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'[EE]: Inverter DC-AC'
2003\02\15@150220 by Ray Gallant

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I would like to build a mostly sinusoidal power inverter for my home forced air heating system now PIC controlled. The application is similar to a UPS where the load is an 115VAC motor and a few other small 115VAC loads. Available DC source will be 1 or more 12 volts batteries similar to a car or marine high duty for now. A PIC is involved for LCD, keypad, Mains AC monitor, alarm, RS232 and other application but not directly for the inverter yet. At this point, I have developed a sinusoidal 24VAC p-p 60Hz with zero DC offset from a matched pair of transistors (push/pull) and an ICL IC. Things OK up to here! Now I need to increase this to 115VAC at about 5 amps or more for a while. I was looking at a Powerex transistor http://www.pwrx.com/pwrx/docs/kd224503.pdf   Should I use a 24:115 transformer and amplify from there? I have difficulty with the power stage, snubber and such. I have large thyristors available if required. Any feedback is much appreciated. Thanks {slewrate}






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2003\02\15@154803 by Peter L. Peres

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My advice would be to purchase a used 600VA-1kVA UPS and find a way to
wire your batteries to it and keep it reasonably cool while running. The
older and bigger the better (the new units are designed very 'tightly' and
will have trouble running for a longer time). Building one from scratch
for a one off is not reasonable imho.

Peter

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2003\02\15@161819 by Ray Gallant

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Thanks for the feedback, but  I insist on building it since it's an addition
to my present Home HVAC System that I built from scratch (all pic driven)
for which I am proud of.  I could have bought one of these also.  {slewrate}

{Original Message removed}

2003\02\15@164946 by Peter L. Peres

picon face
On Sat, 15 Feb 2003, Ray Gallant wrote:

*>Thanks for the feedback, but  I insist on building it since it's an addition
*>to my present Home HVAC System that I built from scratch (all pic driven)
*>for which I am proud of.  I could have bought one of these also.  {slewrate}

The 'easy' way to make a sine converter is to use a class B push-pull
amplifier and a split secondary transformer (2x10V for 12V batteries). All
the other ways involve a DC/DC converter steered with |sin| waveform and
an active H bridge in the high voltage side.  This will give about 70%
efficiency, which is low. Maybe it would be easier to convert the 115Vac
installation to 24Vdc or 48Vdc and save one major part (or pita). What
would it take to convert the blower to a static switched dc motor ? These
are made to specs like you need for servo purposes.

Peter

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2003\02\15@174525 by Ray Gallant

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----- Original Message -----
From: "Peter L. Peres" <.....plpKILLspamspam.....ACTCOM.CO.IL>
To: <EraseMEPICLISTspam_OUTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Saturday, February 15, 2003 5:46 PM
Subject: Re: [EE]: Inverter DC-AC


> On Sat, 15 Feb 2003, Ray Gallant wrote:
>
> *>Thanks for the feedback, but  I insist on building it since it's an
addition
> *>to my present Home HVAC System that I built from scratch (all pic
driven)
> *>for which I am proud of.  I could have bought one of these also.
{slewrate}
{Quote hidden}

Good Idea.  I still need 115VAC to run my automation system and modifying
this is do-able but a pain.  I can get 115VAC from my source through a
transformer (PT) but how do I switch it.  My DC rail is 12VDC.  Are you
advising me to develop a DC-DC to up this rail closer to peak voltage?  I
think I put something together with a large toroidial transformer a long
time ago used in a DC-DC appl.  Should I amplify the AC with a DC offset,
such that 24VAC would be riding on 0VDC as the source?  Thank again.  I
should of mentioned earlier that this application is handy for power outages
as well.

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2003\02\15@174923 by Olin Lathrop

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> The 'easy' way to make a sine converter is to use a class B push-pull
> amplifier and a split secondary transformer (2x10V for 12V batteries).

Not so easy when you realize that class B is not very efficient, and at
his power levels getting rid of the heat will be a significant challenge.

Commercial inverters either alternately drive the transformer primary to
full in each direction producing a sort of rounded square wave, or they
use DC-DC converter techniques to drive the primary with a high frequency
chopped sine wave as you said.  The rounded square wave approach may not
be as bad as it first appears, depending on the loads you plan on driving.
Incandescent light bulbs and electronic equipment with switching power
supplies (like a PC) don't mind this at all.  However, sine wave output is
the only thing guaranteed to work with all standard appliances.

