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 12/120V inverter again
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cyclopsitis
Nobel Prize Winner

Canada
732 Posts

Posted - Dec 12 2006 :  3:43:27 PM  Show Profile  Click to see cyclopsitis's MSN Messenger address  Reply with Quote
Hey audioguru its ken again.

I guess i was a little unclear on what i was asking about so I'll clean it up a bit. I'm taking an electrical class and for it i have to build a circuit for a project (not for marks but for something to do in class to utilize our skills). I want to make a power inverter just for fun. I want to make it able to supply about 3000 watts and a current of about 20A on the AC side of the circuit. I want to team it up with a 3kva transformer.

the next thing i want to do is be able to control the frequency of the AC output wave. I want to be able to select a frequency between 60hz (standard ac) and maybe up to 400hz AC output frequency. I'm doing this for a project and I want some help with the idea. I would like to see if anyone on this site has any ideas for an inverter that would supply this kind of current and what the circuit should look like.

that is my question. So does anyone have a schematic for this?
and
Does anyone know what type of SCR,TRIACS, or transistors i would need??

thanks Ken

ps. How do i reply to just my post?
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Dec 12 2006 :  6:56:39 PM  Show Profile  Reply with Quote
A 3kW inverter will use about 3750W from a battery. The current will be 3750/12= 312.5A which is ridiculous and will cause a car battery to explode.
The circuit will heat with about 750W.
How expensive will its custom-made huge and heavy transformer be? 32 power transistors?

There was a monster inverter like that on the internet. The prototype exploded and set the house on fire.
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cyclopsitis
Nobel Prize Winner

Canada
732 Posts

Posted - Dec 12 2006 :  10:06:08 PM  Show Profile  Click to see cyclopsitis's MSN Messenger address  Reply with Quote
could it be done if I built a power supply?

what would the circuit be like if it was rated at 15 amps standard?? could it be done then? I already have the prototype for the toroid power transformers they are about $400. what about 15 amps can it be done? can i make the inverter with a selectable frequency?
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Dec 13 2006 :  08:39:07 AM  Show Profile  Reply with Quote
Hi Ken,
You don't need a battery operated very high power inverter. You need a mains operated 3kW audio amplifier fed from an oscillator. If it is a linear circuit then it will make about 2000W of heat. If it is a class-D switching circuit then it will be cooler but will be extremely complicated.

Since you talk only about Power and Amps then you don't care about its output voltage. 3kW at 20A is a voltage of 3000/20= 150V. 3000/15= 200V.

What are the voltage ratings for the transformer you have?
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tim
Mad Scientist

198 Posts

Posted - Dec 13 2006 :  2:47:34 PM  Show Profile  Reply with Quote
well i found a winter project fixing a 3200 watt inverter, few mosfets , 4 zeners , 3 resistors, op amp ic, pwm control regulater ic, pwm control ic, and 1 lm339n ic. but it really was not worth it but only paid 10.00 for it and the power goes out all the time where i am .
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cyclopsitis
Nobel Prize Winner

Canada
732 Posts

Posted - Dec 13 2006 :  4:16:30 PM  Show Profile  Click to see cyclopsitis's MSN Messenger address  Reply with Quote
Hi Tim,

Where did you get that power inverter from? Do you have any ideas on what the schematic would look like or have any idea where i would get one from. I was thinking of just buying one and taking it apart.. but hey if you have ideas that would be great.

Audioguru,

Thanks for your help so far. I was thinking of using a DC power supply to supply voltage for this. I realize at 12V the amps would through the roof so i ruled out batteries already. I was going to maybe use a power supply voltage around 48V that would bring down the amps alot.

How do i make this 3KW amplifier if that is best way to do it. I've have only built simple audio circuits before so if you have any pointers that would be great!

As for the transformer the 3KVA was meant for the current the transformer would have to take. 3kilo volt amps is VA=E X I so 120V X 20A= 2400VA or 2.4KVA With transformer you always want to supply a cousion of 120% incase of a fault. so 120V X 20A X 1.20= 2880VA So i stepped it up to 3000VA just because that is a standard size... I think that is right anyway... I double checked this with
http://www.unitedhomeproducts.com/id171.htm
they make high end audio stuff. The two boxes side by side house 3KVA isolation transformers check it out.
http://www.purepoweraps.com/
this one keeps the 120VAC 60hz at 120VAC 60hz no matter what and there are others you can change the frequency on.

