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Rewinding a MOT for 300-450VAC

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GrimX9

Wed Sept 22 2010, 06:31PM

Registered Member #3225 Joined: Wed Sept 22 2010, 02:14AM
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Posts: 24

Removed the magnetic shunts and rewound some of the transformer, currently I am at 54.6 Volts with about 1.2V per winding :)

radiotech

Wed Sept 22 2010, 07:20PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Are you measuring the voltage on the go by connecting the meter to the start and the other end of the wire spool you are winding from?
Is so dont use the 100 ohm resistor I suggested before

GrimX9

Thu Sept 23 2010, 12:38AM

Registered Member #3225 Joined: Wed Sept 22 2010, 02:14AM
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I didn't grind the lamintes open, so I am just taking a long wire and winding it. I measured the voltage once the wire left over was about 6 inches long, a "realistic" output if I wanted to keep the current output. I will attach another wire, insulate it via heatshrink and tape and continue to wind. Each wire when completely wound will yield an additional 50VAC hopefully.

Here's a question for you: If my 1200 Watt transformer is outputting an "Ideal" output wattage of 1200 watts at 300VAC, rectified full wave to about 420VDC (300*squareroot 2), that equals roughly 2.8 amps. If I charge a 15,600 uf bank to 420VDC (That equals 1375J), what can I expect recharge times to be? Obviously greater than any DCDC converter, as an ideal output of 1200W is far greater than any DCDC converter I have seen.

Patrick

Thu Sept 23 2010, 01:11AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

GrimX9 wrote ...

Here's a question for you: If my 1200 Watt transformer is outputting an "Ideal" output wattage of 1200 watts at 300VAC, rectified full wave to about 420VDC (300*squareroot 2), that equals roughly 2.8 amps. If I charge a 15,600 uf bank to 420VDC (That equals 1375J), what can I expect recharge times to be? Obviously greater than any DCDC converter, as an ideal output of 1200W is far greater than any DCDC converter I have seen.

well, you need a ballast with resistence, remember I = V/R so if you have a very high voltage across a very low resistence youll get huge current, if you get too much stuff will melt. so, if you add a resistence to lower the current to a safe level, you have a charge time. I.E. its an RC circuit, so the RC times 5 product is your single charge time.

since 1200W / 420Vdc = 2.85 A if the 2.85 amps is what we dont want to exceed, then 420VDC / 2.85A = 147Ohms...now since you want to charge 15,600uF, and oops letme redo that math... ok... 15,600uF = 0.0156F x 147 ohm =2.3RC x 5 = 11.5 seconds to 99% charge on your cap @ 420Vdc.

so, you should look for 147 ohms or there about.

now, since we have only high current during the first part of RCx5 then the duty cycle might be considered lower, so you could increase current more than that 2.85Amp numberfor a short time. since it takes 1 AmpSecond to raise 1 Farad, to 1 volt, we have F= Q/V which means 0.0156 F = 6.55 ampSeconds / 420VDC
so if 6.55 Amps is tolerable for a short time then 420Vdc / 6.55 Amps = 64 Ohms. so your time constant wouldbe equivelent to 64 ohm x 0.0156 Farads = 1RC x 5 = 5 seconds for 99% of charge.

so if your secondary can put out 6.55 amp for a short time, then you could use 64 ohms fro a charge time of 5 seconds.

GrimX9

Thu Sept 23 2010, 01:14AM

Registered Member #3225 Joined: Wed Sept 22 2010, 02:14AM
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Patrick wrote ...

well, you need a ballast with resistence, remember I = V/R so if you have a very high voltage across a very low resistence youll get huge current, if you get too much stuff will melt.

Yup, I was planning a resistive ballast using a lightbulb. I'm thinking a flood lamp in series with the primary of the MOT to limit the current, I just need to find the correct resistance and lamp wattage.

Patrick

Thu Sept 23 2010, 01:21AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

hold on , hold on, im doing math...stop posting for a minute.

GrimX9

Thu Sept 23 2010, 01:39AM

Registered Member #3225 Joined: Wed Sept 22 2010, 02:14AM
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Wow, Thank you so much, I need to write down those formulas! 11.5 Seconds? That's pretty fast! Thanks again for the help, I'll probably ask another question later :)

How could I draw more than 2.85 amps if 1200W at 420V and thats the max of the transformer?

Patrick

Thu Sept 23 2010, 01:48AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok now i finished that post look again. you could get away with 5 seconds, i think. but try not to die.

i think i did all the math right, if not others will surley crush my ego once again.

Patrick

Thu Sept 23 2010, 01:57AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

GrimX9 wrote ...

How could I draw more than 2.85 amps if 1200W at 420V and thats the max of the transformer?

you just let the transformer heat up abit and brown out your neigborhood.

and yes charging, 0.0156 F to 420V in 11.5 seconds is fast, and thats a lot of joules. so try not to explode your body into bits.

GrimX9

Thu Sept 23 2010, 02:00AM

Registered Member #3225 Joined: Wed Sept 22 2010, 02:14AM
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Posts: 24

Patrick wrote ...

GrimX9 wrote ...

How could I draw more than 2.85 amps if 1200W at 420V and thats the max of the transformer?

you just let the transformer heat up abit and brown out your neigborhood.

and yes charging, 0.0156 F to 420V in 11.5 seconds is fast, and thats a lot of joules. so try not to explode your body into bits.

Hahaha, bits. Repaint rooms in seconds!

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