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  1. #1
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    Default Searching for a small step down transformer.

    I have a machine that's connected to 240v-3phase. There is a onboard DRO that has a small 120v wall-wart type power supply. I would like to find a very small transformer with a 240v primary and a 120v secondary to power that wall-wart. All the control transformers I can find that accept a 3 phase input are a bit to large for my liking.

    Is there a dinky type I'm overlooking?

    Stuart

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    Unless your DRO requires 3 phase (and it doesn't) power you only need a single phase transformer. Look on ebay for control transformers. They are very common. Use the data on the sided of the wall wart to size the control transformer. There are possibly consumer products for using 120V items on European power that would work as well.

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    Bill,

    The little DRO is a Chinese, single axis cheapo that serves a purpose for me on this particular machine..it operates on two watch batteries but has the ability to plugged into its little charger so as not to rely on the batteries. I have seen the travel voltage adapters, but my problem is I don't know if two legs of a true 3 phase system is the equivalent of two legs of 240 volt European power, which I could call '2 phase'?

    Stuart

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    I "think" it is, I had a european piece of test equipment (o-scope) that was 240v 1ph, I had same concerns you do, power input looked like standard 120v socket on back of every computer/tv/etc, I wired it 120-N-120 and it worked fine.

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    If you look at my Avatar you will see a white box in the bottom of the Monarch. That is one of those step/step down chinese transformers. switch select input voltage, output voltage, and directly takes those round pin european plugs, as well as USA.

    I was given a Dewalt reciprocating saw, new but 240V with european plug. Works great, but I can barely pick the saw up.

    Heres one $45 ebay 303123826110

    many others

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    Check your wall wart, many can accept up to 240V.

    What is it's output rated at, a new wall wart of the proper rating might be easier and cheaper?

    SAF Ω

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    One leg of the input to neutral will give the lower voltage. A wall wart won't cause imbalance.


    It works. Unless there is no neutral pulled to the machine. A bad practice not to pull that fourth wire in my opinion, but it saves a length of wire.

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    I'm still a bit confused. The facility my machine will be delivered to will supply 240 volt 3phase which will be 3 wires with a 4th wire as a equipment ground. There is no neutral to my knowledge unless you're calling the safety ground a neutral, which it isn't..IMHO.

    Stuart

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    Quote Originally Posted by atomarc View Post
    I'm still a bit confused. The facility my machine will be delivered to will supply 240 volt 3phase which will be 3 wires with a 4th wire as a equipment ground. There is no neutral to my knowledge unless you're calling the safety ground a neutral, which it isn't..IMHO.

    Stuart
    It is NOT.

    Hence the comment about the "extra run of wire"


    On re-read of my post, I see my error. "the fourth" should be "the fifth". Sorry.

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    Because I don't undertand the finer points of electricity I simply ordered a traditional control transformer with a 240/480 primary and a 120/240 secondary. I've used a ton of them and understand how they work.

    What I don't understand, and what I was trying to ask in my original post is...two legs of 3 phase are out of sync 120 degrees and I think two legs of 240 volt single phase are out of sync 180 degrees. A control transformer would use only two legs on the primary side. Does it matter that the legs are out of phase 120 degrees or 180 degrees..does it matter to the secondary.

    Does this make any sense?

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    Between any two wires the voltage is 180 degrees out of phase. Without a third point reference point there is no 120 degree phase shift. And since you are only using 2 wires it is just single phase power.

    In fact single phase power from the power company is power taken from two wires of the 3 phase power.

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    Bill,

    Thank you. So what I think you are saying is that connecting a transformer to two legs of a 3 phase system is no different than connecting a transformer to two legs of a single phase (240v) system. The transformer doesn't know or care..is that correct?

    Stuart

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    Quote Originally Posted by atomarc View Post
    Because I don't undertand the finer points of electricity I simply ordered a traditional control transformer with a 240/480 primary and a 120/240 secondary. I've used a ton of them and understand how they work.

    What I don't understand, and what I was trying to ask in my original post is...two legs of 3 phase are out of sync 120 degrees and I think two legs of 240 volt single phase are out of sync 180 degrees. A control transformer would use only two legs on the primary side. Does it matter that the legs are out of phase 120 degrees or 180 degrees..does it matter to the secondary.

    Does this make any sense?
    electricity doesn't work that way

    two legs are always 180, any two, any time.

    Magic!

    Don't let the smoke out ;-)

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    Magic aside Cal...looking at two legs of household 240 voltage with a scope, you'll see the sine waves out of phase 180 degrees. Same scope but hooked to two legs of a 3 phase supply, your scope will show the sine waves out of phase the same exact 180 degrees, right. Is that correct?

    If I were a little step down or control transformer and you hooked my two primary leads to either of the above power sources, I wouldn't know the difference, right?