> All
> the other ways involve a DC/DC converter steered with |sin| waveform and
> an active H bridge in the high voltage side.

Really, "all" other configurations than class B are like this!?  C'mon
now.  For one thing, you can use the same center tapped primary you
mentioned above, center to the +DC supply, and each end switched to ground
using PWM so as to produce a sine wave out.  In fact, it was my
understanding that this was a reasonably common arrangement.

> This will give about 70% efficiency, which is low.

A well tuned and tweaked system should be able to do better than 70%, but
that is still a lot better then class B.

> Maybe it would be easier to convert the 115Vac
> installation to 24Vdc or 48Vdc and save one major part (or pita). What
> would it take to convert the blower to a static switched dc motor ?
These
> are made to specs like you need for servo purposes.

That may be a reasonable approach in some cases, but I think the original
poster is doing this partly for the sake of doing and learning this
himself.  Certainly a system that produced normal line power would be much
more flexible, and a much better learning experience.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\02\15@192036 by Vern Jones

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Why not use a PWM either mirrored drive to switching FETs or a single
ended bank of high power switching FETS. Then the transformer will do
the integration.

Vern

Ray Gallant wrote:
>
> I would like to build a mostly sinusoidal power inverter for my home forced air heating system now PIC controlled. The application is similar to a UPS where the load is an 115VAC motor and a few other small 115VAC loads. Available DC source will be 1 or more 12 volts batteries similar to a car or marine high duty for now. A PIC is involved for LCD, keypad, Mains AC monitor, alarm, RS232 and other application but not directly for the inverter yet. At this point, I have developed a sinusoidal 24VAC p-p 60Hz with zero DC offset from a matched pair of transistors (push/pull) and an ICL IC. Things OK up to here! Now I need to increase this to 115VAC at about 5 amps or more for a while. I was looking at a Powerex transistor http://www.pwrx.com/pwrx/docs/kd224503.pdf   Should I use a 24:115 transformer and amplify from there? I have difficulty with the power stage, snubber and such. I have large thyristors available if required. Any feedback is much appreciated. Thanks {slewrate}
>
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2003\02\15@195233 by Peter L. Peres

picon face
On Sat, 15 Feb 2003, Olin Lathrop wrote:

*>> The 'easy' way to make a sine converter is to use a class B push-pull
*>> amplifier and a split secondary transformer (2x10V for 12V batteries).
*>
*>Not so easy when you realize that class B is not very efficient, and at
*>his power levels getting rid of the heat will be a significant challenge.

Class B is the most efficienct way to drive a sine wave with reasonable
distortion and without complications (like high power harmonic filters
whose Q must be kept low and dealing with load dumps with several Henrys
of secondary inductance at tens of A/us slew rates produced by
non-sinusoidal drive). The efficiency is ~70% theoretical and the same as
that of an equivalent powered audio amplifier, and it is feasible in
low-tech low-development-expense mode upto 1kW or so.  Also most older AC
inverters work exactly like this.

*>Commercial inverters either alternately drive the transformer primary to
*>full in each direction producing a sort of rounded square wave, or they

That is not a rounded square wave, it is a square wave. What they do is
drive the primary with fast PWM modulated to make a sine shape after
integration by the big transformer and associated filters.

*>use DC-DC converter techniques to drive the primary with a high frequency
*>chopped sine wave as you said.  The rounded square wave approach may not
*>be as bad as it first appears, depending on the loads you plan on driving.

What in g*d s name is a rounded square wave and how do you make it using
straight switches ? The only way I know if a multiply tapped transformer.
It is very expensive to make. That would approximate a sine in steps.

*>Incandescent light bulbs and electronic equipment with switching power
*>supplies (like a PC) don't mind this at all.  However, sine wave output is
*>the only thing guaranteed to work with all standard appliances.
*>
*>> All
*>> the other ways involve a DC/DC converter steered with |sin| waveform and
*>> an active H bridge in the high voltage side.
*>
*>Really, "all" other configurations than class B are like this!?  C'mon
*>now.  For one thing, you can use the same center tapped primary you
*>mentioned above, center to the +DC supply, and each end switched to ground
*>using PWM so as to produce a sine wave out.  In fact, it was my
*>understanding that this was a reasonably common arrangement.

