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phase converter question

G

garageguy

Super User
Premium Member
I have a 3hp ,3 ph air compressor run off a 3hp static phase converter. If I wanted to run a 1/2 hp 3ph motor do I need a smaller phase converter? I was told that the PC needs to be matched to the motor. Any help appreciated!
 
I don't know the answer to your question, but why not just run a VFD for the smaller motor ?
 
garageguy said:
I have a 3hp ,3 ph air compressor run off a 3hp static phase converter. If I wanted to run a 1/2 hp 3ph motor do I need a smaller phase converter? I was told that the PC needs to be matched to the motor. Any help appreciated!
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So either the 3hp motor was grossly oversized or the 1/2hp replacement motor will be grossly undersized? Or the more likely reason is that I am out to lunch and am missing some important information.o_O
 
what I would like to know is if I can run a 1/2 hp motor with a 3hp phase converter? How are VFD priced compared to PC's ?
 
A static phase converter can only run a 3ph motor at about 2/3 of nameplate hp, and they are voltage balanced with capacitors. Theres a bit of leeway, but you'd probably have a voltage imbalance running a 3/4 hp motor on a 3hp static.
 
as a couple people suggested, I am considering running a VFD . I have never had anything to do with them but they seem fairly simple to install . Are there any decent suppliers in Canada or are you guys using amazon/alibaba etc?
 
garageguy said:
as a couple people suggested, I am considering running a VFD . I have never had anything to do with them but they seem fairly simple to install . Are there any decent suppliers in Canada or are you guys using amazon/alibaba etc?
Click to expand...
I bought a few of them of Amazon and have been happy with them and the price.
 
@garageguy I have an inexpensive VFD I bought from @John Conroy and it looks pretty good. It was sub-75$. I have run a TECO 710 for about 10 years now, and It was pricey, but worth every penny, I paid about 290$ US,

At the current price point these days, I'd give one of the cheap ones a try. If you are into a more 'luxury' solution I can recommend TECO (it is made and supported by GE/Westinghouse).
 
I was told that you need to size the RPC to the load.This is especially important for hard starting loads like hydraulic pumps and compressors. For these loads it is recommended that the RPC be sized to 2x the load. Therefore for a 3 hp compressor you could get away with a 5 hp RPC. Does your air compressor actually have a 3 hp motor or is it rated at 3 hp "peak" and is actually 1 or 2 hp running?

As far as using a larger RPC on a smaller motor, it is really not a problem, it is just a little less efficient since the idler motor on the RPC is over sized for the task.
 
RotoPhase, the maker of my RPC recommends 1 1/3 times the size of the largest motor to be started off of it. This, however doesn't account for the rotating mass of lathes, which requires a larger RPC. 2X is always considered safe, but for lightly loaded motors 1X is fine. the 1.33 recommendation is just a rule of thumb.

I start my 7.5 HP lathe on a 15HP RotoPhase, but American Rotary recommends a 10HP for the same purpose. So nothing is cut and dried. There are ways to start a heavily loaded motor even a 1X rotary phase converter, but that is out of the scope of a 'general discussion'.
 
I'm pretty sure he is talking about a static, not a rotary. With a rotary you also have to factor in the load of the idler motor, so its a bit different.
 
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