Thermal Overload Relays [OLR]

thermal overload relay [olr] is an electrical protection device designed to monitor and protect electric motors from overheating due to excessive current flow. It serves as a safeguard against the risks posed by motor burnouts, which can occur if motors operate for prolonged periods with elevated current levels.

Overload Relay Types. These are classified into two types namely 

thermal overload relay and magnetic overload relay.

A thermal over load relay (OLR) is an electro mechanical relay which is used to protect motors from overloading (or overheating).

How Do Thermal Overload Relays Work?

Thermal overload relays operate on a simple yet ingenious principle. They incorporate a bi-metallic strip that bends when heated. This strip is in direct contact with the motor current. When the motor current exceeds the predetermined safe threshold, the strip heats up, causing it to bend and trigger a mechanical switch. This switch then cuts off power to the motor, preventing further current flow and allowing the motor to cool down.

Importance of Thermal Overload Relays

1-Preventing Motor Damage

The primary purpose of a thermal overload relay is to prevent damage to electric motors. Overheating can lead to insulation breakdown, which can, in turn, result in short circuits, fires, or even complete motor failure. By promptly shutting down the motor when excess heat is detected, these relays contribute significantly to the longevity and reliability of electrical systems.

2-Enhancing Safety

Safety is paramount in any electrical setup. Thermal overload relays play a crucial role in ensuring the safety of personnel and property. By preventing motors from running at unsafe temperatures, the risk of fires or electrical shocks is substantially reduced, safeguarding both people and equipment.

3-Energy Efficiency

Beyond safety, thermal overload relays contribute to energy efficiency. Motors that operate within their optimal temperature range consume less energy and operate more efficiently. This not only reduces operational costs but also has positive environmental implications by minimizing energy wastage.

Applications of Thermal Overload Relays

1-Residential Use

In residential applications, thermal overload relays are commonly found in various appliances, including refrigerators, air conditioners, and washing machines. These relays prevent the motors in these appliances from overheating and ensure their smooth operation.

2-Industrial Sector

The industrial sector heavily relies on electric motors to power machinery. Thermal overload relays are vital in this context, protecting motors that drive conveyor belts, pumps, compressors, and other essential equipment from overheating and potential breakdowns.

3-Commercial Buildings

In commercial buildings, such as office complexes and shopping malls, thermal overload relays are integral to the HVAC (heating, ventilation, and air conditioning) systems. They ensure that the motors driving air circulation remain within safe temperature limits, contributing to a comfortable and secure environment.

types of OLR

There are three types of thermal overload relays –




Bimetallic thermal overload relays (sometimes referred to as heater elements) 

FAQs Exam point-

Can thermal overload relays be reset after activation?

Yes, most thermal overload relays have a reset function. Once the motor has cooled down, the relay can be manually reset to allow normal operation.

Are thermal overload relays suitable for both small and large motors?

Yes, thermal overload relays come in a range of sizes to accommodate motors of various capacities, from small household appliances to large industrial machinery.

Do thermal overload relays require regular maintenance?

Thermal overload relays are designed to function reliably over extended periods. However, periodic checks and tests are recommended to ensure their proper operation.

Can thermal overload relays be used with both AC and DC motors?

Yes, thermal overload relays can be used with both AC and DC motors, provided they are appropriately selected based on the motor type and specifications.

Which relay is used in DOL starter?

 Thermal Overload Relay 

What is 95 and 96 on a contactor?

The contact 95-96 is used to control the load contactor

What is a Class 20 overload relay?

Class 10 overload will trip in 10 seconds or less (depending on the percentage variation of AMP pull) and a Class 20 will trip in 20 seconds or less.

What is the maximum size of a DOL motor?

maximum motor size of 10Kw

What is the formula for overload relay?

Thermal Overload Relay setting = 120%x Full Load Current(Phase) Max. Thermal Overload Relay setting = 120%x4 = 4 Amp

What is the maximum current of relay?

Maximum 2A DC at 110 Volt

What is the amp range of overload relay?

Range Overload Relays (0.10 to 93 AMPS)

What is IEC in motor?

International Electrotechnical Commission (IEC)

What is minimum current relay?

minimum switching current = 10mA

What is relay rating?

Relays often have two ratings: AC and DC. These rating indicate how much power can be switched through the relays. This does not necessarily tell you what the limits of the relay are. For instance, a 5 Amp relay rated at 125VAC can also switch 2.5 Amps at 250VAC.

What is IE1 IE2 IE3 IE4 in motors?

IE1 = Standard EfficiencyIE2 = High EfficiencyIE3 = Premium EfficiencyIE4 = Super Premium Efficiency.

What is the size of relay?

2.8mm, 4.8mm and 9.5mm.

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