What is a Thermal Overload Relay?

What is a Thermal Overload Relay?


Do you know what is inevitable while you operate an electric motor?

It is overheating or thermal overload.

And we all know that overheating results in potential damage to the motor and sometimes surroundings also.

So it is crucial to have safety devices that save electric motors from overheating.

One such device is a THERMAL OVERLOAD RELAY.

This extensive guide will take you deep into the details of the thermal overload relay. What is a thermal overload relay, construction, working, functions, and some more good stuff?

Keep scrolling!

What is a Thermal Overload Relay?

A thermal overload relay is a sensory device that protects a motor from the continuous high current draw. Still, it allows a non-dangerous temporary overload such as high current flow conditions during starting. However, it will turn off the motor if the current is high enough to cause motor damage over time.

Note: A thermal overload relay can operate both automatically and manually. A switch is provided for this purpose.

Thermal overload relay is the simplest type of overload protection relay. It connects to the main circuit and provides reliable protection in case of the following events:

  • Overload
  • Phase imbalance
  • Phase loss

Below are the details of the above events:

1. Overload:

Motor overload occurs when the motor is under excessive load, and the motor starts to consume more current than the rated current resulting in overheating the motor.

The following are the leading causes of motor overloading:

  • The rotor of the motor is mechanically locked and becomes stationary by external forces.
  • The motor is frequently started or stopped.
  • Short circuit between two windings.

Note: Thermal overload relay does not protect against short circuits. It rather protects the motor from overheating caused by short circuits. To protect against short circuits, we need other protection devices such as fuse and circuit breaker.

  • When the motor is put under mechanical overload, i.e., more work is being taken from the motor than the rated load.

The additional causes of motor overload include:

  • Under or over voltage – in this case, the motor de-accelerates when the voltage supply goes off. It accelerates as soon as the voltage reestablishes to maintain the rated speed of the motor resulting in a high current draw.
  • Damaged bearings.
  • Improperly aligned shafts.
  • Poor ventilation.
  • High ambient temperature.
  • Or any combination of the above factors.

2. Phase Imbalance

When one or more of the line-to-line voltages are mismatched in a three-phase system, phase imbalance occurs. The unbalanced situation in the three-phase supply results in a negative sequence current in the stator winding, leading to overheating the motor winding.

The following are the causes of an unbalanced system:

  • Unstable utility supply.
  • Unbalanced transformer bank.
  • Uneven distribution of single-phase loads on the same power system.
  • Malfunctioning overload.
  • Broken wires, damaged contacts, or failed fuses can also cause single phasing.

3. Phase Loss

When one phase is out of service in a three-phase system, it is known as phase loss. ‘Single Phasing’ is another term used for phase loss. When phase loss occurs, the other two phases draw more current to maintain the rated load torque. This results in overheating of the motor.

The causes of single phasing include:

  • Broken wire.
  • Thermal overload.
  • Worn contact.
  • Blown fuse.
  • Mechanical failure.

Purpose/Application of Thermal Overload Relay

The primary purpose of the thermal overload relay is to protect the motor from getting overheated. It trips the motor when it overheats. The overheating of the motor causes insulation failure and results in unnecessary repairing expenses. It also reduces the life of the motor, which leads to premature failure.

Following are the conditions against which a motor should be protected:

  • Unbalancing of the three-phase system.
  • Phase loss or single phasing of the supply main.
  • Low voltage supply.
  • Sudden loss and reestablishment of voltage supply.
  • Motor shaft stalling.
  • Mechanical overload.

Another application of the thermal overload relay is to detect overload conditions as well as fault conditions.

Construction of Thermal Overload Relay

There is no rocket science behind the construction of a thermal overload relay. It is simple to understand. Following are the parts of which the relay is constructed:

1. Heating Element

The thermal overload relay consists of a heating element that is responsible for producing heat. The element is connected to the transformer and is placed right next to the bimetallic strip.

2. Bimetallic Strip

This tripping mechanism consists of a bimetallic strip. The strip is made up of two different metals (usually manganese nickel and copper plate) that are welded together along their lengths. Both the metals have different coefficients of expansion. One layer of metal has a higher coefficient of expansion and is known as the active layer, while the other one has a lower coefficient of expansion and is known as the passive layer.

3. Contacts

The thermal overload relay comprises a pair of contacts. One of the contacts is fixed while the other one moves. These contacts are responsible for tripping the current flow when it exceeds the rated value.

4. Auxiliary Contacts

There are two auxiliary contacts in a thermal overload relay. One is normally closed NC (95-96), and the other is normally open NO (97-98). The NC contact disconnects the contactor while the NO contact is used for signaling trips. NC contacts are capable of direct switching of contactor coils.

5. Terminals

The overload relay has two sets of terminals. One set of terminals is known as L1, L2, L3, and these input terminals are mounted to the contactor. The other set of terminals is T1, T2, T3, and these are output terminals that supply power to the motor.

6. Adjustable Current Setting

The overload relay comes with a rotary knob that is used to set the rated current for the motor. The upper and lower limits, which are the maximum and minimum current ratings of the relay, are provided on the relay and the rotary knob can be set in between them accordingly.

