An actual design question...

[Howhurley]

I am designing an interesting power converter and would like to see if
I missed anything overt...

150-100VDC to 120 Vac, but the kicker is it is exclusively being used
to power a 4.5A peak compressor motor. I realize the 600 or so VA at
the output (highly reactive) could be problematic, but here goes:

I think I can get to the power throughput with a reasonable sine
wave. Do I just correct and match impedances with a huge cap and try
to bring everything into resistive impedance? Or can I just come out
with a squarish wave and correct that, knowing it will shape the
waveform by doing it?

I realize that's a lot of power for a PCB. Any suggestions?

Thanks!

 

[Ecnerwal]

I am designing an interesting power converter and would like to see if
I missed anything overt...

150-100VDC to 120 Vac, but the kicker is it is exclusively being used
to power a 4.5A peak compressor motor.

First would be to consider if it makes more sense to buy a system off
the shelf - 12, 24, 48 VDC in to 120/220 VAC out inverters of that
capacity and quite a bit more are an off-the shelf item, if you are not
tied to 100-150 VDC as your input. True sine out will result in less
heating of the motor than square out (misnomered mod-sine in the
industry) will. You'd have to check to be sure it would work with your
motor, but there's at least one true sine 300W (continuous) with 600W
peak out there for $250 retail - 12V input. Reinventing the wheel can be
expensive. Using an appropriate cap to correct the power factor should
help, of course, since you have a specific dedicated load.

Second would be to swap the motor out for a 90VDC motor (again, off the
shelf item) and simplify your controller problem to PWMing (or otherwise
dropping your input voltage) to 90 VDC.

As for the PCB, if there's too much current to keep your traces happy,
use big fat wires for those parts. Before that point, use short, wide
traces for the power stuff.

 

[Ecnerwal]

Compressors usually present a very high surge load upon start-up.

Depending again on what the overall design parameters are, a compressor
with an unloader (mechanical device that disables compression) can help
a LOT with startup - you get the motor and compressor up to speed under
no load, then drop the unloader and start compressing, with the
rotational inertia of the system (already started and running full
speed) helping to get it over the initial hump. More often seen on big
compressors, but if it's a product you are building, might be worth
including on your small compressor.

 

[Joerg]

Depending again on what the overall design parameters are, a compressor
with an unloader (mechanical device that disables compression) can help
a LOT with startup - you get the motor and compressor up to speed under
no load, then drop the unloader and start compressing, with the
rotational inertia of the system (already started and running full
speed) helping to get it over the initial hump. More often seen on big
compressors, but if it's a product you are building, might be worth
including on your small compressor.

Yes, a clutch would help but I've never seen one on a compressor this small.

Cats, coffee, chocolate...vices to live by

The Shepherd here next to me has voiced a complaint about item number
one ;-)

Item number two has just been consumed, item number three is in the
fridge and when my wife isn't here later today I might happen to find a
piece that miraculously "fell out of the fridge".

 

[Ecnerwal]

Yes, a clutch would help but I've never seen one on a compressor this small.

Not a clutch, usually - rather, a valve that opens the cylinder (thus,
"unloads the system" so that rather than compressing, it's just wiffing
air in and out. You might find one on a small compressor designed to be
run by a gasoline motor, for instance, since automatically stopping and
starting the motor is excessively complex in that situation, and of
course they are common on large compressors. A clutch would fail a lot
faster, and present more of a load to the motor on starting to compress
(with the unloader valve, the whole rotating mass of the
compressor/motor system is moving.) Without knowing how much of the OPs
system is actually open to design, just tossing out options.

Yet another one being to use a 180V DC motor and jut run it at less than
full speed, directly (well, fused and switched) connected to the
100-150VDC source, if a particular compressor speed is not too critical.

 

[Ecnerwal]

Sounds expensive and special order though.

Nope - both 90 and 180 VDC motors are as common as dirt in industrial
supply outlets. How expensive varies with how good (or how good a brand
used to be, in a few cases of running things in the ground) but they are
extremely off-the-shelf. Both MSC and McMaster could have one on your
desk tomorrow, if you wanted it that badly. In many cases they'll have
it on your desk tomorrow without even making it a rush order or paying
extra for shipping, but they usually don't guarantee that it gets there
that fast - it just does, anyway. They don't tend to stock the crummy
ones, though, nor to they seek to be the lowest price suppliers.

If seeking low price for a one off, electric treadmill motors range from
$40 (surplus outlets) down to free (free usually involves prying it out
of a treadmill that provides an unpleasant reminder of an exercise
program that didn't last) and they are often 90VDC.

Contraiwise, if the OP is designing some product to be manufactured, I'm
sure the factory suppliers can find a source for motors priced to suit
whatever market it's aimed at (ie, quality or cheap).

