I have a 7.5 HP vane type compressor that I am researching how to run on single phase power. Swapping to a single phase motor is not an option. Anyone have any knowledge of sizing a phase converter? It looks like I can find rotary phase converters that say 7.5 HP for around $600. But I don't know if this is one of those situations where I really should be getting a 10 HP converter for the 'extra overhead power'. Advice?

That's going to draw LOTS of amps.

The final answer is up to you, but if it is really rated for 7.5HP, then it should run the 7.5HP motor. You may need to look at duty cycles of both the compressor motor and the phase converter.

Im no electrical wiz, but...

You can simply get an additional 3 phase motor, mount it on the floor.
Start that motor with a single phase, once running you use that as a convertor for the compressor..

A good industrial electrician can explain it...
It is the cheapest way to go, just requires a bit of work on the slave motor to get it to start. It just gets re-wired somehow...
I had one in my shop many years ago...much cheaper than a phase convertor,,,,yet its a phase convertor...go figure...just not in a pretty box.

Rich

This is a quote from a web site I found..
I had the capacitor type rotary pahse convertor (another motor)
I just came in, turned on the power and this motor sat in the corner and ran all day long (you can turn it on and off as needed, just wait for it)

Again, this is really normal practice for an industrial electrician that does alot of commercial work....3 phase power is not everywhere...so they all have run across this issue. I hired a guy, he made it happen.

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Question
I would appreciate any information regarding building a rotary phase converter.

Forum Responses
Do you mean, like, from scratch? I don't think you mean that!

What you could do is get a motor and generator and connect them together. Is that what you mean? What capacity do you need (how many HP is your load)? Is it for just one machine or several? I assume you are going from 1 phase to 3 phase. What voltage--same on both sides?



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There are 2 basic routes you can take. One is to use a small hp single phase motor to get a large 3 phase motor spinning. Once spinning, the 3 phase motor will continue to spin by feeding 220 single phase to 2 of the 3 legs. The 3rd leg will produce the 3 phase 3rd leg you need for your 3 phase machinery. Once the 3 phase motor (the one generating the 3rd leg) is spinning, you will need to disconnect it from the small hp single phase motor that started it spinning. One way people typically do this is by having the small motor on a slide base that you can move toward the larger motor, subsequently slacking the belt that connects the two.
The other method is a bit more complex, and takes more electrical skills. You can build a capacitor start system for the 3 phase motor, eliminating the need for the small hp motor to get it spinning. These capacitors will have to be matched to the 3 phase motor you will be using to generate the 3rd leg. By being capacitor started, the rotary converter will be "instant on", versus the slow "up to speed" arrangement that results from the small motor start up system.

I will emphasize this now: if you don't completely understand this brief explanation, then you probably don't have the electrical background to proceed. If that is the case, either purchase a commercially made rotary converter, or hire an industrially experienced electrician to put the converter together for you.

I had a factory that had no 3 phase power at one point and a had a lot of equipment that was 3 phase and one big wet sanding mill that had a 7.5 HP head. I used a bunch of small low cost $125.00 phase converters boxes but found then not to work right with the big motor. They where fine for the buffers thought. The motors did not give you the true power with them and you had to spin then by hand some times.. I bought a set up that was a 10 HP motor on a base that turned all the time and it made 3-phase power by changing the phase to a box /panel that was part of it . I was able to run the full shop, all 3 phases on this panel just like it was
3 –phase coming in by the city. In the morning I would just turn it on. It did not make any noise, as it was just the motor turning. This was 17 years ago. They may have come up with something different in this time. I paid $600 for the 10hp used. And it will suck amps if you are doing this at home. It was 220V.

Bob Burton

I just went to the web to look and it was a Rotary Phase Converter. The small low cost boxes are Static Phase Converters. They do not give you true power and I was told they would be a problem for big equipment.

The web page was http://www.phase-a-matic.com/ Call these people and ask them!!!!

I did not read it just saw it and it came back to me. You need to do the homework. I hope this helps. I see ebay has them for a low price. My unit worked great. I used it for 2 years and sold it when I moved to a unit with 3 ph.

Bob

I have done this in the past and made a mistake .. the converter will only cover all three legs at start up for a limited time ... then it cuts out to two legs ... so you get 2/3 of the available hp to continue to run the motor ... heres the problem I encountered : Cold january night, I forgot to turn off the compressor ... air pressure dropped over time, motor tried to start but due to having a "head" of pressure and thick oil the motor did not get up to speed before the converter cut out due to build in timer for start-up ... now the bad part ... I went down to the barn the next am to get my truck and the compressor motor was on fire due to the two legs overheating the windings to try to compensate for the missing leg ...moral of the story not a big enough converter you need 1/3 more hp then the motor calls for to compensate for the extra "pull" on the system ... I.E. my motor was 5hp I should have had a 7.5hp converter, I only had a 5 hp converter. anyway 2 fire extinguishers and $650 later (new motor and switch gear)and I was back in buisness.Learn from others mistakes ... you never live long enough to make them all yourself ...

I believe that is the difference between the static. It only tweaks the third leg. The rotary has its own motor and is making true 3phase all the time it is running. You need to run the motor all the time on the converter if you want the compressor to be on demand. For my factory like I said, opened up and started up the power.

Bob

I'm in agreement ...

I have freinds (I pay them well) that use the rope start method very succesfully but I guess thats up to the individual and their preference ...

Static phase converters are not a GOOD option for a compressor... PERIOD...!!!!!!!!!!!!!!!!

Especially if the compressor head bleed valve is a tad sticky. That will smoke a Static converter AND the motor very quickly.
(ask me how I know)

The statics are great for anything other than a HARD starting piece "O" machinery like a compressor or a big mill.

Why is a single phase motor not an option?

I am guessing you have a wire size or breaker/panel threshold that doesn't allow you to use a single phase 220 motor..... Corrrect?

C

Mark I posted a similar thread a yr or so ago.
After doing the reasearch I opted to sell my rotery phase converter and dump the 3 phase comp. motor and replace with a single phase (of correct frame) on my compresor. At the end of the day the expense was a wash. and the complexity was (single phase) was eliminated.

I'll look into a new motor, but this is not a belt driven compressor. the drive shaft from the motor hooks directly into the compressor so I don't know how hard it will be to find a motor that will bolt right in.

Or I'll sell the whole setup and buy a different compressor.

before you make a decision, open up the yellow pages, call a commercial electrician, or better yet, go to the closest machine shop..
They have 3 phase converters running.
Ask the lead machinist/tool guy what to use, and who to call...
That is how I found my guy, there was a machine shop down the street...I asked them...

Rich

If you want to talk to someone that knows, call Brad Nelsen of Nelson Electric Motor in Salt Lake City, he's been building them for years. We used one of his on a punch press and it worked great. 801-262-9530 He can point ya in the right direction. Dave Pigott

A single phase 7.5HP compressor duty motor will set you back some money, but I'll bet you'll be ahead in the long run. IIRC, the single phase 7.5 motor will be around 43 amps. It's probably a "C" face motor if it bolts directly to the input of the compressor. You should be able to find one.

The nameplate information should provide the mounting style of the motor for a supplier to match it with a single phase unit.

My compressor motor is only 5HP, 220VAC, 1phase and it was expensive.