What HVAC part or switch uses back EMF?

[sevensages]

TL;DR

What HVAC part or switch uses back EMF?

There is a switch for a type of motor or some sort of part in HVAC that closes due to back EMF, but I don't remember what type of motor or switch in HVAC closes due to back EMF.

What type of HVAC part or switch closes due to back EMF?

 

[Baluncore]

The potential or voltage relay detects back EMF from the motor.

 

[DaveE]

After about 1 minute with google search:
https://literature.rockwellautomation.com/idc/groups/literature/documents/pp/safety-pp006_-en-p.pdf

There are others, you should try it too.