8mm Forum: TRANSFORMER FAILURES - how to predict them

This is topic TRANSFORMER FAILURES - how to predict them in forum 16mm Forum at 8mm Forum.

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Posted by David Pannell (Member # 300) on April 02, 2007, 06:08 AM:

Thought I'd give this topic a new title, rather than people having to wade through all the posts leading up to this question.

A couple of members have asked this question, as many of us own older machines, and it would be useful to be able to predict when a transformer is on its way out.

This is not an easy one, as there are so many variables and contributing factors, that some failures are fairly easy to predict, whilst others are not.

As I said in my other thread entitled 'Answer to general transformer question', I don't want to be accused of starting an 'Open University' forum within this Forum, so I will try to keep things as straight forward as I can.

Firstly, it is important to realise that heat is public enemy No.1 of all electrical and electronic devices. It destroys the integrity of components and insulation materials (apart from those designed to operate in a hostile environment). There are three major component parts to a transformer. The iron core, the windings, and the insulation. First to suffer is usually the winding insulation. Age can be a contributing factor, but not always. Depending upon the type of insulation used for the windings, it either stays supple, or else becomes brittle. Even if brittle, the actual insulating properties do not tend to deteriorate unless disturbed. When this happens, then the insulating material disintegrates, and we are left with bare conductors, which inevitably results in shorted turns in the transformer, leading to overheating and burn-out. If there are shorted turns in the primary, then the output voltage will increase. If there are shorted turns in the secondary, then the output voltage will decrease. This will be evidenced in the change of brightness of the lamp. In either case, this failure will be accompanied by a smell of heated formaldehyde, from which the insulating shellac and varnish is made. You will then know to switch off.

Another cause of transformer failure is by running it on too high an input voltage. The primary windings are usually made of very fine wire, and cannot accommodate a heavier current than they were designed for. Therefore, always choose the tapping nearest to the supply voltage. There is always some tolerance, and in certain instances it may be necessary to select a tapping slightly lower than the supply, in order to allow the transformer to deliver the exactly correct voltage to the lamp. This will still be within the design parameters of the transformer, as there is always a percentage tolerance.

Even if there isn't any smell of heating, check the appearance of the transformer regularly, to ensure there are no dark or brown areas on the winding insulating tapes. This will have been caused by overheating over a prolonged period of time, and the shellac will have almost certainly deteriorated.

Fractured wires are also sometimes the cause of failure, though not easily predicted. This can occur due to mis-handling or constant flexing of the windings where they exit the core, and make their way to the terminal strip. Also if the transformer has been dropped, then the windings can shift on the iron core, and their insulation can be scraped off on a sharp edge - another cause of shorted turns.

Here, a lot depends on quality control during manufacture, and it must be remembered that transformers are relatively cheap devices, not rocket science, and therefore quality may not always be up to scratch. Again, if the windings or the iron core laminations are not clamped tightly, every time the transformer is switched on, the magnetostriction will cause the laminations and windings to move, thereby exascerbating wear on the varnish or shellac insulation. This poor quality of construction will be evidenced by an accompanying sound of a 'bonk' or loudish hum, when first switched on, and then which gradually dies away, as the magnetising current settles down. In this event, the magnetising current starts off higher than it should, as it tries to align the flux density of the laminations, which should be tightly clamped together. This high inrush current can also be the cause of premature failure in the primary winding.

Finally, just to correct a point made in an earlier post, it is fine to switch on a transformer with no load on the secondary. In fact it is preferable to do so, in order that the magnetising current can establish itself before taking the load.

This is quite different from not running your stereo without the speakers connected, or, indeed from using a 'current transformer' as opposed to a 'voltage transformer', which is the subject of this topic.

By operating a current transformer with an open circuit secondary, excessively high voltages can be induced in the secondary windings which can spark and cause flashover - but that's a completely different scenario which I do not propose to discuss here.

So therefore in conclusion, keep a judicious eye on the state of the appearance of the transformer, a judicious nose on any odours, and a judicious ear on any magnetostriction 'bonks'. Inspect the transformer for any signs of looseness of any part of its construction, and don't wiggle the wires where they exit the windings.

