Author Topic: Transformer in reverse  (Read 2483 times)

Offline John Rudd

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Transformer in reverse
« on: December 13, 2015, 11:48:14 AM »

For the sparkie types among us.....I need a 400v ac supply.....

I have a toroidal transformer with 240v ac pri and 110v ac secondary....
If I feed it via variac on the 110v side to give me around 400v ac on the primary any problems in doing this?  :zap:
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Offline chipenter

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Re: Transformer in reverse
« Reply #1 on: December 13, 2015, 12:02:39 PM »
Step up step down workes both ways , step up can get hot if you draw to mutch current .
Jeff

Offline Joules

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Re: Transformer in reverse
« Reply #2 on: December 13, 2015, 12:04:07 PM »
I would seriously downrate the transformers capacity if you use it in step up mode.  Your old primary winding was wound with lighter gauge wire.  My guess would be the 110v side will also have a lower impedance than desirable, could result in a high inrush current.  Play carefully. 
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline John Rudd

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Re: Transformer in reverse
« Reply #3 on: December 13, 2015, 12:20:59 PM »
Thanks for the advice guys....

I have a 3ph inverter that obviously needs a 415ac supply, if I can generate 415v single phase then I should be able to test it.... :zap:

I have a test rig that comprises a light bulb in series with the Live leg, then a switch, then a 13A socket, using my variac ( 500w rated) I can gradually wind the juice up using the lamp bulb as a current limiter....ok back to it.... :bugeye:  :zap:
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Offline awemawson

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Re: Transformer in reverse
« Reply #4 on: December 13, 2015, 12:43:08 PM »
John, if I understand what you are doing correctly you are going to feed the 110 winding with pretty well twice the voltage it was designed for and it WILL draw too much current and over heat.

Although the transformer ratio is about right, when they designed the primary and secondary windings it was done to avoid magnetically saturating the core. If it saturates, impedances drop dramatically. fuses blow and the cursing starts.  :zap:

Some useful guidance here:

http://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/practical-considerations-transformers/


If you don't need the transformer to isolate, then you can connect primary and secondary in additive series (ie start of the 110 to the finish of the 240). Then applying 240 to the 240 winding will give you 240 + 110 = 350 across all windings which may be in the range of operation of your VFD ?
« Last Edit: December 13, 2015, 01:26:09 PM by awemawson »
Andrew Mawson
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Offline Joules

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Re: Transformer in reverse
« Reply #5 on: December 13, 2015, 01:13:51 PM »
Sounds much more sensible using it as an autotransformer.  I don't really understand how you're going to power your VFD with only equivalent of 2 phases though, surely it will just flag an error.  It's dark now so will watch for the flash on the horizon   :zap:


Be careful.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline awemawson

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Re: Transformer in reverse
« Reply #6 on: December 13, 2015, 01:24:58 PM »
Joules, most VFD's take in three phase and rectify to some DC voltage using a three phase bridge rectifier. If you apply single phase 400  between two phase inputs then you still get the DC rail established, you just have to de-rate the VFD a bit.

The Gotcha is that some, but not many, VFD's have 'phase failure detectors' embodied that will fail out if you do this. You're fine in most cases though and I've done it many times reliably.
Andrew Mawson
East Sussex

Offline Joules

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Re: Transformer in reverse
« Reply #7 on: December 13, 2015, 01:34:52 PM »
Cheers Andrew, didn't know that.
Just get doing and make swarf, you can decide what its going to be later.   :thumbup:

Offline philf

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Re: Transformer in reverse
« Reply #8 on: December 13, 2015, 01:36:07 PM »
Thanks for the advice guys....

I have a 3ph inverter that obviously needs a 415ac supply, if I can generate 415v single phase then I should be able to test it.... :zap:

I have a test rig that comprises a light bulb in series with the Live leg, then a switch, then a 13A socket, using my variac ( 500w rated) I can gradually wind the juice up using the lamp bulb as a current limiter....ok back to it.... :bugeye:  :zap:

John,

There was some discussion a while back on modifying a 415v input 3 phase VFD to run on 240v single phase. Have a look in this thread:

http://madmodder.net/index.php/topic,10679.msg123558.html#msg123558

I successfully converted an ABB VFD which is now happily powering my Fobco Seven-Eight drill.

Phil.
Phil Fern
Location: Marple, Cheshire

Offline John Rudd

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Re: Transformer in reverse
« Reply #9 on: December 13, 2015, 01:36:39 PM »
Andrew,
Good idea about the tranny windings....I should have thought of that in the first instance.... :doh:

I'll give that a try...need to find the transformer out first....hidden in a pile of other stuff.

I'm limiting my chances of something going  :zap: all of the plugs have 3A fuses in them... :lol: and my nerves dont like big bangs...
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Offline John Rudd

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Re: Transformer in reverse
« Reply #10 on: December 13, 2015, 01:41:01 PM »
Phil,
After reading your post( while I typing my previous reply ) I 've had a light bul mome.... :scr

Oh dammit....

But just another thought....the majority of inverters I've repaired/ tested have undervolts lockout on them....if the dc bus isnt high enough they wont run....
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Offline philf

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Re: Transformer in reverse
« Reply #11 on: December 13, 2015, 01:57:22 PM »
John,

To quote HopefulDave:

.....older inverters (ABB ACS300 series, for example) have two banks of reservoir capacitors, a little rewiring to put the neutral between the banks and live to two of the rectifier's three phase inputs and it thinks it has 480v in when you feed it 240... Again, derating by about a third is in order!

So, no undervoltage problems.

When I modded mine I ran it up via a Variac and an internal relay clicked in at 190v input.

Phil.
Phil Fern
Location: Marple, Cheshire

Offline John Rudd

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Re: Transformer in reverse
« Reply #12 on: December 13, 2015, 02:01:32 PM »
Phil,
Yup you are right....what was I thinking! :Doh:

I can cobble up a bridge rectifier and a couple of largish caps to double the juice....connect this lot across the existing caps and away we go..... :zap:

Ok, seems a bit safer than the transformer route.....mebbe :coffee: :smart:

I'll sleep on it....
eccentric millionaire financed by 'er indoors
Location:  near Hull

Skype: chippiejnr