how warm/hot is safe for a transformer

[rjvh]

hi i have a aplication that becuase of the power draw warms up the transformer
the transformer is roughly 50 degree Celcius in use with an ambient temp of 27 degrees Celcius no airflow

can this be seen as acseptable or do i have to increase the transformer

please advice

Robert-Jan

 

[dknguyen]

Sounds okay. I assume a transformer can handle much more heat than a power MOSFET and a 20 degree rise is pretty darn good for a power MOSFET (at least in my circuits anyways).

 

[mneary]

Transformers vary. But 23 C rise seems pretty reasonable to me.

You do waste a bit of power in heating the transformer, but most systems don't care.

 

[Roff]

I have an isolation transformer that gets so hot you can't keep your finger on it, even with no load. IIRC, the temperature was about 60C, a 30-35C rise. I called the mfr and spoke to an engineer, who said it was normal. Pissed me off, but it works, so what the hey.
My assumption is that the primary inductance is low, which saves on copper. This is why it was cheap (literally). Low inductance causes higher magnetizing current, with accompanying I^2*R loss.

 

[dknguyen]

I have an isolation transformer that gets so hot you can't keep your finger on it, even with no load. IIRC, the temperature was about 60C. I called the mfr and spoke to an engineer, who said it was normal. Pissed me off, but it works, so what the hey.
My assumption is that the primary inductance is low, which saves on copper. This is why it was cheap (literally). Low inductance causes higher magnetizing current, with accompanying I^2*R loss.

Lol..
"It burns when I touch it!"
"It's supposed to. THat's normal."

 

[Roff]

Lol..
"It burns when I touch it!"
"Don't worry, tHat's normal."

He stopped just short of saying "tough ****, sucker!".

 

[rjvh]

thanks for the conformation that everything is OK

Robert-Jan

 

[dknguyen]

Yeh, for the motor circuit I'm building I'm predicting the FET will be running at 130C room temperature. They are supposed to be able to handle 150C though, and I over assumed an ambient temperature of 50C like I usually do. I figure it should maybe be okay since full-load is stall.

I wish I could get a 20C rise out of that lol.

 

[Diver300]

I have an isolation transformer that gets so hot you can't keep your finger on it, even with no load. IIRC, the temperature was about 60C, a 30-35C rise.
My assumption is that the primary inductance is low, which saves on copper. This is why it was cheap (literally). Low inductance causes higher magnetizing current, with accompanying I^2*R loss.

Even a poor transformer will take a lot more primary current when loaded than when not. The I^2*R loss will be far bigger when loaded, so if that was what was making it hot when unloaded, it will burn out when loaded.

It is the iron loss that is causing the unloaded heating. That is due to not enough iron, so magnetic field is too intense, driving the core into saturation. With cheap iron, that is also where there is hysterysis, so there is energy loss as the iron is "permanently" magnetised forward and backwards, 50 or 60 times a second.

There is also eddy current loss, if the laminations aren't done correctly.

 

[stevez]

The manufacturer should be able to provide an indication of the limits and conditions that apply to their product. While at a basic level transformers might be similar a simple thing like insulation on the wire could affect allowable temperature rise. As Ron indicated, his transformer runs hot and the manufacturer said it's ok. The manufacturer might have constructed the transformer so that it will provide reliable service for a long time under those conditions - or they might have overstated their specifications. That's a game that's played constantly.

 

[audioguru]

I have a Chinese Ni-MH battery and a Chinese AC-DC charging adapter for it.
Boy oh boy oh boy do they get hot, very very hot.

I threw out the 9V/200mA adapter that was about 22VDC without a load and made my own charger. Luckily the battery didn't explode.

 

[grim]

it's hard to find materials to make a transformer that are rated at less than 130'C (class B). Winding wire is pretty much all 155 or 180'C rated.

23'c rise is very very very cool.

 

[carmusic]

most small transformer have a temp rise rating of 55 to 85 C over ambient temperature at full load. if you take big transfromers (1kw or more) often they have a 125 or 155 rise temp!!

 

[crutschow]

Yeh, for the motor circuit I'm building I'm predicting the FET will be running at 130C room temperature. They are supposed to be able to handle 150C though, and I over assumed an ambient temperature of 50C like I usually do. I figure it should maybe be okay since full-load is stall.

Is that 150C allowed for the case or 150C for the junction? Did you take into account the thermal resistance from the junction to the case?

130C for the case is very hot and likely will reduce the operating life of the transistors. How long does this circuit need to operate?

 

[Roff]

Never mind!

 

[Sceadwian]

Keep in mind, even if it's within design parameters pretty much everything will run better/longer at a lower temperature.

 

[Martel]

I generally had bad experience with Hammond transformers and try to avoid them.

All Hammonds i saw do heat up a lot even when unloaded. I can't keep my fingers on them for more that a second or two.

You should plug your transformer UNLOADED for a while to see if it heats-up by itself. If it heats too much, try another one.

Also, stupid comment, get sure you hook-up the transformer correctly ! Many transformers do not have anything printed on them. generally, i say, generally, Black wires are for primary.

A good way to test a transformer is to use an incadescent light bulb is series with the transformer. If you hook-up a low voltage secondary winding to the mains side, the light bulb will glow white and protect the transformer and your mains circuit breaker. If the transformer is plugged correctly, the bulb will either dim very weakly or not at all. Use a light bulb with it's wattage accordignly to your transformer.

 

[ben7]

I have some transformers that will give you blisters while unloaded!
SUCH a waste of energy!!!

 

[audioguru]

I have some transformers that will give you blisters while unloaded!
SUCH a waste of energy!!!

Transformers are not supposed to consume power, they are supposed to pass power.

 

[Sceadwian]

No transformer should be THAT hot unloaded.