OT: Phase Conversion, For running a compressor Options

[McMark]

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?

[r_towle]

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.

QUOTE

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.

[doper]

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 ... (IMG:style_emoticons/default/aktion035.gif)

[burton73]

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 ... (IMG:style_emoticons/default/aktion035.gif)

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

[doper]

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 ...