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The_Davster
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[*] posted on 30-7-2005 at 11:48
Rewiring a transformer


So here is the background info behind this thread...
I recently bought a hotplate/mag stirrer from a lab supplier in the states. When I recieve it, however, the damn thing takes 240V(did not say that on the item description). Seeing as I am in Canada and not europe this poses quite the problem to me. I have went around and checked travel stores for a transformer that will convert 120V to 240V, unfortunatly they only have ones that will convert 240 to 120V:mad:.

Since I can buy this (see edit) product locally, would it be posible to crack it open and switch the input and output so it converts 120 to 240V?

In terms of grounding as the transformer does not have a ground plug hole, would it be sufficient to run a wire from the ground hole on the European plug on the hotplate directly to the ground socket in the wall plug?

Also why is there no ground pin on european plugs?


Thanks all.

EDIT: Wrong transformer I want to modify, the one I want to use is this.



[Edited on 30-7-2005 by rogue chemist]




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bio2
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smile.gif posted on 30-7-2005 at 12:18


........unfortunatly they only have ones that will convert 240 to 120V. ..........

This could be connected in reverse to double the voltage providing the wire size is not too small on the secondary which becomes primary. Choose a transformer in which the 240V winding current is greater than or equal to the hotplate. The kVA rating should be a little more than the load for cool long term operation.

A single phase 480/240V control transformer would also work. They are available at industrial electrical suppliers like WESCO or GESCO.

European standard is 380/220V or 415/240V and there is a neutral and ground in the house plug. The convertor you showed is thyristor based and can't be reversed and a 50watt hotplate ain't much use anyway is it? :o

Another possibility is that if the heating element is not a single piece wire then it could be modified. If you plug into 120V now you get 1/4 wattage and the controls/motor won't work!
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[*] posted on 30-7-2005 at 13:15


Transformers can work equally well in either direction , just make sure to keep the amperage within rating .

Almost certainly you have 240 available even though there isn't an outlet for it ,
and this is one way to get to it .

At your breaker panel look and see which wall outlets in different areas are on different branches . See it is 240 in the panel , and 120 is split from the 240 into two branches . The 240 feed is 3 wires ,
240 volts between the outside pair of hot wires , and 120 between each of them to the center wire which gives your two 120 branches . If your breaker panel has any
unganged small dual breakers they are very likely on separate feeds and that may narrow your search for which served outlets will have the separate branches where you can get 240 , half from one outlet and half from another outlet :D

Confirm using a test meter and some extension cords and just locate which 120 wall outlets are on different 120 branches so that there is 240 between their hot wires . You can make up a special 240 extension cord which has its feed for one hot leg from one wall outlet plug and another plug for a second wall outlet on a separate branch , Y connected to give you 240 . Y connect the neutrals to make your neutral for your 240 . You can Y connect the grounds from each wall outlet to provide a separate ground for your 240 if you want it or have a 4 conductor cord to your 240 outlet , having a separate ground .

I deliberately have separate branch 120 outlets in my shop , and a couple of 240
outlets dropped locally from those separate branches this way . It may not be code to do this sort of nigger rigging ,
but it is a technically sufficient way to get 240 where you need it in a pinch , if you have nearby two different 120 branches .

It's also cheaper than big transformers or
running separate wiring for that oddball whatever that needs the 240 where you don't have it handy .
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[*] posted on 30-7-2005 at 13:34


Rosco, so as I understand it the 240V line comes into the house with essentially a +120V line, a -120V line, and a neutral line. Use of the +120 and -120V lines creates 240V of potential difference while using either the +120 or the -120V line with the neutral creates just 120V potential difference. But seeing as the 120V lines are already 6.3A, and the hotplate takes 3.6A, would this not work?

Bio2, could you explain "Choose a transformer in which the 240V winding current is greater than or equal to the hotplate. The kVA rating should be a little more than the load for cool long term operation." a bit more, I am not sure what you mean

[Edited on 30-7-2005 by rogue chemist]




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[*] posted on 30-7-2005 at 14:20


Rosco's idea is a good one and certainly cheaper if you will not be moving the hotplate very far. However don't apply a much higher voltage than rating to a transformer as it may saturate.

Only I might add that for safeties sake put a 2pole common trip breaker at your tie in point. This is not code either but if you don't do this then one side is still hot if it trips and it will appear dead to the unenlightened! IIRC technically it is only allowed when accessible to authorized personnel like a locked control panel etc.

All I mean for the xfmr rating is to look at the nameplate amps to make sure it is enough on the side which is 240v which will carry the hotplate amps when connected reverse. This also applies to a 480/240V xfmr which won't be cheap in a 1-2kVA capacity.