    I'm not being argumentative..just electrically challenged.

    Stuart

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    Quote Originally Posted by atomarc View Post
    Magic aside Cal...looking at two legs of household 240 voltage with a scope, you'll see the sine waves out of phase 180 degrees. Same scope but hooked to two legs of a 3 phase supply, your scope will show the sine waves out of phase the same exact 180 degrees, right. Is that correct?

    If I were a little step down or control transformer and you hooked my two primary leads to either of the above power sources, I wouldn't know the difference, right?

    I'm not being argumentative..just electrically challenged.

    Stuart
    Hi,

    On single phase power 240v with two hot wires, you have 120v
    To ground on each and the two hots are 180degrees out of phase.

    On 3phase delta, you have 120degree phase angle between the 3 hot
    Wires.

    The voltage between any 2 hot wires on 3ph is sqrt(3) * V where V is the ground to hot voltage of any one hot leg.

    E.g. With typical 3ph delta in the US you would have 120v to ground, 208v
    Between 2 hots.

    so, if you want a transformer, get one that is 208v to 120v. For some decent wallwart transformers now they are auto-sensing 120v-240v, in which case you can just use one of them between two hot legs.

    Note the above is for delta 3ph. I would suggest using a voltmeter to verify hot to ground on all legs is 120v and hot to hot is 208v.

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    Quote Originally Posted by Brandenberger View Post
    Hi,

    On single phase power 240v with two hot wires, you have 120v
    To ground on each and the two hots are 180degrees out of phase.

    On 3phase delta, you have 120degree phase angle between the 3 hot
    Wires.

    The voltage between any 2 hot wires on 3ph is sqrt(3) * V where V is the ground to hot voltage of any one hot leg.

    E.g. With typical 3ph delta in the US you would have 120v to ground, 208v
    Between 2 hots.

    so, if you want a transformer, get one that is 208v to 120v. For some decent wallwart transformers now they are auto-sensing 120v-240v, in which case you can just use one of them between two hot legs.

    Note the above is for delta 3ph. I would suggest using a voltmeter to verify hot to ground on all legs is 120v and hot to hot is 208v.
    This is incorrect. However what you say would be correct if you substitute "WYE" for "Delta".
    Wye has a neutral in the center. Delta has no center. Even with a WYE in which there is a phase shift relative to the neutral to each of the hot leads one can not tell that the 208 from any two wires is anything other than single phase unless you reference to the neutral point.

    On a delta there is not 120 degrees between each of the three wires. There is 120 degrees between the voltage of any two pairs of two wires, ie, AB and AC and BC have 120 degrees of shift between them but not A to B or A to C or B to C.

    Stuart said he had 240V 3 phase so if that is correct he has a DELTA source.

    But the solutions outlined by myself and others will all work on one pair. After reading the responses I think that a 240V wall wart may be the simplest and least expensive solution.

    Bill

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    Bill,

    Just to crystal clear, using a wall wart rated for 120-240 volt input, connected to two legs of a 240v-3 phase circuit will get me what I want..without smoke!

    Stuart

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    Stuart,

    Yes it will work great. No smoke.

    Three phase power is just three sets of single phase power with a time difference between each of the three phases. All that matters is the voltage across any pair of wires. No matter what path it takes, taken two wires at a time it is single phase.

    Now I will make a recommendation on which two wires to use. Most three phase delta installations provide 120V by center tapping one of the transformers and thus two of the wires will measure 120V to neutral (or ground) and one wire will measure a much higher voltage to ground. Use the two wires that measure 120 volts to ground. This will provide a little bit of saftey as the insulation of the device will be stressed less.

    Bill

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    Bill,

    Thank you.

    Stuart

    Edit. Further scrutiny of the little 5v wall wart thingy lists its input voltage as 120-240volts.
    Last edited by atomarc; 09-14-2019 at 05:36 PM.

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    Much as I hate to do this, I'm going to flog this dead horse a little bit more. Looking at the stuff in front of me, I see the little 5 volt wall wart charger for the digital scale and it's tagged for input voltage of 100-240 volts.

    I'm making the assumption that this makes it more global friendly as Europe and others use household power in the 220 volt range. Looking around the net tells me that European residential power consists of a single leg of 220 volt and a single leg which is neutral.

    The power that's available to me is standard, run of the mill 240 volt 3 phase which has 3 wires and a safety ground, but no neutral.

    The little wall wart transformer can use 240 volts but its two prong plug would need 240 volts on a single prong and a neutral on the other..just like European power.

    I'm confused..am I missing something?

    Stuart


    Edit. Is this a 'stinger' configuration, with one leg 240 and the other neutral..I think it is.
    Last edited by atomarc; 09-15-2019 at 11:58 PM.


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