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Forum Index : Electronics : Materials for main transformer?

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SmartLikeTruck
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Joined: 22/04/2020
Location: Canada
Posts: 3
Posted: 05:38am 25 Apr 2020
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Is there a particular reason that laminated silicon iron cores wound clock spring style are chosen for the main power handling transformer on a low frequency DC to mains AC inverter?

Is there an advantage to the ones that are wound with many multiple "leafs" of silicon steel?

What advantages or disadvantages are had by powdered molded ferrite cores of the same size if used in the same inverter application?

Looking inside my new to me Connext SW4024 vs. the inside of the Trace SW4048 that I had is what has nudged me in this direction. I've got ClockmanFRA's book and boards en-route and was thinking I could perhaps cast a torrid core of iron powder filled geopolymer ceramic.
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3254
Posted: 06:46am 25 Apr 2020
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Two main advantages of wound grain oriented silicon steel strip at mains power frequencies.

Very high flux density before saturation, which means you need far fewer turns to support a given voltage.  Fewer turns means for a given sized core, thicker wire can be used which maximises the power rating.

The second advantage is very high permeability, which gives a high inductance per turn, and that reduces the zero load idling power. The very thin strip also reduces eddy current loss at low frequencies which also minimizes idling power.

Ferrite is pretty useless at low frequencies, very early saturation, so it needs to be made vastly larger, and its extremely expensive for a given size.

If your sine wave inverter was for 50Hkz and not 50Hz, then ferrite would be a better choice because the combined losses at higher frequencies would be lower. But at 50Hz grain oriented silicon steel wins.
Cheers, Tony.
 
BenandAmber
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Joined: 16/02/2019
Location: United States
Posts: 845
Posted: 02:27pm 25 Apr 2020
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You can also use a square Transformer

One of the world's greatest use square

You never know whe you may be talking to on this site

You could be talking to the world's most experienced or you could be talking to a monkey see monkey do like me

There are a lot of incredibly talented people on here


Pay attention to this and you will know who's words you can take to the bank!
 
SmartLikeTruck
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Joined: 22/04/2020
Location: Canada
Posts: 3
Posted: 04:05pm 25 Apr 2020
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Thanks for the quick response y'all.

Since DIY from scratch is out, salvage is the route for me, not sure if I need another excuse to visit the scrapyard ;)
 
SmartLikeTruck
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Joined: 22/04/2020
Location: Canada
Posts: 3
Posted: 06:03pm 27 Apr 2020
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What ratings for transformers should I be watching for to build a 4000W constant inverter?

I would have assumed VA rating generally a 1:1 ratio with watts but that doesn't seem to be what I've gathered by skimming the forum.
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3254
Posted: 09:52pm 27 Apr 2020
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There are no hard and fast rules to this game.
 
Power ratings are tied to temperature rise, and how hot is too hot is debatable as well.
But mostly it depends on how much copper you can pack onto a particular sized core.

Commercial designs are always machine wound, and a toroid winding machine needs a fair working space, so there is always going to be a fair bit of hole remaining before it gets too tight for the machine to work without fouling the winding.

A home brew winding will be less neat, and the wire cannot be packed as tightly as with a machine, but its possible to thread many extra final turns through the hole manually as the hole gets much smaller, so you might even end up better off.

Another aspect of the design are the tradeoffs.  
Which would you prefer, a 2Kw transformer that has 50 watts of no load idling power, or a temperature limited 1.5Kw rated transformer that has 20 watts of idling power?
Its possible to argue either way which would be best for a particular inverter, depending on the application.

If its to run just a single irrigation pump, and inverter and pump are both switched on and off together, for say an hour a day, being able to safely drive a 2Kw pump would probably be more useful than being limited to a 1.5Kw pump.  
But if the inverter has to run all night off a small battery to power your home, 20 watts if idling power beats 50 watts of idling power hands down.
That example is a bit extreme, but you can see what I am getting at...

So you cannot really compare a commercially rated transformer design to a do it yourself back yard design, or maybe even two different designs by different people for very different applications.  Power rating is just one of several characteristics, and may end up being the least important.

So basically, you start off looking at a bare steel toroidal core, or perhaps two stacked together, something you salvaged free from the rubbish tip or bought from a metal scrap yard, or even purchased brand new.
Then you design your new inverter transformer around that.
Final design is a trial and error process, repeating the calculations over and over several times until you end up with numbers that look promising, and are practical to build, and hopefully it will all fit onto the core without running out of space.

You could be very conservative in your design and finally call it an XXX Kw transformer, or you can push the limits a bit and rate it for YYY Kw, depending on your own outlook.

There is no right or wrong solution, and the final numbers can vary all over the place. Its what is right for you that matters. Power rating is very important to transformer and inverter manufacturers, watts per dollar is everything.

Its only when you finally test it and come up with some real world results that you can decide if the safe power rating, efficiency, and no load idling power are going to be useful to you, or meet what you originally aimed for in the first place.

So you won't find any consistency in the numbers which can be a bit confusing when starting out.



.
Edited 2020-04-28 09:14 by Warpspeed
Cheers, Tony.
 
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