Classification and Function of Relays

Classification and function of relaysA protection relay is a device that senses any change in the signal which it is receiving, usuallyfrom a current and/or voltage source. If the magnitude of the incoming signal is outside a presetrange, the relay will operate, generally to close or open electrical contacts to initiate some furtheroperation, for example the tripping of a circuit breaker.

3.1 Classification:Protection relays can be classified in accordance with the function which they carry out, theirconstruction, the incoming signal and the type of functioning.

3.1.1 General function:· Auxiliary.· Protection.· Monitoring.· Control.

3.1.2 Construction:· Electromagnetic.· Solid state.· Microprocessor.· Computerized.· Nonelectric (thermal, pressure ......etc.).

3.1.3 Incoming signal:· Current.· Voltage.· Frequency.· Temperature.· Pressure.· Velocity.· Others.

3.1.4 Type of protection· Over current.· Directional over current.· Distance.· Over voltage.· Differential.· Reverse power.· Other.

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Figure 1 Armature-type relayIn some cases a letter is added to the number associated with the protection in order tospecify its place of location, for example G for generator, Τ for transformer etc.Nonelectric relays are outside the scope of this book and therefore are not referred to.

3.2 Electromagnetic relaysElectromagnetic relays are constructed with electrical, magnetic and mechanical

components, have an operating coil and various contacts and are very robust andreliable. The construction characteristics can be classified in three groups, as detailedbelow.

3.2.1 Attraction relaysAttraction relays can be supplied by AC or DC, and operate by the movement of a

piece of metal when it is attracted by the magnetic field produced by a coil. There are twomain types of relay in this class.

The attracted armature relay, which is shown in figure 1, consists of a bar or plate ofmetal which pivots when it is attracted towards the coil.

The armature carries the moving part of the contact, which is closed or openedaccording to the design when the armature is attracted to the coil. The other type is the pistonor solenoid relay, illustrated in Figure 2, in which α bar or piston is attracted axially within thefield of the solenoid. In this case, the piston also carries the operating contacts.It can be shown that the force of attraction is equal to K1I2 - K2, where Κ1 depends upon thenumber of turns on the operating solenoid, the air gap, the effective area and the reluctanceof the magnetic circuit, among other factors. K2 is the restraining force, usually produced by aspring. When the relay is balanced, the resultant force is zero and therefore Κ112 = K2,

So that

In order to control the value at which the relay starts to operate, the restraining tensionof the spring or the resistance of the solenoid circuit can be varied, thus modifying therestricting force. Attraction relays effectively have no time delay and, for that reason, arewidely used when instantaneous operations are required.


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3.2 .2 Relays with moveable coilsThis type of relay consists of a rotating movement with a small coil suspended or pivoted withthe freedom to rotate between the poles of a permanent magnet. The coil is restrained by twosprings which also serve as connections to carry the current to the coil.The torque produced in the coil is given by:

T = B.l.a.N.i

Where:

T= torqueB = flux densityL =length of the coila = diameter of the coilN = number of turns on the coil i = current flowing through the coil

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Figure 2 Solenoid-type relay


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Figure 3 Inverse time characteristicFrom the above equation it will be noted that the torque developed is proportional to thecurrent. The speed of movement is controlled by the damping action, which is proportional tothe torque. It thus follows that the relay has an inverse time characteristic similar to thatillustrated in Figure 3. The relay can be designed so that the coil makes a large angularmovement, for example 80º.

3 .2 .3 Induction relaysAn induction relay works only with alternating current. It consists of an electromagnetic systemwhich operates on a moving conductor, generally in the form of a disc or cup, and functionsthrough the interaction of electromagnetic fluxes with the parasitic Fault currents which areinduced in the rotor by these fluxes. These two fluxes, which are mutually displaced both inangle and in position, produce a torque that can be expressed by

T= Κ1.Φ1.Φ2 .sin θ,

Where Φ1 and Φ2 are the interacting fluxes and θ is the phase angle between Φ1 and Φ2. Itshould be noted that the torque is a maximum when the fluxes are out of phase by 90º, andzero when they are in phase.

Figure 4 Electromagnetic forces ininduction relays

It can be shown that Φ1= Φ1sin ωt, and Φ2= Φ2 sin (ωt+ θ), where θ is the angle bywhich Φ2 leads Φ1. Then:

And


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Figure 4 shows the interrelationship between the currents and the opposing forces. Thus:

F= (F 1 -F 2 ) α (Φ2 iΦ1+ Φ1 iΦ2 )

F α Φ2 Φ1 sin θ α T

Induction relays can be grouped into three classes as set out below.

· Shaded-pole relayIn this case a portion of the electromagnetic section is short-circuited by means of acopper ring or coil. This creates a flux in the area influenced by the short circuited section(the so-called shaded section) which lags the flux in the nonshaded section, see Figure 5.

Figure 5 Shaded-pole relay

Figure 6 Wattmetric-type relay

In its more common form, this type of relay uses an arrangement of coils above and below thedisc with the upper and lower coils fed by different values or, in some cases, with just one supplyfor the top coil, which induces an out-of-phase flux in the lower coil because of the air gap. Figure6 illust rates a typical arrangement.

· Cup-type relay

This type of relay has a cylinder similar to a cu which can rotate in the annular air gap between the


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poles of the coils, and has a fixed central core, see Figure 7. The operation of this relay is verysimilar to that

Figure 7Cup-type relayOf an induction motor with salient poles for the windings of the stator. Configurations withfour or eight poles spaced symmetrically around the circumference of the cup are often used.The movement of the cylinder is limited to a small amount by the contact and the stops. Αspecial spring provides the restraining torque.

The torque is a function of the product of the two currents through the coils and the cosineof the angle between them. The torque equation is

T= ( KI1I2 cos (θ12 – Φ) – Ks ),

Where K, .Κs and Φ are design constants, Ι1 and I2 are the currents through the two coilsand θ12 is the angle between I1 and I2.

In the first two types of relay mentioned above, which are provided with a disc, the inertia ofthe disc provides the time-delay characteristic. The time delay can be increased by theaddition of a permanent magnet. The cup-type relay has a small inertia and is thereforeprincipally used when high speed operation is required, for example in instantaneous units.

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Classification and Function of Relays