When PWM is used normally there is no tap, and either a half bridge or a
full bridge drive is used. Switches are cheaper than transformers. There
are some designs with tapped transformers but the core space is used
inefficiently like that.

*>> This will give about 70% efficiency, which is low.
*>
*>A well tuned and tweaked system should be able to do better than 70%, but
*>that is still a lot better then class B.

Class B is rated 70% efficient (at 5% or 10% distortion - I do not
remember).

*>> Maybe it would be easier to convert the 115Vac
*>> installation to 24Vdc or 48Vdc and save one major part (or pita). What
*>> would it take to convert the blower to a static switched dc motor ?
*>These
*>> are made to specs like you need for servo purposes.
*>
*>That may be a reasonable approach in some cases, but I think the original
*>poster is doing this partly for the sake of doing and learning this
*>himself.  Certainly a system that produced normal line power would be much
*>more flexible, and a much better learning experience.

The op is trying to buy insurance against ice storms probably. Imho for
his needs the best way would be to get a genny that runs on the same fuel
as the furnace and concentrate on making an automatic fail-over circuit
(with temporary power dropout while switching). At least that would be
what I would do. The kind of failover without power drop in the middle is
very hard to do for mains.

Peter

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2003\02\16@112624 by Ray Gallant

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----- Original Message -----
From: "Peter L. Peres" <TakeThisOuTplpEraseMEspamspam_OUTACTCOM.CO.IL>
To: <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Saturday, February 15, 2003 8:51 PM
Subject: Re: [EE]: Inverter DC-AC


> On Sat, 15 Feb 2003, Olin Lathrop wrote:
>
> *>> The 'easy' way to make a sine converter is to use a class B push-pull
> *>> amplifier and a split secondary transformer (2x10V for 12V batteries).
> *>
> *>Not so easy when you realize that class B is not very efficient, and at
> *>his power levels getting rid of the heat will be a significant
challenge.
{Quote hidden}

frequency
> *>chopped sine wave as you said.  The rounded square wave approach may not
> *>be as bad as it first appears, depending on the loads you plan on
driving.
>
> What in g*d s name is a rounded square wave and how do you make it using
> straight switches ? The only way I know if a multiply tapped transformer.
> It is very expensive to make. That would approximate a sine in steps.
>
> *>Incandescent light bulbs and electronic equipment with switching power
> *>supplies (like a PC) don't mind this at all.  However, sine wave output
is
> *>the only thing guaranteed to work with all standard appliances.
> *>
> *>> All
> *>> the other ways involve a DC/DC converter steered with |sin| waveform
and
> *>> an active H bridge in the high voltage side.
> *>
> *>Really, "all" other configurations than class B are like this!?  C'mon
> *>now.  For one thing, you can use the same center tapped primary you
> *>mentioned above, center to the +DC supply, and each end switched to
ground
{Quote hidden}

but
{Quote hidden}

original
> *>poster is doing this partly for the sake of doing and learning this
> *>himself.  Certainly a system that produced normal line power would be
much
{Quote hidden}

24VDC is no problem.  I have batteries from big farm tractor available for
test trial and re-charging these babies are no problem.  The forced air
system is heated via wood.  A steam type genny is not an option.  I want to
say that I appreciate all your feedback and that the info to date has been
informative.  I am googling on class B and high voltage DC-DC which is
actually very interesting and with your help, I have a design concept.
Still worried about things like harmonics, load monitor and snubber but it's
a comin'. Thanks, {slewrate}

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2003\02\16@112632 by Olin Lathrop

face picon face
> Class B is the most efficienct way to drive a sine wave with reasonable
> distortion and without complications (like high power harmonic filters
> whose Q must be kept low and dealing with load dumps with several Henrys
> of secondary inductance at tens of A/us slew rates produced by
> non-sinusoidal drive). The efficiency is ~70% theoretical and the same
as
> that of an equivalent powered audio amplifier, and it is feasible in
> low-tech low-development-expense mode upto 1kW or so.  Also most older
AC
> inverters work exactly like this.

300W is still a lot of heat to get rid of.