7. Auto/Manual Reset Selection

This selection button allows you to pick your priority whether you want your overload relay to reset automatically or manually after a trip.

8. Test Button

The control wiring is tested using the test button.

Working Principle of Thermal Overload Relay

Thermal overload relays have an interesting yet easy to understand the working principle. They simply work on the principle of the thermal effect of electrical energy.

The feedback from an overload relay usually takes the form of a normally closed contact in the control circuit. When the overload is deactivated or when the motor is running under normal conditions, the contact stays in the closed state. When the current sensitive thermal element senses a continuous overload inside the primary circuit, the contact opens and the contactor coil de-energizes, and the connection breaks. 

For this purpose, the current-sensitive thermal element used is a bimetallic strip, and it is placed in the motor circuit in such a way that the flow of current to the motor passes through its poles. When the current exceeds the rated value, the bimetallic strip gets heated and bends, resulting in contact breakage.

The thermal overload relay cannot run the motor alone. It works together with the contactor. Once it is connected with the contactor, it forms two necessary components for a motor starter.

The Contactor – it is starting or stopping the motor.

The Overload Relay – it is protecting the motor from continuous overload conditions.

The auxiliary contacts in the overload relay are used for feedbacking elements inside the control circuit. These auxiliary contacts tell the contactor to break the connection when they sense continuous overload.

The overload heater elements are placed in series with the contactor and the motor that is supposed to be protected. It is rated to carry the primary current of the motor.

When the trip contact is activated, the power supply to the contactor coil breaks, resulting in de-energizing the coil. The current flow towards the motor is then broken.

The higher the flow of current is, the quicker the relay trips (less tripping time as the current increases). So the time the relay takes to trip is inversely proportional to the current flowing through it. These relays are therefore known as inversely time-delayed and current dependent relays.

Internal Working of Thermal Overload Relays

By now, we already know that the working of an overload relay depends on the deflection of the bimetallic strip. We will now see what triggers the bimetallic strip to deflect:

1. Normal Conditions

Under normal conditions, i.e., when there is no overload, the heat produced inside the heating element is not enough to bend the bimetallic strip. As a result, the relay does not work.

2. Overload Conditions

When an overload occurs due to any of the aforementioned reasons, the motor draws more current than the rated current, and the temperature of the heating element increases.

The increase in the temperature of the heating element results in the deflection of the bimetallic strip. When the bimetallic strip bends, the tripping mechanism of the relay gets activated. The moving contact and the fixed contact come in connection to one another, sending a signal to the contactor to trip the connection.

Since the bimetallic strip takes time to bend, therefore relays are designed in a way that permits overcurrent for some time.

How and Why is Overload Relay Connected to the Contactor?

Overload relays have a carrier. The carrier allows it to be tightly paired with the contactor for a motor starter. An overload relay by itself cannot be considered a motor starter. Since it cannot establish or interrupt current delivered to a motor provided by the contactor. The overload’s only purpose is to protect the motor from continuous overload conditions. This is the reason a motor starter is formed of two elements. They can both make and break the connection and detect continuous overload conditions.

What is the Class Number of Thermal Overload Relay?

Overload relays are known by their class number. The class number indicates how long it takes for an overload to trip when carrying a current equal to six times its rated current.

For example, a class 10 relay will take 10 seconds or less to trip when carrying a current six times its rated value.

Similarly, it would take 20 seconds or less for a class 20 overload to trip when carrying a current equal to six times its rating.

Class 10 and class 20 overloads are the most common types of overloads used. A class 20 overload would be suitable for applications with high initial loads. A class 30 overload is suitable for motors that operate high inactivity loads, whereas; the class 5 overloads come in handy when fast tripping is required.

Manufacturers should provide a tripping graph with tripping time as the function of current. 

What is Thermal Memory and Reset Problem of Overload Relay?

There is a problem with overload relays that can affect the tripping time of the overload relays. That problem is known as ‘Thermal Memory.’ To define thermal memory, we say that,

Due to the increasing nature of the heat, if the overload was previously ever triggered, it will trip sooner because of something commonly called thermal memory. If the relay is repeatedly triggered and gets tripped, and is set to reset without correcting the fault (cause of overload), it can result in motor damage.

Ambient temperature is another factor that affects the tripping time of the overload relay greatly unless the overload includes some means to balance the ambient temperature.

Is Thermal Overload Relay Handy to Protect Against Short Circuits?

The short answer to this question is: No.

The purpose of a thermal overload relay is to protect the motor against:

  1. Phase imbalance
  2. Single phasing or phase failure
  3. Overheating due to overload

Devices like ‘Circuit Breakers’ should be used along with overload relays to provide protection against short circuits.

Note: No single device can serve the purpose of both, so it is important to install both the circuit breaker and the overload relay to ensure maximum safety from every aspect.

How Does Thermal Overload Relay Work In Case of Phase Failure?

Phase failure, aka single phasing, occurs when one of the three-phase is interrupted. Under normal circumstances, the current flowing through the overload relay to the motor is normal. But as soon as one of the phases is interrupted, the current increases to 1.73 times in the other two phases. In this situation, the motor starts to overheat. The heating element in the relay heats up and trips the connection protecting the motor against potential damage.

