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Thread: Question about transformers

  1. #1
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    Question Question about transformers

    Hi. I have a question about transformer capacity. If I have a transformer rated for 200VA (~1.74A), and I hook up a 2A load to it, what is gonna happen? Is the load gonna get starved of current? Basically is the capacity a cap? Or will the load still get all the current it wants, function as it should, at the expense of the transformer's health?

  2. #2

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    At that rating/load, probably not much. It might get uncomfortably warm. Depends on the load voltage too.

    Its practically no different than putting 2a down a wire rated for 1.75.
    It is a mistake to think you can solve any major problems just with potatoes.

  3. #3

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    It is unlikely that that small an increase will do much. Things are usually designed with some head room. It is hard to say what is the limiting factor. The core or the wire. In the end it will be the wire. At some point as you increase the load, the core will saturate. When this happens, the current in the primary goes up quickly and the wire fails, hopefully before starting a fire. Of course, all the current will heat the wire and all the core losses will heat the core. The insulation between the windings could fail from heat. The question is, is the load really 2 amperes or is the load just rated at 2 amperes and really only 1.4 amperes or such.
    Dwight

  4. #4
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    I deal more with larger transformers, and I don't have my copy of the NEC in front of me, but for larger transformers at least it's allowed to run one at 125% of the continuous rating. That would be 250VA. But there are different rules for smaller transformers, as I recall, and I work more with transformers in the 50KVA to 1,000KVA range at $dayjob.
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  5. #5
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    Just tossing my .02 here. You're dealing with small transformers (unless you're talking about 12KV class voltages). The tendency of consumer-grade transformer suppliers is to design their products such that open-circuit voltages are much higher than nameplate and the voltage drops as the load is increased. That is, a certain minimum current draw is anticipated, so as to allow the secondary winding resistance to compensate a bit.

    I discovered this when converting from low-voltage incandescent light strings to LED. With a full incandescent load, the RMS voltage pretty much matched the nameplate spec of say, 12Vrms. When the incandescent lamps were substituted by LEDs (AC units with rectifier and filter fitted into the lamp itself), the transformer voltage rose to about 18Vrms, which was not so good for the LEDs. A series resistor brought things back into the desired range. Fortunately, I had a good supply of vitreous enamel wirewound power resistors.

    FWIW, these were not cheap transformers--they were US-branded screw-terminal control units. You'd initially think that using a unit with a much higher nameplate current rating would be a good thing, but it's not the case.

    My take is that with a heavier load, the output voltage will drop somewhat, but at only 125% of nameplate rating, you probably won't notice it.

  6. #6
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    If it's an older unregulated linear type transformer, check the voltage with a multimeter before you plug it into the device because they have a tenancy to drift up over time.

    The cause of the voltage drift is the windings shorting together; Which can be caused by the 50/60Hz AC making the coils vibrate, heat and the nature of them being tightly wrapped together. The transformer doesn't need to be powered either to have voltage drift because the tight winding wrapping will eventually make the varnish migrate.

    I found out the hard way some years back when I got my Sega Genesis out of storage, I used the original transformer that came with it and the console started behaving erratically. Games would crash, the console would spontaneously reboot and eventually smoke, the LM7805s burned up. I checked the voltage on the transformer, which was supposed to be 9v was actually around 17.8v. Alarmed and curious, I pulled out all of the linear transformers I had and tested them and sure enough, they were all way out of spec.

    I smashed a few of them to inspect the windings and found that the thin varnish on the copper wire had clumped up in several areas and you could get continuity between the terminals on the transformer and brushing across the windings with the other probe.

    Making sure it wasn't just my stuff, I got a friend who had literally hundreds of linear transformers of all varieties of all ages to test his and sure enough they were all out of spec. The older they were, the worse they were out.

  7. #7
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    One issue here is where (and by whom) the transformers were made. I've got US-manufactured 60+ year old filament transformers that perform just as well as the day they were new. But things weren' manufactured for the throwaway consumer market much, then--and really good transformers were (and still are) expensive.

  8. #8
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    The quality ones are probably wound by hand, or at least by a human operator on a winding machine with decent sized wire.

    All of the bad transformers I have seen go bad use thin whispy wire wound so tight that unwinding the coils usually ends up breaking sections of wire because the clumps fused together. I've seen videos of the automated winding machines doing it at a blistering pace.

  9. #9
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    That--and the old ones soaked each winding in varnish--you can tell this when you unwind one--bits of extra varnish come off with the wire. Varnish and fishpaper... That's not including the high-end potted ones.

    They don't make 'em like they used to.

  10. #10

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    Even back then they didn't always. Zenith really seems to have started the drive to low cost consumer grade. But I don't know that I've ever worked on a Crosley, I expect those to be worse, and really with all the other odd brands back then there should be others, too.

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