Electromagnetic induction relays are the most widely used relays for protective relaying purposes involving only ac quantities. These relays operate on the simple principle of split-phase induction motor. Actuating force is developed on a moving element, that may be a disc or other form of rotor of non-magnetic current-conducting material (such as aluminium), by the interaction of electromagnetic fluxes with eddy currents that are induced in the rotor by these fluxes.
Figure 2.6 illustrates how force is developed in a section of a rotor that is pierced by 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 shown in Fig. 2.6 may be expressed as –
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ɸ1 = ɸ1max sin ωt
ɸ2 = ɸ2max sin (ωt + θ)
where, θ is the angle by which ɸ2 leads ɸ1
It may be assumed with negligible error that the paths in which the rotor currents flow have negligible self-inductance, and therefore rotor currents are in phase with their respective induced voltages.
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Now-
i1 ∝ e1 ∝ d ɸ1/dt ∝ ɸ1max cos ωt
i2 ∝ e2 ∝ d ɸ2/dt ∝ ɸ2max cos (ωt + θ)
Since the two forces (F1 and F2) developed are in opposition, as illustrated in Fig. 2.6, therefore net force acting on the movable element is given as –
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F = (F2 – F1) ∝ ɸ2 i1 – ɸ1 i2
∝ ɸ1max ɸ2max [cos ωt sin (ωt + θ) – sin ωt cos (ωt + θ)]
or F ∝ ɸ1max ɸ2max sin θ …(2.2)
The following points may be noted from Eq. (2.2):
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(i) The net force is same at every instant as the term to r is not involved in the expression for the force developed. It is most significant. This fact does not depend upon the assumptions made in arriving at Eq. (2.2). The action of a relay under the influence of such a force is positive and free from vibration.
(ii) The greater the phase angle 0 between the two fluxes, the greater is the net force applicable to the movable element. Obviously the force developed will be maximum when phase angle θ is 90°.
(iii) Also the force developed will be more when the resistance R of the annular ring is low because i ∝ v/R i.e., the movable element must be of low resistance material such as copper or aluminium. From torque-weight ratio point of view the movable element should be of aluminium alone.
(iv) The direction of net force and hence the direction of motion of movable element depends upon which flux is leading.
ADVERTISEMENTS:
The different types of structures that have been used for obtaining the phase difference in the fluxes and hence the operating torque in induction relays are:
(i) Shaded pole structure (disc type)
(ii) Watt-hour meter or double winding structure (disc type) and
(iii) Induction cup structure.
High, low and adjustable speeds are possible in induction type relays. The ratio of reset to pick-up is inherently high in case of induction relays as compared to attraction armature relays as their operation does not involve any change in the air gap of the magnetic circuit as it is in the case of latter. The ratio lies between 95% and 100%. This is not perfectly 100% because of the friction and imperfect compensation of the control spring torque.
The accuracy of an induction type relay recommends it for protective relaying purposes. Such relays are comparable in accuracy to meters employed for registration of electrical energy consumption. This accuracy is not a consequence of the induction principle, but because such relays invariably employ jewel bearings and precision parts that minimize friction.
Induction type relays are employed extensively for protection against overloads, short circuits, and earth faults on transmission/distribution lines and in industrial plants.
(i) Shaded Pole Structure:
The shaded-pole structure, illustrated in Fig. 2.8, is usually actuated by current flowing in a single coil wound on a magnetic structure containing an air gap. The air-gap flux produced by the actuating current is split into two fluxes displaced in time and space by a so-called shading ring, generally of copper, that encircles part of the pole face of each pole, as shown in Fig. 2.8.
The disc is normally made of aluminium so as to have low inertia and, therefore, needs less deflecting torque for its movement. The two rings have currents induced in them by the alternating flux of the electromagnet and the magnetic fields developed by these induced currents cause the flux, in the portions of the iron surrounded by the rings to lag in phase by 40° to 50° behind the flux in the un-shaded portions of the pole. The driving torque is given as –
T ∝ ɸS ɸu sin θ
Assuming that the flux in the shaded portion of the pole, ɸS and the flux in the un-shaded portion of the pole, ɸu to be proportional to the actuating current in the coil, I.
T ∝ I2 sin θ
i.e., the driving torque varies as the square of the current flowing through the relay coil.
