The is a rotary phase converter question

[Alphawolf45]

 Likely not the correct category to ask this but-- I have ordered a 20 hp rotary phase converter to power my big sinker EDM as it was trying to kill my 5hp rpc … But long range plan is to buy a second 20 hp rpc to team with this one to feed the minimum required amps to my plastic injection molding machine that has sat unused since I got it.. The question then is how to wire two RPCs outputs together to get the phases synced ?
     I would have real 3 phase brought into the shop but have fought off cancer and think I may not live long enough to justify a big price tag for two service poles and transformers , 3 phase panel and all else. I am just doing this work for fun and side money to finance the fun.

[awemawson]

I'm sure that it must be possible, but personally I wouldn't countenance trying to parallel two RPC's. 
I have a feeling that it would be far too easy to let the magic smoke out 

Why not just dedicate one to one machine and the other to the other?

[Alphawolf45]

 Awesomeman  There is as you know a lot of bullhockey on the net . From the stuff I read , if you need 40hp rotary phase converter then two 20 hp RPCs gives an advantage in starting them up..Start one at a time and the surge current is more easily managed.

  20hp RPC will not run the plastic injection molding machine...Even 40 hp total is pushing it...The molding machine is rated at 150 amps full load . Fortunately the motors on the machine never operate all at the same time.

[awemawson]

I've made several static phase converters, including an extremely big one, but have only ever bought Transwave commercial rotary ones. Firstly  I had a 3 kW static one that I used to power my Bridgeport Mill, and Colchester Student lathe. Then I bought a Transwave 5 kW rotary to run the whole workshop, then a 15 kW one to run the  10 kW chiller unit for my 100 kW induction furnace (furnace ran off a  generator).

All the commercial ones managed their own start up sequences, and there was no need to interfere. (Though I did once manage to blow the 100 amp 240 volt company fuse with too much running at once)

When I was running off static converters the trick to starting big machines was to have a large (or several large) unloaded motors run up to speed before trying to start the machine in question

Amusingly my first really big static that I made, which was to run my Bridgeport Moog Hydropoint 1000 NC mill and it's huge compressor, I sold to the late Tim Leech (of Dutton Dock fame) and then later saw it advertised on eBay 

[vintageandclassicrepairs]

Hi Alpha,
Not knowing whether you live in an urban or rural location may render my suggestion useless ??
The price of two 20hp RPC's is not to be sneezed at !!!
Would a diesel generator be an alternative ? if it is not to be run full time it should be economical enough.
They come up secondhand regularly

I do not think its very feasible to parallel 2 RPC's
Unless you could split the electrical load between them ?

John

[mc]

Paralleling two RPCs could be done.  Ensure phases are matched, run both up to speed, then connect them together.
The input frequency takes care of ensuring speeds match, you'd just need to ensure the correct phases are matched.

If you're unsure, I'd wire in some relatively low value fuses (5A would probably be good enough for testing) the first time you try connecting both, as I'd rather blow some small fuses if phases are mismatched, than rely on the main breaker.

[awemawson]

You will get large circulating currents between the RPCs as the voltages when open circuit will never be the same.

[mc]

There will be, but on two identical RPCs they shouldn't be that different. Once you get over the initial connection, in reality there should be no difference between two RPCs running in parallel, and adding in an extra motor and capacitors.

[Alphawolf45]

I got the idea for two rotary phase converters from the suggestion of a major rotary phase converter manufacturers website..
    I wouldn't like using a 40hp rotary phase converter every time I wanted to use the edm . The diesel or propane generator idea has real merit for running the injection molding machine.. I've seen decent old biguns for 3 or 4 thousand dollars.. 'Believe I'll start looking for one in another month after I have tests at the oncologist saying my cancer is indeed in remission. I have real 3 phase on a pole in my yard but would be a problem getting it into my shop because of big trees that would need to be taken down. And too, the cancer I had usually returns in 5 to 8 years so I don't need to be initiating another electric bill with cost of poles and transformers added in every month for who knows how long.

[seadog]

There will be, but on two identical RPCs they shouldn't be that different. Once you get over the initial connection, in reality there should be no difference between two RPCs running in parallel, and adding in an extra motor and capacitors.

The problem is synchronising the two outputs. Unless they peak and trough at the same time they cancel out.

[John Rudd]

The rpc in essence seems simple enough....treat it as a 3 phase supply.
Now add a 3 phase generator.....The issue now is how to synchronise the 2 supplies...

My experience with 3ph, we had to sync up a generator to the local supply. This was done by speeding up or slowing the prime mover..( a 3.6 MW gas generator...)

So now to paralleling 2 rpc's.... I would expect there to be a slight phase difference between them due to tolerance allowances in the capacitors and also the motors....the cap values wont be exact and the motor speeds wont be exact....slip, frictional losses etc...

I wouldnt do it...easier just to get a bigger rpc or a diesel gen set...

[awemawson]

I've seen pictures of the effect of a large turbine set being loaded onto the National Grid when not in phase. The main turbine shaft was bent then sheared and at a guess it was about 15" diameter !

I assume that their synchronising equipment was faulty.

[timby]

I think it would be best to ask the manufacturer, someone will have tried that before.

[John Rudd]

I've seen pictures of the effect of a large turbine set being loaded onto the National Grid when not in phase. The main turbine shaft was bent then sheared and at a guess it was about 15" diameter !

I assume that their synchronising equipment was faulty.

I'm surprised any shears pins would have failed at a coupling, if fitted...maybe not....

We actually sheared a coupling on one of our 3.6 MW gen sets, starting a 2.1 MW Regrigeration compressor...even though the motor was on an autotransforner starter...something went wrong, I forget what!..

[Alphawolf45]

 Well I read it on the internet so you know it has to be right
  Anyway you have all convinced me that it can be done but it is beyond my set of skills...
      The generator offers bonus that we sometimes lose power to the house and wife has been on me to get a generator......Uhmmm Now can somebody give me a ballpark figure on kilowatts required to feed a healthy 200 amps of 220 3 phase to my shop? Pulling a number out of the air I am guessing that 100 KW would be necessary.
   By the way , I have an even dozen 3 phase machines in my shop if I only count the ones that work.. And for those I manage nicely with a 5 hp rotary phase converter aside from that power sucking EDM.

[John Rudd]

3phase power= V x A x 1.73
 Ergo 220 x 200 x 1.73 = 76130 watts....
Your ballpark seems good to me...