type of relays

7/27/2019 Type of Relays

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Attracted Armature Type Relays



• In such relays the operation is obtained by virtue of anarmature being attracted to the poles of an electro-

magnet or plunger being drawn into a solenoid.• The force exerted on the moving element is proportional

to the square of the flux in the air-gap or the square of

the current flowing through the coil.• Single actuating quantity relay

• Respond to both AC and DC.

• In DC the torque developed is constant.• In case of AC, there are two parts (i) Constant (ii)

depending upon time and pulsating with double thesupply frequency.



• Second part is responsible for the vibration.

• It also causes the relay to hum and produce noise andcan damage the contacts of the relay.

• Such relays are usually instantaneous type but can be

made a definite time lag or inverse time lag by using an

oil dash pot or an air-escapement chamber.

• They do not have directional feature.

• Polarized coils are used to provide directional features.



Directional Relays of the Electromagnetic

Attraction Type



Induction Type Relays



Two forces are in opposition, so the net force is



• The actuating force is produced in the presence of out of phasefluxes.

• The direction of the force and hence the direction of motion of therelay’s movable member depends on which flux is leading the other.



• The sum of the instantaneous values of the two waves is 1.0at every instant.

• If θ were assumed to be less than 90 degree, the effect onFig would be to raise the zero-force axis, and a smaller per-unit net force would result.

• When θ is zero, the two waves are symmetrical about thezero-force axis, and no net force is produced. If we let θ benegative, which is to say that f 2 is lagging f 1, the zero-forceaxis is raised still higher and net force in the opposite

direction is produced.



Types of Actuating Structures

• Shaded pole structure

• Watthour-meter structure

• Induction-cup and "double-induction-loop"

structures• Single-induction-loop structure.



Shaded pole structure



• The shaded-pole structure, illustrated in

Fig. is generally actuated by currentflowing in a single coil on a magneticstructure containing an air gap. The airgap

flux produced by this current is split intotwo out-of-phase components by a"shading ring," generally of copper, that

encircles part of the pole face of each poleat the air gap



Watthour Meter Structure



Induction-cup and "double-induction-loop"

structures



Torque Relation in terms of Actuating

Quantities

• Current-Current relay: Actuated from two different current

transformer sources.

To have maximum torque angle other than 90°, one of the coil

is shunted by a resistance or a capacitor



since it is the value of θ at which maximum positive torque occurs.



• Current-Voltage Relay:• A current-voltage relay receives one actuating quantity from a

current-transformer source and the other actuating quantity from a

voltage-transformer source.

By inserting a combination of resistance and capacitance in series with

the voltage coil, we can change the angle between the applied voltage

and IV to almost any value either lagging or leading V without changingthe magnitude of IV.



Time-Current Curve

• Definite time lag

• Standard IDMT• Very inverse

• Extremely inverse

type of relays

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