General question about transformers.

Discussion in 'General Electronics Chat' started by THCynical, May 5, 2014.

  1. THCynical

    Thread Starter New Member

    May 5, 2014
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    I have a few general questions about transformers from a couple of basic experiments I have conducted. I can find very specific and detailed information online, but not a general rough explanation of what going on. I’ll try to keep things as clear as possible.

    I have recently been mucking about with transformers and I’m just wondering;
    Why do larger Microwave oven transformers, when powered from a 12V 100W supply, have a higher output drop off than smaller transformers when loaded?

    For example, MOT 1 produced 227V from 12V, but when I tried to power a small 18W 120VAC MO fan It barley rotated output down to just 38V, and with MOT 2 107V output down to 21V and the fan didn't move.
    But when I use a midsized centre taped step down (wired in reverse) A 190V output drops only to 90V and the fan rotates with vigour, drawing 3A from the 12V supply.
    I use this transformer and fan when soldering to blow away the fumes, the transformer gets quite warm after extended use.
    The diagram also includes the coil resistances.

    What factors are at play? And what information is needed to estimate this effect?

    But from these very basic experiments I’m also wondering why doesn’t wiring transformers secondary into the primary of another and so on produce stable very high voltage?
    CTSD transformer, stepping up into MOT1, doesn’t work, MOT1 has a similar/same output as if it’s connected directly to 12V, upon investigation the first transformer has it output dropped right down to the 12V on the input. Only when I use the two MOTs one stepping up into the next to I get around 2kV put it drops sharply with any type of loading.

    Finally, and this is the one that has me most baffled, when using a small wall plug adapter 12VA AC wired in reverse as a step up 12V input to 207V unloaded. Loading with the fan gives 70V output, this is enough to power the fan but there is almost 3 times the rated current trough the transformer, it gets hot and begins to fail.
    It’s the nature of the failure I don’t understand. The fans speed after a small amount of time beings to fluctuate eventually slowing right down, But voltage remains constant!
    The 12V in and the 70V output slowly drops to 64V, but not necessarily at the same time as the fluctuations.
    I’m assuming it has something to do with the increase in temperature but I have no idea how this is happening and would not know how to begin to ask it.
    Both the fan and the power supply seem robust.

    If this type of effect has been discussed before point me it that direction, if there are external links that are relevant include them.
    My first question is the main one. If you have any opinion any way related feel free to chime in!
    Thank you for your time and wisdom.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    One thing to keep in mind when dealing with M.W.T. is that there is often a magnetic shunt inserted between the windings.
    When rewiring these TXFR's for other uses the magnetic shunt is usually punched out.
    Max.
     
  3. PackratKing

    Well-Known Member

    Jul 13, 2008
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    For example, MOT 1 produced 227V from 12V, but when I tried to power a small 18W 120VAC MO fan It barley rotated output down to just 38V, and with MOT 2 107V output down to 21V and the fan didn't move.


    12 v will not induce an adequate magnetic flux in the MW xfmr core, to transfer enough energy to run your motor...

    OTOH< I have some huge UPS xfmrs, [pic] that will produce 48 volts, in the #8 wire coil shown on the top, when using the black / Orange leads as 120 v Primary... That puts up enough power, just due to its size, to run a 1/3 Hp 120v induction motor under a moderate load...

    The xfmr in the pictures normal primary, is the blue / white on the left, though I have dozen ~ options of combinations for the primary coil, counting the 5-color coil with black as common..with voltage outputs up to 156... with output current equal to input... All leads -- primary / 5-wire coil, are #16

    That #8 coil, is never used as a primary, though with the right primary combo, I gan get 59 volts @ input amperage, all day long... which is being built into one kick-ass bench supply.

    The xfmr shown, is a 1000 watt... I have four - 600 watt models available for sale, { previously listed in the Flea Market } though shipping to Dublin would be utterly ridiculous, as they weigh ~ 30 pounds...
     
    Last edited: May 6, 2014
  4. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    A microwave oven transformer is probably not the best thing to play with. Certain transformers, like the microwave oven, energy limiting transformers and constant voltage transformers have very different constraints.