> *>Commercial inverters either alternately drive the transformer primary
to
> *>full in each direction producing a sort of rounded square wave, or
they
>
> That is not a rounded square wave, it is a square wave. What they do is
> drive the primary with fast PWM modulated to make a sine shape after
> integration by the big transformer and associated filters.

I was talking about driving the primary without PWM, just banging
alternate ends of the center tapped primary to ground with a little dead
time in between.  The square wave gets rounded a bit because of the
filtering effect of the transformer.  Years ago my parents had such an
inverter in a motor home.  It converted the 12V DC to 115V AC up to 300W
if I remember correctly.  It allowed the refrigerator to be run while
driving.  This was the kind that had no compressor and worked just from
heat.  It would be run from a propane flame when parked, but you can't do
that when moving.  In electric mode, the place right above where the flame
would be was heated resistively, which is why it didn't care about square
wave versus sine wave.

I looked at the output of this thing on a scope once, and the wave shape
was a sort of a square wave with rounded corners.  Of course that did
change a bit over the load range.  It didn't have very good output
regulation.  This was late 1960s technology, and the switching elements
were 2 or 3 large TO-3 NPN transistors on each end of the primary.

My point was that this is a relatively easy thing to do if you can
tolerate a square wave output.  The drawback is you do need a large 60Hz
power transformer with low voltage center tapped primary and 115V
secondary.

> When PWM is used normally there is no tap, and either a half bridge or a
> full bridge drive is used. Switches are cheaper than transformers. There
> are some designs with tapped transformers but the core space is used
> inefficiently like that.

I agree this sort of scheme can be very effective, but is less forgiving
and more tricky to get right.  I've seen professional power supply
engineers take months to get something like this just right.  They
eventually did and it was a successful product, but it might not be the
best path for a one-off home built unit.  A lot of things get simpler once
you buy into the big fat 60Hz power transformer, although the unit cost
will be higher.  I figured this wasn't so important if you are only
building one unit.

> Class B is rated 70% efficient (at 5% or 10% distortion - I do not
> remember).

The maximum theoretical efficiency for a sine output from a class B output
stage is 1/sqrt(2) = 71%.  This assumes that the output is a pure sine
wave (no distortion) with the peaks driven exactly to the power supply
rails, meaning the pass elements drop no voltage when fully on.  If you
are willing to tolerate some distortion, you can clip the peaks of the
sine wave a bit.  This increases the RMS output voltage as a fraction of
the power supply voltage, thereby increasing efficiency.  This is because
the power supply and the output current are the same, and the inefficiency
is due to the different voltages those currents are delivered at.

A theoretical class B output stage is 100% efficient at producing a square
wave, since it's RMS voltage is the same as the power supply voltage.
This fact is the basis for PWM and switching techniques.

In practise of course there are no such things as perfect pass elements.
For example, assume a -6V to +6V power supply and pass elements that
saturated to 500mV at load.  The maximum sine output is therefore 11Vpp,
which is 5.5Vp, which is 3.89V RMS, which results in an efficiency of 65%.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\02\16@115308 by Marcelo Puhl

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FYI, stay away from APC. They don't support products older than four years.

Mark

On 15 Feb 2003 at 22:45, Peter L. Peres wrote:

{Quote hidden}

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2003\02\16@145420 by Herbert Graf

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I'm not sure why that would be a problem, repairing most UPSs costs more
than buying new, and the most common mode of failure is the battery, which
is easily replaceable. Or are you talking about software support?

> {Original Message removed}

2003\02\16@152537 by Vern Jones

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face
If you would like a commercial unit to emulate, look at BEST--
http://www.mcct.com/best.html when I have used their systems, the only
failures have been the batteries after several years. As their name
implies, they are the best in the smaller UPS systems. Even large
systems fail, I had a 80 KW 440 Inverter fail when one of the phases
switched out of sync. What a mess that was, and another that had a
failure that doubled the output voltage taking out a very large disk
array in the process, so take care in your design..


Ray Gallant wrote:
I was looking at a Powerex transistor
http://www.pwrx.com/pwrx/docs/kd224503.pdf   Should I use a 24:115
transformer and amplify from there? I have difficulty with the power
stage, snubber and such. I have large thyristors available if required.
Any feedback is much appreciated. Thanks {slewrate}
>
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2003\02\16@163048 by Peter L. Peres

picon face
On Sun, 16 Feb 2003, Marcelo Puhl wrote:

*>FYI, stay away from APC. They don't support products older than four years.

