208/240V Transformer

[Van Vanderford]

After having several (like 4) thermostats fail in a 6-year period, it finally dawned on me to check the voltage from the transformer in the air handler and I found it to be a bit over 28 volts (when it should be 24V). My condo building has 208V service. The transformer in my air handler is a 208/240V one and was wired to the 208V terminal. I removed the wire from the 208V terminal and connected it to the 240V terminal and then found the voltage to be 24.2V. This makes sense when you do the math: 208/24 equals (approximately) 240/28. A transformer just reduces ("transforms") voltage based on a ratio, correct? And that ratio is determined by the windings in the primary and secondary, yes?

Strange that connecting 208V to the 240V terminal would produce the correct 24V, but it could just be something funky about the transformer. And BTW of the 61 units in the building, MOST have had at least one thermostat fail so there is a common thread.

Anyway, it appears I have possibly solved the problem of the thermostats failing but my question is, will this cause any damage to the primary (or secondary) of the transformer? Thanks in advance!

 

[CDRIVE]

Based on what I understood you to say... No worries.

Cheers,
Chris

 

[Tha fios agaibh]

will this cause any damage to the primary (or secondary)

Not the primary, but the secondary could potentially be a problem if it had other loads on it (causing it to run undervoltage).
Imo, 28v is not terribly high for that transformer. I guess the million dollar question is what else runs on that transformer? If for instance, it also runs a damper motor or something, I'd put the primary taps back the way they were.
It is generally better to be a little overvoltage than undervoltage.
Running undervoltage causes current and heat to go up.

 

[CDRIVE]

It is generally better to be a little overvoltage than undervoltage.
Running undervoltage causes current and heat to go up.

For this to be true we would have to know what the load or loads are. It certainly would be true for some motors, especially if stalled but the opposite would be true for most purely resistive loads.

Chris

 

[hevans1944]

Interesting that your condo is wired for 208 instead of 240 distribution. The 208 is a common phase-to-phase voltage for a three-phase wye distribution because it yields 120 from any of the three phases to neutral. The 240 is a common single-phase center-tapped "pole pig" distribution because it also yields 120 from either "phase" to neutral. Given my druthers, I would love to have a three-phase distribution in my house, but it ain't gonna happen.

I suspect the reason your thermostats are failing lies in the construction of the thermostats, rather than in a slight increase in voltage from a nominal 24 VAC to 28 VAC. But only time will tell if your suspicions are correct. In any event, it will not harm the transformer to supply the 240 V tap with 208 V power. Four failures in a six year period, out of sixty-one units in service, does sound a bit high though. What does the manufacturer say about their life expectancy?

 

[Van Vanderford]

It was discovered that we had 208V instead of 240V when one of the condo owners replaced his water heater with a tankless type. We contacted FPL (power company) to make sure that was what we were supposed to be getting and they answered yes. The tankless heater people sent out different elements rated for 208V.

Besides having problems with thermostats burning out, everyone in the building complains that it takes almost 30 minutes for the oven to reach 400 degrees. Sure, the rating plate says it is good for 208 but with lower voltage, of course it will take longer to reach a given temperature because the heating element is getting less voltage than it was designed for.

Back to the transformer, it is simply a transformer in an air handler for central electric heat and A/C. In addition to the thermostat, it also powers relays for the fan and compressor.

Perhaps I should check the voltage when the A/C is on? That way both relays will be energized, but since those are resistive loads I should be okay, right?

 

[hevans1944]

Well, it is what it is. I hate electric heat. Natural gas is for heat; electric is for light (as Tesla and Edison intended). Yeah, measure the transformer voltage under load with everything energized and working properly.

Another possibility is transients on the electric power lines caused by nearby lightning strikes causing thermostat failure, especially if these are programmable thermostats. If it's a problem, you might be able to suppress transients with MOVs installed in the thermostats. You probably don't want to go to the expense of doing so, but you can rent digitally recording power-line-quality monitors. It wouldn't hurt to ask the FPL utility if you could borrow one of theirs for a week. They will probably say no, but it will put them on notice that you are trying to discover why your building thermostats are dying at an early age. These monitors record "events" like power surges and sags and time-stamp them so you can compare with the weather and other related events.

Good luck with this!

 

[Tha fios agaibh]

For this to be true we would have to know what the load or loads are. It certainly would be true for some motors, especially if stalled but the opposite would be true for most purely resistive loads. Chris

Agreed, thats why I said Generally.
I wouldn't expect to see resistive loads on a hvac control transformer.

