Power transformer heat vs loading

[beatnik]

I have recently completed a tube limiter build, everything works as expected but strangely the power transformer gets a bit hot (around 45 deg C / 111 deg F)

The PT is way oversized, the 6.3V tap and HT tap are twice the current rating that is required for this circuit, in fact I added resistors to the 6.3V output because without those the voltage was more like 6.8V
I just used this transformer because I had it laying around unused. Since it had plenty of headroom I thought the transformer would run cooler, not heating up.
Also expected there should be less hum radiation because the transformer is not operating near the limit, but in fact I quickly swapped a smaller transformer and there was 10dB less noise in the audio path.

Doing some research about the subject I found out that transformers can generate extra heath also when they are underloaded. I thought this would happen only when the transformer is overloaded.
It seems the transformer should be able to live with this kind of temperatures, anyways I would be interested in knowing your opinion about this.

 

[scott2000]

Can we see the details and specs of the transformers? How many toobs? What is the dropping resistor?AC?

 

[beatnik]

6.3V tap = 4A / actual load 2A
550V tap = 140mA / actual load 70mA
5V tap = 3A / actual load 2A

basically this transformer was meant to be for a dual channel unit.

I have left the unit powered on overnight and the temperature of the transformer did not increase further than what I listed above.

I am on 230V mains but I have tried paralleling the primaries and powering with a variac at 115V, there isn't any change in either noise or temperature.

 

[Rusan]

Don't forget the variable of which lamination material is being used, as M19 will get much hotter than M6.

 

[JohnRoberts]

The reasons why transformers get hot are: Resistive losses in the copper windings. Eddycurrents that are induced in the core of the transformer. Hysteresis loses. An excessive load applied to the transformer. Environmental factors such as heat, dirt, or water.

JR

 

[user 137017]

i have a hunch that you ave a short somewhere

 

[sahib]

It is normal. I have just done a bench test on a job using +-8V @800mA. Transformer is 2x9V @2A and the temperature settled at just under 40 degrees C

 

[user 133392]

May not apply in this situation but to add what JR quoted: Transformers can also heat up if DC or excessive DC is allowed to occur in the windings. Core saturation in one axis can also occur with DC in the windings which will increase harmonic content.

National Semiconductor Audio Handbook Chapter 6 Power Supply Design: https://www.proaudiodesignforum.com/images/pdf/NSC_Audio_Handbook_Chapter6_PSUDesign.pdf

Page 6-2 has a table to VA-rate transformers and diodes based on rectifier type and load.

Bipolar Power Supply Load Current Imbalance And Transformer Core Saturation - Pro Audio Design Forum

 

[totoxraymond]

It reminds me my Avantone power transformers. These monitors have an external tranformer that will get quite hot even when not loaded. (I mean mains applied but not connected to the speaker).

 

[SoundBlaster]

50Hz? Does make transformers run hotter than 60Hz.

 

[JohnRoberts]

50Hz? Does make transformers run hotter than 60Hz.

Properly designed, 50Hz transformers are larger than 60 Hz transformers (more iron).

JR

 

[SoundBlaster]

Key word.. "Properly". I've come across a lot of transformers that had a nearly double the magnetizing current at 50Hz than 60Hz which usually indicates not enough copper.. Regulation spec comes into play.. Copper losses vs core capacity.

 

[CJ]

measure the dcr on the 550 wind

 

[FunkyTek]

Another thing to consider, although probably not as significant as 50 vs 60Hz, is if the transformer is older, and designed for lower primary voltage, then you can get early winding/core heating. In the US, I have seen this with some of my older transformers, especially when potted in hard shellac or similar.

 

[Tubetec]

I noticed a similar effect in a smaller transformer today ,
Under 50% load it ran warm , under no load ,noticably hotter ,
A transformer that has audible buzz in the lams will also get hotter as some energy is wasted as heat in the process .
Meggering the insulation resistance across the windings might show up something ,
A spray with transformer laquer all over the bobbin and core can help glue any loose lams or wires down and perhaps reduce vibrations and wasted power especially when unloaded .

 

[CJ]

Elliot Sound Products blurb:

"The flux density in a transformer is greatest at no load, and it decreases as load is increased.​

When you test a transformer with no load, the primary current is solely due to magnetising current and an additional current caused by partial saturation (almost all mains transformers will show some evidence of saturation current. Let's assume that the transformer is operating from 230V on the primary, and it has a primary winding resistance of 10Ω. If we now connect a load to the secondary that causes the primary current to rise to 1A, the effective primary voltage is reduced by 10V (10Ω × 1A), so it falls to 220V. The flux density is reduced proportionally, and with a lower effective voltage, flux density must be lower when current is drawn from the secondary."