Information Regarding Thermal Overload Relays

Circuit breakers and thermal overload relays 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, a thermal overload relay is 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 over current 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. Instead, by observing the motor’s starting and running currents, the relay determines 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. As the probe reaches its nominal temperature, its resistance quickly rises. A threshold circuit then notices this rise and opens the relay connections.

A heating coil, a eutectic alloy, and a mechanical circuit breaker comprise a melting alloy (or eutectic) overload relay. By monitoring the amount of current being drawn by the heater coil, the relay measures the temperature of the motor.

Features

Customers can pick an overload thermal 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.

Several levels of phase monitoring are included in certain relays. Using these items, phase loss, reversal, or imbalance may be examined. If any phase problems were discovered, the relay would trip and cut off power to the engine. Phase imbalance in particular can harm a motor by causing risky changes in the voltage or current flow.

Under load detection is the ability of an overload relay to detect a drop in current caused by unloading. For example, if a pump starts to lose capacity, this may occur. These relays are designed to detect these fluctuations and trip as if they detect an overload. Light-emitting diodes (LEDs) or other status indicators are used in relays with visual indications.