Arkansas HVACR NewsMagazine November 2022

HVACR NewsMagazine November 2022

Tech News

I encourage caution because I have seen many junior techs condemn good compressors due to a “leg -to- leg” short— just because the ohm reading between Run and Common appeared low. The only way to know if a single-phase compressor is shorted “leg to leg” with an ohmmeter is to know what the windings should read in the first place. On a three-phase motor, all three legs should read the same ohms leg to leg, making it considerably easier. When you do encounter leg-to-leg (only) short circuits, it is more often on fan motors than on compressors. Breaker Issues Because most breakers trip due to heat, anything that causes the breaker to get hotter than normal can result in tripping. That can be due to a poor connection inside the breaker itself, but it's often due to a poor wire connection on the breaker or a poor connection between the breaker and the bus bar.

Techs will often look for short circuits from “leg to leg” or “winding to winding” in a compressor or a motor without first measuring to ground.

THIS IS NOT A GOOD IDEA.

Even when a motor does short “winding to winding,” it is rare that it just stays shorted. Usually, it will ALSO be shorted to ground, or it will be open after the arc flash that resulted from the short.

Usually, these types of breaker issues are caused by installation problems such as a loose connection, wrong breaker type, failure to use anti-oxidation paste on aluminum-to copper connections, excessive tripping, or using the breaker as a switch.

Think of a circuit board. Circuit boards short out all the time. The result is a big black spot on the board, and nothing works anymore (open). It rarely results in a continued short circuit because the arc from the short blew the connection apart.