The shading rings may be replaced by coils if control of the operation of a shaded-pole relays is desired. If the shading coils are short circuited by a contact of some other relay, torque will be developed; but, if the coils are open circuited, no torque will be developed because there will be no phase-splitting of the flux. Such torque control is used where directional feature is required.
The control torque is provided with the help of a control spring attached to the disc spindle. With the movement of the disc towards closing of the contacts, spring torque increases slightly with the winding of the spring. The relay disc is so shaped that as it turns towards the pick-up position (closing of contacts), there is increase in the area of the disc between the poles of the actuating structure which causes increase in eddy currents and, therefore, increase in electrical torque that just balances the increase in the control spring torque.
The shape of the disc usually is that of a spiral. Since the shape of disc is not perfectly circular, suitable balance weight is provided on the part of the disc which has smaller area. A permanent magnet of high retentivity steel is provided to give eddy current braking to the disc. This is necessary to reduce to a minimum the over-run of the disc in case the current or voltage providing the driving torque stops before the operation has been completed. A modern induction disc relay will have an over-run of not more than 2 cycles on interruption of 20 times the setting quantity.
Modern induction disc relays are robust and reliable. VA burden depends upon rating. It is of the order of 2.5 VA. The time-current characteristics of induction relays are inverse characteristics, as shown in Fig. 2.9, i.e., the time reduces as current increases. The current setting can be changed by taking the suitable number of turns. Higher current setting will require smaller number of turns so as to give fixed ampere-turns required for developing the minimum torque for the movement of the disc.
The time setting can be changed by changing the relative position of contacts by adjusting the length of travel of moving contacts. This is known as time multiplier setting. The higher the time multiplier setting the greater is the operating time. The effect of dc offset may be neglected with inverse time single quantity induction relays, because they are generally slow. The dc offset may affect fast relays. Ratio of reset to pick-up is high (above 95%) because operation does not involve any change in air gap.
(ii) Watt-Hour Meter Structure:
Watt-hour meter type induction disc relay gives the same results as given by a shaded pole type. The construction of this relay is similar to the watt-hour meter commonly used everywhere. It consists of an E-shaped electromagnet and a U-shaped electromagnet with a disc free to rotate in between. A phase displacement between the fluxes produced by the two magnets is obtained either by having different resistance and inductances for the two circuits or by energizing them from two different sources whose outputs are relatively displaced in phase.
Many variations in the design and construction are possible to suit the required conditions. The E-shaped electromagnet carries two windings; the primary and the secondary. The primary winding carries relay current I1 while the secondary winding is connected to the windings of U-shaped electromagnet (Fig. 2.10).
The primary current induces emf in the secondary and so circulates a current I2 in it. The flux ɸ2 induced in the U-shaped or lower magnet by the current in the secondary winding of the E-shaped or upper magnet will lag behind flux ɸ1 by an angle θ. The two fluxes ɸ1 and ɸ2 induced in upper and lower magnets respectively differing in phase by angle θ will develop a driving torque on the disc proportional to ɸ1. ɸ2 sin θ.
Most of the modern induction relays are of this type. The advantage of this type of construction is that it can provide larger phase angle between ɸ 1 and ɸ 2 and thus higher torque, being proportional to sin θ. An important feature of relay of this type is that its operation can be controlled by opening or closing the secondary winding circuit. If this circuit is opened, no torque will be developed and thus the relay can be made inoperative.
(iii) Induction-Cup (Electromagnetic) Relay:
These relays operate on the same principle as the induction motor. The relay has two, four or more electromagnets energized by the relay coils. A stationary iron core is placed between these electromagnets (four in number) as illustrated in Fig. 2.11.
The rotor is a hollow metallic cylindrical cup which is free to rotate in the gap between the electromagnets and the stationary iron core. The rotating field is produced by two pairs of coils wound on four poles as shown. The rotating field induces currents in the cup causing it to rotate in the same direction.
The rotation depends on the direction of rotation of the field and the magnitude of the applied voltage and/or currents and the phase angle between them. A control spring and the back stop or closing of the contacts carried on an arm are attached to the spindle of the cup so as to prevent the continuous rotation.
The operating time characteristic depends on the type of structure. These relays have inverse-time characteristics.
Induction cup structures are more efficient torque producers than either the shaded-pole or the watt-hour-meter structures. Therefore, such relays are very fast in operation and may have an operating time of less than 0.01 second.