noisy filament transformer

[Craig405]

Hi all,

I am slightly concerned with a transformer of mine.
I have a pair of very nice potted Gardners GR94171 6.3v @2A filament transformers. Im using them for independant heating supplies per channel in my Aikido (PCB's on the way !!)

One of them hums loudly off load, i havent loaded either yet but ill show you what i have measured..

Quite transformer:
primary current off load = 9.8ma
primary resistance = 46ohm
secondary resistance = 0.6ohm

buzzy transformer:
primary current off load = 21.6ma
primary resistance = 46ohm
secondary resistance = 0.48ohm

Whats going on? i can see the noisy traffo draws 2x magnetising current but thats all i notice?

Oh yes one more thing both are 115V primary traffos with a screw-in 500mA fuse. I m running them from 240V via a 115v autotransformer.

[nhuwar]

You could have a few shorted windings in the transfomer primary.

Nick

[Craig405]

Yea i thought that might be that case but i wasnt sure, is this likely to be dangerous?
i can put up with a bit of hum from the case, generally music will drown it out but i dont want anything failing on me a this is going to be my 'good preamp' built to last.

[HollowState]

Funny, I just ordered a pair of mono Aikido boards myself just because they were on sale.

Anyway, personally I would not live with or use that transformer. From your resistance measurments, I 'd guess that the secondary had at least one shorted turn somewhere. Either to itself or perhaps to the core. I'd give it a hypot test if I could. I'd also return it if it was new.

Edit: Does the output voltage differ? You didn't say, but that will tell you something. In any event, it will probably run hotter too.

Victor

[nhuwar]

Some one will correct me if I'm wrong. I was always under the impression that the transformer will eventually fail. The is a high heat build up in the area of the shorted winding that will eventually cause more insulation to fail making it a cascade failure.


Nick

[Craig405]

Thanks for the advice people, i guess it looks like ill only use one of them .
Real shame, one last observation in case it means somthing to someone... when my multimeter is measuring current in the noisy transformer (i.e. in series with primary) i get no buzzing at all.

The transformers were second hand so i wont take them back.

P.S. the output voltage off load was the same (about 6.8V +/- less than 0.1v difference) i guess loading might yield a better difference.

[kevinkr]

It might be that "cost" engineering accounts for the difference in primary excitation current. Since the output voltage is the same the transformer is possibly not bad, just flawed. Excessive temperature after 30 minutes should be pretty conclusive. To know whether that power consumed is real or not you need a watt meter, just the excitation current may not tell the whole story depending on the nature of the losses. (May be in the real or imaginary plane and real = heat, imaginary doesn't. Phase angle between current and voltage.)

Running transformers narrowly designed for 60Hz on 50Hz means that both core and copper losses increase (insufficient inductance), reducing operating margin, (probably requiring you to derate) and resulting in more heat and more noise. Some are rated for operation down to 50Hz and these generally run fine at the lower line frequency.

[Johan Potgieter]

Really a little strange.

The buzzing transformer draws 18% of the full-load current (in the primary) on no-load? No go; something is wrong. (The 9% of the other transformer for mainly iron-loss is already somewhat high.)

Buzzing in itself can be as simple as an un-tight core. But then I am intrigued by the effect of the series current meter. They usually drop no more than a volt. Could someone relieve the tension please?

But unless the buzzing is an easy loose piece of core. I would not use them.

[Craig405]

Hi,

Ill give it a good burn in for 30 minutes tonight, that may make things clearer.
not sure i follow about the imagimary losses. The transformers are both 50hz which is the correct frequency for UK so this shouldnt be an issue.

As for the magnetising currents being high, yea i agree, i though they would be very low when the field had been established. but i f i read the meter wrong and the measurements were in uA that seems too low to me?

If i get time tonight ill stick on a 1 amp load to make them sweat a bit and see if the buzzing gets worse, or i get strange voltage behaviour.

[Craig405]

Left it on for a good 30 minutes, didnt get warm at all. I also loaded it with 4.7ohms (about 1.37amp) and the output voltage was stable at 6.4Vrms.

Wierd.. it seems to work absolutely fine apart from the loud buzzing!