An electromagnetic relay is a switch operated by a magnetic coil. The relay switch consists of a solenoid that has a fixed iron core and a part which is movable. A spring is often used to provide the force to hold the movable portion (the armature) away from the fixed portion (the stator or core) when the solenoid is de-energized.
When the coil is energized, the armature is pulled against the stator, closing the magnetic circuit and mechanically closing (or opening) one or more sets of contacts.
Relay switches operate electrical contacts for closing or opening a circuit, while solenoids operate a plunger to effect a mechanical operation.
Electromagnetic Relay Construction
Figure 1 shows a simple attracted-armature type of electromagnetic relay (relay switch) used to open or close an electrical circuit.
Figure 1 Electromagnetic Relay Working Principle
When current flows in the operating coil, a magnetic flux is created in the soft iron core and around the magnetic circuit, including the armature and the air gap. If the air gap between the core and the armature is not too large, most of the core flux will pass through the armature and induce polarities in the pole faces of the armature and core, creating a magnetic force that will attract the armature to the pole face.
The force of attraction between the armature and the core is greater than the force holding the armature in the open position (due to the spring). The armature will close and hold for as long as the magnetic flux is applied and is strong enough.
The force exerted on the armature in the closed position (minimum air gap) will be many times greater than when the armature is in the fully open position (maximum air gap).
For a coil connected to a direct current supply, the current will be constant for all positions of the armature, but the reluctance of the magnetic circuit will change with the length of the air gap, as the reluctance of air is much greater than the reluctance of the iron core.
For an alternating supply, conditions are somewhat different due to the coil current being dependent on the flux and the reluctance of the magnetic circuit. If the ampere-turns are great enough to create the tractive force necessary to close the armature through a large air gap, then this same force will often leave a residual flux in the magnetic circuit. This residual magnetism may be strong enough to keep the armature closed even when the coil current is switched off.
This problem can be overcome by using a non-magnetic spacing piece on one pole face, to ensure that a certain minimum air gap is left in the magnetic circuit when the armature is in the fully closed position. The length of this gap must be such that the residual magnetism is not sufficient to maintain the armature in the closed position.
Electromagnetic Relay Types
Some electrical machines are prone to failure when the supply voltage is lower than the value machine was designed for. No-volt relays, low-volt relays or brown-out relays, as they are also called, are circuit-closing relays with an operating coil connected across the supply voltage. The armature and contacts close when the supply is energized at the expected level, and are held closed as long as the voltage across the coil is above a designed value, which is some percentage of the normal circuit voltage.
Figure 2 No-Volt Relays
A brown-out is where a supply fault causes the supply voltage to be reduced, but it is not zero as it in a black-out.
When the circuit voltage falls below the minimum allowable value, the armature is released and the relay contacts open. Therefore the control circuit of the apparatus being protected stops the machine, to prevent damage.
Another common electromagnetic protective relay is the overload relay. The operating coil of this relay is connected in series, so the flux comes from the current flowing in the circuit. When the circuit current exceeds a designed value, the armature is attracted and the relay pulls a trigger to disconnect the contacts, thus disconnecting the overloaded circuit from the supply. Once triggered, an operator may need to reset the relay before the machine can be re-started.
Figure 3 Three single-phase overload relay Diagram
In this type of relay, the important feature is the length of the air gap between the core and armature poles, because this controls the value of the ampere-turns necessary to attract the armature and therefore operate the relay.
Polarized relays operate only when the polarity of a DC supply is correct. A polarized relay might be used to prevent a battery charger from attempting to charge a battery in the wrong polarity, which would not be good for the battery. Polarized relays can also be used to protect electronic equipment from reverse polarity, which can destroy electronic components. Typically, Polarized relays have an electronic diode in series with the coil, thus only allowing the coil to operate when the polarity of the voltage on the coil is correct.
Figure 4 Polarized Relay Diagram
Many other types of relay exist, for a wide range of specialized uses. Most are described in the packaging of the relay, or in some cases in a special manual.