Information about the Thermal Overload Relays

Circuit breakers and thermal overload relay are similar in design and operation; however, most circuit breakers vary in that they interrupt the circuit if an overload occurs, even for a little period. Contrarily, thermal overload relays are built to monitor a motor’s heating profile; as a result, overload must persist for a long time before the circuit is shut off.

Type

Relays come in a variety of varieties, such as bimetallic thermal, solid state, and temperature control versions.

Bimetallic thermal relays, as their name suggests, open the connections mechanically using a bimetallic strip. Bimetallic strips are made of two linked metal segments that expand at various speeds in response to heat. When heated, this disparity makes the strip bend. A spring holds the strip to contact in a thermal relay. The contacts are forced apart, the circuit is disrupted, and the extra heat from the overcurrent bends the strip and pulls the spring. The strip assumes its former shape after cooling.

Electronic devices with no moving or mechanical parts are called solid-state relays. The relay instead computes By observing the beginning and running currents of the motor, the thermal overload relay can determine the average motor temperature. Relays made of solid state also have programmable set points and trip times and are often quicker than electromechanical ones. They can be utilized in explosive conditions since they cannot produce a spark.

Thermistor or resistance thermal device (RTD) probes placed in the motor winding are used by temperature control relays to measure the temperature of a motor directly. When the probe reaches its nominal temperature, its resistance quickly rises. A threshold circuit then notices this rise and opens the overload thermal relay connections.

A heating coil, a eutectic alloy, and a mechanical circuit breaker comprise a melting alloy (or eutectic) overload relay. The relay gauges the temperature of the motor by keeping track of the amount of current being pulled via the heater coil.

Features

Customers can pick a relay with a variety of unique features.

After a predetermined amount of time, a relay with automatic reset will revert to its initial “closed” position. The relay will trip once again if the engine is still overloaded after the reset.

Relays that compensate for ambient temperature function effectively under a variety of environmental conditions.

Various levels of phase monitoring are included in certain relays. Phase loss, reversal, or imbalance may be checked for with these items. The relay would trip and turn off power to the engine if any phase issues were found. Phase imbalance in particular can harm a motor by causing risky changes in the voltage or current flow.

A relay’s capacity to recognize a dip in current brought on by unloading is known as under load detection. This may happen, for instance, if a pump starts to lose its capacity. These relays are made to recognize these variations and trip as if they were sensing an overload relay. Light-emitting diodes (LEDs) or other status indicators are used in relays with visual indications.