 

[Joerg]

Nope - both 90 and 180 VDC motors are as common as dirt in industrial
supply outlets. How expensive varies with how good (or how good a brand
used to be, in a few cases of running things in the ground) but they are
extremely off-the-shelf. Both MSC and McMaster could have one on your
desk tomorrow, if you wanted it that badly. In many cases they'll have
it on your desk tomorrow without even making it a rush order or paying
extra for shipping, but they usually don't guarantee that it gets there
that fast - it just does, anyway. They don't tend to stock the crummy
ones, though, nor to they seek to be the lowest price suppliers.

If seeking low price for a one off, electric treadmill motors range from
$40 (surplus outlets) down to free (free usually involves prying it out
of a treadmill that provides an unpleasant reminder of an exercise
program that didn't last) and they are often 90VDC.

Contraiwise, if the OP is designing some product to be manufactured, I'm
sure the factory suppliers can find a source for motors priced to suit
whatever market it's aimed at (ie, quality or cheap).

Motors, sure, but this would mean he'd have to build his own compressor.
Usually small compressors come with an encapsulated motor inside. Even
the big 4kW one in our AC is like that, you'd need a cutting torch to
get to it.

 

[[email protected]]

Compressors usually present a very high surge load upon start-up. Your
design needs to deliver that surge reliably. Also, it may not like
"modified sine" which, as Lawrence said, is just glorified marketing
speak for a square wave with a pause around the zero transitions.

That's not entirely fair. A proper "modified sine" has a pause (0V
out) that runs from exactly 330 dgrees to 30 degrees, and from 150
degrees to 210 degrees. This means that it has no third harmonic
content at all and a fairly low fifth harmonic content, which you can
filter out, along with the higher harmonics with LC low pass filter.
Even without the filter, you've got rid of a lot of eddy current
heating.

<snipped the rest>

 

[Joerg]

That's not entirely fair. A proper "modified sine" has a pause (0V
out) that runs from exactly 330 dgrees to 30 degrees, and from 150
degrees to 210 degrees. This means that it has no third harmonic
content at all and a fairly low fifth harmonic content, which you can
filter out, along with the higher harmonics with LC low pass filter.
Even without the filter, you've got rid of a lot of eddy current
heating.

<snipped the rest>

Well, yeah, it'll run. All I can tell you that I ran quite a few smaller
motors on modified sine and none of them was too happy about it. Heat
wasn't a huge issue but reduced power was.

 

[Joerg]

Do you know why?

Haven't looked into that much. For example, in a power outage I cannot
run the AC fans of our wood stove at high speeds using modified sine.
They just won't go there. With a real sine I can but that inverter is
very old, heavy and quite the guzzler. In this case it's ok because we
never really run them full bore unless we have a hard freeze. So I
prefer the modified sine inverter because it is small and more efficient
(battery lasts longer).

On pump motors that's another matter.

Years ago in a power electronics class that I took we did various SPICE
designs of motor controllers, and while we just generated "modified" square
waves, the inductance of the modeled motors made the output waveforms look
*very* nice.

AFAIK most motor controllers use modified square waves?

Not the ones I designed

But that was usually because the client wanted a quiet operation. On
steppers you often slam them in order to ramp up the current as fast as
possible (limited by the voltage you have at hand). So those are rarely
sine-driven.

 

[Joerg]

Speaking of wood stoves and fans... do you point your fan at the wood stove,
or have the fan facing outward from the wood stove?

It's an insert (Quadrafire 3100i) and they are built in. No intake
filters <grumble, grumble>. Air gets sucked in on both sides, cirulated
and out the top front. A ceiling fan also runs in the living room,
direction up, which pushes the air back down at the walls and windows
which avoids a draft. Extremely nice compared to central heat.

 

[Howhurley]

A Sola ferroresonant transformer might be the ticket.

Cheers

Phil Hobbs- Hide quoted text -

- Show quoted text -

Well, thank you for your input. I think I will do it this way:

I will design a modified sine wave version with an unfinished output
stage and adjust the parts to optimize according to the impedance of
the compressor, and looking at the waveform, I will be able to see the
effect relationship and get as close as I can to a good sine.
(minimize noise and reflected emf)

Thanks again, guys.

 

[Joerg]

Ah, ok, thanks. We have a standalone wood stove at present, although it's in
the living room around a corner and a good 100' I'd guess from the office, so
it's supplemented with the regular electric furnace. (...which blew a fuse
and quit working a few weeks ago! They had two 5kW coils plus the blower on
one fuse and just a single 5kW coil on another fuse. Both were blown, which
worried me for awhile until I realized that in all likelihood the single 5kW
coil's fuse had been blown for ages and no one realized the furnace was just
operating at 10kW rather than 15kW. I replaced both fuses and so far it's
been perfectly happy... unlike me, watching the meter spin, when it's on...)

I think outdoor wood furnaces are rather cool:
http://www.centralboiler.com/models.php ... but the prices are such that I
don't expect the actual payback occurs for many, many years. (They seem to
appeal to people who have large yards with lots of their own wood to burn...)

If you ever get one make sure it's a clean burner. Either multi-stage or
catalytic. I hate it when people just chuck in wood, walk away and smoke
crawls out for hours. That's the reason why politicians are banning wood
burning in a lot of places and put us at the mercy of some racket or
monopoly. The majority of wood stove owners in our neck of the woods
simply does not know how to operate them properly.

