Tag: common


I walked into my first real job interview in the HVAC trade. 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 a dude named Ernie.

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.

Now, of course, this only applies to single-phase compressors and this leg to leg reading is helpful for identifying terminals but tells you very little about the condition of the windings unless you know the resistance in the first place or have historical readings or another identical compressor to compare to.

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. Many will tout this as a diagnostic reading you should check. it’s more a mathematical fact than something useful to check. If you did see a higher reading Start to Common + Run to Common vs. Run to Start it could really only indicate an increased resistance through the motor thermal overload that breaks common.

Here is how I remember which winding resistance is which (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.

Also…

The orientation when read like a book (top left to bottom right) is usually… if not always Common, Start then Run. Many techs remember that with the phrase “Can She Run”.

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

OK, I’m done.

— Bryan

Grounded_240v

One of the most common questions we get from techs is about using a voltmeter to diagnose a high voltage circuit. It’s especially tricky when a tech is used to working on Low voltage or 120V circuit where there is a clear “hot” side of the circuit and a clear “grounded” side of the circuit. In 120V you have one hot leg and the other side is neutral which is actually connected to ground back in the panel. Most (but not all) 24v transformers have one hot leg and the other leg is grounded. A car has one 12VDC Hot and the other side is grounded to the chassis.

All of these cases cause techs to get used to putting one meter lead to ground and “walking” the other lead through the circuit, looking for where the voltage is lost. While this is still not the best idea even on these circuits, it usually works.

However…

in 240v or 3 phase diagnosis it doesn’t work. Here is why –

The other “side” of the completed circuit is not grounded at all. So when you check to ground, you are checking to a point that has literally NOTHING to do with the completed circuit you are diagnosing. Even more important is the fact that you will often read “120v to ground” even when the leg you of power you are attempting to diagnose is open.

Here’s an example

Simple_Schematic_240v

Let’s say you are trying to see if the IFR contact is open. So you put your meter from L1 to ground. Good news you have 120v. So now you are feeling confident and you read from IFR terminal 2 to ground and you still have 120v. So now you think, “The IFR terminals are closed because I have 120v on each side”…

WRONG!

You will have 120V to ground on IFR terminal 2 regardless of whether the contacts are open or closed. If they are open you will be reading 120v backfed through the motor from L2, if they are closed then you will read L1.

In other words, it’s a pointless test.

Take a deep breath…

This next part is gonna take some focus to understand. If you don’t intend to pay careful attention to these next paragraphs you won’t benefit.

Instead read from L1 to L2 and confirm 240V then read from IFR1 to L2 and from IFR2 to L2. If you have 240v on IFR1 and not IFR2 then you know IFR is open…

An alternate method if you are DEAD SET on reading to ground is to check IFR1 to ground. If you have 120V then check from IFR1 to IFR2. If you read anything across the contacts you would then know they were open.

Remember…

You will read potential (voltage) so long as a path and difference in charges exist, across a load and across an open switch. You will not read potential (voltage) across a closed switch because a closed switch has no potential difference across it.

Final notes –

You are encouraged to check both legs to the ground for safety purposes to confirm the disconnect is actually off and open.

Checking to the equipment ground can be a way to check the ground itself, although in that case, a de-energized ohm or megohm test can often be a better test.

–Bryan


You have seen the C terminal on a dual run capacitor before. You have also seen the C terminal on a compressor.

It stands to reason that they would both connect together right?

Wrong, They don’t connect together and they aren’t even related, at least not in the way that you think.

In both cases, the C denotes a “common point” in the dual capacitor it is the common point between the fan capacitor (fan) and the compressor capacitor (herm). In the compressor it is the common point between the run and start windings (this is why R+C + S+C = R+S if you ohm a compressor)

The C terminal of a dual capacitor is actually fed from the OPPOSITE leg of power as the C terminal on the compressor. This is because you must power the start and run windings with the same leg and common with the other leg.

The way I always said it was “The same leg that feeds start feeds run” and the C terminal on a capacitor is actually the common feeds for the start winding of the compressor and fan (OPPOSITE side from the fan and herm plates on the capacitor)

So compressor terminals

C goes to one leg of power

R goes to the other

S goes to the HERM terminal on a capacitor with the other side of that capacitor (C) going to the same leg that feeds R.

C what I’m saying? Confusing

If you are new to the trade and you see the designation C or the word common don’t assume it is the same as other C and common terminals and start connecting stuff together… Unless you like creating smoke.

— Bryan

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