Editing Relay (section)

=== Overload protection relay ===

Electric motors need [[overcurrent]] protection to prevent damage from over-loading the motor, or to protect against short circuits in connecting cables or internal faults in the motor windings.<ref>{{ cite book | last = Zocholl | first = Stan | title = AC Motor Protection | publisher = Schweitzer Engineering Laboratories | year = 2003 | isbn = 978-0972502610 }}</ref> The overload sensing devices are a form of heat operated relay where a coil heats a [[bimetallic strip]], or where a solder pot melts, to operate auxiliary contacts. These auxiliary contacts are in series with the motor's contactor coil, so they turn off the motor when it overheats.<ref>{{cite web|title=Working Principle of Thermal Motor Protection Relay|url=http://electrical-engineering-portal.com/working-principle-of-thermal-motor-protection-relay|website=Electrical-Engineering-Portal.com|publisher=Electrical Engineering Portal|author=Edvard|date=2013-03-09|access-date=2017-12-30}}</ref>

This thermal protection operates relatively slowly allowing the motor to draw higher starting currents before the protection relay will trip. Where the overload relay is exposed to the same ambient temperature as the motor, a useful though crude compensation for motor ambient temperature is provided.<ref>{{cite journal|title=Coordinated Power Systems Protection|issue=811–814|journal=Department of the Army Technical Manual|publisher=[[United States Department of the Army]]|date=1991|pages=3–1}}</ref>

The other common overload protection system uses an electromagnet coil in series with the motor circuit that directly operates contacts. This is similar to a control relay but requires a rather high fault current to operate the contacts. To prevent short over current spikes from causing nuisance triggering the armature movement is damped with a [[dashpot]]. The thermal and magnetic overload detections are typically used together in a motor protection relay.{{Citation needed|date=December 2017}}

Electronic overload protection relays measure motor current and can estimate motor winding temperature using a "thermal model" of the motor armature system that can be set to provide more accurate motor protection. Some motor protection relays include temperature detector inputs for direct measurement from a [[thermocouple]] or [[resistance thermometer]] sensor embedded in the winding.<ref>{{Cite web |date=2020-02-15 |title=Overload relay - Principle of operation, types, connection |url=https://www.electricalclassroom.com/overload-relay-principle-of-operation-types-connection/ |access-date=2022-06-13 |website=www.electricalclassroom.com |language=en-US}}</ref>