Maybe Roscoe can explain to you why there is no +/- line in AC which goes positive/negative at the line frequency (60Hz).

[edit - double post]

Whoops, I almost forgot this VERY important point.

If you do use a 240v (2 hot lines) circuit from the house wiring then examine very carefully the circuit in the heater as it was intended for a neutral line and 1 hot line! This is 380/220V Wye connected. In North America this is equivalent to our 208/120V or if Delta 240/120V.

[Edited on 31-7-2005 by Ramiel]
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[*] posted on 30-7-2005 at 14:33


Yeah, I get that there is no static +120 or -120, they always switch as they are AC, I just meant it figurativly.

I understand about the amps in the transformer now Bio2, thanks.

[Edited on 30-7-2005 by rogue chemist]




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Rosco Bodine
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[*] posted on 30-7-2005 at 15:41


At the street pole is a 15Kv maybe more distribution line to a transformer . For a residential service , the output side is single phase AC 240 V ( RMS ) center tapped and the center is also grounded at the pole and at the power plant and everywhere else . The center tap conductor is usually a bare stranded cable around which is twisted the two insulated conductors , and this twisted three conductor is what goes to the power meter and from there to your breaker panel .

The center tap bare conductor is the common neutral for the 120 branches .
Either of the insulated end tap conductors
down from the service pole transformer will form a 120 V circuit branch with respect to the center tap or neutral ,
which definitely carries current , even though it is also a ground . In fact if both
120 branches are fully loaded the neutral
wire carries the sum of the amperage of both the outer conductors .

Normally a 240 volt appliance heating element is what is powered by the 240 in order to use a smaller gauge wire for the supply cord and not have the supply cord overheat from IR losses . On something like an electric stove , the heating elements are supplied 240 from the two hot wires , and the oven light and clock are supplied 120 by tapping power from
one of the hot wires , and then from the neutral , to provide a local 120 source from the 240 outlet . Air conditioners may be the same way with the compressor running off 240 , but the fans running 120 which is split off in the unit .

Many of your dual breakers have a hole through the toggles where you can gang them together with a cotter pin so they will trip together or remove the pin so they trip separately as you wish .

Anyway if you see the two branches of 120 as simply each being the top half or bottom half of a center tapped 240 transformer , how all this works should become clear .

Your green wire or bare copper wire in your wall outlets is a " backup ground "
a redundant safety conductor that is never energized unless something else
is compromised .
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[*] posted on 31-7-2005 at 08:06


Thats a 220V european plug. The ground is connected to the 2 bands you see on the side of it.
The transformer you linked is the opposite of what you need. Thats something you need when you go to europe on a trip and need to plug a 120V american appliance on the 220V receptacle.

Probably something like this should work:
http://cgi.ebay.com/1000-w-up-down-voltage-converter-transfo...



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[*] posted on 11-8-2005 at 23:38


So here is the background info behind this thread...
I recently bought a hotplate/mag stirrer from a lab supplier in the states. When I recieve it, however, the damn thing takes 240V(did not say that on the item description). Seeing as I am in Canada and not europe this poses quite the problem to me. I have went around and checked travel stores for a transformer that will convert 120V to 240V, unfortunatly they only have ones that will convert 240 to 120V.

Since I can buy this (see edit) product locally, would it be posible to crack it open and switch the input and output so it converts 120 to 240V?
------------------------------------------------------------------------
Easy. Place 120VAC across the output and measure the input side. It should read 240VAC. However...ther is only one problem....what is the transformer rated for interms of VA or Volt/Ampere rating? The transformer may not be suited for high VA draw in which case you will smoke the transformer.

PS: Reversing the voltage feed on the transformer will NOT damage the transformer windings.




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The_Davster
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[*] posted on 12-8-2005 at 00:00


Quote:
Originally posted by ((Blasta))
what is the transformer rated for interms of VA or Volt/Ampere rating?

I am not sure, but oh well, I got the product in Archimede's link. I lost 2 auctions on the same product so I just broke down and paid full price for it.




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[*] posted on 12-8-2005 at 03:36


Voltage/Ampere gives you the resistance and VA gives you the power. I guess that most of the transformers are rated in terms of VA because the power in a transformer is always constant. It is the voltage and the current which are changing in a transformer (normally shown as VpAp = VsAs). The input voltage and the output voltage depends on the number of coils on each side. A step-up transformer converts a low input voltage and high current to a high output voltage and low current. Therefore, it is relevant to rate transformer in terms of VA because consumers want to know how much power is involved in the transformer as both the voltage and the current can be changed accordingly.
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