Is this knick-knack useful?

bmetbmet Solar Expert Posts: 630 ✭✭
I picked up an small autotransformer at a thrift store today for $4. It's 120VAC in, 120VAC (75VA) out. Is this useful for very small devices on the inverter output? Thank you.

Comments

  • boBboB Solar Expert Posts: 995 ✭✭✭✭
    Re: Is this knick-knack useful?

    An autoformer would have a different output voltage than its input voltage.
    Is it maybe a 1:1 ratio isolation transformer ?? If so, it could be wired for a 2:1 or 1:2 autoformer
    for 120VAC to 240VAC or isolation or different things. If so, I'd say it was well worth 4 bucks !!

    boB
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Is this knick-knack useful?

    Are there two separate windings, not connected to each other? If it's a true autotransformer, the voltages don't make sense, but if it has two separate windings you could use it as a isolation transformer, meaning the output would not be connected in any way other than magnetically, to the input. Or, you could wire it as an autotransformer to supply 240 volts to a small load, or in reverse it could supply 60 volts to a small load. Not really a big call for any of these though.
  • bmetbmet Solar Expert Posts: 630 ✭✭
    Re: Is this knick-knack useful?

    Those are the only numbers on it. The input is listed as VAC, and the output is listed as VA. It must be a 1:1 transformer, with output limited to not more than 75VA. The wattage is something like 60% of VA value, so could it be that the isolated output is limited to 45 [email protected]?
    Are there two separate windings, not connected to each other? If it's a true autotransformer, the voltages don't make sense, but if it has two separate windings you could use it as a isolation transformer, meaning the output would not be connected in any way other than magnetically, to the input. Or, you could wire it as an autotransformer to supply 240 volts to a small load, or in reverse it could supply 60 volts to a small load. Not really a big call for any of these though.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this knick-knack useful?

    If it is not an autotransformer, don't try to use it as one.
    I think it's probably an isolation transformer like Bob said.
  • bmetbmet Solar Expert Posts: 630 ✭✭
    Re: Is this knick-knack useful?

    It has the word 'Autotransformer' etched into the case. The values are the only numbers printed with it. I just wanted to know if it could be useful for solar with small devices after the inverter. Thank you.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this knick-knack useful?

    That's the definitive answer, then! Are the wires colour-coded?
    Do you have anything that needs 240 VAC?
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Is this knick-knack useful?

    Strange--I am not sure what a 1:1 autotransformer would do (except keep itself warm)...

    http://en.wikipedia.org/wiki/Autotransformer

    An Autotransformer has only one winding, and at one lead has to be to a different tap than the other three (typically step up or step down). There is no isolation between input and output (should be quick to check with an ohm meter--be careful the transformer does not "bite" you when you use the ohm meter leads and inductive kickback).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bmetbmet Solar Expert Posts: 630 ✭✭
    Re: Is this knick-knack useful?

    It has a standard Edison 3 prong plug, and a standard Edison 3 prong outlet. I'll try to submit a picture of it.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Is this knick-knack useful?

    Definitely an isolation transformer then. I have one rated 100VA and another rated 1000va and a couple rated @ 1500va. If wired correctly at the job site, they can be used as an autotransformer to either up, or down the voltage. Case in point, neighbour of mine was building a large shop but had no electricity at the time, Was too far to run a 120 volt cable from my place, so I ran 240V and used one of the 1500 VA isolation x-formers as a step down autotransformer to give them 120VAC to run their saws etc. Worked perfect and they used it that way all one Summer. Did the same thing for another friend who was building a new home about 800ft from his old home. Ran 240 from his old place and stepped it down at the new, where he used it for almost a year till he got grid there.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Is this knick-knack useful?

    Does it look like this? http://www.allaboutcircuits.com/vol_2/chpt_9/3.html#52006.jpg

    A 1:1 autotransformer would literally be two wires connected straight through, with a winding connected between that does nothing but waste power, so the "autotransformer" labelling doesn't make sense. It is possible its a reverse-phase transformer, designed to output an opposite waveform from what is input. If it shows a wiring diagram, look for dots on the input and output. If they are directly across from each other on the winding its the same phase, if one is on top and the other on the bottom it is opposite phase. Or maybe it is a variable transformer, do you see a knob or lever somewhere?
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Is this knick-knack useful?

    Actually autotransformers are 1:1, wound in phase. You connect two of the leads together, feed power to one side, and get a "mirror" image of that power on the other side. Since the windings are wound in series, you get 240 VAC with a center-tapped "neutral". Or you can feed it 240 VAC and get a balanced pair of 120 VAC connections. If you don't connect the two leads the windings aren't in series and you have a 1:1 isolation transformer.

    This is not the same as a 1:2 step-up/step-down transformer nor an isolation transformer that does not necessarily have its windings in phase.

    Sounds like it's either internally coupled (in which case the use of standard outlets for in and out is wrong) or mis-labeled as something it's not.
  • techntrektechntrek Solar Expert Posts: 1,372 ✭✭✭
    Re: Is this knick-knack useful?

    No, autotransformers - or isolated transformers - will not output 240 from 120 input in a 1:1 configuration. That is a 1:2 configuration. Or, for a standard North American 120/240 split-phase output its 1:2 with center tap. Only difference is that an autotransformer has a single coil instead of two.

    Attachment not found.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
  • bmetbmet Solar Expert Posts: 630 ✭✭
    Re: Is this knick-knack useful?
    Sounds like it's either internally coupled (in which case the use of standard outlets for in and out is wrong) or mis-labeled as something it's not.

    I think you're correct. The standard cord doesn't look to be of the same industrial design as the rest of the box. I hadn't scrutinized it in the store, someone must have modded it for their needs. Thanks for bringing it to my attention.
  • RCinFLARCinFLA Solar Expert Posts: 1,394 ✭✭✭✭
    Re: Is this knick-knack useful?

    A 1:1 isolation transformer is used for what its name implies. It isolates the secondary from the grounded neutral of the power lines.

    They are good for troubleshooting circuits that have direct connection to 120vac like a switching power supply or electronic fluorescent light. With an isolation transformer feeding a switching power supply you can then connect the ground probe of your scope to sections of the primary side switching circuitry without shorting things out due to a direct connect grounded neutral feed that is flipped back and forth with hot side due to full wave rectifier.

    Yes, you can connect them as an auto-transformer to accomplish non-isolated 120vac to 240 vac or 240vac to 120 vac. You have to get the phasing connection correct. Connect the right wire from secondary side to primary and you get 240 vac out. Connect the wrong one and you get near zero.

    Isolation transformers usually have slightly greater then a 1:1 turns ratio. They can be 1:1.02 to 1:1.05 to make up for transformer losses to accomplish correct output voltage under loading.

    I use two 2kVA isolation transformers to isolate my inverter-generator which has an H-bridge output driver so I can ground the secondary neutral connection.

    Many small portable true sinewave inverters do not allow neutral side of AC out plug to be neutral grounded. You can use an isolation transformer if you need to feed into the house wiring which has a grounded neutral. You should not use a transformer on the output of a modified sinewave inverter.
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