# Why does high voltage side of transformer tend to burn out first?

## Main Question or Discussion Point

In residential split-system air-conditioners and heat pumps, there are transformers inside both the condenser and the air-handler that step the voltage down from 220/230/240 volts to 24 volts. Someone once told me that in residential air-conditioners/heat pumps, the high voltage side of a transformer tends to burn out more often than the low voltage side of a transformer. Both the high voltage side and the low voltage side of a transformer deal with the (approximately) same amount of wattage. The low voltage side of a transformer has lower current, and the high voltage side of a transformer has higher voltage. Is higher voltage more likely than high current to cause a transformer to burn out?
In a residential air-conditioner/heat pump, why does the high voltage side of the transformer tend to burn out more often than the low voltage side of the transformer?

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berkeman
Mentor
Interesting question. It would be better if we had some articles or links to places that claim this, instead of just your friend saying this. But if he is a tradesperson who works with those kinds of systems, it might be valid information.

Since such transformers are generally monolithic, how can you tell if one side is "burned out" versus the other? With resistance checks on the windings (looking for opens or inter-winding shorts)?

We can discuss the theoretical reasons for such a phenomena (I can think of several reasons it might be true), but in the end it would depend on the wire gauge sizes used and transformer construction and a number of other things that the manufacturers control beyond any transformer theory.

But if he is a tradesperson who works with those kinds of systems, it might be valid information.
The guy who told me this is an expert HVAC technician.

Since such transformers are generally monolithic, how can you tell if one side is "burned out" versus the other? With resistance checks on the windings (looking for opens or inter-winding shorts)?
He told me about this, but I was not taking notes or anything, so I cannot be 100% sure what he said.

However, I'm 99% sure that he checks if one side is "burned out" by checking the resistance of the wires coming out of the transformer, not the windings inside the transformer.

We can discuss the theoretical reasons for such a phenomena (I can think of several reasons it might be true), but in the end it would depend on the wire gauge sizes used and transformer construction and a number of other things that the manufacturers control beyond any transformer theory.
He told me that the wires on the low voltage side are bigger (as in thicker) than the wires on the high voltage side because the low voltage side wires carry a higher current. My impression is that manufacturers of residential HVAC transformers universally choose to make transformers with thicker wires on the low voltage side to accommodate the higher current on the low voltage side. That is probably the key characteristic of residential HVAC transformers that the manufacturers control beyond any transformer theory.

You said that you can think of several reasons that it might be true. That is the key to answering this thread. Assuming that the manufacturers of residential HVAC transformers universally make the transformers with thicker wires on the low voltage side of the transformer (to accommodate the higher current on the low voltage side), why does the high voltage side of the transformer burn out first?

berkeman
Mentor
The guy who told me this is an expert HVAC technician.
Awesome, my wife talks with these folks every day to help resolve issues.
However, I'm 99% sure that he checks if one side is "burned out" by checking the resistance of the wires coming out of the transformer, not the windings inside the transformer.
Well, sorry, that kind of doesn't make any sense. If you could ask him what he checks in his failure analysis, that would be a big help in answering your question. And does he have specifications from the manufacturer on what the winding resistances should be for a good transformer? At my work, we publish those specs.
He told me that the wires on the low voltage side are bigger (as in thicker) than the wires on the high voltage side because the low voltage side wires carry a higher current.
Yeah, that follows directly from how a transformer works.

Probably your question will boil down to which side of a practical power transformer will fail first given an output fault (short?). I would probably design my transformers to fail equally likely on primary or secondary to minimize the cost of the transformer, but maybe some of our experts like @anorlunda can offer other insights.

EDIT / ADD -- And the fuse or breaker should fail before the transformer fails That is their purpose...

Well, sorry, that kind of doesn't make any sense. If you could ask him what he checks in his failure analysis, that would be a big help in answering your question.
I thought about this. You're right. What I wrote there does not make sense. If he checks the resistance by connecting the leads of his multimeter to the wires coming out of the transformer, that will automatically check the resistance of the windings.

Let me rephrase this. I'm 99% sure that he checks the resistance of the windings by connecting the leads of his multimeter to the wires coming out of the transformer. The wires coming out of the transformer are connected to the windings, so this checks the resistance of the windings as well.

And does he have specifications from the manufacturer on what the winding resistances should be for a good transformer? At my work, we publish those specs.
I doubt he or any other HVAC technician keeps written specifications from the manufacturer on what the winding resistance should be for a good transformer. I believe what he does is that he knows what a ballpark figure of what the resistance should be and compares the resistance figures to that. For instance, it might be the case that the resistance should be something like, say, 0.2 ohms, and if his multimeter says something like "OL" for open line or if it says infinite resistance, he knows that it's a defective transformer.

I would probably design my transformers to fail equally likely on the primary or secondary to minimize the cost of the transformer...
Perhaps you would, but most or all residential HVAC transformer manufacturers don't.

EDIT/ADD---And the fuse or breaker should fail before the transformer fails That is their purpose...
I agree, but in the real world what should happen is not always what happens.

Last edited:
berkeman
Mentor
Yeah, if the fault is open circuit on a winding, that's easy to check for. Shorted windings usually just lower the winding resistance, so unless you have typical winding resistance specification from the manufacturer, that's harder to isolate.

Do you know which it was? Can you say which unit it was? Even if it's not available via a regular internet search, my wife my have access to the specs via her work...

Yeah, if the fault is open circuit on a winding, that's easy to check for. Shorted windings usually just lower the winding resistance, so unless you have typical winding resistance specification from the manufacturer, that's harder to isolate.

