What are the primary working methodologies of the overload relay?

The overload relay is an electronic device that turns OFF the motor or the heater automatically when there is an overload. The overload relay turns OFF the engine when a higher voltage is applied; this is more than the suited voltage of the motor pump. The application of the overload relay can prevent the coil inside the motor from getting burnt due to the higher voltage.

Many people are in a big misconception that it is normal for the motor pump coil to get burnt with time. Each rewind of the overload relay can cost up to 7000 – 10000 INR, and hence it is difficult to change the coil of the overload relay regularly.

The burning of the coil can easily be prevented by applying an overload relay, which can turn-OFF the motor when it senses a massive electrical voltage input than usual.

The advantages of using the overload relay

• The overload relay can ensure that the motor is safe and runs for longer life and can also protect the engine from any potential damage.
• The overload relay can also prevent the loss, which can occur due to the breakdown of the water pump.
• The overload relay can protect the motor from being damaged, and hence all the people can save on their yearly budget by minimizing the cost of maintenance and repairs which they spend every year on the motor.
• As the overload relay protects the motor from variant flaws, it is evident that it increases the life-span of the engine.

What are the major types of overload relay?

There are two types of overload relay

1. Thermal overload relay and
2. Digital overload relay

The thermal overload relay trip off the motor when a higher temperature is observed, which is higher than the average temperature.

The digital overload relay monitors the input voltage and current through a digital meter and hence trip off the motor when the voltage value exceeds the average value.

The thermal overload relays were a popular choice of vendors for a more extended period. The only problem with the thermal overload relay is they are climate dependent. On the other digital overload relay does not provide any room to the factors, and hence they can be considered as a good option for modern motors.

Thermal overload relays are useful electrical devices which are used to protect the functioning of motors. It mainly prevents the damage of motor due to overload. Thus thermal overload relays are constructed based on the principle that it would cut power to the motor when too much of current flows for a particular period. Thus thermal overload relays are mostly closed relay. In a motor circuit when there is an increase in temperature, due to excessive current flow, then the relay opens based on the relay type.

Thermal overload relays are similar to the circuit breakers that you install for overload. The main difference between overload relays and circuit breakers is that circuit breakers would interrupt the circuit at the very instant when the overload occurs. But a relay waits for a considerable period and then interrupts the circuit if the overload continues. Thus relays permit overload for an extended period.

Things To Know about Thermal Overload Relays

Overload Relay Types

There are three different types of overload relays. They are detailed below.

Bimetallic Thermal Relays: This type of relays make use of a bimetallic strip which opens mechanically. This includes two types of metallic strips with different expand rates when exposed to heat. Thus the metallic strip bends when the temperature increases. A spring is attached to the metallic strip. The spring gets pulled when the excess heat makes the metallic strip to bend. Thus the contacts are pulled, and the circuit is broken. When the bimetallic strip gets cooled, then it gets its original shape.

Solid State Relays: This is an electronic device which does not contain any moving mechanical parts. The relay operates by calculating the average temperature of the motor. This is done by tracking the starting and running currents. It is better to make use of solid state relays as it is faster other than other types of relays. It also features adjustable set points and trip times. Thus they are used in explosive environments.

Temperature Control Relays: The temperature of the motor is sensed by a thermistor or resistance thermal device. These are embedded in the motor winding — the resistance of the device increases as the temperature increases. The threshold circuit detects the increase, and this opens the relay contacts.

Electrical Rating

The following are some of the electrical relay specifications.

Tripping: This is the general term used when the circuit interrupts due to overload.
Full load current range: This is the current range to be set with the relay
Temperature trip Range: This term applies to the relays which measure current instead of temperature.
Trips Class: This is the minimum time that a relay can withstand overload.

Features

Some of the essential features of thermal overload relays are listed below.

• The automatic reset is the feature of the overload relay that returns to the original position after some specified time.
• Ambient temperature compensation is a feature that works efficiently for ambient temperatures.
• Phase monitoring is done to avoid motor damage.
  The above are some of the things to know about thermal overload relays.

Read More : A Detailed Overview Of Thermal Relay

Thermal relay is an electrical component which is used in various electrical appliances. It is used for specific applications. The thermal relay works on a particular principle which one should be aware before knowing about its construction. It is used primarily in electrical fixtures to protect from overcurrent. Mostly thermal relays are installed in motors to avoid damage of motor due to power overload. Here you would know about the principle, construction, and applications of Thermal Relay.

