Saturday, 29 February 2020

How works Electromechanical Relay? Construction, Types, Applications

 Electromechanical Relays

  • These relays are constructed with electrical, magnetic & mechanical components & have an operating coil & various contacts.
  • If a relay has moving parts, it is called electromechanical relay.  
  • These are very robust & reliable. Based on the construction, characteristics, these are classified in three groups.
Electromechanical Relay

Electromechanical Relay

Construction And Work Of Electromagnetic Relay

  • Relay are typically used when it is necessary to switch a small amount of power. Relay contains several electronic parts to make them work.
  • These includes an electromagnet, which controls opening and closing of the relay.
  • Next is the armature, or the moving part, which is the electronic part that opens and closes.
  • A spring is also used in relay. This is the part that forces the relay back to its original position after each revolution.
  • In addition, a set of electric contacts is needed in order to transfer the power.
  • The power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. 
  • Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
  • As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.
Construction, Working of Relay

Construction, Working of Relay

Types Of Electromagnetic relay

There are three types of elctromagnetic relay
  1. Attraction relays
  2. Relays with movable coils
  3. Induction relays

1.Attraction Relays

  • Attraction relays can be AC & DC and operate by the movement of a piece of iron when it is attracted by the magnetic field produced by a coil. 
  • There are two main types of relays:

                        1. The attracted armature type
                        2. Solenoid type relay

1. Attracted armature relays
  • Consists of a bar or plate (made of iron) that 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.

2. Solenoid type relays
  • In this a plunger or a piston is attracted axially within the field of the solenoid. In this case, the piston carries the moving contacts.
  • In order to control the value of current at which relay operates, the parameters resultant force and restraining force  may adjusted. Attraction relays effectively have no time delay and are widely used when instantaneous operation is required.

Relays with movable 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 special springs which also serve as connections to carry the current to the coil.
  • The relay has inverse type characteristic.

Induction relays
  • An induction relay works only with AC.
  • It consists of an electromagnetic system Which operates on a moving conductor, generally in the form of a DISC or CUP.
  • Classification of induction relays
  1. Shaded pole relay
  2. Watthour- meter type relay
  3. Cup type relay

1.Shaded pole relay

  • The air gap flux produced by the current flowing in a single coil is split into two out of phase components by a so called „Shading Ring‟ generally of copper, that encircles part of the pole face of each pole at the air gap.
  • The shading ring may be replaced by coils if control of operation of the shaded pole relay is desired.
  • The inertia of the disc provides the time delay characteristics

2. Watt hour –meter structure

  • This structure gets its name from the fact that it is used in watt hour meters.
  • As shown in the top figure below, it contains two separate coils on two different magnetic circuit, each of which produces one of two necessary fluxes for driving the rotor, which is also a disc.


  • This type of relay has a cylinder similar to a cup which can rotate in the annular air gap between the poles & the fixed central core. 
  • The operation of this relay is similar to that of an induction motor with salient poles for the windings of the stator. 
  • The movement of the cup is limited to a small amount by the contact & the stops. A special spring provides restraining torque.
  • The cup type of relay has a small inertia & is therefore principally used when high speed operation is required, for example in instantaneous units.

Use of Electromechanical Relay
  • Relay typically are used in modern household appliances such as hair dryers, kitchen appliances, and lights that need to be switched on and off. 
  • They are also used in cars where things need to be turned off and on.
  • In fuel pumps. 
  • Industrial Applications- Where the control of high voltage and current is intended.
  • Controlling large Power loads.

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