What are Some Other Kinds of Overload Relays?

By now, we know that overload relays protect the motor against overheating. There are other types of overload relays other than the bimetallic thermal overload relay, that are used for similar purposes. The difference between bimetallic thermal overload relays and these other types is their principle of working.

The following are other most commonly used relays.

1. Electronic Overload Relays

Electronic overload relays are also known as solid-state relays (SSR). These relays do not have a bimetallic strip. They instead use semiconductor components, temperature sensors, current transformers, switching transistors, bidirectional silicon-controlled rectifiers, etc., to make or break the circuit without contact and spark. These are, therefore, also known as CONTACTLESS SWITCH.

The microprocessor-based technology uses PTC to sense temperature and interrupt the circuit in case of overloads.

Some electronic relays come with Hall Effect sensors that sense the current amount directly.

The advantage of bimetallic strip absence is less heat dissipation inside the relay. Another advantage electronic relays have over bimetallic relays is precision. They are more precise than the conventional thermal overload relays.

Earth fault protection, motor stall protection, and data provision for diagnostics and maintenance are some other extensive features that make electronic relays stand out from the crowd.

They have a better design, and they allow starting current of the motor for a limited time period like 15 to 30 seconds without tripping them. This current is 6 to 10 times more than the rated current.

Solid State Relays are divided into different types according to their categories:

  • For Power Supply Type: The relays come in AC and DC type.
  • For Switch Type: The relays are divided into a normally open and normally closed switch.
  • For Isolation Type: The relays are classified into hybrid type, transformer isolation type, and photoelectric isolation type.

2. Eutectic Overload Relays

The tripping mechanism of eutectic overload relays comprises an eutectic alloy, a winding heater, and a mechanical device. Eutectic is an alloy of metals that melts and hardens at a specific temperature.

This alloy is enclosed in a tube, and it is connected to the heater winding. When the overload occurs, the heater winding heats up, and as a result, the eutectic alloy melts. When the alloy liquefies, the mechanical device releases and opens up the contact resulting in tripping the relay.

The eutectic overload relay is reset when the alloy cools down completely and solidifies to its original state.

3. Fridge Overload Relays

Relays used in refrigerators are known as fridge overload relays. The purpose of these relays is to provide a safe start-up to the refrigerator until the compressor starts to run at normal speed.

What are the Differences Between Thermal Overload Relay, Circuit Breakers, and Fuse?

The differences between circuit breakers, fuse, and overload relays are based on multiple characteristics. The below table explains in detail.

CharacteristicsOverload RelayCircuit BreakerFuse
PrincipleThis is a sensory device that sends signals to the circuit breaker as soon as it senses some fault in the power supply system.As soon as the circuit breaker receives a signal from the overload relay, it breaks the circuit.Fuse is similar to the circuit breaker, and its objective is to break the current supply. It works on the electrical and thermal properties of the conducting material, whereas circuit breakers work on the principle of switching and electromagnetism.
OperationRelay’s function is to send fault signals to the circuit breaker. It does not interrupt the current. It also helps in detecting the fault.The purpose of the circuit breaker is to break the circuit. It does not help in detecting the fault.Fuse does the job of both detecting the fault and interrupting the current as well. It cannot sense the overload, though.
ConstructionRelay is constructed using a bimetallic strip or sensory devices whose purpose is to send signals to the circuit breaker.A circuit breaker is constructed by a combination of electromechanical and switch and relay mechanisms. It interrupts the current when it receives signals from the relay.Fuse is constructed very simply. It is simply a piece of wire which melts when it gets overheated and, as a result, interrupts the current.
Type of DeviceRelay is used as a sensory device and a switch.A circuit breaker is used as a disconnecting device and as a switch.Fuse is just a disconnecting device and cannot be used as a switch.
UsabilityA relay can control one among many circuits.A circuit breaker can only be used one per circuit.A fuse is also used one per circuit.
As an AmplifierA relay also acts as an amplifier. It turns high-voltage signals into low-voltage signals and vice versa.A circuit breaker receives signals from a relay and cannot be used as an amplifier.Fuse cannot act as an amplifier either.
Short Circuit ProtectionAn overload relay cannot protect against short circuits.A circuit breaker protects against both overload and short circuits.A fuse interrupts the circuit in case of any fault that can harm the circuit (overload and short circuit).
Tripping TimeOverload relay provides a greater time delay in tripping. That is why they allow temporary overload, especially when the motor is starting.A circuit breaker works faster than an overload relay and slower than a fuse.Fuse is an instantaneous device that interrupts current in just 2 ms.

Summing Up

No one wants burnt-up insulation or a damaged motor. And why would anyone? After all it’s the unnecessary expenses and the consumption of precious time. This is where the thermal overload relays come in handy. They sense overload and other faults that cause the motor to overheat and trip in return to protect the motor from potential damage.

But remember, they cannot protect against short circuits, and a circuit breaker must be used for this purpose.

Multiple types of overload relays are used, with thermal overload relay being the most common one.


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