Introduction to Industrial Automation and Process Control: Relays

Relay

A relay is an electrical switch that uses an electromagnet to move the switch from the off to one position instead of a person moving the switch. It takes a relatively small amount of power to turn on a relay but the relay can control something that draws much more power. Ex: A relay is used to control the air conditioner in your home. The AC unit probably runs off of 220VAC at around 30A. That's 6600 Watts! The coil that controls the relay may only need a few watts to pull the contacts together.

A relay is defined as an electrically controlled device that opens and closes electrical contacts, or activates and deactivates operation of other devices in the same or another electrical circuit. Two types of relay technology are available, mechanical and solid state. A mechanical relay is essentially a combination of an inductor and a switch, where the electromagnetic force of the inductor causes a switch to change position. A solid state relay accomplishes the same function with semiconductor devices changing impedance to effectively activate or deactivate a circuit open or closed.

Relays are amazingly simple devices. There are four parts in every relay:

· Electromagnet

· Armature that can be attracted by the electromagnet

· Spring

· Set of electrical contacts

The following figure shows these four parts in action:

In this figure, you can see that a relay consists of two separate and completely independent circuits. The first is at the bottom and drives the electromagnet. In this circuit, a switch is controlling power to the electromagnet. When the switch is on, the electromagnet is on, and it attracts the armature (blue). The armature is acting as a switch in the second circuit. When the electromagnet is energized, the armature completes the second circuit and the light is on. When the electromagnet is not energized, the spring pulls the armature away and the circuit is not complete. In that case, the light is dark.

When you purchase relays, you generally have control over several variables:

The voltage and current that is needed to activate the armature
The maximum voltage and current that can run through the armature and the armature contacts
The number of armatures (generally one or two)
· The number of contacts for the armature (generally one or two -- the relay shown here has two, one of which is unused)
· Whether the contact (if only one contact is provided) is normally open (NO) or normally closed (NC)

Operating Principles

There are really only two fundamentally different operating principles:

(1) Electromagnetic attraction, and

(2) Electromagnetic induction.

Electromagnetic attraction relays operate by virtue of a plunger being drawn into a solenoid, or an armature being attracted to the poles of an electromagnet. Such relays may be actuated by d-c or by a-c quantities. Electromagnetic-induction relays use the principle of the induction motor whereby torque is developed by induction in a rotor; this operating principle applies only to relays actuated by alternating current, and in dealing with those relays we shall call them simply "induction-type" relays.

Poles & Throws

Relays have the exact working of a switch. So, the same concept is also applied. A relay is said to switch one or more poles. Each pole has contacts that can be thrown in mainly three ways. They are;

· Normally Open Contact (NO) – NO contact is also called a make contact. It closes the circuit when the relay is activated. It disconnects the circuit when the relay is inactive.

· Normally Closed Contact (NC) – NC contact is also known as break contact. This is opposite to the NO contact. When the relay is activated, the circuit disconnects. When the relay is deactivated, the circuit connects.

· Change-over (CO) / Double-throw (DT) Contacts – This type of contacts are used to control two types of circuits. They are used to control a NO contact and also a NC contact with a common terminal. According to their type they are called by the names break before make and make before break contacts.

Relays are also named with designations like

· Single Pole Single Throw (SPST) – This type of relay has a total of four terminals. Out of these two terminals can be connected or disconnected. The other two terminals are needed for the coil.

· Single Pole Double Throw (SPDT) – This type of a relay has a total of five terminals. Out of these two are the coil terminals. A common terminal is also included which connects to either of two others.

· Double Pole Single Throw (DPST) – This relay has a total of six terminals. These terminals are further divided into two pairs. Thus they can act as two SPST’s which are actuated by a single coil. Out of the six terminals two of them are coil terminals.

· Double Pole Double Throw (DPDT) – This is the biggest of all. It has mainly eight relay terminals. Out of these two rows are designed to be change over terminals. They are designed to act as two SPDT relays which are actuated by a single coil.

Click here to learn about timer relays

Before going on to a deeper classification of the relays there are some basic relay circuits that must be kept in our mind. They are:

· Voltage Suppression Relays Image

As relays are used in industrial purposes very often, they are mostly controlled with the help of computers. But when relays are controlled with such devices, there will surely be the presence of semi-conductors like transistors. This will in turn cause the presence of voltage spikes. As a result, it is really necessary to introduce voltage suppression devices; otherwise they will clearly destroy the transistors.

This voltage suppression can be introduced in two ways. Either the computer provides the suppression or the relay provides the suppression. If the relay provides the suppression they are called voltage-suppression relays. In relays voltage suppression is provided with the help of resistors of high value and even diodes and capacitors. Out of these diodes and resistors are more commonly used. Whatever device is used, it will be clearly stated in the relay. Take a look at the diagram of a voltage suppressed relay with the help of a diode.

