The main purpose of a thermal overload relay is to protect a motor from excessive heating because of overcurrent conditions. As soon as a motor is overloaded due to mechanical binding, excessive loads,
imbalance, or insufficient cooling, this excess current increases in the motor. As a consequence, the motor winding and insulation get heated because of this excess current. As a result, if this problem continues without being interrupted, they can get damaged or even cause a disaster in form of a fire hazard. A thermal relay is designed to stop this problem before a disaster takes place.

In contrast to short-circuit faults, which require very high levels of current and hence an immediate disconnect via a circuit breaker or a fuse, an overload fault grows in a more gradual manner. As such, thermal overload relays have a time-dependent response curve very much attuned to the thermal curve of a motor. Such a coordination is critical in an Australian power system, where working in accordance with AS/NZS wiring standards demands a coordinated function for both short-circuiting and overload protection.

Operating Principle

The working principle of a thermal overload relay is based on the heating effect of an electric current, which is explained by the I²R law in most descriptions. Current passing through a conductor heats it in proportion to the square of the current and the resistance of a conductor. The law is applied in thermal overload relays by passing a motor current through a heating element or a current-sensitive device.

One of the important characteristics of thermal overload relays is their inverse time-current curve. This implies that as the overload current grows in intensity, the time taken for tripping will be less. For instance, small overloads may take a number of minutes to trip, but large overloads will have a very short tripping time. Such a characteristic is particularly important during motor starts, where inrush current may be a short time multiple of the normal current. The thermal overload relay can withstand such normal inrush currents and, at the same time, react to abnormal overcurrents.

Characteristics of Thermal Memory

In many thermal overload relays, thermal memory can be observed. When a relay is subjected to an overload current, making it hot, it does not readily return to room temperature when the motor is stopped. If, in such a case, a start button is pressed before it gets a chance to cool down, it will readily trip if an overload existed. Such a characteristic replicates the nature of a motor.

Construction and Internal Features

A thermal overload relay consists of a heating element, a temperature-sensitive device, a mechanical trip, auxiliary contacts, and a reset circuit. In a conventional thermal relay, a bimetallic strip with two different metals bonded together is used. The two metals have different thermal expansion coefficients. As a result, when heated, this device will curve.

This deflection is mechanically coupled with a trip mechanism, which alters the status of the relay_contacts when a given temperature is attained. The auxiliary contacts are wired into the motor control circuit and not into the main power circuit. With the relay tripped, this alters the status of these contacts, which in return de-energizes the coil of the related main contactor, thus opening it and thereby disconnecting the power supply to the motor.

Reset Techniques & Safety

Reset functions are considered a critical part of thermal overload relay functions. After an overload trip, it is necessary to reset the relay before attempting to start the motor. Manual reset of the relay is considered preferable in most Australian industries since it calls for an acknowledgment of the problem before the motor can be restarted.

automatic reset can be applied in a situation where continuity of operation is necessary and rebooting does not pose a risk. Nevertheless, automatic reset can be considered critical because continuous rebooting without correcting a fault can cause constant degradation of a motor.

Types of Thermal Overload Relays

There exist different thermal overload relay systems used in Australian electrical systems, which are applied in different settings. The most common thermal overload relay is called a bimetallic thermal overload relay. Such devices are much appreciated for their simplicity, ruggedness, and economy. Although such devices can have accuracy affected by temperatures, this impact is limited by current technology.

Eutectic or melting alloy thermal over load relays are another type. Such relays function on the principle of an alloy melted at a definite temperature. When an overload takes place, this alloy melts and liberates a spring-loaded device, which in turn trips this relay. Although this relay gives precise tripping characteristics, it is less used in present-day systems and is mainly used in old systems.

Electronic thermal overload relays

A relatively recent innovation in motor protection relays is the electronic thermal overload relay. Instead of utilizing a heating effect and a moving part, this type of relay employs a current sensor and an electronic circuit to simulate a heating effect in a motor. Such relays have improved accuracy and are not influenced by a temperature effect, and they have other functions such as a phase loss and a phase imbalance.

Simultaneous Use with Other Protecting Devices

Thermal overload relays are not standalone devices in a complete protection system for a motor. In an Australian electrical system, thermal overload relays are integrated with circuit breakers or fuses that offer short-circuit protection based on AS/NZS specifications. The circuit breaker or fuse is intended to react quickly to high fault current, but not to continuous overcurrent, which is the function of a thermal relay.

Thermal overload relays are usually mounted in conjunction with contactors in motor control circuits such as direct-on-line, star-delta, and soft starters. When this relay trips, it breaks a control circuit and opens a contactor to disconnect a motor from a supply.

Selection and Adjustment Issues

In order to have efficient motor protection, proper selection and adjustment of thermal relay overload protection are critical. The relay used should have a rating suitable for the motor’s full-current load; in addition, it should be adjusted to a level indicated by the name plate of the motor.

The type of application and the starting parameters of the motor will need consideration in this case. Some motors are used with heavy inertia loads, wherein overload relays with a higher permissible time to trip will be necessary in case of a prolonged start time. The presence of a high level of ambient temperature, dust, or vibration in an application will impact the selection and application of an overload relay.

Applications in Australian Industry

Applications in Australian Thermal overload relays are widely used in a variety of industries in Australia. In mining environments, thermal overload relays protect motors that drive conveyors, pumps, and processing systems operating under heavy load conditions. Water and wastewater treatment plants employ thermal overload relays to protect pump motors which are critical in continuous operations. Agricultural settings employ thermal overload relays to protect irrigation pumps and grain handling systems operating in rugged environments. Commercial buildings employ thermal overload relays in HVAC systems to protect fans, compression systems, and air handling units.

Advantages and Disadvantages

Thermal overload relays are still in common usage, mainly because they offer cheap and efficient protection for an electric motor. The working curve of thermal overload relays is very close to thermal responses in an electric motor. They work well in situations where motor protection against overheating is a major requirement. The disadvantage is that they do not offer short-circuit protection and take a longer time to respond when compared with devices using electronic protection.