes involving A.C. quantities.
• An induction relay essentially consists of a pivoted aluminium
disc placed in two alternating magnetic fields of the same
frequency but displaced in time and space.
• The torque is produced in the disc by the interaction of one of
the magnetic fields with the currents induced in the disc by the
other.

The following points may be noted from exp. (i) :
(a) the maximum force will be produced when the two fluxes
are 90 out of phase.
(b) The net force is the same at every instant.
(c) The direction of net force and hence the direction of motion
of the disc depends upon which flux is leading.
The following three types of structures are commonly used
for obtaining the phase difference in the fluxes and hence the
operating torque in induction relays :
(i) shaded-pole structure
(ii) watt-hour-meter or double winding structure
(iii) induction cup structure

(i) shaded-pole structure
• It consists of a pivoted aluminium disc free to rotate in the air-
gap of an electromagnet.
• One-half of each pole of the magnet is surrounded by a copper
band known as shading ring.

• The alternating flux ϕs in the shaded portion of the poles will,
owing to the reaction of the current induced in the ring, lag
behind the flux ϕu in the un-shaded portion by an angle α.
• These two A.C. fluxes differing in-phase will produce the
necessary torque to rotate the disc.

(ii) watt-hour-meter or double winding structure
• It consists of a pivoted aluminium disc arranged to rotate
freely between the poles of two electromagnets.
• The upper electromagnet carries two windings; the primary
and the secondary.
• The primary winding carries the relay current I1 while the
secondary winding is connected to the winding of the lower
magnet.
• The primary current induces e.m.f. in the secondary and so
circulates a current I2 in it.
• The flux φ2 induced in the lower magnet by the current in the
secondary winding of the upper magnet will lag behind φ1 by
an angle α.

•The two fluxes φ1 and φ2 differing in phase by α will produce
a driving torque on the disc proportional to
• Its operation can be controlled by opening or closing the
secondarywinding circuit.

(iii) induction cup structure
• It most closely resembles an induction motor, except that the
rotor iron is stationary, only the rotor conductor portion being
free to rotate.
• The moving element is a hollow cylindrical rotor which turns
on its axis.
• The rotating field is produced by two pairs of coils wound on
four poles as shown.
• The rotating field induces currents in the cup to provide the
necessary driving torque.
• Induction cup structures are more efficient torque
producers than either the shaded-pole or the watt hour
meter structures. Therefore, this type of relay has very high
speed and may have an operating time less then 0·1 second.

Important terms
(i) Pick-up current. It is the minimum current in the relay coil
at which the relay starts to operate. So long as the current in the
relay is less than the pick-up value, the relay does not operate
and the breaker controlled by it remains in the closed position.
(ii) Current setting. It is often desirable to adjust the pick-up
current to any required value. This is known as current setting
and is usually achieved by the use of tapings on the relay
operating coil.
Pick-up current = Rated secondary current of C.T. ×
Current setting

(iii) Plug-setting multiplier (P.S.M.):Itis the ratio of fault
current in relay coil to the pick-up current.
P.S.M. = Fault current in relay coil
Pick - up current
(iv) Time-setting multiplier. A relay is generally provided with
control to adjust the time of operation. This adjustment is
known as time-setting multiplier.

Reference:
• V.K.MEHTA,ROHIT MEHTA, “Principles of Power System”,
S.Chand,2018.
• B.Rabindranath and N.Chander, ‘Power System Protection and
Switchgear’, New Age International (P) Ltd., First Edition
2011.

Electromagnetic relay

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