Rofl. There is an APC UPS under my table. I don't know if they support it,
however I do mine ;-)

Peter

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2003\02\16@163050 by Peter L. Peres

picon face
On Sun, 16 Feb 2003, Herbert Graf wrote:

*>I'm not sure why that would be a problem, repairing most UPSs costs more
*>than buying new, and the most common mode of failure is the battery, which
*>is easily replaceable. Or are you talking about software support?

There are certain designs of UPSes that fail in, er, interesting modes
after a few years of use. I do not know if the op referred to this.

Peter

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2003\02\16@163054 by Peter L. Peres

picon face
On Sun, 16 Feb 2003, Olin Lathrop wrote:

*>> When PWM is used normally there is no tap, and either a half bridge or a
*>> full bridge drive is used. Switches are cheaper than transformers. There
*>> are some designs with tapped transformers but the core space is used
*>> inefficiently like that.
*>
*>I agree this sort of scheme can be very effective, but is less forgiving
*>and more tricky to get right.  I've seen professional power supply
*>engineers take months to get something like this just right.  They
*>eventually did and it was a successful product, but it might not be the
*>best path for a one-off home built unit.  A lot of things get simpler once
*>you buy into the big fat 60Hz power transformer, although the unit cost
*>will be higher.  I figured this wasn't so important if you are only
*>building one unit.

That is what I had in mind when I said the 'easy' way is the class B
amplifier type driver.

*>> Class B is rated 70% efficient (at 5% or 10% distortion - I do not
*>> remember).
*>
*>The maximum theoretical efficiency for a sine output from a class B output
*>stage is 1/sqrt(2) = 71%.  This assumes that the output is a pure sine
*>wave (no distortion) with the peaks driven exactly to the power supply

The distortion I had in mind was crossover distortion. Which can disable
carrier current communication (don't ask how I know this).

Otherwise I agree with you. The 'rounded square wave' could be a solution
since he only needs to drive his controller and the air circulation
blower. Maybe it would be enough.

Peter

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2003\02\16@163505 by Herbert Graf

flavicon
face
> On Sun, 16 Feb 2003, Herbert Graf wrote:
>
> *>I'm not sure why that would be a problem, repairing most UPSs costs more
> *>than buying new, and the most common mode of failure is the
> battery, which
> *>is easily replaceable. Or are you talking about software support?
>
> There are certain designs of UPSes that fail in, er, interesting modes
> after a few years of use. I do not know if the op referred to this.

       True, but what does this have to do with "support". If the UPS is under
warranty APC will replace it. I don't see why anyone would need "support"
for a UPS out of warranty??

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2003\02\16@164132 by Marcelo Puhl

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face
I'm talking about an EEPROM 93C46 or the file to program it. APC doesn't
sell the EEPROM nor the file. So I had to loose hours hacking the EEPROM
content to get my UPS half-running.

Even the local APC service center didn't get my UPS repaired. They told me
APC doesn't send them the file or a programmed 93C46.

I'll never buy anything from APC again. Neither my clients, the old ones and
the future ones, depending on me.

Mark

On 16 Feb 2003 at 14:52, Herbert Graf wrote:

> I'm not sure why that would be a problem, repairing most UPSs costs more
> than buying new, and the most common mode of failure is the battery, which
> is easily replaceable. Or are you talking about software support?
>
> > {Original Message removed}

2003\02\17@003810 by John Dammeyer

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face
Hi Ray,

Give me a few more days to test the concept and I should be able to
demonstrate a fairly efficient inverter circuit.  A simulation using
Microsoft C and Excel to plot it looks really good.  PIC 16F87x series
will do the job.  The code will be written for MPC but hand compiling
into assembler if that's your flavour of choice would be fine too.

Cheers,

John Dammeyer


Wireless CAN with the CANRF module now available.
http://www.autoartisans.com/products
Automation Artisans Inc.
Ph. 1 250 544 4950


> {Original Message removed}

2003\02\17@012307 by Dmitriy A. Kiryashov

picon face
Hi Peter and guys.

What is the most difficult part with PWM type of DC-AC with transformer
converter design? Either regular PWM or some sort of magic sine waves.
(minimal efforts to receive optimal output sine wave with minimal
distortions)

Negative feedback in addition thru say some opto couple to make sure that
output AC signal is sine and required voltage and frequency.

