Overload Relay: Types, Functions & Its Applications

Electrical systems must be protected in robust ways that will guarantee the safe and continuous running of the system. Of these, the overload relay is an important device. These relays have the crucial task of protecting electrical motors and circuits against the adverse impact of overload conditions. In this blog, we will look at the various categories of overload relays, what they do, where they are used, and the protection principle that makes the electrical system secure. We will also look at features like the price of an overload relay and the quality of the overload relays available today from reputable brands.

What is Overload Relay?

Before diving into the specifics, let’s first address the basic question: What is overload relay? An overload relay can be defined as an electromechanical or electronic device which is used to protect an electric motor from overload. In other words, when the current passing through a motor is beyond the rated current for an uninterrupted period, the overload relay operates to protect the motor, apparatus or fire hazards. Besides being vital in households where motors are used, these relays are used widely in industries and business establishments.

An overload relay is a protective device in a system that regularly checks for the current. When the current is above its permissible limit, it cuts off the motor from the power supply to avoid damage to the motor and its surrounding systems.

Types of Overload Relay

As explained below, overload relays are available in different types to suit specific functions and applications.

Thermal Overload Relay

The most widely used variety is the thermal overload relay, which incorporates a bimetallic strip measuring the current amount. In the event the current rises beyond the allowed maximum, the bimetallic strip becomes hot, bends and pulls the trip. After it has become hot, these relays can be reset by hand or automatic. They are commonly applied in motor protection schemes and dependable in under-average over load situations.

Magnetic Overload Relay

Magnetic overload relays work on electromagnetic force, not heat, as most thermal relays do. A magnetic field in a coil is created when an overload is detected; this pulls a plunger to interrupt the circuit. These are renowned for their quicker response to any overload conditions, making them suitable for use in areas where quick protection is needed.

Electronic Overload Relay

Electronic overload relays provide accurate overload protection by constantly measuring motor current and evaluating it against set values. They are more accurate, and extra options like phase loss protection or temperature measuring can be added.

Function of Overload Relay

Having reviewed several types of overload relays, it is time to learn about the purpose of an overload relay as a part of the electrical system. The main role of the overload relay is to guard electrical motors and circuits against harm resulting from current overloads.

Here’s how it works:

Current Sensing: The overload relay constantly checks how much current passes through the motor. The relay notices the overload when the current increases beyond the stipulated limit.

• Delay Mechanism: In contrast with short-circuit protection devices, overload relays give a time delay before the trip. This delay is important because motors mainly give high inrush currents at starting, and this is not detrimental. The overload relay is designed to be aware of this temporary condition and does not cause a tripping effect.

• Tripping Mechanism: If the excessive current continues to flow beyond the permitted time, the relay operates a tripping device which isolates the motor from the electricity supply. It also saves time and prevents the motor from getting too hot, thus damaging the motor winding.

• Manual or Automatic Reset: The relays can be self-restoring after the overload situation has been coped with or require that it be reset manually, depending on the design.

Overload Relay Working principle

Let’s take a closer look at the overload relay working principle.

• Thermal Overload Relay Working Principle: Due to excessive current, the bimetallic strip in the relay gets heated up and bends to activate the system’s mechanism. The higher the current, the faster it bends, and the sooner the relay trips.

• Magnetic Overload Relay Working Principle: The architecture of these relays is to produce a magnetic field that is directly proportional to the current in the motor. If the current exceeds the predetermined value, the bigger force attracted to the coil moves a plunger or armature that opens the trip.

• Electronic Overload Relay Working Principle: Electronic overload relays depend on sensors and microcontrollers to detect current circulation. The overload condition occurs when the current or voltage exceeds the threshold value in a circuit. These individual relays provide the finest security and can also allow other sophisticated attributes, such as monitoring and testing from distance.

Each type of overload relay has its unique working principle. Still, the ultimate goal remains the same: to limit the effect of overload conditions on the motor and other systems associated with its operation.

Application of Overload Relay

The overload relay application is versatile and used in most manufacturing sectors. Here are some key areas where these relays are crucial:

Industrial Automation

Electric motors are the most widely used across factories, manufacturing plants and industries. These motors are protected from the effects of the constant application of heavy loads by overload relays. Thus, motors are vulnerable to overheating when not properly protected; such a case requires repair or replacement, which might be very expensive.

HVAC Systems

Electric motor control is also prevalent in heating, ventilation, and air conditioning (HVAC) equipment, and it is applied to the fan, compressor, or pump. This overload relay arrangement ensures that motors in these systems operate effectively without being subjected to conditions of overload that would otherwise compromise the systems.

Pumps and Compressors

In the pumping and compressor industries, overload relays protect various mechanisms from being overloaded and, hence, getting burnt, resulting in halted production.

Commercial Buildings

Overload relays are utilised in commercial premises concerning motors driving elevators, escalators, and ventilating equipment. The continued functioning of such motors is crucial for the safety and comfort of the inhabitants of high-rise constructions.

Agriculture

Irrigation pumps, threshers, and other agricultural equipment use overload relays to extend the motor’s life, especially when unstable power supplies.

Overload Relay Price

Overload relay price depends on the type, function, and manufacturer. Simple protective relays, such as thermal overload relays, are cheaper than such complex electronic overload relays, equipped with many functions, and provide a higher level of protection.

Also Read - What is a Thermal Overload Relay? Types and Use Cases

Conclusion

In conclusion, it is worth explaining what an overload relay is. It is an essential part that guards motors against damage due to overloads. It is very important to learn the various types of overload relay and how they operate when choosing one for use.