Do you know which it was? Can you say which unit it was? Even if it's not available via a regular internet search, my wife my have access to the specs via her work...
I just texted him about this. This is how he tests a transformer when the air-conditioner/heat pump is turned off: First, he tests the resistance of each wire to ground (ground being the metal casing of the transformer). When he tests the resistance of each wire to ground, he gets an "OL" (open line or open circuit) reading if the transformer is good. If he gets any reading other than OL, the wires are touching the casing and the transformer is bad.

Next he uses his ohm meter to check the resistance of the secondary side and the primary side. If he gets an OL reading, the winding is open (meaning the wire is broken or burnt and there is no path). If he gets resistance on both, then he ensures that the primary has more resistance than the secondary. If the primary has more resistance than the secondary, the transformer is good. If the secondary has more resistance than the primary, then the transformer has a shorted primary winding or he has a step up transformer rather than a step down transformer.
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So does that tell you why the high voltage side of the transformer tends to burn out first?

Tom.G
Some small transformers such as these have an internal fuse or thermal overload in the primary side. This is done more often with enclosed transformers but I have seen some open-frame transformers with them as well. The fuse or overload is at or near the outer surface of the primary, with another layer of paper covering; so even if present you won't see them.

First of course is the smell test. If it smells burned, get rid of it.

Under the special conditions of a tripped thermal overload due to a temporary fault, sometimes it can be reset by spraying it with Freeze Spray, available at electronic supply stores. But considereing the cost of a service call, you are probably ahead to just replace the transformer and check for overloads or shorts.

(the resettable thermal overloads {not all are resettable} are a snap-action thermostat with a wide deadband. barring physical damage, get them cold enough and they switch back on.)

Cheers,
Tom

Tom G, what are you saying? That the residential HVAC transformers tend to burn out on the primary side because they usually have a fuse on the primary side? If not, I don't understand your post.

Tom.G
Tom G, what are you saying? That the residential HVAC transformers tend to burn out on the primary side because they usually have a fuse on the primary side? If not, I don't understand your post.
Yup.
An open fuse or tripped thermal overload in series with the primary is electrically identical to an open winding.

This is how he tests a transformer

These cheap, small ones has a big contact area inside the primary winding, all covered with only a thin layer of insulation, almost always heated up. Also, the wire of the primary is really thin, almost like the wire in a small amp fuse. A shorted turn can easily cause enough current to break the wire, an due the cheap, high turn count winding it is easy to get a shorted turn when the insulation gets worn down by the heat.

davenn
Gold Member
2019 Award
Some small transformers such as these have an internal fuse or thermal overload in the primary side. This is done more often with enclosed transformers but I have seen some open-frame transformers with them as well. The fuse or overload is at or near the outer surface of the primary, with another layer of paper covering; so even if present you won't see them.
Tom G, what are you saying? That the residential HVAC transformers tend to burn out on the primary side because they usually have a fuse on the primary side? If not, I don't understand your post.

Internal fuses are very common in transformers and I in personal experience have found many failed transformers because of the fuse blowing ... resulting in an open circuit primary
( ALL on the primary side)

So a typically "HVAC" transformer will be from Line to some lower ( often control voltage) - but I would agree the High side of transformers probably do have a higher failure rate:

1) Higher voltage stress on the insulation. The insulation on the high side will typically be closer to the "spec" - for example a 480 VAC primary, will be 600V Class... guess what the insulation class of a 120/240 LV side is? Typically still 600V.... Even for control power transformers, like 24V - there will be a lot of margin in the insulation since there is little savings in using the minimum.

1A ) Affect of line disturbances on the primary windings. All of the disturbances coming in the facility from the utility connection, switching events, lightning, voltage dips & the associated recovery, and then even just the turning on and off of the transformer locally, stress the primary windings much more than the secondary. ( Both result in both over voltage and higher frequency elements)

2) Overloading - will cause the overall unit to heat up, what does heat do to a transformer do? It ages the insulation faster. But there can definitely some hot-spot issues that can occur on the LV side - I^2*R type issues, become more prevalent if the unit is frequently overloaded, or it was marginally sized. ( Also note - as the low side leads to much of the heating, the windings are often built with a little better spacing, for cooling, but this also helps reduce insulation stress - I have seen secondary faults become re-energized and "live" - but never a primary.)

But.. this leads many lay-persons astray... a majority of these issues lead to insulation failure, leading many to "blame" the insulation, the transformer, and thus the manufacturer. But a lot of the stress can be outside of the design parameters, or the transformer is not applied or protected properly. Still - in a mass produced consumer / commercial products like HVAC transformer, I'll bet your customer knows some brands that fail more frequently than others...

The question for him is -- when he has to replace a transformer, does he use the better brand - or the lesser one for "job security"?? (;-)

jim hardy
Gold Member
2019 Award
Dearly Missed
I think the answer to his question is
"the smaller primary wire acts like a fuse"
I've opened many "Wall Wart" transformers and found the primary open right where it solders to the transformer connector tab.
I suspect (but do not know for certain) that manufacturers of inexpensive consumer equipment use small fragile wire there as a 'fusible link'.

I have noticed high end consumer and industrial manufacturers are more likely to protect their transformers with a fuse.
I guess that's a tradeoff - protect the transformer or eliminate a failure point ?
As a DIY'er i'd vote to protect the transformer
but a business analysis might justify saving the cost of a fuse and a service call to replace it .