    The "Normal Transformer" follows the Vp*Ip = Vs*Is or power in = power out rule. Transformers also have an inherent regulation spec from no-load to full load and generally you can switch the primaries and secondaries.

    The primary is wound closer to the core.

    Frequency plays a role and transformers that operate at higher frequencies are usually smaller. A transformer designed for 50 Hz will generally work fine at 60 Hz. Sometimes transformers might be marked 50/60 Hz.

    In an "energy limiting" transformer, the magnetics are very lossy. A direct short forever on the secondary will do no harm whatsoever.

    The microwave transformer is somewhat like this.

    Constant voltage transformers have a winding that a capacitor is connected to and this "squares up" the voltage.

    The diameter of the wire plays a role as to how much current can be drawn or supplied.
    Sometimes a transformer may be made from wire with a square cross-section rather than round. This is rare, but the magnetic coupling is better.
     
  5. THCynical

    Thread Starter New Member

    May 5, 2014
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    Thanks max i'll keep that in mind.

    "12 v will not induce an adequate magnetic flux in the MW xfmr core"
    Is that due to the size of the core? is there a rule of thumb for voltage against xfmr weight?
    I connected it to mains a while back (the only weekend i ever used mains voltage) when i turned the power on it tripped the house MCB. had i known i would have used the second MOT as a ballast.

    why does it draw so much current when the secondary is open?
    i'm presuming it would not have done so if the secondary was shorted as in the case of a ballast.

    When my exams are over I reckon i'll just raid the local recycling center and harvest boot load of MOTs and other goodies.

    R.
     
  6. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    See why the microwave transformer has series issue: http://www.microtechfactoryservice.com/doubler.html

    It doesn't take much of gap for 2800 VAC to jump.

    When motors and/or transformer are first turned on, they act pretty much like a very low value resistor hence there is a large surge.

    The voltage across an inductor is defined to be V(t)=L*di/dt ; At the instant of turn on di/dt is very large and somewhat dependent on where in the mains cycle power was applied.

    Open or unloaded secondaries should generally create higher voltages.
     
  7. PackratKing

    Well-Known Member

    Jul 13, 2008
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    "12 v will not induce an adequate magnetic flux in the MW xfmr core"
    Is that due to the size of the core? is there a rule of thumb for voltage against xfmr weight?
     
  8. THCynical

    Thread Starter New Member

    May 5, 2014
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    "draws a short pulse of line amperage, to magnetize the core... likely enough to blow a breaker... it is wise to place fusing in the line when setting up a test.."

    I have my own 10A MBC on the bench, which tripped, but so did the main house one think 40/60A or somthing.
    oven heating element sounds like the right type of load, no chance of doing one of those in!

    removed, the MOT has no ground and is isloaded, but when its in the actual MO the secondary is on the chassis.

    So does that mean that both the primary and secondary are earth grounded?
    why is that not a short?

    presumably, the MO does not draw huge current when connected for the first time, does loading the xfrm reduce the line draw?

    thanks for the replies, much appreciated.
     
  9. #12

    Expert

    Nov 30, 2010
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    Some of this just seems backwards to me. (I guess that's why we are discussing it.)

    First, the primary and secondary coils stand alone until you connect them to something. Attaching one end of two isolated coils to ground does not form a closed circuit. I use a MOT on a Variac to get 300 volts for my vacuum tubes, and it works just fine with a low voltage given to the primary.

    Second, it seems to me that powering up an unloaded transformer should do just about nothing, as long as the powering voltage is at or below the design voltage. Attaching 12 VAC to the primary should only cause the secondary voltage to be about a tenth of what the label says (assuming you have 120 volts at the wall outlet). (You being in Ireland, you probably have 220 VAC at the wall outlet.)

    In other words, I disagree with Packrat.
     
  10. THCynical

    Thread Starter New Member

    May 5, 2014
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    "First, the primary and secondary coils stand alone until you connect them to something. Attaching one end of two isolated coils to ground does not form a closed circuit"

    I have seen schematics where both primary and secondaries are tied common ground, and because the MOT has its secondary fixed to the xfrmr core/ Microwave chassis, i assumed the primary side would be also.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If your transformer has a magnetic shunt, the normal predicted results are not going to occur.
    Max.
     
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