An electromagnetic relay (electromechanical relay) is a type of relay that can be used in either normally open or normally closed mode. Based on the design or based on the number of pins they can be of four types. They are Single Pole Single Throw (SPST), Single Pole Double Throw (SPDT), Double Pole Single Throw (DPST), and Double Pole Double Throw (DPDT) switch. We will here discuss these switches.
Electromechanical relay is mainly used for the protection of the various types of AC or DC components. Similarly, it is useful for turning ON or OFF the circuit using a low power signal. Also, it can be useful to control numerous circuits using a single signal. You can find it in instruments such as incubators, stability chambers, freeze, etc, in the health sector. However, these kinds of relays are slowly being replaced by solid-state relays, contractors, etc.
Design of the Electromagnetic Relay (EMR)
The major components of the relay are electromagnetic coil, movable armature, spring and switch point contacts.
The relay consists of two circuits. One of them is the control circuit and the other one is the load circuit. The control circuit consists of the electromagnetic coil. When the DC voltage is supplied to the circuit, the current flows through the coil causing the formation of the magnetic field.
The load circuit consists of the armature and the spring. The movement of the armature is controlled by the magnetic field of the coil and the spring attached to it. Besides these, it also consists of load and the power supply (AC/DC) to supply voltage to the load.
Working of the Electromagnetic Relay (EMR)
The relay can be operated in two ways. One of them is the normally open (NO) mode. The other one is the normally closed (NC) mode.
1. Normally Open (NO) Mode
The normally open (NO) means that when the system is not energized the circuit is open. Here in the figure, the movable armature is raised up. So, there is no contact between the two switch contacts.
When the DC power supply is given to the control circuit, the magnetic coil gets energized. Thus, this pulls the movable armature towards itself. So, the two switch contacts come in contact with each other and the load circuit becomes closed.
Now, the current flows in the load circuit, and the load gets activated.
One end of the spring is fixed at a fixed point. Similarly, the other end is in contact with the movable armature. When the power in the control circuit is cut off, the magnetic coil gets demagnetized. So, the spring pulls back the armature. Thus it goes back to the normally open condition.
2. Normally Closed (NC) Mode
The normally closed (NC) means that when the system is not energized the circuit is closed. Here in the figure, there is contact between the two switch contacts.
When you supply DC voltage to the control circuit, the magnetic coil gets energized. Thus it pulls the movable armature towards itself and the load circuit becomes open. Similarly, when you cut off the DC voltage from the control circuit, the coil becomes demagnetized. Thus, the armature returns back to its original stage.
Types of Electromagnetic Relay
Based on the number of pins present, the relay can be further classified into the following types.
1. Single Pole Single Throw (SPST)
SPST consists of 4 pins in total. Two pins come from the coil. The remaining two pins come from the point A and B in the given figure. On applying the voltage across the coil, the magnetic field is created. Thus, the movable armature changes its direction from ON to OFF position or vice versa.
2. Single Pole Double Throw (SPDT)
SPDT consists of 5 pins in total. Two pins come from the coil. The remaining three pins come from the point A, B and C in the given figure. Here a common terminal A comes in contact with either terminal B or C based on the activation or the deactivation of the relay.
3. Double Pole Single Throw (DPST)
DPST consists of 6 pins in total. Two pins come from the coil. The remaining four pins come from the point A, B, C, and D in the given figure. DPST is equivalent to two SPST switches activated by a single coil.
4. Double Pole Double Throw (DPDT)
DPDT consists of 8 pins in total. Two pins come from the coil. The remaining six pins come from points A to F. DPDT is equivalent to two SPDT switches activated by a single coil
Advantages of Electromagnetic Relay
Some of the advantages of the electromagnetic relay are as follows.
- It is simple and easy to operate.
- It is the cheapest type of relay available in the market.
- It can operate and reset quickly.
- Since the electromagnetic relay does not undergo much voltage drop, it does not produce much heat. So, you do not need to add a heat sink.
- Unlike solid-state relay, there is no chance of leakage current. Thus, there is a low chance of electric hazards to the operator.
- The contact can be both AC or DC.
- It is robust, compact, and small in size.
Disadvantages of Electromagnetic Relay
- The continuous physical contact within the relay for the purpose of activation and the deactivation can cause wear and tear in the long run.
- It has got a low life span as it undergoes wear and tear.
- In comparison to other types of relays, it has got a lower efficiency.
- A single relay can have a single function. In other words, multifunctioning is not possible.
- The directional feature is absent in the electromechanical relay.
- Periodic maintenance and calibration of the spring are needed.
There are many variations in the normal relay. Some of them operate in the normally closed mode while the others operate in the normally open mode. Similarly, they can have 4, 5, 6, or 8 pins. So, I will not be able to precisely tell you the method of testing. For this, you need to see your electromechanical relay. On one of the sides, you will find the circuit diagram. Based on that, you need to carefully test the condition of the relay.
However, I will tell you some general ideas to check it.
1. Noise Test
- The shape of the input pins is generally different from that of the output pins, Here in our figure, the bottom two pins are for the supply of 5 VDC. On supplying the required DC voltage, the relay gets activated. During this process, there will be physical contact within, leading to the production of noise. In other words, you will hear a click sound. It means that the load circuit has been closed. Thus, it means that it operates in a normally open mode.
If your relay operates in normally closed mode then on supplying the voltage the load circuit opens. Then when you remove the voltage from the control circuit, the armature comes back to its original stage during which it makes a click sound.
2. Continuity Test
- The Single Pole Single Throw (SPST), Single Pole Double Throw (SPDT), Double Pole Single Throw
(DPST) and Double Pole Double Throw (DPDT) is in the normally open mode in the given figure.
For the SPST switch, check the continuity between A and B terminals when the relay is energized.
For the SPDT switch, check the continuity between A and C terminals when the relay is not energized. When it is energized, check the continuity between A and B terminal.
For the DPST switch, check the continuity in the AB terminals and CD terminals when the relay is energized.
For the DPDT switch, check the continuity in the AC terminals and DF terminals when the relay is not energized. When it is energized, check the continuity in the AB terminals and DE terminals.
Thus, whenever you need to check the electromechanical relay, supply and remove the DC voltage from the load circuit simultaneously, and hear the click sound. When you are able to hear a click sound, then check for the continuity in the remaining terminals. For that see the circuit diagram shown in your relay.