Classification and function of relays

A protection relay is a device that senses any change in the signal which it is receiving, usually from a current and/or voltage source. If the magnitude of the incoming signal is outside a preset range, the relay will operate, generally to close or open electrical contacts to initiate some further operation, 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, their construction, 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.

Figure 1 Armature-type relay

In some cases a letter is added to the number associated with the protection in order to specify 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 relays

Electromagnetic relays are constructed with electrical, magnetic and mechanical components, have an operating coil and various contacts and are very robust and reliable. The construction characteristics can be classified in three groups, as detailed below.

3.2.1 Attraction relays

Attraction 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 two main types of relay in this class.

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

The armature carries the moving part of the contact, which is closed or opened according to the design when the armature is attracted to the coil. The other type is the piston or solenoid relay, illustrated in Figure 2, in which α bar or piston is attracted axially within the field 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 the number of turns on the operating solenoid, the air gap, the effective area and the reluctance of the magnetic circuit, among other factors. K2 is the restraining force, usually produced by a spring. 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 tension of the spring or the resistance of the solenoid circuit can be varied, thus modifying the restricting force. Attraction relays effectively have no time delay and, for that reason, are widely used when instantaneous operations are required.

 3.2.2 Relays with moveable coils

This type of relay consists of a rotating movement with a small coil suspended or pivoted with the freedom to rotate between the poles of a permanent magnet. The coil is restrained by two springs 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


 T= torque

B = flux density

L =length of the coil

a = diameter of the coil

N = number of turns on the coil

 i = current flowing through the coil

Figure 2 Solenoid-type relay


Figure 3 Inverse time characteristic

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

3.2.3 Induction relays

An induction relay works only with alternating current. It consists of an electromagnetic system which operates on a moving conductor, generally in the form of a disc or cup, and functions through the interaction of electromagnetic fluxes with the parasitic Fault currents which are induced in the rotor by these fluxes. These two fluxes, which are mutually displaced both in angle and in position, produce a torque that can be expressed by

T= Κ11.Φ2 .sin θ,

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

ure 4 Electromagnetic forces in induction relays

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


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

F= (F1-F2) 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 relay

In this case a portion of the electromagnetic section is short-circuited by means of a copper 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 the disc with the upper and lower coils fed by different values or, in some cases, with just one supply for the top coil, which induces an out-of-phase flux in the lower coil because of the air gap. Figure 6 illustrates 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 poles of the coils, and has a fixed central core, see Figure 7. The operation of this relay is very similar to that

Figure 7Cup-type relay

Of an induction motor with salient poles for the windings of the stator. Configurations with four 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. Α special spring provides the restraining torque.

The torque is a function of the product of the two currents through the coils and the cosine of 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 coils and 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 of the disc provides the time-delay character­istic. The time delay can be increased by the addition of a permanent magnet. The cup-type relay has a small inertia and is therefore principally used when high speed operation is required, for example in instantaneous units.