The AM Forum

I have and am using a home made converter (ala Lindsay) to run 3 phase electric motors in my shop...The converter is a 15 hp 3 phase motor that I run on single phase (2 legs) and use the 3rd leg to generate the missing phase...For motor use this setup works very well....Now I have a transmitter with 3 phase input that I would like to run...Are there any reasons why this rotary converter can't be used to feed the primary of a 3 phase plate transformer? 

I think as long as the waveform would look right (everything clean and in the right phases) then it should work. There are some broadcast sites that only have single phase power so run 3 phase transmitters off of a rotary converter.

I had a setup similar to your's to drive a 3-phase milling machine motor, but running with only a 1 HP Y-connected pilot motor, and it worked great. I think you will be better off with your rotary converter than with an elecrtronic converter -- and at no additional cost.

Do you have any balancing capacitors in your set-up to make the three phase voltages equal? I found it necessary to adjust capacity on two phases to get equal voltage on all three lines under load.

Bob - NE

Most three phase converter schemes result in low voltage on at least one phase.  You will probably have increased ripple on your dc.  Try it and let us know your results.
73,
--Chuck K7MCG

Hi Steve,

Interesting topic. I have been interested in 3 phase for a long time and would like to build something like you have. We'll have to chat one eve when I get the antenna project finished. Just dumped 3" of snow here last night so there will be a minor delay.

When I converted my log splitter to electric from gas, I used a 3 phase 7.5HP motor with a Baldor variable speed drive, which inputs either 3 or single phase and outputs 3 phase, which I found cheap at a local auction. Works like a hot damn and is quiet and no more gas, oil or warm up or exhaust.

Keep us posted.

Paul

The rotary converters used by broadcasters are true motor-generator types, making true 3-phase power via mechanical means.  Phasemaster is one company that builds them.

Using a motor and caps is an old trick for driving 3-phase motors.  It doesn't work out well when trying to excite a multiple core plate transformer.

Broadcasters being the usual cheap types would've used the motor-N-cap trick if it worked.

Transmitter manufactures even went so far as to absolutely prohibit open-delta source for use, as the harmonic content wiped out components.

73GD

Thanks...I'm a cheap type as well....So what could I damage?   ..What is the easiest way to couple a scope to the individual legs of the 3 phase so I can view the wave form(s)?This could be fun...

Not so much waveform distortion, it is the 120-120-120 degree phase relationship that is not possible to make correctly out of 0-180-360 single phase.

Motors can be sloppy, single-core transformers are pickier.

73DG

I ran my automotive machne shop here for many years on a rotophaze converter. The guy who bought the complete shop machine tools does the same. Now 20 plus years of using that particular converter. My PAW a P Eng with both experience in power (Hydro when first graduated) and in RF(Aircraft radio design (Collins) told me that while the first conversion may be a bit rough every motor you add running in the system makes it better. I asked  even if I put a motor in the system just running does it help with the phasing. He said yes. With the rortophase you already have one running and from what I have seen it works by far the best. Have also seen the cap conversions and read a very interesting book called running three phase motors on single phase which while I didnt use that certainly gave me a much better understanding of what I had to to. One phase remains the same. One leads. One drags behind. Feed one windig direct, one through a cap and one through an inductor.and all that is accopmplished .
Don Ve3LYx

i have had perfect results using Phasemaster rotary converters on *big* and smaller (5 KW) broadcast transmitters over many years. The third, or manufactured C leg doesn't need to be perfectly in phase,and it's usually not 120 degrees out, but close enough. You can adjust the 3-phanse transformer primaries for balanced secondary voltage. I have set plate transformers for unequal primary power.Yes, you always won't get perfect balance with a manufactured phase, but I don't see why your setup won't work.

