Anover current protection device such as a circuit breaker or fuse protects against excessive currents such as a short circuit and generally operates instantly. However, an overload relay protects against a running over current that would cause overheating of the protected equipment eventually leading to damage. The protected equipment includes the power conductors as well as the motor.
The overload relay monitors the current flowing through the power conductors leading to the motor. If the current exceeds a certain amount for a certain period of time, the overload relay will interrupt the motor control circuit which will cause the power to be removed from the motor. Once the motor has sufficiently cooled, the overload relay may be reset allowing the motor to be restarted.
There are three types of overload relays - melting alloy, bimetallic and electronic.
The melting alloy overload relay employees a spring loaded lever which is held in place by a solid plug of melting, or eutectic alloy. By definition, the eutectic alloy is a material which, at a given temperature, changes instantly from solid to liquid state. When enough heat is generated, this change occurs, releasing the spring loaded mechanism and opening the contact. Once the melting alloy cools enough to return to solid state, the spring loaded triggering mechanism must be manually reset by turning against a ratchet. The motor current flows through heater elements within the overload relay which heats up the melting alloy. The heater elements are sized according the motor full load amps (FLA) and are usually ordered separately from the overload relay.
A bimetallic overload relay consists of a small heater element wired in series with the motor and a bimetal strip that can be used as a trip lever. The bimetal strip is made of two dissimilar metals bonded together. The two metals have different thermal expansion characteristics, so the bimetal strip bends at a given rate when heated. Under normal operating conditions, the heat generated by the heater element will be insufficient to cause the bimetal strip to bend enough to trip the overload relay. As current rises, heat also rises. The hotter the bimetal strip becomes, the more it bends. In an overload condition, the heat generated from the heater will cause the bimetal strip to bend until the mechanism is tripped, stopping the motor. Some overload relays equipped with a bimetal strip are designed to reset the circuit automatically when the bimetal strip has cooled and reshaped itself, restarting the motor. If the cause of the overload still exists, the relay will trip again and reset at given intervals. Care must be exercised in the selection of this type of overload relay, since repeated cycling will eventually damage the motor.
Non-Ambient Compensated Bimetal Overload Relays
In certain applications (such as a submersible pump), the motor may be installed in a location having a constant ambient temperature. However, the motor control and overload relay may be installed in a location with a varying ambient temperature. In such cases, the trip point of the overload relay will vary with the temperature of the surrounding air as well as current flowing through the motor, which can lead to premature and nuisance tripping. Ambient compensated bimetal overload relays are designed to overcome this problem. A compensated bimetal strip is used along with a primary bimetal strip. As the ambient temperature changes, both bimetal strips will bend equally and the overload relay will not trip the motor. However, current flow through the motor and the heater element will affect only the primary bimetal strip. In the event of an overload condition, the primary bimetal strip will engage the trip unit. Siemens Class 48 ambient compensated bimetal overloadrelays are available in single-pole or three-pole designs and can be set for manual or self-resetting operation. An adjustment dial located on the unit allows the ampere trip setting to be adjusted by ±15%. A manual test button is provided to test the operation of the overload relay control contacts. The ambient compensated bimetal overload relay heater elements are available in Class 20 or Class 10 ratings. A normally open or normally closed auxiliary contact is available as an option.
Ambient Compensated Bimetal Overload Relays
Class 48 Ambient Compensated Bimetal Overload Relay
Unlike the other twp types which senses motor current thermally, electronic overload relays senses (measures) current electronically. The features and benefits of electronic overload relays vary, but there are a few common traits. One advantage offered by electronic overload relays is a heaterless design, reducing installation cost and the need to stock a variety of heaters to match motor ratings. Heaterless design also allows the electronic relay to be insensitive to the ambient temperature, minimizing nuisance tripping. Electronic relays also offer phase loss protection. If a power phase is lost, motor windings can burn out very quickly. Electronic overload relays can detect a phase loss and disconnect the motor from the power source. Phase loss protection is not available on mechanical types of overload relays.
A single Siemens ESP100 electronic overload relay replaces at least six size ranges of heaters and is available in both single phase and three phase. Instead of installing heaters, the full load amperes (FLA) rating of the motor is set with a dial. NEMA Class 10, 20, and 30 trip curves are available for a variety of applications. A manual test button is provided to test the operation of the overload relay contacts. One normally closed auxiliary contact is included as a standard feature.
Overload relays typically operate on an inverse time curve where the tripping time becomes less as the current increases. They are rated by trip class. Trip class specifies the length of time it will take for the relay to open in an overload condition. Classes 5, 10, 20 & 30 are the most common. Class 5, 10, 20 & 30 overload relays will trip within 5, 10, 20 & 30 seconds respectively at 600% of motor full load amps. Class 5 is usually used for motors requiring extremely fast tripping. Class 10 is commonly used to protect artificially cooled motors such as submersible pump motors of low thermal capacity.. Class 20 is usually sufficient for general purpose applications. Class 30 is usually required for high inertial loads to help prevent nuisance tripping. Refer to the figure below for trip curves.
Class 48 ESP100 Electronic Overload Relay