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Attracted Armature Type Relays
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• 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.
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• 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.
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Directional Relays of the Electromagnetic
Attraction Type
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Induction Type Relays
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Two forces are in opposition, so the net force is
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• 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.
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• 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.
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Types of Actuating Structures
• Shaded pole structure
• Watthour-meter structure
• Induction-cup and "double-induction-loop"
structures• Single-induction-loop structure.
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Shaded pole structure
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• 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
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Watthour Meter Structure
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Induction-cup and "double-induction-loop"
structures
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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
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since it is the value of θ at which maximum positive torque occurs.
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• 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.
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Time-Current Curve
• Definite time lag
• Standard IDMT• Very inverse
• Extremely inverse
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