There is a guy selling a book on ebay to use a 3 phase transformer to
run as a converter from single phase using capacitors to help start the
motors. Any thoughts on this? Advantages / Disadvantages? I want to run
a 7.5 hp 3 phase compressor motor but the cost to run a 15 hp converter
continually is too much.
If you want to say that you do not want to pay the cost of running the
phase converter when the compressor is not running, it is not too hard
to rig a system involving one (I think) time delay relay that would
start the phase converter, and then start the compressor after, say,
10 seconds, when the pressure switch decides it needs to run the
Then your phase converter runs only as needed. You can also buy a 5 HP
single phase motor and use a smaller pulley.
The way it works would be that you would use one pole of your
compressor's starter to send a signal to the main contactor of the
phase converter to start it.
There would also be a contactor on the output side of the phase
That contactor would be actuated by a time delay on relay, wired to
close 10 seconds after the phase converter starts.
This way, the compressor would come up 10 seconds after the phase
converter starts. The above assumes that your compressor is the only
load. If it is not, it would need another contactor near the
compressor motor, turned on by another pole of the pressure switch.
Your expense would be a time delay relay $30, and a pair of contactors
-- also $30. A simple solution.
do not forget that the compressor runs intermittently, but the phase
converter would need to run continuously -- unless it is switched on
on demand. (which is what I suggested, to run a regular RPC but switch
it on demand based on pressure switch, see my another post).
So, 600 watt continuous could even exceed the draw of the compressor
on a daily basis.
Ah, but based on the comment that his compressor would be running
continously, that's not an issue.
Did he make that comment? Nope. But I've found that the quickest
way to get a poster to provide more information about the project is
to simply make wild guesses about what he's doing. Then everyone
including him, every other contributor on this board, and their
brothers-in-law, chime in to say I'm wrong.
But at least the details come out that way.
I am curious if anyone tried to build a homemade VFD (perhaps without
the "V" part), using a rectifier, some way of smoothing the DC, a PWM,
and a bunch of mosfets or IGBTs that would create 3 phase output. It
seems doable in principle, although I cannot see how it would be
superior to a regular rotary phase converter. The only advantages I
see is less noise and less idle opwer consumption.
"> I am curious if anyone tried to build a homemade VFD (perhaps without
So Iggy, it seems you must have gotten a new electronic dictionary for
Christmas! As usual, you are brimming with buzzwords in new combinations
that support the totally inane questions you impose on the good readers of
And rather than being "curious" (the entree to many of your troll-posts) why
don't you try to recall some of the answers you've already gotten from well
meaning members of RCM. How many times do you have to be told you aren't
going to be able to build a VFD? And, I don't care how sophisticated you'd
have us believe you've become as an accomplished "e-bayer", you aren't
liable to find the components and instructions (those you could understand)
on the construction of a VFD.
Give it up, Iggy-troll. Why don't you go back to your idea of using a a
variable frequency square wave generator as the driver/trigger for a VFD of
your own design? Give us a rest, already! Why not experiment on that idea,
and keep quiet until you have something of substance to report.
I am not sure what made you fume at the mouth so much, Bob.
My inverter was working, until I miswired the snubber circuit and
fried the IGBTs. I posted pictures to that effect. I am going to get
some parts this week and put it back together in a little bit more
compact form (because I will use 400 amp IGBTs instead of paralleling
200 amp ones).
Actually, as it turns out, VFDs are implemented using pulse width
modulation (if you do not know what it is, it is your problem and not
``The latest method used for adjusting the motor voltage is called
pulse width modulation PWM. With PWM voltage control, the inverter
switches are used to divide the quasi-sinusoidal output waveform into
a series of narrow voltage pulses and modulate the width of the
"> I am not sure what made you fume at the mouth so much, Bob."
Aw Jeeze, Iggy-troll. I just wanted to give you another chance to come back
with one of your half-baked retorts; you know, collect one more appearance
of seeing your name in print on RCM. That is what you are doing, isn't it?
PS: I think you meant "foam" not fume at the mouth. Grammar, spelling and
good use of the English language is not the hallmark of an accomplished
troll. See, you scored again, when you weren't even trying.
I had a lengthy discussion about this several years ago with a professor
of electrical engineering (a guy who really knows his stuff when it
comes to power electronics). His view was that it was entirely possible,
but not a simple project. Areas of concern were acquiring semiconductors
and heatsinks which would cope with the power required, minimising the
losses, and also the design and calibration of the control system, which
is not as simple as it seems because it must reduce the voltage as the
frequency is reduced. I decided to build a static phase convertor
instead, partly because of the cost of the semiconductors and the
doubtful reliability of a home made VFD, and also because I was busy
with other things at the time.