I have a 100 KW ERP FM (50 KW AC draw) that's been using mechanical phase converters for some 30 years. Same Phasemaster (now a backup) since 1983. Just grease gun them once a year. Beats the cost of running three phase up a 9,000' mountain. Only thing to watch for is the startup current for the motor, which you prolly have a handle on. My biggest one hits >1,000 amps for a second or two from the 240 volt single phase source.

Check out the website for the Phasemaster for some good info, look to hear that peanut whistle on the air!

http://www.kayind.com/

Bill

The plate transformer  has 3 cores...one for each leg....each core has 4 taps on the primary side..  That should allow me  to balance each  legs output somewhat to get close to balance..I have a clip over the wire amprobe that I use to check the current in each leg...  Is there a way to check the lead and lag on the 3 legs?

 

Would a dual trace scope running on an isolation transformer, allow you to look at 2 pair at a time?

Just thinkin out loud.

Paul

Maybe I'm missing something and if so, I'm open to correction. Isn't the phase angle of voltages from the converter determined by the mechanical construction of the pilot motor? It is operating as an alternator from which the phase of output lines is fixed by the construction of the motor/alternator if it is working into a balanced load, although the amplitude of each phase can be different in this case. That's what I went by to capacitively-balance balance my small system.

i would imagine that an application wound 3 phase generator driven mechanically would be balanced in all three phases and inductive until capacitatively loaded.

Consolidated Gas, CNG, now Dominion Resources used Add a Phase units at their remote natural gas compressor stations almost exclusively for many years.
Nice units.

Current supplier I think.
http://www.ronkelectrical.com/add_a_phase.html (http://www.ronkelectrical.com/add_a_phase.html)

Here is the REAL deal:

http://www.georator.com/ProductRotaryMotorGenerator.html

Idler-motor jobs are for wimps.

And the poor.

73DG

Tres interessant.

'Also
http://www.ronkelectrical.com/rotoverter.html (http://www.ronkelectrical.com/rotoverter.html)

Most for motor loads only as the add'l motors are integral to the 3 phase operaton.

I guess if I had a really neat transmitter requiring 3 phase, I'd look to see if 3ph power was hung nearby and inquire into rates.  Sometimes a power co. might run a three phase, lightly loaded and short line, but at commercial demand rates, for "free."  umhmmm..  yeah right.  Guess one better plan on 24/7 operation, heh, heh. 

Don't assume polyphase power is out of your reach.  Lots of utilities sell power and will work with you.  When we worked up the deal, I asked not to be set up with a demand meter, they said not to worry as they don't use them on residential accounts.

Our electrical co-op set us up with 480V wye and subsidized the upgrade from 120/240.  It took three 50 kVA pigs and a 700' run of 12.7 kV.

We pay the same residential rate as everyone else in the area.  We're even on a TOU plan...

Every device here that can be is 3-phase is now.

The voltage makes for cheap distribution around the 'ranch'.

73DG

There are some good sites on the internet dealing with the ins and outs of doing the 3 phase rotary converter. I seem to recall the deal with the simple version, when putting the cap in to make the third leg on a pony motor that is doing the phase conversion is that the value of the cap has to be tweaked for the load. (is that english?)

There are some "fancier" arrangements that do a somewhat better job with the pony motor rotary converter. I didn't need that to run my milling machine so I didn't deal with it, but I remember downloading some stuff...

Seems to me that perhaps interposing another three phase 1:1 transformer between the rotary thingie and the transmitter's iron might balance the thing a bit too... not sure about that though.

                                 _-_-bear

Hmm, we have 3 phase lines running right past the house, like only 40-50 feet away.....

And sounds like some sort of 3 phase isolation transformer would work, you could homebrew one using 3 identical single phase isolation transformers rated at the proper voltage and wired correctly.

Be careful when setting the taps on the primary of any triple-core transformer.

I had to replace one in a 20 kW transmitter because the station engineer "set the taps according to what the individual leg voltages were measured at".

It burned to a crisp.... :o

The factory (Collins Radio) said it was a no-no, what was he thinking...

73DG