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Need help with finding a transformer

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tkc100

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I have a power converter designed for a recreational vehicle (see attachment) that works in parallel with one or more 12 volt batteries to delivery 12 volts at 50 amps.
Unfortunately it uses a ferroresonant transformer and is a good stable power supply but a lousy charger.
I would like to replace the ferroresonant transformer with a simple power transformer. I have looked on a number of websites but I am not an electrical engineer and I got lost.
I need help in selecting and finding a source for an appropriate transformer.
I need a power transformer with a 120 volt primary and a secondary voltage output of 15 to 16 volts. It needs to be able to supply between 50 and 60 amps.
In order to properly charge a 12 volt lead acid battery I need a voltage of about 14.8 volts.
There will be some voltage lose over the diodes but fortunately I can work with a rather wide range of secondary output voltages as the output in further control by a charge controller. The controller input can be anywhere between 12 and 50 volts.
Any assistance you all can give will be greatly appreciated
Thanks!
 

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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
How much current do you need for charging?

I'm not thinking replacing at this point. 12 A*50 V is only a 600 VA transformer. There is at least two 600 VA or greater torroidal transformers here, but no price. http://www.desertelectric.com/surplus_inventory.html

Custom is possible. I did it for quantity 1 before. Cost me $120 USD 25 years ago for a 300 VA transdormer.

Hear me out for a moment. basically what I'm going to suggest is to "wind your own" or add to an existing.

You can easily remove windings from a torroid. You can wind them with a stick slotted at two ends and can fir through the core.

It should not be too hard to wind a few bifilar turns of #8 wires to get a couple of volts. You only need about 1.5V to get a total of 3 volts. Bi-filar, means you wind two windings at the same time.

You essentially need to increase your current secondary a few volts in either direction since it's center tapped.

I can provide better details if you don't quite understand what I'm saying.
 
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Mosaic

Well-Known Member
This is a bit off topic but...I plan on using a microwave transformer for a 12 to 20 Amp 20VDC power supply. I can separate the E and I laminations, then remove the HV winding and rewind. There is this problem of the transformers being saturated under no load and heating up. If I place an air gap ( 2mm or so?) in the rebuild between the E & I lamiations, shouldn't this solve the saturation issue? Would their be other negative effects?

thx
 

Diver300

Well-Known Member
Most Helpful Member
You will get more current if you have an air gap. The frequency is fixed, and the saturation level of the core is fixed, so the back-emf from the winding only depends on the number of turns. If that is less than the supply voltage, you will get a lot of current even with no load.

I suggest that you add a few more turns on the primary. If you are using second-hand microwave transformers they should be cheap if not free, you could find two identical ones and put the primaries in series. If you make the secondaries identical, and put them in series, you will be running a long way from saturation.
 

tkc100

Member
You are right I don't completely understand.
The converter with the ferroresonant transformer supplies all of the 12 volt circuits in the coach when attached to 120v shore power. In this configuration it does quite well in parallel with the 12v coach batteries. It provides a stable clean source of DC power. I don't understand all the theory behind this type of transformer but it seems to be self limiting and does not require any additional circuitry to monitor the voltage.
The problem for me arises when we are operating on a generator. The ferroresonant transformer never supplies adequate voltage to quickly or properly charge the batteries. Ideally the converter/charger would bring the coach batteries back to at least a 90% state of charge in the smallest amount of time possible. Say two hours, that way the generator can be shut down. I have a Xantrex charge controller that I can use in conjunction with the charger so it would provide a three stage charge and control the charge voltage. It is rated a 60 amps. So the quest here is to raise the voltage and it could be anywhere between 15 to 50 volts while maintaining adequate current to quickly bring the batteries up to a decent state of charge. Presently I have about 240 amp hours on batteries so 50 to 60 amp would suffice.
I hope this better explain the situation.
As far as modifying the current transformer or another transformer I am game but will need a bit more explanation.
Thanks for taking the time to assist me!
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
A couple of bit of info to see if you unserstand.

1. A toroidal transformer looks like a donut and mounted using two large washers and two rubber washers. The primary is always first, so you can relatively easily remove windings and add windings as long as you have a 600 VA core (12 *50)

2. Bifilar wound means you take two wires at a time to create 2 windings or a center-tapped winding.

3. AC voltages can add or subtract when in series. It just depends on the phase. Parallel windings MUST be indentical and they would increase current.

4. DC voltages can add or subtract in series. Parallel DC sources have other issues.

5. the AC voltage of the secondary does not mean the same DC. For a capaitor filtered power supply 70 VAC center tapped will yield 100 VDC

So, some options:

1. Use two of what you have. 12+12 = 24 VDC as input to your charge controller
2. Add a little bit of AC to each half of your center tapped transformer. (make it from a 600 VA torroidal transformer)
3. Add a bit of DC to your output. Again, make it from a torroidal transformer.
4. Custom transformer.

Two more of what you have to increase the voltage to 24VDC might not be a bad way to go.

Reconfiguring with relays may prove to be more difficult.

Make sense?

1.
 

Mosaic

Well-Known Member
You will get more current if you have an air gap. The frequency is fixed, and the saturation level of the core is fixed, so the back-emf from the winding only depends on the number of turns. If that is less than the supply voltage, you will get a lot of current even with no load.

I suggest that you add a few more turns on the primary. If you are using second-hand microwave transformers they should be cheap if not free, you could find two identical ones and put the primaries in series. If you make the secondaries identical, and put them in series, you will be running a long way from saturation.
Ok, basicaly using the 2nd MOT as an inductor. Well, since I can separate the I & E sections I suppose I can test the saturation by shimming the I from the E and testing no load currents. Keep doing that until the current suddenly drops off.

What about this, adding iron to the core? I can use headless soft iron nails (1.6 mm thick) dipped in with some paint/clear coat and then stacked to form 3 matching blocks of iron, the varnish should hold em together. Use these 3 blocks to extend the 'legs' of the E laminations and then reassemble. Assembling the nails parallel to the laminations should manage the eddy currents as well. maybe a quarter pound or so might do it?
 
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