Probably main problem is to drive high current into primary windings
but with on/off type control it will actually save you on heatsink
and will increase? converter efficience as well.

Anybody was successfull with doing this type of design with PIC ?
(different than class B)

WBR Dmitry.


"Peter L. Peres" wrote:
{Quote hidden}

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2003\02\17@021640 by Mike Singer

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Dmitriy A. Kiryashov wrote:
...
> Probably main problem is to drive high current into primary
> windings but with on/off type control it will actually save you
> on heatsink and will increase? converter efficience as well.
>
> Anybody was successfull with doing this type of design
> with PIC ? (different than class B)

What do you mean "successful"?

- not got killed playing with dangerous circuits;

- succeeded in making a circuits that did some voltage
 conversion without any standard product-proof tests
 to make sure device is safe, reliable and meets sine
 or similar requirements;

- developed a device better then TrippLite converters;

  Mike.

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2003\02\17@035238 by Andre Abelian

picon face
Mike,

I got 5kw inverter working fine without the processor
And now I am working on same design to replace with
Pic micro pic18Fxxx the only problem is the analog section is
Slaw on pic we need faster conversion for current limiter.

Andre Abelian




{Original Message removed}

2003\02\17@051502 by Mike Singer

picon face
Andre,
I don't mind at all if XXX Kwatt converters are designed
professionally. I just think the main point to keep in mind
when home-brewing and using such things should be safety.

Don't you see some discrepancy in your post:
"I got 5kw inverter working fine" and " we need faster
conversion for current limiter".

Sounds like: My car is fine; just little problem: over 50 mph
it gets uncontrolled.

  Mike.
----------------------
P.S. Sorry for being slightly religious about safety.



Andre Abelian wrote:
{Quote hidden}

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2003\02\17@104209 by Ray Gallant

flavicon
face
xtra cool.  I am anticipating to see your results.  Assembler is great and
deadline is not a factor.  I'm working on healthy DC_DC component for this
project.  TX 1Meg$.

----- Original Message -----
From: "John Dammeyer" <RemoveMEjohndEraseMEspamEraseMEAUTOARTISANS.COM>
To: <RemoveMEPICLISTspam_OUTspamKILLspamMITVMA.MIT.EDU>
Sent: Monday, February 17, 2003 1:35 AM
Subject: Re: [EE]: Inverter DC-AC


{Quote hidden}

> > {Original Message removed}

2003\02\17@123002 by Andre Abelian

picon face
Mike,

I never said having problem with existing working unit
I mean having problem converting over to pic design.
My car is fine and I can go over 50 mph and every thing is
Under control and no smoke " just kidding"

Andre



{Original Message removed}

2003\02\17@160543 by Peter L. Peres

picon face
On Mon, 17 Feb 2003, Dmitriy A. Kiryashov wrote:

*>Hi Peter and guys.
*>
*>What is the most difficult part with PWM type of DC-AC with transformer
*>converter design? Either regular PWM or some sort of magic sine waves.
*>(minimal efforts to receive optimal output sine wave with minimal
*>distortions)

The load dump condition. Assume a load is attached and suddenly turns off,
f.ex. at voltage zero crossing. If the UPS reactive elements contain X
Joules and are not unity damped these X Joules will become voltage that
keeps rising until it finds a place to put some current through. The usual
PI filter, VDRs, spark arrestors, and fuses at the input of any circuit
connected to mains are there to prevent this, and sometimes succeed. Now
if you drive a transformer with relatively clean sine (class B or better)
there will be no load dump (well, a little) even if there is no feedback
loop since the transformer output impedance is hard-coupled to the driver
stage. The voltage will not rise (well, a little). If the transformer is
driven with a switch then even if there is feedback the energy in the
transformer will become a fearsome spike and thefeedback circuit can do
nothing to stop it. There are several ways to deal with this (like dynamic
crowbars etc) but they add complications to the high voltage side.

