Theoretically possible, but why? You can buy a speed controller with
soft start, dynamic braking, and dozens of features, that also converts
single phase to to 3 phase, for $200 or so. Why bother to make your
own, which will take many hours, might or might not work well, might or
might not fail catastrophically, and will never have all the features of
a commerical speeed controller?

Richard


Ignoramus17647 wrote:
I am thinking about my high capacity transistors. Supposedly they can
be used for making inverters.

What if I build something like this:

1) a 220V to 208 V transformer

fed into

2) a rectifier

fed into

3) three DC to AC inverters that have their outputs shifted by 120
degrees relative to one another.

Would that not become a phase converter (1 - 3 phase)?

i

He is only a little closer to being able to make a VFD from those
transistors than if he'd start with a bucket of sand..

Bob Swinney
"Richard Ferguson" wrote in message
...
Theoretically possible, but why? You can buy a speed controller with soft
start, dynamic braking, and dozens of features, that also converts single
phase to to 3 phase, for $200 or so. Why bother to make your own, which
will take many hours, might or might not work well, might or might not
fail catastrophically, and will never have all the features of a
commerical speeed controller?

Richard


Ignoramus17647 wrote:
I am thinking about my high capacity transistors. Supposedly they can
be used for making inverters.

What if I build something like this:

1) a 220V to 208 V transformer

fed into 2) a rectifier

fed into

3) three DC to AC inverters that have their outputs shifted by 120
degrees relative to one another.

Would that not become a phase converter (1 - 3 phase)?

i

"Robert Swinney" wrote: He is only a little closer to being able to make a
VFD from those transistors than if he'd start with a bucket of sand..
^^^^^^^^^^^^^^
I had a friend who used to say, "Give me enough sand, and I'll build you a
golden castle."

Ignoramus17647 wrote:
I am thinking about my high capacity transistors. Supposedly they can
be used for making inverters.

What if I build something like this:

1) a 220V to 208 V transformer

fed into

2) a rectifier

fed into

3) three DC to AC inverters that have their outputs shifted by 120
degrees relative to one another.

Would that not become a phase converter (1 - 3 phase)?

i

To build your own inverter type phase converter you have to start with
building a big DC power supply. It's much harder than it looks. What would
very likely happen is that you'd go spend $40 on big power rectifiers and
as soon as you fired it up they'd go "BOOM!" and you'd be wondering why.

There is a design for an SCR-based variable frequency power supply in the
back of the old GE SCR Handbook. I did a lot of design work planning to
build one until I found out more about the failure mechanism of large SCRs
(roughly analogous to a hand grenade) and decided I didn't want them in my
house. You can pursue this if you want to and you will likely learn a great
deal but I can tell you this with great certainty: you will never cobble up
something cheaper and more robust than the inexpensive inverters you can buy
now from places like dealerselectric.com - but do keep us posted.

If you decide to press on, make sure you google on the term "snubber" because
a snubber circuit is what you'll have to design and test to keep your power
rectifiers from going boom.

GWE

Grant Erwin wrote:


If you decide to press on, make sure you google on the term "snubber"
because
a snubber circuit is what you'll have to design and test to keep your
power
rectifiers from going boom.

GWE


Then there was the talk a friend of mine gave at work one time "How to
build a power amplifier without going blind". I had the good fortune to
be wearing goggles when the power fet in the circuit I was working on
fried. It didn't explode, but the 5V chip driving it did when it got hit
with 300VDC.

Steve

By the way, you can't build an inverter with SCRs. An SCR can only be
turned on, not off, on command. Once you turn it on, it will not shut
off until the current through it goes to zero. For building an
inverter, you want power devices that can switch on _and_ off in the
kHz range in order to PWM all the phases you are trying to drive. SCRs
are fine for dimmers and heaters, and if you look at designs for those
you will see that they rely on triggering the SCR at a variable phase
angle, but it is always variable moving back from the zero crossing.

So you want to be using IGBTs probably, although you can also do it
with FETs. As other people have mentioned, there are _lots_ of details
to take care of in this seemingly simple system. Try building a good
variable speed DC motor controller first, since that is sort of like
1/3 of what you are eventually going to get to.

-Holly

Holly,
Maybe someone "lost the recipe" since SCR circuits have been used ever
since they were invented with commutation circuits. There are even
application notes on using SCR drives for DC motors driven from
batteries. My brother used to have an inverter that took 24 volts and
generated 240 volts AC and the active devices were in fact just two
SCR's.
That was his power source in the Alaskan wilderness. It had to be
reliable.

Now, in todays modern world, there are much better solutions, but don't
believe that they can't be used for inverters or commutated once turned
on.

Pete

"Richard Ferguson" wrote in message
...
Theoretically possible, but why? You can buy a speed controller with
soft start, dynamic braking, and dozens of features ...

I'm working on an induction heater (just blew two more transistors, pffbt),
I could just as well buy one, with all the features, temperature control,
ability to run any coil, and be completely load protected, but I won't. I
have more incentive because these things run upwards of $10k, but that's not
my point.

Some people like to build a Gingery lathe too...

Tim

--
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

"Ignoramus17647" wrote in message
...
I am thinking about my high capacity transistors. Supposedly they can
be used for making inverters.

What if I build something like this:

1) a 220V to 208 V transformer

fed into

2) a rectifier

fed into

3) three DC to AC inverters that have their outputs shifted by 120
degrees relative to one another.

Would that not become a phase converter (1 - 3 phase)?

i

Sure, but how do you plan on ensuring the three inverters stay in sync??
Greg

"Greg O" wrote in message
...

"Ignoramus17647" wrote in message
...
I am thinking about my high capacity transistors. Supposedly they can
be used for making inverters.

What if I build something like this:

1) a 220V to 208 V transformer

fed into

2) a rectifier

fed into

3) three DC to AC inverters that have their outputs shifted by 120
degrees relative to one another.

Would that not become a phase converter (1 - 3 phase)?

i

Sure, but how do you plan on ensuring the three inverters stay in sync??
Greg

Use a tone wheel and 3 pickups 120* apart. Amplify the pickups to control
the inverter circuit. Control the frequency by varing the motor speed.
Old Tube theory stuff.
Glenn
"Greg O" wrote in message
...

"Ignoramus17647" wrote in message
...
I am thinking about my high capacity transistors. Supposedly they can
be used for making inverters.

What if I build something like this:

1) a 220V to 208 V transformer

fed into

2) a rectifier

fed into

3) three DC to AC inverters that have their outputs shifted by 120
degrees relative to one another.

Would that not become a phase converter (1 - 3 phase)?

i

Sure, but how do you plan on ensuring the three inverters stay in sync??
Greg