Modifying generator windings to convert output from 220V to 55V.

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  • #1
GSPL
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Sirs,

I have two honda clone 2.5KW/220VAC gasoline generators, one with AVR and another with Capacitor excitation. I want to modify alternator windings of one of them so that it can directly output around 55VAC which shall then be rectified and use it to charge 48V batteries. I have fair knowledge of Electronics and so building a customized AVR (if there is a need) should not be issue. My questions are:

1. For 48V battery charging, the stator windings (probably rotor winding also) has to be changed in a way that existing numbers of winding turns are reduced by 1/4 (for voltage reduction from 220V to 55V) and increase coil diameter effectively by 4 times (to increase current capacity).
2. For the purpose of direct battery charging (48V) with winding modified, which excitation method (AVR or Capacitor) would be simple and efficient?

I am aware that there are other options available for battery charging but they are either costly (220V to 48V charger) or needs mechanical modification in the engines to drive alternator through pulleys. So i am inclined towards the option of changing the windings itself.

May i have suggestions from experts if this idea sounds good ?

Best,
GSPL
 

Answers and Replies

  • #2
Windadct
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As a mass produced item - IMO - you will not be able to modify it and get it to work reliably. Build a charger with a Stepdown transformer.
 
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berkeman
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  • #4
DaveE
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Much easier to just put a 4:1 step down transformer (or autotransformer) after it. Yes it's big and heavy, but it's easy, reliable, and clean.
 
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  • #6
anorlunda
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Although possible, I would not take on such a DIY project unless I had a lot of schooling on motor design and a lot of experience with motor rewinding. Even then you may fail because many mass produced items are not intended to be disassembled/reassembled.

Since you do have experience with electronics, perhaps a DIY charger as @Windadct suggested is a better fit for your skills.
 
  • #7
jrmichler
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I once spent a summer working in a motor rewinding shop. There are many details that are not in the books, but are absolutely necessary in order that the finished product actually work. Things like voltage between the first and last turn of a coil, burrs on the stator laminations, how to get the wires into the slots, managing the end turns, stator varnish (or epoxy), and much much more.

How much current do you need? Can you adjust the engine governor to run at 25% speed? Or mount the generator separately from the motor and drive it with 4:1 ratio belt drive? Either approach would be much less work than rewinding the generator, and would have a far better chance of succeeding.
 
  • #8
sophiecentaur
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Much easier to just put a 4:1 step down transformer (or autotransformer) after it. Yes it's big and heavy, but it's easy, reliable, and clean.
I can't think of any reason why this isn't the obvious and best solution. An autotransformer could be cheaper and lighter, too.
There are many details that are not in the books, but are absolutely necessary in order that the finished product actually work.
I can well believe this; winding coils for a given current capacity requires much thicker gauge. 10 gauge would handle 50A but, wound in a motor, you would need around 8 gauge (if I have my swg and awg sorted out correctly). 4mm wire is thick!

At least, if you buy a transformer, someone else will have taken care of that problem and it will have a guarantee too.
 
  • #9
artis
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@GSPL Since isolation transformers are heavier and bigger I would suggest in this case an autotransformer.

I assume you need to step down to 55 volts AC in order to have some margin for voltage sag , because if you want to charge 48 volt batteries then rectifying 55v AC to DC with a full bridge rectifier and smoothing caps will give you a no load voltage of 55*1.414=77.77 volts DC, I'm not sure what voltage regulation you will use but that seems like a bit too much for 48 volt batteries you will have to drop 30 volts DC.
Unless of course I'm missing something in how 48 volt batteries have to be charged.
Anyway if the amperage is not that high you could also build a capacitor voltage divider out of some old caps or whatever, then use the stepped down voltage through a even smaller autotransformer to drop the extra voltage.
like a capacitor divider would say give you 220/2=110 , or adding more in series would drop the voltage even more but then I suspect capacitor series resistance and maximum current might become a problem.


Maybe an even easier solution might simply be to use thyristors, with thyristors you can simply cut the AC at whatever voltage level you wish, thyristor are rather rugged and can handle high power. Also they are easy to implement on AC. Then simply rectify whatever comes after the thyristor to DC, also you could then use the same thyristor for voltage control of your DC output.
Many vacuum cleaners have them for example to reduce motor RPM.
 
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