3 phase converter using a transformer?

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 compressor.

Then your phase converter runs only as needed. You can also buy a 5 HP single phase motor and use a smaller pulley.

i

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 converter (RPC).

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.

i

You don't really understand how rotary converters work. A 15 hp idler motor will consume about 600 watts of power, which is small compared to the 7.5 hp load motor draw.

Jim

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.

sosc

i

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.

Jim

I wonder why the OP is not coming back with more details... Like the planned use of the compressor.

i

I have been following the responses here, thanks. But I was more interested in the use of the Transformer to get a 3 phase voltage.

It won't work.

Jim

Do you have the item number of that ebay auction that you mentioned?

i

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

If someone did there would be another name for it, A welder =)

Iggy-troll sez:

"> 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 RCM.

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.

Bob Swinney

Give him a break Bob. At least his ramblings keep him out of hospital dumpsters (:

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). i

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 mine).

``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 pulses.''

i

Iggy-troll goes:

"> 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?

Bob Swinney

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.

You are right. It is foam at the mouth. Do you have anything of substance to say?

i

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.

Best wishes,

Chris

Thanks Christopher. That was interesting. Semiconductors and heatsinks are available these days, but your point on control is a very good one.

i