Thermal overload relay

Types of thermal overload relay

There are several heat radiators. The choice should depend on the features of any specific one. Thermal overload relays are manufactured in the following types:

• Biometric

  In these models, the mechanics technologically developed are moisture-sensitive and thermally expanded. The selection of its sensitivity depends on the material assigned. Such a relay is very effective when the drive engines work at a stable frequency and load. The advantage of this type is that it does not require additional power supply, and it is very easy to set up and install.

• Electronic

  Such overload protection relays are more accurately given. There is a high level of sensitivity. Feedback is ensured, and smart microphones can be used to detect abnormal work of motors. An electronic thermal relay has a broader range of controlled currents and higher accuracy due to the constructions of amplifiers, filters, AD converters, etc. Such relay systems consume much energy because the setting parameters are usually changed through control panels.

• With single heat sensors

  There is one heat sensor fastening on the pole of the contactor, and there are relays that contain one heat sensor on each pole. One applies a single thermal overload relay where low accuracy is not required. Its cheapness and ease of installation make it very popular. A double relay is used in machines operating at a higher overload risk.

• With dual temperature

  These models combine a bi-metallic design with one that contains temperature-sensing elements. A dynamic temperature change is more sensitively detected. So, they make the relays very useful for applications with variable motor loads. And while fairly complex regarding construction and installation, these are the most accurate.

Industrial applications of thermal overload relay

• Induction motors

  Thermal overload relays are mostly found in three-phase induction motors. These motors are spread widely in many industries and are strategically important. About half of the electric motors in the European Union are three-phase induction motors. In this case, the main task of the relay is to protect the motor from overheating due to excessive load.

• Fans

  Industrial fans play a vital role in the cooling system of production cycles. They are used, e.g., in the electronic cooling process. Many systems employ thermal overload relays as protection devices against fan failure. This is often related to mechanical jamming or motor overload. With such a setup, overheating is prevented, and system downtime is avoided.

• Pumps

  Heating, ventilation, and air conditioning systems use pumps to circulate coolant, water, and refrigerant. For other industries, there is a requirement for the same thermal overload protection because of varying load conditions. For example, a pump in a reservoir can suddenly receive a high flow rate from a nearby process within a short period. A relay stops this situation from resulting in motor damage.

• Compressors

  Thermal overload relays are also used in thermal protection for mechanical heat pumps and chillers. Compressors work with variable loads, which gives a big chance for both under- and overloading. Thermal overloads help recognize overheating with electronic relays, which are more accurate. Thus, an important process is being prevented from failing - the production of refrigerants.

Product specifications and features of thermal overload relay

Technical specifications

• Current rating

  The key difference between the various thermal overload relays is the range of the rated currents. Some models are designed for low currents, while others are suitable for high-power motors. The selection of the relay highly depends on the specific application. So, it should be done with caution.

• Reset

  Manual resetting is typical for some models, where the operator has to reset the system when tripping. Relays that are automatic and self-resetting are also produced. Such units' operation is resumed after overload cessation without the need for a manual reset. This feature is crucial for unattended facilities.

• Temperature coefficient

  This indication shows how sensitive the relay is to temperature changes. It is given in percent per degree. This means that relays with a smaller coefficient are less sensitive to ambient temperature changes. Thus, they provide correct operation under conditions of fluctuating temperature.

• Delay

  There can be a built-in short-circuit protection delay in some models. This delay can be from a few seconds up to a few minutes. At this time, the relay is not deactivating even if there is an overload. Temporary overloads are usually normal. They often occur at the motor start or during a process peak.

How to install

• Preinstallation checks

  Establish construction continuity by isolating all equipment on which the relay is to be fitted. Determine the relay location where the incoming current will be monitored.

• Mounting

  Mount the relay with the relevant instructions on the materials, if any. There should be at least 50mm spacing around the relay to allow airflow.

• Wiring

  There should be wiring done. Connect the power supply cables to the terminal labeled L1, L2, and L3. Connect the motor cables to the terminal marked T1, T2, and T3. In this way, the relay and disconnecting devices are wired in series. Check whether the wiring follows the manufacturer.

• Setting

  Set the current range using the adjusting knob. This range should correspond to the full load current of the motor. Now, the thermal settings should be done. The relay manuals indicate the required temperature ranges.

• Testing

  Turn on the system after installing and setting everything up. Check the relay operation by monitoring the motor. If there is an overload, the relay should trip, cutting off the supply. With a multimeter, it is also possible to check out the relay functioning correctly.

How to use

• Monitoring

  Keep constant observation on the motor operating conditions and ambient environment. If an overload occurs, the relay will recognize it and disconnect the supply.

• Automatic reset (if applicable)

  If it is an autocorrecting relay, it will automatically reset after returning to the permissible temperature range. Manual trips need manual resets.

• Alarm systems

  Many modern relays provide feedback not only to disconnect the supply but also to indicate or log the event on a connected system. It can be an audible alarm or an interface with a control system.

Maintenance

• Regular inspections

  Check the relay regularly. Inspect working conditions and signs of physical damage or wear and tear.

• Dust and debris

  Eliminate all dust and debris on the relay and surrounding area. Use a dry cloth or compressed air for this.

• Functional tests

  Do regular functional tests to check if the relay operates correctly. The device may be faulty if there is any failure.

Quality and safety considerations of the thermal overload relay

Quality considerations

• Temperature-sensitive elements

  It can be materials that change shape or size when heated, e.g., bimetallic strips. They should not be affected by temperature changes.

• Precision

  There should be high accuracy in the relay current settings. This means that the fuses must be of good quality.

• Mechanical durability

  The relay should be vibration- and shock-resistant since they are in some critical industrial environments. Besides, all components must be robust and withstand wearing.

• Corrosion resistance

  Corrosion can cause damage to the electrical contacts and impact the thermal sensing mechanisms. It can be eliminated by using coatings and corrosion-resistant materials.

• Humidity seals

  High-quality relays are sealed against moisture ingress. This is done by gaskets or epoxy coatings. Humidity can cause internal short circuits and reduce reliability.

Safety consideration

• Electrical insulation

  Proper electrical insulation is a must between live parts and the relay housing. This will avoid electric shocks. To achieve electrical safety, the relay must be properly grounded.

• Overvoltage and undervoltage conditions

  Thermal relays must also withstand overvoltage and undervoltage. Excessive voltage or low voltage can endanger electronic components inside the relay and cause malfunction.

Q&A

Q1. Which materials are commonly used in thermal overload relays?

A1. Those materials frequently used in heat sensor components of thermal overload relays include steel, copper, brass, and aluminum. Bi-metallic strips are normally made of two different metals, e.g., copper and steel, which are bonded together. The different expansion coefficients of these metals allow the strip to bend when the temperature increases. This trips the overload condition.

Q2. How does one reset a manual thermal overload relay?

A2. First, the system has to be isolated from the electric current and allow the motor to cool down. This person will then use the reset tool or knob. The reset button should be pressed so that the contacts are closed again. The system will then be reconnected to the electric current and allowed to function as normal.

Q3. Do thermal overloads have an expiration date?

A3. There are some which are autocorrecting. They reset themselves after the relay cools down to an acceptable temperature. Manual resetting relays require human intervention to reset the overload. A thermal overload relay should generally last between five and ten years. Its lifespan will, however, depend on how overloaded the motor frequently is and the working environment condition.

Q4. Is it necessary to use a thermal overload?

A4. A thermal overload is normally crucial in motor-driven equipment. These ensure that the motor does not damage from overheating due to overloading or mechanical failures. They disconnect the power and allow the motor to cool down whenever an overload condition is registered.