I didn't know there are motors that are not inductive. Can you elaborate on this Chris?

John

 

[CDRIVE]

I didn't know there are motors that are not inductive. Can you elaborate on this Chris?

John

I wasn't implying that there are but then I gave it more thought and found this.

Chris

 

[Van Vanderford]

So, hevans1944, could I place the MOV downstream of the transformer? Or maybe directly on the secondary terminals? Or on the primary?

Keep in mind I am a homeowner (or rather condo owner) but I do have a fair amount of experience with residential wiring, so I would need a bit of guidance on the size (if that's the correct terminology) of the MOV. And with the 5 minutes of MOV research I was able to do, it looks like it would be connected across hot and neutral, correct?

 

[CDRIVE]

It is generally better to be a little overvoltage than undervoltage.
Running undervoltage causes current and heat to go up.

When I made my last post I wasn't quite sure why you thought I said that there are non inductive motors because I never said that. That said it should be pointed out that a brushed DC motor is predominantly a resistive load. The resistive elements are far greater than the reactive byproducts. Relay coils, both DC & AC are also predominantly resistive. Heck a DC relay only shows its reactive properties upon energizing and d energizing.

I think my main issue with your statement is a brushed DC Motor (Not Stalled), Relay or Solenoid will not draw more current and produce more heat when the supply voltage is reduced. Ohm's Law dictates the opposite is true. Even an AC Relay coil will prove to draw less current when the supply voltage is reduced.

Chris

 

[CDRIVE]

Van, I wouldn't hang an MOV across an un-fused circuit. Your mains breaker doesn't count. Also, be aware that when you mess with the Mains your putting yourself in a precarious legal position. Insurance companies will deny an electrical fire claim in heartbeat if they get wind that "Unlicensed" maintenance was performed on the Mains.

BTW: If your power utility is FP&L then you're a neighbor!

Chris

 

[Tha fios agaibh]

Might want to check the voltage requirements of the heat also.

a brushed DC motor is predominantly a resistive load. The resistive elements are far greater than the reactive byproducts. Relay coils, both DC & AC are also predominantly resistive Chris

Chris;
We are talking about AC loads here driven by a transformer so the DC point is sort of moot.
Since you brought it up, ...I've always consider all motors it to be more inductive than resistive for the simple fact that the motor windings create magnetic fields where energy is stored.
Wouldn't a DC motor still draw more current (creating heat) trying to drive/maintain the same load at a lower voltage?
If this is getting off topic we can move to another thread. I don't want you to get another spanking.

John

 

[hevans1944]

So, hevans1944, could I place the MOV downstream of the transformer? Or maybe directly on the secondary terminals? Or on the primary?...

It would probably be more effective on the secondary, installed in the same case as the thermostat. If installed in the primary at the transformer, you need a fuse in the hot lead that will blow if the MOV shorts failed. It might be difficult to find an MOV with a low enough "trip" voltage to be effective on a 24 VAC secondary. And there is that pesky insurance question if you install it on the primary side, an essentially unauthorized modification of UL approved equipment whether performed by a licensed electrician or not. Catch 22, that one.

... Keep in mind I am a homeowner (or rather condo owner) but I do have a fair amount of experience with residential wiring, so I would need a bit of guidance on the size (if that's the correct terminology) of the MOV. And with the 5 minutes of MOV research I was able to do, it looks like it would be connected across hot and neutral, correct?

Yeah, you want one that will tolerate a moderate amount of energy, perhaps ten to fifty joules or so, for several thousand events Unfortunately, it is hard to tell when the MOV has done its job and failed "open circuit". There are probably millions of "surge protected" outlet strips in use with failed MOV surge protectors. This Google page has a few links to MOV surge protection circuits you should look at.

 

[CDRIVE]

Chris;
If this is getting off topic we can move to another thread. I don't want you to get another spanking.

John

John, go ahead and start the topic. I think it will generate much banter by more than just the two of us. Right or wrong I'm lining up my arguments as I type this. Put it in the Electronics Chat section.

Chris

 

[CDRIVE]

Members that love to read mea-culpa's can go Here where I lay my neck bare for the axe!

Chris

 

[Van Vanderford]

Hey guys, I really appreciate all the guidance provided. Bottom line for me, at least for now, is to just leave it as it is, connected to the 240V terminal so that it outputs 24.2V. I checked the transformer on a friend's house (with 240V service) and his transformer was putting out 24.6V so I think my 24.2V is good.

If I run into any issues, I can always purchase a new transformer that puts out correct voltage when supplied with 208V.

Thanks again!