Amp pull at startup

[ironmonger]

<snip>
As far as amp draw goes, Lets say that the amp draw from a machine is 30 amps, It'll be roughly the same whether its single phase or 3 phase. The only difference is that in 1 phase circuit the amperage gets divided between 2 legs (15 amps per leg),and on a 3 phase circuit the amp draw would be divided up by 3 legs (10 amps per leg), <snip>
Marcel

Current is a series circuit is equal. The 220V circuit must supply 30 amps, the current in each 'leg' (see * below) is 30 amps. The neutral is irrelevant in this example as the voltage is measured between L1 and L2 of the mains circuit, but the equivalent would be two 110v circuits in series, in each of which 30amps would flow and would be 220V at 30 amps when series connected.

paul

*(the 'legs' represent L1 and L2, which are the secodary winding leads of the distribuition trnasfomer that sits on the ground or or your pole)

 

[JimDawson]

Hmmmm... 30 HP is a lot.


In your shoes, yes, I'd get a dedicated panel box for this and depending on how much you load the machine, you may need to look into "slow-blow" type breakers or possibly a VFD to use as a "slow-start" device if at all possible.

Ray

I remember way back an old guy(about my age now I suppose) ran his 3 phase machines from a single phase supply. He did have a RPC that consisted of a large 3 phase motor. The difference was that he used a small single phase motor to spin up the 3 phase motor before applying power to it. Had it on a rocker mount with a belt drive and a foot pedal, start the motor, apply foot pressure. Seemed to work pretty well, and eliminated the start up load. I guess this is a pretty good way of doing a soft-start.

 

[Phils69]

Thanks for all the feedback guys. I know it's more than I will probaly ever need but It was one of those deals that I felt like I couldn't pass up. I was thinking of the same soft start idea using a small motor to spin it up to operating speed then hitting the breaker on the RPC. I'll probaly try it if I have any issues. My friend has a 20 hp idler on his and is running it off of 100 amp service and it starts fine so maybe the 30 won't be too bad on the 200 amp panel. Btw I went to a one man shop in VA last year that was closing and he had some big machines, full size Carlton radial and a King VTL, that he was running off a RPC. It was a dubious looking setup but he was running a 60hp idler, it was an old motor also so it was huge! I would love to have heard that one fire up. I am scheduled to close on the house end of June so it will probaly be end of July or August before I have the 30hp RPC running. I'll do a temporary setup with my 7.5 so I can put power to my small mill and lathe. I feel naked without them operable LOL. On another note I am thinking of mounting the RPC outside of the building, along with a air compressor, under a attached shelter behind it. Other than possibly having to upsize the gage wire for the extended run would there be any other issues to worry about? Would the moisture be too bad for it? I'd sure like to have as little noise as possible but I could put it up in the rafters if I have to have it inside.

 

[John Hasler]

I remember way back an old guy(about my age now I suppose) ran his 3 phase machines from a single phase supply. He did have a RPC that consisted of a large 3 phase motor. The difference was that he used a small single phase motor to spin up the 3 phase motor before applying power to it. Had it on a rocker mount with a belt drive and a foot pedal, start the motor, apply foot pressure. Seemed to work pretty well, and eliminated the start up load. I guess this is a pretty good way of doing a soft-start.

That's a reasonable way to start an RPC. If the motors are matched for speed there's no need to disconnect the single phase one. Use a time delay relay or centrifugal switch to apply power to the three-phase machine.

 

[John Hasler]

Thanks for all the feedback guys. I know it's more than I will probaly ever need but It was one of those deals that I felt like I couldn't pass up. I was thinking of the same soft start idea using a small motor to spin it up to operating speed then hitting the breaker on the RPC. I'll probaly try it if I have any issues. My friend has a 20 hp idler on his and is running it off of 100 amp service and it starts fine so maybe the 30 won't be too bad on the 200 amp panel. Btw I went to a one man shop in VA last year that was closing and he had some big machines, full size Carlton radial and a King VTL, that he was running off a RPC. It was a dubious looking setup but he was running a 60hp idler, it was an old motor also so it was huge! I would love to have heard that one fire up. I am scheduled to close on the house end of June so it will probaly be end of July or August before I have the 30hp RPC running. I'll do a temporary setup with my 7.5 so I can put power to my small mill and lathe. I feel naked without them operable LOL. On another note I am thinking of mounting the RPC outside of the building, along with a air compressor, under a attached shelter behind it. Other than possibly having to upsize the gage wire for the extended run would there be any other issues to worry about? Would the moisture be too bad for it? I'd sure like to have as little noise as possible but I could put it up in the rafters if I have to have it inside.

The RPC should be fine in the shelter as long as it's protected from the weather. It shouldn't be very loud, though.