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Author Topic: PWM into a laminated transformer?  (Read 3147 times)
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solutions
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« on: March 13, 2011, 12:08:27 12:08 »

Guys,

I have had thoughts of building a PWM inverter that generates a variable amplitude sine wave (on time axis, constant amplitude, integrating the energy over a cycle) and feeding the raw PWM at 20kHz rate into a 50Hz transformer with standard iron laminations.

Thoughts?  Done it?

I'm pretty sure I can pull it off if I low pass filter the waveform before it goes into the primary, but any other ideas as far as getting it into the transformer as a raw waveform, since the core losses may be rather horrible?

thanks
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dennis78
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« Reply #1 on: March 13, 2011, 02:02:24 14:02 »

Without low pass filter before primary on laminated transformer you will have many losses Laminated transformers are designed for 50/60Hz 20kHz is much more. You know it Smiley  Why you don't use switching transformer? In that way you will have much smaller device for same power. For quality wave, low pass filter is necessary, but if you want make inverter from ex. 12/220V, on secondary side filter's inductor will be much smaller than filer on primary side.
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Walkura
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« Reply #2 on: March 13, 2011, 06:35:52 18:35 »

In itself its a strategy you can find in inverters from India etc.
I don't know, if You know those old color tv's from Philips (K9 to K11) they used a powersupply with a standard ironcore but it was running as a flyback at 18 Khz .
(just look up the Philips k9 (k10 was short lived ,K11 was a pain in the ass (but not due to the PSU) schema at the internet there must be plenty around)
Those tv's been around in Europe for over 25 years and most people threw them away because they wanted a new one (not because it was broken)
You can use iron cores on high frequency's ,just adjust the amount of windings and keep an eye on temperature.
Over time the core might degrade due to metal fatigue,although they make special iron cores for this sort of purpose (i believe it was a Czech company that was making ironcores with additional silicon)
You will have some increased corelosses (thats why i say keep an eye on the temperature)
If You filter before the transformer there is in itself no difference compared to standard 220 Volt ,but i also tryed it with PWM aswell (we had the ringcore transformer wound for this) and we could put 500 Watt's through a core suitable for 120 VA at 50Hz without any problem or excessive heating)
« Last Edit: March 13, 2011, 06:38:11 18:38 by Walkura » Logged
carbontracks
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« Reply #3 on: March 18, 2011, 07:20:30 19:20 »

Steel laminated transformers are pretty poor at high frequencies due to eddy current losses.  Hence why high frequency transformers use core materials with much higher resistivity.

I know that in large motor drives, PWM frequencies between 2 and 4 KHz are used with steel inductors, but I'm pretty sure that if you go much higher, losses will become extreme.
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Walkura
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« Reply #4 on: March 19, 2011, 03:24:48 15:24 »

Although i agree with your point about eddy currents.
But the amount of eddy currents depends on the thickness of the "layers"
Also they can add upto 3% of silicon which increases the resistivity upto 4 times.
Have a look at the following schema (sorry its djvu) http://www.eserviceinfo.com/downloadsm/828/Philips_K-9.html
In the powersupply You will find T182 which is a laminated core (2 C-cores) and it runs at 18 or 15 Khz (sorry its decades ago i been fixing those tv's i am not sure on the frequency)
This powersupply was very reliable and those tv's sometimes made over 25 service years .
During my own experiments with laminated iron cores losses were acceptable upto 30 Khz.
(do keep in mind this transformer (and core) was specially wound for us it wasn't a standard of the shelf 50Hz core)
Ofcourse You shouldn't push the limits because the losses would be horrible and ferrite would be much more efficient.
« Last Edit: March 19, 2011, 03:27:48 15:27 by Walkura » Logged
DreamCat
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« Reply #5 on: March 26, 2011, 02:02:08 14:02 »

Here is a document about it, but I forgot where I download.

http://www.easy-share.com/1914398524/MQP_D_1_2.pdf
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May be I expressed the wrong meaning, sorry for my bad english. Please correct it for me if you can.
solutions
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« Reply #6 on: March 26, 2011, 08:04:13 20:04 »

Thanks for your replies so far.  The laminated transformer is there for a reason, which I should have spelled out for you in greater detail. 

100 turn primary, 3 turns secondary (not a typo)....25kVA, and a 240V single phase primary:

If you do the math and understand what's out there for silicon and the size of "wire" needed, you'll see why I need to play with the primary side to control the secondary side current and why I can't easily get rid of that 50Hz laminated transformer. You can also see that a 20% eddycurrent loss is core meltdown territory.

I have a good idea on how to do this, just looking to see if there's alternate perspective on the problem before I spend the money to make it.
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mrpicing
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« Reply #7 on: March 27, 2011, 07:45:43 19:45 »

very interesting topic for me.
Here is a document about it, but I forgot where I download.

http://www.easy-share.com/1914398524/MQP_D_1_2.pdf
i m unable to download from given address.
can you upload to any other site?
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DreamCat
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« Reply #8 on: March 30, 2011, 08:38:32 08:38 »

very interesting topic for me. i m unable to download from given address.
can you upload to any other site?

here: http://www.4shared.com/document/uXGtGk0F/MQP_D_1_2.html
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May be I expressed the wrong meaning, sorry for my bad english. Please correct it for me if you can.
asrarulhaq
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« Reply #9 on: April 08, 2011, 01:10:29 13:10 »

Here is the source of original article:

http://www.wpi.edu/Pubs/E-project/Available/E-project-042507-092653/

Asrar
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