How does a class D function differently to produce less heat?

Thanks guys!

Edited by - cyclopsitis on Dec 13 2006 4:43:10 PM
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tim
Mad Scientist

198 Posts

Posted - Dec 13 2006 :  4:40:05 PM  Show Profile  Reply with Quote
hi cyclopsitis, i find these inverters on ebay some just need fuses and others need alot more. i bought one on ebay for 99 cents and it only needed 3 fuses because the polariry was hooked up wrong.
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cyclopsitis
Nobel Prize Winner

Canada
732 Posts

Posted - Dec 13 2006 :  4:44:34 PM  Show Profile  Click to see cyclopsitis's MSN Messenger address  Reply with Quote
Thanks Tim i did'nt think of looking at ebay! many thanks!!

Ken
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tim
Mad Scientist

198 Posts

Posted - Dec 13 2006 :  4:49:49 PM  Show Profile  Reply with Quote
i have 2 inverters and one is strange it say 200 watts but i kind of got like 300 watts out of it its new in the box and its from a few years ago but it has a huge transformer that looks like it can be modified to put out alot more then 300 watts . i may put it on ebay though cause i dont have the time to play with it.
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tim
Mad Scientist

198 Posts

Posted - Dec 13 2006 :  4:52:21 PM  Show Profile  Reply with Quote
and the other one is an old radio shack from the 1980s i had put more powerfull transistors in it and its rated for only 100 watts but now i get about 200 watts.
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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Dec 13 2006 :  6:22:20 PM  Show Profile  Reply with Quote
Most inexpensive inverters have a square-wave output that works fine for heates and incandescent lights. Many electronic products and electric tools with motor speed controls won't work. The circuit is simple.

Then there is "modified sine-wave" inverters that produce a square-wave with a step in it. More things work with it. The circuit is complicated.

The best is a pure sine-wave output inverter that use PWM or class-D to stay cool because their transistors switch fully on and fully off at a very high frequency and their average output is controlled by the duty-cycle of the output pulses. The circuit is extremely complicated.

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tim
Mad Scientist

198 Posts

Posted - Dec 13 2006 :  7:06:30 PM  Show Profile  Reply with Quote
yea well the one im fixing is enough for me to handle as far as complicated.
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petermotosh
New Member

Nigeria
1 Posts

Posted - Dec 14 2006 :  06:59:38 AM  Show Profile  Reply with Quote
quote:
Originally posted by Techoduro

No big deal, Aaronīs design ???, a simple oscilator seems to me more like a challenge to make it work as inverter than a serious project, since elementary in circuits told us you could not use it that way but as a driver to other circuit to reach the goal. I donīt think you would use a fork to have soup, all you have to do is using a spoon.
By the way thanks for all the information provided by some of you, which is being very usefull to me.





Petrotech
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SAE140
Apprentece

United Kingdom
6 Posts

Posted - Dec 15 2006 :  11:03:43 AM  Show Profile  Reply with Quote
quote:
Originally posted by cyclopsitis

Hi Tim,

Where did you get that power inverter from? Do you have any ideas on what the schematic would look like or have any idea where i would get one from. I was thinking of just buying one and taking it apart.. but hey if you have ideas that would be great.



If you are looking for BIG inverters, then checkout:
http://www.qsl.net/dg5sga/inverter.htm
http://technology.niagarac.on.ca/staff/mcsele/i2k.htm

Both of those guys know how to drive Power Mosfets.

This guy doesn't:
http://www.cadvision.com/blanchas/hexfet/h-bridge2.gif

Getting Mosfets quickly through their linear region when switching large currents is critical to avoid excessive heating - so use a dedicated chip, or at least a totem-pole push-pull driver.

Colin

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audioguru
Nobel Prize Winner

Canada
4214 Posts

Posted - Dec 15 2006 :  12:19:24 PM  Show Profile  Reply with Quote
Hi Colin,
How about a lousy inverter circuit that has its capacitors backwards and its transistors operating with hardly any base current and having avalanche breakdown like this one?
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