Construction and Working of Thermal Relay  

Basic Principle

The principle on which the thermal relay works is the thermal effect with electrical energy. Different element possesses a different coefficient of thermal expansion. Thus when two metals are heated together, then one with a higher coefficient of thermal expansion will expand more than that of the other metal. This is the basic principle used with a thermal relay.

Construction And Working

The essential parts of the thermal relay required for its construction are bimetallic strips, heating coils, and current transformers. The current transformer serves as a source of electrical supply to the heating coils which generate thermal energy. This would, in turn, heat bimetallic strips. Nickel and steel alloys are used to create the bimetallic strips. These alloys are steel resistive, and it prevents from thermal aging.

The construction of the thermal relay simple; thus it is used in motors and other devices for preventing it from overload. You need a bimetallic strip of two different metals each with different coefficient of expansion. Consider metal A with a lower coefficient of thermal expansion and metal B with a high coefficient of expansion. Thus metal B would expand more when it gets heated, and the metallic strip would bend towards the metal A. Thus the No contact is closed, and ultimately the circuit breaker works as the trip coil gets energized.The heating effect created is not instantaneous, and it is based on Joule’s law of heating.

H the amount of heat is proportional to the square of current and resistance concerning time. Thus there would be a prolonged time delay for the operation of thermal lag. This type of relay is used in devices where the overload is allowed for a particular period. If the current falls in between the normal value, the relay would be not be operated. The main application of thermal relay is the protection of an electric motor from overload.

The overload relay is installed in the motor in places where temperature varies. The overload trip point would change based on the temperature of the surrounding. This would result in premature tripping. Special overload relays called as ambient compensated bimetal overload relays are used to combat this issue.

The overload relay is generally used on an inverse time curve where the tripping time becomes less when the current flow increases. They are measured based on trip class. It is nothing but the length of time it takes for the relay to open when an overload occurs. Some of the common trip classes are class 5,10,20,30. Class 5 is used for motors.
The above offers a detailed overview of thermal relays.

Read More : Guide to Understanding Switching Systems

A relay is a device used for switching and is used in many electrical as well as electronic circuits. It acts as shield guard against extra voltage, current or even heat and protects the gadget as well enable the gadget to perform with great efficiency. As per the expert’s relays are types of switch controlled by an electric current. Basically, relays can be utilized safely switch from low to high voltages and currents. Though there are many types of relays used in the various application, this short article is primarily written to educate the readers and other enthusiasts about the Thermal Relay which is widely used in the areas such as motor control, temperature control devices both in homes as well as in large factories.

Working Principle of Thermal Relays and its Construction

In general, a thermal relay also known as Thermal Overload Protection Relay (TOPR) is a simple device which offers over current protection for all motors used in various applications in homes or in the industries. Basically, this TOPR prevents the motor when it gets overloaded and produces heat as a result. Such events occur when there is an excess current drawn by the motor, and if continued for long, it can ruin the motor itself. In this context, a TOPR is installed which can sense the excess heat and switch off the motor circuit and thereby prevents serious damages.

Let us know what a thermal overload relay is all about. As the general law in physical science, various metals have a different coefficient of thermal expansion. When two different metals which are joined together are heated the metal which has a higher value of the coefficient of thermal expansion will expand more than the other one, and it causes a bend in this bimetallic strip. All the TOPR works in this principle and the switches used in these relays are fitted with the bimetallic strips so that it can trip off when the temperature goes high in the applied motor.

Let us know what a thermal overload relay is all about. As the general law in physical science, various metals have a different coefficient of thermal expansion. When two different metals which are joined together are heated the metal which has a higher value of the coefficient of thermal expansion will expand more than the other one, and it causes a bend in this bimetallic strip. All the TOPR works in this principle and the switches used in these relays are fitted with the bimetallic strips,o that it can trip off when the temperature goes high in the applied motor.

The design of the TOPR is known to be critical, and the relays are made for various specifications and hence used in different applications. Here one should remember the fact that a motor during its general operation produces a certain amount of heat due to self-induction. Hence while designing these thermal sensitive relays, a designer has to consider the optimum temperature generated by the motors and accordingly the suitable relays are to be added to the circuit. This factor is considered to be critical, and hence the buyers need to use the correctly rated relays that can produce the expected results by switching off the motor when there is an excess heat than the desirable thermal level.

Read Also : The Latest Solid State Relays