What is relay in electrical engineering?

An electromechanical relay (EMR) is best defined as a switch that is operated by an electromagnet. The relay turns a load circuit ON or OFF by energizing an electromagnet, which opens or closes contacts connected in series with a load.

A relay is made up of two circuits: the coil input or control circuit and the contact output or load circuit, as illustrated in Figure 1.

what is relay in electrical engineering
Figure 1

Relays are used to control small loads of 15 A or less. In motor circuits electromechanical relays are often used to control coils in motor contactors and starters. Other applications include switching of solenoids, pilot lights, audible alarms and small motors ( 1 ∕8 hp or less).

Operation of a relay is very similar to that of a contactor. The main difference between a control relay and a contactor is the size and number of contacts. Control relay contacts are relatively small because they need to handle only the small currents used in control circuits.

The small size of control relay contacts allows control relays to contain multiple isolated contacts. A relay will usually have only one coil, but it may have any number of different contacts. Electromechanical relays contain both stationary and moving contacts, as illustrated in Figure 2. The moving contacts are attached to the armature. Contacts are referred to as normally open (NO) and normally closed (NC).

what is relay in electrical
Figure 2

Operation of Relay

When the coil is energized, it produces an electromagnetic field. Action of this field, in turn, causes the armature to move, closing the NO contacts and opening the NC contacts. The distance that the plunger moves is generally short—about ¼ inch or less.

A letter is used in most diagrams to designate the coil. The letter M frequently indicates a motor starter, while CR is used for control relays. The associated contacts will have the same identifying letters.

Normally open contacts are open when no current flows through the coil but closed as soon as the coil conducts a current or is energized. Normally closed contacts are closed when the coil is deenergized and open when the coil is energized. Each contact is normally drawn as it would appear with the coil deenergized.

Relays are used to control several switching operations by a single, separate current. One relay coil/armature assembly may be used to actuate more than one set of contacts. Those contacts may be normally open, normally closed, or any combination of the two.

A simple example of this type of application is the relay control with two pilot lights illustrated in Figure given below. The operation of the circuit can be summarized as follows:

what is relay
Figure 3

With the switch open, coil CR1 is deenergized.

  • The circuit to the green pilot light is completed through normally closed contact CR1-2, so this light will be on.
  • At the same time, the circuit to the red pilot light is opened through normally open contact CR1-1, so this light will be off.
  • With the switch closed, the coil is energized.
  • The normally open contact CR1-1 closes to switch the red pilot light on.
  • At the same time, the normally closed CR1-2 opens to switch the green pilot light off.

Relay Applications

Relays are extremely useful when we need to control a large amount of current and/or voltage with a small electrical signal. The relay coil, which produces the magnetic field, may consume only a fraction of a watt of power, while the contacts closed or opened by that magnetic field may be able to conduct hundreds of times that amount of power to a load.

You can use a relay to control a high-voltage load circuit with a low-voltage control circuit as illustrated in the circuit of Figure 4. This is possible because the coil and contacts of the relay are electrically insulated from each other.

Figure 4

The relay’s coil is energized by the low- voltage (12-V) source, while the contact interrupts the high- voltage (480-V) circuit. Closing and opening the switch energizes and deenergizes the coil. This, in turn, closes and opens the contacts to switch the load on and off.

You can also use a relay to control a high-current load circuit with a low-current control circuit. This is possible because the current that can be handled by the contacts can be much greater than what is required to operate the relay coil. Relay coils are capable of being controlled by low-current signals from integrated circuits and transistors, as illustrated in Figure 5.

Figure 5

The operation of the circuit can be summarized as follows.

  • The electronic control signal switches the transistor on or off, which in turn causes the relay coil to energize or deenergize.
  • The current in the transistor control circuit and relay coil is quite small in comparison to that of the solenoid load.
  • Transistors and integrated circuits (ICs, or chips) must be protected from the brief high-voltage spike produced when the relay coil is switched off.
  • In this circuit a diode is connected across the relay coil to provide this protection.
  • Note that the diode is connected backward so that it will normally not conduct. Conduction occurs only when the relay coil is switched off; at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode.

Relay Specifications

Relay coils and contacts have separate ratings. Relay coils are usually rated for type of operating current (DC or AC), normal operating voltage or current, permissible coil voltage variation (pickup and dropout), resistance, and power.

Coil voltages of 12 V DC, 24 V DC, 24 V AC, and 120 V AC are most common. Sensitive relay coils that require as little as 4 mA at 5 V DC are used in relay circuits operated by transistor or integrated circuit chips.

Relays are available in a wide range of switching configurations. Figure 6 illustrates common relay contact switching arrangements. Like switch contacts, relay contacts are classified by their number of poles, throws, and breaks.

Figure 6
  • The number of poles indicates the number of completely isolated circuits that a relay contact can switch. The single-pole contact can conduct current through only one circuit at a time while a double-pole contact can conduct current through two circuits simultaneously.
  • A throw is the number of closed contact positions per pole (single or double). The single-throw contact can control current in only one circuit while the double-throw contact can control two circuits.
  • The term break designates the number of points in a set of contacts where the current will be interrupted during opening of the contacts. All relay contacts are constructed as single break or double break. Single-break contacts have lower current ratings because they break the current at only one point.
  • In general, relay contact ratings are rated in terms of the maximum amount of current the contacts are capable of handling at a specified voltage level and type (AC or DC).
  • The load-carrying capacity of contacts is normally given as a current value for a resistive load.
  • Inductive loads, such as transformers, act as energy storage devices and can cause excessive contact arcing when the relay breaks the circuit. For inductive type loads contacts are normally derated to 50 percent of their resistive load capacity.
  • Relay contacts often have two ratings: AC and DC. These ratings indicate how much power can be switched through the contacts. One way to determine the maximum power capacity of relay contacts is to multiply the rated volts times the rated amperes. This will give you the total watts a relay can switch.
  • For instance, a 5-A relay rated at 125 V AC can also switch 2.5 A at 250 V AC. Similarly, a 5-A relay rated at 24 V DC can switch 2.5 A at 48 V DC, or even 10 A at 12 V DC.

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