Uncommon start and run


I walked in to my first real job interview in the A/C business. The manager was a guy named Ernie and he walked me out to the warehouse.

Quick warning.. guys named Ernie are tough. Don’t mess with dude named Ernie.

Anyway..

He walked up to a box, snatched a pen out of his shirt pocket and scribbled a circle, 3 dots and three numbers on it while grunting “which is common, start and run”

I was in luck….

While I may have had almost zero practical knowledge of air conditioning, this was one thing I HAD actually learned in school.

I marked the terminals and I got the job.

Before you say that this information is useless let me stop you. 

It isn’t useless. It may not be something you use every day, but I have needed to ohm out a motor or compressor a handful of times and it got me out of a pinch.

So here it goes

The lowest ohm reading is between Common and Run

The middle ohm reading is between Common and start 

The highest ohm reading is between start and run

Common is just a point between start and run and therefore the common to start and run to start readings will add up to the run to start reading.

Here is how I remember this (let the mockery begin)

Starting is hard… so it has the highest resistance 

Running is hard also… but not as hard as starting, so it has a resistance less than start.

Common is easy… being common requires the lowest resistance

So common to run is the least and start to run is the most.

Understanding common, run and start is uncommon… so it requires a lot of resistance… so start… knowing it

OK I’m done. 

Happy Thanksgiving ? 

— Bryan

One comment

  1. Chris says:

    There are a lot of seasoned techs who don’t know CSR mostly because they just test for ground and continuity. But it is very helpful to see if a motor is on its way out which could also be a symptom of other issues. Adding to that it’s also helpful to know 3 ph motors should all read equal ohms within mfr specs and CR + CS should = SR in single phase motors.

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