· De-spiking Diode Relays Image

A diode in the reverse-biased position is connected in parallel with the relay coil. As there is no flow of current due to such a connection, an open circuit of the relay will cause the current to stop flowing through the coil. This will have effect on the magnetic field. The magnetic field will be decreased instantly. This will cause the rise of an opposite voltage with very high reverse polarity to be induced. This is mainly caused because of the magnetic lines of force that cut the armature coil due to the open circuit. Thus the opposite voltage rises until the diode reaches 0.7 volts. As soon as this cut-off voltage is achieved, the diode becomes forward-biased. This causes a closed circuit in the relay, causing the entire voltage to pass through the load. The current thus produced will be flowing through the circuit for a very long time. As soon as the voltage is completely drained, this current flow will also stop.

· De-spiking Resistor Relays Image

A resistor is almost efficient as that of a diode. It can not only suppress the voltage spikes efficiently, but also allows the entire current to flow through it when the relay is in the on position. Thus the current flow through it will also be very high. To reduce this, the value of the resistance should be as high as 1 Kilo Ohm. But, as the value of the resistors increases the voltage spiking capability of the relay decreases.

Various Relay that are used in Industry

1. Latching Relay:- Latching relays are also called impulse relays. They work in the bi-stable mode, and thus have two relaxing states. They are also called keep relays or stay relays because as soon as the current towards this relay is switched off, the relay continues the process that it was doing in the last state. This can be achieved only with a solenoid which is operating in a ratchet and cam mechanism. It can also be done by an over-center spring mechanism or a permanent magnet mechanism in which, when the coil is kept in the relaxed point, the over-center spring holds the armature and the contacts in the right spot. This can also be done with the help of a remnant core. In the ratchet and cam method, power consumption occurs only for a particular time. Hence it is more advantageous than the others.

2. Reed Relay:- These types of relays have been given more importance in the contacts. In order to protect them from atmospheric protection they are safely kept inside a vacuum or inert gas. Though these types of relays have a very low switching current and voltage ratings, they are famous for their switching speeds.

3. Polarized Relay:- This type of relay has been given more importance on its sensitivity. These relays have been used since the invention of telephones. They played very important roles in early telephone exchanges and also in detecting telegraphic distortion. The sensitivity of these relays are very easy to adjust as the armature of the relay is placed between the poles of a permanent magnet.

4. Buchholz Relay:- This relay is actually used as a safety device. They are used for knowing the amount of gas present in large oil-filled transformers. They are designed in such a way that they produce a warning if it senses either the slow production of gas or fast production of gas in the transformer oil.

5. Overload protection Relay:- As the name implies, these relays are used to prevent the electric motors from damage by over current and short circuits. For this the heating element is kept in series with the motor. Thus when over heat occurs the bi-metallic strip connected to the motor heats up and in turn releases a spring to operate the contacts of the relay.

6. Mercury Wetted Relay:- This relay is almost similar to the reed relay explained earlier. The only difference is that instead of inert gases, the contacts are wetted with mercury. This makes them more position sensitive and also expensive. They have to be vertically mounted for any operation. They have very low contact resistance and so can be used for timing applications. Due to these factors, this relay is not used frequently.

7. Machine Tool Relay:- This is one of the most famous industrial relay. They are mainly used for the controlling of all kinds of machines. They have a number of contacts with easily replaceable coils. This enables them to be easily converted from NO contact to NC contact. Many types of these relays can easily be setup in a control panel. Though they are very useful in industrial applications, the invention of PLC has made them farther away from industries.

8. Contactor Relay:- This is one of the most heavy load relay ever used. They are mainly used in switching electric motors. They have a wide range of current ratings from a few amps to hundreds. The contacts of these relays are usually made with alloys containing a small percentage of silver. This is done so as to avoid the hazardous effects of arcing. These types of relays are mainly categorized in the rough use areas. So, they produce loud noises while operated and hence cannot be used in places where noise is a problem.

9. Solid State relay:- SSR relays, as its name implies are designed with the help of solid state components. As they do not have any moving objects in their design they are known for their high reliability.

10. Solid State Contactor Relay:- These relays combine both the features of solid state relays and contactor relays. As a result they have a number of advantages. They have a very good heat sink and can be designed for the correct on-off cycles. They are mainly controlled with the help of PLC, micro-processors or microcontrollers

Click here to learn about timer relays

Links to download PDF files

Understanding Relays by Kevin R. Sullivan

References(for further clarification)

How Relays Work @
www.pcbheaven.com
How Relay Switch Works by Ashok Sharma

Video Tutorials

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Relays