*>Negative feedback in addition thru say some opto couple to make sure that
*>output AC signal is sine and required voltage and frequency.
*>
*>Probably main problem is to drive high current into primary windings
*>but with on/off type control it will actually save you on heatsink
*>and will increase? converter efficience as well.
*>
*>Anybody was successfull with doing this type of design with PIC ?
*>(different than class B)

Not with PIC, with logic circuits (divide by 4 and gates, driving
00-nothing 01-one phase 10-nothing 11-other phase), after a scheme from a
book. The secondary had to be damped with a string of neon lamps to
prevent it from doing stupid things when not loaded. It had a capacitor
across it. The voltages that appear when undamped are quite enough to
break down the insulation of just about any transfomer and will arc over
5mm spark gaps easily.

Peter

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2003\02\17@234820 by Harold Hallikainen

picon face
Speaking of UPSs and inverters, what's the current technology on the DC to AC conversion? Are they using some high frequency PWM to simulate the 60 Hz allowing for smaller magnetics? I built an inverter about 40 years ago with a big 60 Hz transformer that was especially for that purpose (included feedback windings to drive the switching transistors). Seeing the low cost and small stuff available now, I wonder how work.

Thanks!

Harold




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2003\02\18@161549 by Mike Singer

picon face
Andre Abelian wrote:
> I never said having problem with existing working unit
> I mean having problem converting over to pic design.

Sorry for misunderstanding; it was my language problem.
Just haven't got yet into the way of English grammar
punctuation:

You wrote earlier:
> I got 5kw inverter working fine without the processor
> And now I am working on same design to replace with
> Pic micro pic18Fxxx the only problem is the analog section is
> Slaw on pic we need faster conversion for current limiter.


"...the only problem is the analog section is slaw.
On pic we need faster conversion for current limiter."

(The reason why we are shifting to PIC is that we need
faster conversion for current limiter.)

  Mike.

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2003\02\18@163044 by Andre Abelian

picon face
Mike,

No Problem. Thanks for your replay.

Andre

-----Original Message-----

Andre Abelian wrote:
> I never said having problem with existing working unit
> I mean having problem converting over to pic design.

Sorry for misunderstanding; it was my language problem.
Just haven't got yet into the way of English grammar
punctuation:

You wrote earlier:
> I got 5kw inverter working fine without the processor
> And now I am working on same design to replace with
> Pic micro pic18Fxxx the only problem is the analog section is
> Slaw on pic we need faster conversion for current limiter.


"...the only problem is the analog section is slaw.
On pic we need faster conversion for current limiter."

(The reason why we are shifting to PIC is that we need
faster conversion for current limiter.)

  Mike.

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2003\02\20@122653 by Dwayne Reid

flavicon
face
At 04:46 AM 2/18/03 +0000, Harold Hallikainen wrote:
>Speaking of UPSs and inverters, what's the current technology on the DC to
>AC conversion? Are they using some high frequency PWM to simulate the 60
>Hz allowing for smaller magnetics? I built an inverter about 40 years ago
>with a big 60 Hz transformer that was especially for that purpose
>(included feedback windings to drive the switching transistors). Seeing
>the low cost and small stuff available now, I wonder how work.

The little Statpower inverters that I have looked at (300 VA and smaller)
use a nice little 140 Vdc dc-dc converter followed by a H-bridge operating
at 60 Hz.  There is significant dead time in the 60 Hz transitions so as to
create their "modified sine wave" output.  This results in the peak and
average voltages being similar to those obtained from a true sine wave source.

They are inexpensive and reliable little units.  I purchase the 300 VA
units and tap into the 140V supply for a couple of products - its by far
the cheapest 12V - 140V dc-dc converter that I could find.

dwayne

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2003\02\21@000000 by Harold Hallikainen

picon face
    That's pretty clever! I didn't think they were using 60 Hz transformers!
    On the front end inverter... We licensed a 75 watt DC to DC converter module from Cuesta Systems, one of the early manufacturers of UPSs for personal computers. This DC to DC converter took 12V in and output about 150VDC (or could be set up as a voltage doubler to do 300VDC). In our product, we connected this across the high voltage DC capacitor (or capacitors if there was a voltage doubler 120/240VAC input) of a 75 watt switcher. This powered our product when AC was lost. We also used the +5 and -12V outputs of the switcher to charge the Gel Cell battery, shutting down the charger when the inverter input current went up, indicating the loss of AC.
    A couple weekends ago I helped a 12 year old that I help with his projects wire one of those inverters up so he could play video games in the car on long trips. He'd already wired up a color LCD to it (he finds a lot of stuff dumpster diving).
    So, hearing about the inverter below brought back some memories...

