Electromechanical Relays (EMR)

Electromechanical relay (EMR) is a device that uses an electromagnet to provide the force to close or open switch contacts i.e. it is an electrically powered switch.

When the electromagnet (also called coil) is energized, it pulls down on the spring-loaded armature. Relay contacts can be described as normally open contacts (NO), which are open in the unenergized state, and normally closed contacts (NC) which are closed in the unenergized state.

The figure (b) shows a schematic symbol of a relay. By convention the symbol always shows the relay in the unenergized state; so you can easily determine NC and NO contacts from the schematic symbol.

The electrical specifications for the contacts are different from those of the coil. For the contacts, the maximum and voltage for DC and AC operation is specified. For the coil, the anticipated voltage and coil resistance are normally specified. The coil voltage and resistance can be used to calculate the steady-state coil current. In practice, it takes more voltage and current to pull in the relay contacts than it does to hold them there because the armature must be pulled in across the air gap. Therefore these quantities are called, pull-in voltage and pull-current respectively. For example the contacts of a specific 6 V relay actually close at 2.2 V and stay closed until the voltage is decreased to 1 V. The values of the voltage and current needed to keep the relay energized are called the minimum holding voltage and sealed current. You can note that, the actual pull-in voltage is much less than the rated coil voltage in this case the pull-in voltage is 2.2 V while the rated voltage is 6 V. This is to assure that the relay will pull in quickly and reliably when operated at the rated voltage.

Coil voltages are specified to be AC or DC. The different is that AC coils are constructed with shaded poles to prevent ‘buzzing’ with 60 cycle power. A shaded pole relay has a metal ring around the pole face of the electromagnet. The magnetic flux induced in this ring keeps the relay closed when the AC cycles through the 0 V.

Relays are available in various sizes, contact configurations (i.e. SPST, SPDT, DPST and DPDT) and power handling capabilities. Very small relays can plug into an IC socket and be powered directly from a digital logic gate while power relays often called a contactor is used to switch the current directly to larger machines and can handle 50 A.

Shortcomings of Electromechanical Relays (EMR)

• Since electromechanical relays are mechanical devices, the moving parts eventually wear out.
• The electrical contacts can become pitted because of arcing. The contact wear is very dependent on the electric current that is being switched. The relay contact life also depends on the type of the load being controlled e.g. inductively loads such as motors cause much arcing and pitting than resistive loads such as light and heating elements.

Related: Solid-state Relays (SSR) Features and Operation