24V Transformer R and C Phasing

Teemok

If using a 24v transformers R and C for a door bell or a light the polarity of R and C is unimportant. But with flame rectification or I suppose other uses, the polarity needs to be correct.
If you're color blind or the wire colors don't make sense, terminals are unmarked or there is only one secondary wire color how do you test to find R? C is often grounded but not always.

EBEBRATT-Ed

Good information. That has been posted here before

Teemok

Originality is more than I hope for.

realliveplumber

Thanks for sharing. Very useful information.

I thought that I read somewhere one time that you can simply ground one side of the secondary and that would suffice.?

Teemok

I though exactly the same thing for years. And it's true you can ground either side if it's a simple circuit. But it matters when dealing with more complex? subtle?, I don't have the right term...... circuits.

Jamie Hall

realliveplumber said:

Thanks for sharing. Very useful information.

I thought that I read somewhere one time that you can simply ground one side of the secondary and that would suffice.?

That depends. If there is one, and only one, transformer involved, then you can ground either side of the secondary in most cases. You should, however, double check: the odd design already has one side of the secondary tied to one side of the primary. This will be the neutral side --if the neutral is marked. A very few older transformers -- you are not likely to see them, and even less likely to buy one -- had both sides of the "secondary" tied to both sides of the "primary" -- these are autotransformers and are a rare and curious breed. Your best bet is to get a plain vanilla transformer from the supply house...

Now if you have two transformers involved, things get more interesting -- as it is quite possible to hook them up so they are, in fact, out of phase. You can only check that by hooking both up to a power supply and then connecting one of the two secondaries together from each transformer. Then measure the voltage between the other two secondary wires (one from each transformer). If they are in phase, that voltage will be very close to zero. If they are out of phase, that voltage will be close to the rated secondary voltage.

EBEBRATT-Ed

Another topic that relates to this is switches etc in the "neutral" conductor.

When I started a lot of the old commercial burners, I worked on had control circuits that were 208 volt or 240 volts.

There was a job somewhere where one hot wire on an oil solenoid valve chafed and became grounded. The solenoid was only getting 120 volts instead of 208 but while the 120 couldn't open the solenoid it was enough to keep it open.... burned down the building. Sometime late 60s early 70s all oil burners the controls must be 150v to ground or less now.

How this relates to the 12/24 volt transformers is if one side of the 24 volt transformers is grounded which most are now, and any safeties are wired in series with the 24 volt common(neutral) a short can cause a path through the metal and the safety is bypassed.

This is the reason switches are not allowed in neutrals with 120 volt wiring.

Although motor starter overload switches are usually factory wired to break the neutral wire

109A_5

Jamie Hall said:

If they are in phase, that voltage will be very close to zero. If they are out of phase, that voltage will be close to the rated secondary voltage.

If they are out of phase, wouldn't it be twice the rated secondary Voltage of one transformer (assuming the secondary's are the same Voltage) or the sum of the two secondary Voltages.

Phasing issues affecting flame rectification may be a bit of a stretch. If the ground side is compromised, impressing an AC Voltage from the 120 VAC side in the flame rectification circuit, you have other issues.

Often the equipment providing the flame rectification detection also provides the 24VAC ground bonding.

Jamie Hall

Yes. My bad. Should be twice the rated secondary voltage if they are out of phase.

The general issue of grounding is very important and not to be overlooked. It usually is.