Electromagnetic relays automatically sense faults and isolate faulty sections by opening circuit breaker contacts. There are two main types of electromagnetic relays: attracted armature and solenoid/plunger. Attracted armature relays use a counterweighted armature that bridges contacts when a fault increases current enough to attract the armature. Solenoid/plunger relays use a solenoid to attract a plunger that closes trip contacts when current exceeds the pickup value. Induction relays use electromagnetic induction to rotate a disc between electromagnets producing torque from alternating fluxes. Thermal relays use a bimetallic strip heated by overcurrent in a coil; the different expansion rates of the metals cause the strip to

1. RELAY

2. WHAT IS ELECTROMAGNETIC RELAYS
•The relay is a device that automatically senses
the fault condition and closes its contacts. This
in turn the contacts of the circuit breaker to
open and isolates the faulty section.

3. The various type of electromagnetic attraction type
relays are:
•Attracted armature type relay
•Solenoid and Plunger type relay

4. ATTRACTED
ARMATURE
TYPE RELAY

5. • It consists of a laminated electromagnet M carrying a coil C and a pivoted
laminated armature. The armature is balanced by a counterweight and
carries a pair of spring contact fingers at its free end. Under normal
operating conditions, the current through the relay coil C is such that
counterweight holds the armature in the position shown. However, when a
short-circuit occurs, the current through the relay coil increases sufficiently
and the relay armature is attracted upwards. The contacts on the relay
armature bridge a pair of stationary contacts attached to the relay frame. This
completes the trip circuit which results in the opening of the circuit breaker
and, therefore, in the disconnection of the faulty circuit.
• The minimum current at which the relay armature is attracted to close the
trip circuit is called pickup current. It is a usual practice to provide a number
of tappings on the relay coil so that the number of turns in use and hence the
setting value at which the relay operates can be varied

6. SOLENOID
AND
PLUNGER
TYPE RELAY

7. • It consists of a solenoid and movable iron plunger arranged
as shown. Under normal operating conditions, the current
through the relay coil C is such that it holds the plunger by
gravity or spring in the position shown.
•However, on the occurrence of a fault, 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
opening the circuit breaker and disconnecting the faulty
circuit.

8. INDUCTION
RELAY

9. WHAT IS INDUCTION TYPE RELAY
• The induction type relays are also called magnitude
relays. These relays work on the principle of
electromagnetic induction. In these relays, a metallic
disc is allowed to rotate between the two
electromagnets are energized with the help of
alternating currents. The torque is produced due to
the interaction of one alternating flux with the eddy
currents induced in the rotor by another alternating
flux.

10. The various type of the induction relays
•Shaded Pole Type
•Watt hour Meter Type
•Induction Cup Type

11. THERMAL
RELAYS

12. • Construction of Thermal Relay
• The construction of thermal relay is quite simple. As shown in the figure
above the bimetallic strip has two metals – metal A and metal B. Metal A
has lower coefficient of expansion and metal B has higher coefficient of
expansion.
• One heating coil is would on the bimetallic strip. When over current flows
through the heating coil, it heats up the bimetallic strip.
Due to the heat generated by the coil, both of the metals are expanded.
But expansion of metal B is more than expansion of metal A. Due to this
dissimilar expansion the bimetallic strip will bend towards metal A.

14. • The strip bends, the No contact is closed which ultimately energizes the trip
coil of a circuit breaker.
• The heating effect is not instantaneous. As per Joule’s law of heating, the
amount of
Where, I is the over current flowing through the heating coil of thermal relay.
R is the electrical resistance of the heating coil. t is the time for which the
current I flows through the heating coil. Hence from the above equation it is
clear that, heat generator by the coil is directly proportional to the time
during which the over current flows through the coil. Hence there is a
prolonged time delay in the operation of thermal relay.
That is why this type of relay is generally used where over load is allowed to
flow for a predetermined amount of time before it trips. If overload or over
current falls down to normal value before this predetermined time, the relay
will not be operated to trip the protected equipment.
A typical application of thermal relay is overload protection of electric motor.