You won't 'win 'em all', but I hope the above will result in a higher degree of predictability.

Thanks for your patience if you have managed to get to the end!!!

Best to all,

Posted by Mike Peckham (Member # 16) on April 02, 2007, 07:43 AM:

David, this is all fascinating stuff - it prompts me to ask one question though. On my GS1200 after it has been running for a while I get a buzzing noise which appears to becomeing from the transformer. It doesn't appear to vary and once it's come on it will keep on buzzing until the machine is switched off.

I was wondering if this was an early sign of impending doom?

On the pluss side, the buzz isn't transmitted through the amplifier so normal service can be maintained.

Mike [Cool]

Posted by Kevin Faulkner (Member # 6) on April 02, 2007, 07:58 AM:

Great stuff David.

I have the same sort of buzz on my Eiki 16mm machine. I have tried tightening all the screws holding the transformer together but to no avail. Fortunately it doesnt get any worse the longer its on and it cant be heard over the normal running of the machine.

I take it that this sort of thing is caused by a poorly designed transformer. All voltages are as they should be and dont vary.

Kev.

Posted by David Pannell (Member # 300) on April 02, 2007, 08:15 AM:

Hi Mike,

Do you know if the transformer is running hot? As a very rough guide, if you can just about, but barely, keep your hand on the transformer, you can reckon it's around 50 deg.C. DON'T TRY THIS WHILE SWITCHED ON! [Eek!]

This should be OK for most classes of insulation used on commercial/domestic type transformer applications.

If you reckon it's hotter than this, the shellac/varnish could be softening/melting, and as the whole transformer is usually dipped in varnish after manufacture, you could be experiencing lamination vibration due to the soft/melted varnish allowing the magnetostriction at 50Hz to cause the lams to vibrate.

When the projector is cold on start-up, the lams are held fast by the hardened shellac, and this cannot happen. Sounds like the lams were not clamped tightly enough during manufacture - a quality control issue.

However, I shouldn't worry about it too much unless the hum becomes annoying, or you think it is so severe that the vibration could be chaffing the winding insulation.

I'm not too familiar with the GS, but if the transformer mounting is by screws through the core laminations, try really tightening them up hard, without stripping the thread, of course, while the transformer is hot. SWITCH OFF FIRST, though! [Eek!]

Best,

Posted by Mike Peckham (Member # 16) on April 02, 2007, 02:03 PM:

Dave

Many thanks for your very informative reply! I shall try hat you suggested and report back, it's reassuring to know that this buzzing doesn't necessarily mean the transformer is on its last legs, so to speak.

It'll probably be the week end before I get a chance to run it for a while with the back removed and check the temperature and the torque on the mounting screws, but I'll report my findings on here when I have.

Thanks again

Mike

ps. what a helpful lot there is on here... [Smile]

Posted by David Pannell (Member # 300) on April 03, 2007, 04:36 AM:

Kevin,

I think our posts may have crossed or something. It might be an idea to try what I have suggested to Mike, ie. try tightening up when the transformer is really hot.

Also, for Mike, if the tightening exercise doesn't work, you might be interested to read the following:

Other than that, the only other cause is the coil former on the central yoke of the iron core not being tight enough to begin with. As copper has a greater coefficient of expansion than iron, the coil can become loose on the former as it heats up. If there is a real gap, you can try wedging some paper or card between the iron core and the coil former, but other than that I'm afraid there's nothing you can do in this case. Again, it's a question of poor quality control in the manufacturing process.

Good luck.

Posted by Tassos Laudas (Member # 588) on April 09, 2007, 04:54 PM:

David i returned home today just after a week's vacation and was surprised to read your helpful message. Great job is the least i can say...

[ April 09, 2007, 06:25 PM: Message edited by: Tassos Laudas ]

Posted by David Pannell (Member # 300) on April 10, 2007, 03:24 AM:

Thanks Tassos, just trying to help. [Wink]

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