 

[Joerg]

A Sola ferroresonant transformer might be the ticket.

If those work like the resonant transformer they used in former East
Germany (to make their TVs work on a lousy power grid) then they would
be energy hogs.

 

[[email protected]]

Ah, ok, thanks.  We have a standalone wood stove at present, although it's in
the living room around a corner and a good 100' I'd guess from the office, so
it's supplemented with the regular electric furnace.  (...which blew a fuse
and quit working a few weeks ago!  They had two 5kW coils plus the blower on
one fuse and just a single 5kW coil on another fuse.  Both were blown, which
worried me for awhile until I realized that in all likelihood the single 5kW
coil's fuse had been blown for ages and no one realized the furnace was just
operating at 10kW rather than 15kW.  I replaced both fuses and so far it's
been perfectly happy...  unlike me, watching the meter spin, when it's on...)

I think outdoor wood furnaces are rather cool:http://www.centralboiler.com/models.php... but the prices are such that I
don't expect the actual payback occurs for many, many years.  (They seem to
appeal to people who have large yards with lots of their own wood to burn....)

---Joel

I have an outdoor wood furnace (it came with the house) and would not
advise anyone to buy one. Their one advantage is that the ashes are
outside. Disadvantages; You are heating the outside air and ground
where the water lines are buried, You need to keep this thing running
all winter, (or the water will freeze), When something breaks you are
stuck doing plumbing outside in the middle of a blizzard....

George Herold

 

[Ecnerwal]

Thanks for the information, George.

"You need to keep this thing running
all winter, (or the water will freeze)"

I thought they used anti-freeze to avoid this problem, at least until it gets
really, REALLY cold?

---Joel

Usually not. Given the typical volume of water (hundreds to thousands of
gallons), that would first be very expensive, and after a few years, (as
it breaks down with age and needs to be dumped) a large disposal
problem. On the third hand, 99% of "outside wood burners" are designed
exactly wrong, and rather than producing less pollution than a
woodstove, produce much more - to the point where Vermont instituted
fairly harsh restrictions on them because of air quality impacts
(filling valleys with smoke).

If you prefer to have a wood boiler out in a shed, fine. Buy a wood
boiler that could be installed in the house (it will burn clean,
particularly if it's a "gasifier"), and build a nice insulated shed for
it. Insulate the lines to it, and avoid self-defeat by insulating the
lines from each other, too. Several otherwise clever folk have built
unintentional heat exchangers and keep getting cool water out the far
end, even though they pump out hot water and get warm water back ;-)

If all is insulated properly, you should be able to set a circulator
control so your alternate heat source will keep the wood boiler and its
shed from freezing (say, when you go on vacation) without wasting too
much oil, gas, or whatever else you heat with.

Proper setup of a wood boiler should also have a large heat storage
tank, but that should be inside the heated space, ideally - the idea
being that the storage will absorb all the heat of a full load of wood
burning full blast (thus cleanly), and then the stored heat can be used
to heat the house, domestic hot water, etc.

 

[JosephKK]

Do you know why?

Years ago in a power electronics class that I took we did various SPICE
designs of motor controllers, and while we just generated "modified" square
waves, the inductance of the modeled motors made the output waveforms look
*very* nice.

AFAIK most motor controllers use modified square waves?

Naw. All the better ones use PWM to induce sinusoidal currents. With
all the added circuitry that entails. Especially VFDs.

 

[JosephKK]

The added circuitry actually isn't all that much. You need a half- or
full-bridge converter before the communtation bridge anyhow. All you
have to do is modulate the pre-converter with a sine. Either from a
table or, if the last penny counts, from an oscillator. Of course, to do
a thorough job you wouldn't be able to play the usual trick to add the
output onto the incoming DC-voltage in order to save a few pennies in
copper.

Most significant to serious VFDs are 3-phase input and output. Some
of the single phase assumptions don't work for them.

 

[[email protected]]

I am designing an interesting power converter and would like to see if
I missed anything overt...

150-100VDC to 120 Vac, but the kicker is it is exclusively being used
to power a 4.5A peak compressor motor. �I realize the 600 or so VA at
the output (highly reactive) could be problematic, but here goes:

I think I can get to the power throughput with a reasonable sine
wave. �Do I just correct and match impedances with a huge cap andtry
to bring everything into resistive impedance? �Or can I just comeout
with a squarish wave and correct that, knowing it will shape the
waveform by doing it?

I realize that's a lot of power for a PCB. �Any suggestions?

Thanks!

Find yourself a vfd on ebay of a suitable rating. Remove the control
board, that will leave you with all the power electronics you need.
All you would need to build is a board to supply a pwm signal to 4 of
the driver opto couplers.

 

[Joerg]

Most significant to serious VFDs are 3-phase input and output. Some
of the single phase assumptions don't work for them.

Yeah, it'll be a little more effort. But the OP's case does not sound
like more than one phase.