All the relays operate in
response to one or more electrical quantities either to close or to open
contacts. There are really only two fundamentally different operating
principles:

- Electromagnetic Attraction, and

- Electromagnetic Induction.

Electromagnetic attraction
relays operate by virtue of a plunger being drawn into a solenoid, or an
armature being attracted to the poles of an electromagnet. Such relays may be
actuated by DC or by AC quantities. Visit Induction Relay - Construction Detail to know about constructional feature of Induction Relay.

Electromagnetic-induction
relays use the principle of the induction motor where torque is developed by
induction in a rotor; this operating principle applies only to relays actuated by
Alternating Current, and called Induction Type relays.

Induction-type relays are
the most widely used for protective-relaying purposes involving AC quantities.
They are not usable with DC quantities, because of their principle of
operation.

An Induction Type Relay is a
split-phase induction motor with contacts. Actuating force is developed in a
movable element that may be a disc or other form of rotor of non-magnetic
current-conducting material by the interaction of electromagnetic fluxes with
eddy currents that are induced in the rotor by these fluxes.

Figure below shows how force
is produced in a section of a rotor that faces two adjacent AC fluxes. Various
quantities are shown at an instant when both fluxes are directed downward and
are increasing in magnitude. Each flux induces voltage around itself in the rotor,
and currents flow in the rotor under the influence of the two voltages. The
current produced by one flux reacts with the other flux, and vice versa, to
produce forces that act on the rotor.

The quantities involved in an
Induction Type relay may be expressed as follows:

Ø

_{1}= Ø_{1m}sinwt
Ø

_{2}= Ø_{2m}sin (wt + Ɵ)
Where Ɵ is the phase angle
by which Ø

_{2}leads Ø_{1}. It may be assumed with negligible error that the paths in which the rotor currents flow have negligible self-inductance, and hence rotor currents are in phase with their voltages:
iØ

_{1}∝ d Ø_{1}/ dt ∝ Ø_{1}coswt
iØ

_{2}∝ d Ø_{2}/ dt ∝ Ø_{2}cos (wt+Ɵ)
As clear from the figure
shown above, the two forces F1 and F2 are in opposition, and consequently we
may write the equation for the net force (F) as follows:

F = (F

_{2}– F_{1})
∝ (Ø

_{2}i_{Ø1}– Ø_{1}i_{Ø2}) …………………..(1)
Putting the values of the
quantities into equation (1), we get

F ∝ Ø

_{1m}Ø_{2m}[sin (wt + Ɵ)coswt - cos (wt + Ɵ)sinwt ] ………….(2)
Therefore,

**F**

**∝**

**Ø1mØ2mSinƟ**

Thus the actuating force in
an Induction Type Relay is directly proportional to the phase displacement
between the two fluxes. For maximum actuating force, the phase displacement Ɵ
must be 90°.

Thus to summarize, actuating
force is produced in the presence of out-of-phase fluxes. One flux alone would
produce no net force. There must be at least two out-of-phase fluxes to produce
any net force, and the maximum force is produced when the two fluxes are 90° out
of phase. Also, the direction of the force-and hence the direction of motion of
the relay’s movable member-depends on which flux is leading the other.

A better insight into the
production of actuating force in the induction relay can be obtained by plotting
the two components of the expression inside the brackets of equation (2).
Figure below shows such a plot when Ɵ is assumed to be 90°. It shall be observed
that each expression is a double-frequency sinusoidal wave.

The two waves are displaced
from one another by 90° in terms of fundamental frequency, or by 180° in terms
of double frequency. The sum of the instantaneous values of the two waves is
1.0 at every instant. If Ɵ were assumed to be less than 90°, the effect on Figure
above would be to raise the zero-force axis, and a smaller per-unit net force
would result. When Ɵ is zero, the two waves are symmetrical about the
zero-force axis, and no net force is produced. If we let Ɵ be negative, which
is to say that Ø

_{2}is lagging Ø_{1}, the zero-force axis is raised still higher and net force in the opposite direction is produced. However, for a given value of Ɵ, the net force is the same at each instant.
Check out Electrical PowerSystem by C.L Wadhwa. The language used in this book is quite easy and
illustration is superb.

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