Starter Overload Relays
Overload relays protect the motor and its components from an overload situation, which can result in excessive heat and damage. The overload relay is wired in series with the motor, so that the electrical current that flows to the motor will flow through the overload relay as well. When the electrical current flowing in the monitored circuit exceeds a certain preset limit over a certain period of time, the overload relay will trip, causing an auxiliary contact to interrupt the motor control circuit. This de-energizes the contactor, removing power to the motor, avoiding overheating and damage. The overload relay will reset automatically after a certain period of time or can be reset manually after the cause of the overload has been rectified.
When selecting an overload relay for electric motors, it is important to take into consideration time, as well as current. During start up, a motor will draw significantly more current than it is rated for, in a short period of time. Therefore, a relay would trip immediately when the overload current occurred, making starting the motor difficult. Thermal overload relays are rated by their trip class (5, 10, 20, and 30 seconds), which defines the amount of time an overload can occur before the relay trips.
The three types of thermal overload relays are: bimetallic, eutectic and electronic. Bimetal overload relays contain a bimetal strip that when overheated, bends to close and trip the circuit. Eutectic overload relays contain a eutectic alloy that when overheated, melts, releasing a mechanical device that opens the overload contacts, tripping the circuit. Electronic overload relays do not rely on a heater mechanism to detect an overload, but rather use temperature sensors or current transformers to sense the amount of current flowing to the motor. Temperature is sensed using positive temperature coefficient (PTC) thermistors and used to trip the circuit in overload situations.
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