Harold


---------- Dwayne Reid <RemoveMEdwaynerKILLspamspamPLANET.EON.NET> writes:


The little Statpower inverters that I have looked at (300 VA and smaller)
use a nice little 140 Vdc dc-dc converter followed by a H-bridge operating
at 60 Hz.  There is significant dead time in the 60 Hz transitions so as to
create their "modified sine wave" output.  This results in the peak and
average voltages being similar to those obtained from a true sine wave source.

They are inexpensive and reliable little units.  I purchase the 300 VA
units and tap into the 140V supply for a couple of products - its by far
the cheapest 12V - 140V dc-dc converter that I could find.

dwayne





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2003\02\21@200524 by Dave Tweed

face
flavicon
face
"Dmitriy A. Kiryashov" <spamBeGonevze27bymSTOPspamspamEraseMEVERIZON.NET> wrote:
> What is the most difficult part with PWM type of DC-AC with transformer
> converter design? Either regular PWM or some sort of magic sine waves.
> (minimal efforts to receive optimal output sine wave with minimal
> distortions)

See Don Lancaster's magic sinewaves at http://www.tinaja.com/magsn01.asp

-- Dave Tweed

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2003\02\26@015206 by Dmitriy A. Kiryashov

picon face
Hi Peter.

Let's assume that PWM frequency is 256 times by 60 Hz which is 15.36 KHz
PWM switches simply add and/or deduct more current (different polarity) into
primary windings. ( Like approximating curve with very short lines method )

What kind of load dump condition we are talking about in this example ?

There is no abrupt current changes in primary wirings on frequency close
to 60 Hz. ( since PWM frequency is much higher and it assumed to be filtered
by inductance to smoothly changed sine shape of AC current. )

What is wrong with such approach ? Effectiveness ?
( There is feedback also that watches load condition and keeping
output AC wave in certain amplitude and frequency range. )


WBR Dmitry.


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2003\02\26@015218 by Dmitriy A. Kiryashov

picon face
Mike, being very sarcastic doesn't mean informative ;)

WBR Dmitry.


Mike Singer wrote:
{Quote hidden}

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2003\02\26@152157 by Peter L. Peres

picon face
On Wed, 26 Feb 2003, Dmitriy A. Kiryashov wrote:

*>Hi Peter.
*>
*>Let's assume that PWM frequency is 256 times by 60 Hz which is 15.36 KHz
*>PWM switches simply add and/or deduct more current (different polarity) into
*>primary windings. ( Like approximating curve with very short lines method )
*>
*>What kind of load dump condition we are talking about in this example ?

None. We were talking about directly driving the primary with a switch at
60Hz in that thread. Not PWM. If you use PWM at high frequency then your
feedback loop can react fast enough to compensate for a load dump. Since
the energy in the coil is Wl = L*I^2/2 with your method you have at most
one PWM step 'load dumps', so your delta-I will be many times ^2 smaller
than any load dump that can be produced by a direct switching 60Hz system.
There you have the energy of a whole half period stored in the coil. If
the load goes away it can turn several tens of Joules into voltage. With
your PWM method this cannot happen.

*>There is no abrupt current changes in primary wirings on frequency close
*>to 60 Hz. ( since PWM frequency is much higher and it assumed to be filtered
*>by inductance to smoothly changed sine shape of AC current. )
*>
*>What is wrong with such approach ? Effectiveness ?
*>( There is feedback also that watches load condition and keeping
*>output AC wave in certain amplitude and frequency range. )

You now have a high quality switching regulator driving the same large and
heavy and expensive transformer with better efficiency than before. You
can go all the way, use simpler switching+transformer to make 310V (160V)
amplitude modulated with 100Hz (120Hz) |sin| and a H switch on the high
voltage side. Now you have a smaller transformer with less copper on it
(only 2 windings), only one high current switch (or two if driving
push-pull), and a controlled H switch at relatively low current in the
secondary, that is switched with a 100Hz digital signal and has low
losses (it makes sin from |sin| dc).

Most (many) UPSes from about 100W up are built like this, except some
older ones use iron core transformers and low switching frequency for cost
reasons.

Peter

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