I have two options for a 3 phase motor for an RPC. I was originally going to go VFD for my Clausing DP (3/4hp) but in the last week a surface grinder (2hp), dust collector (3/4hp) and a mill (2 hp) have found their way into the garage. I've been looking for a lathe for awhile but have not settled on one yet. The first idler motor is 20 hp and recently rebuilt for a phase converter. The 2nd motor is 7.5hp, both are Baldors. Is there a drawback to going to the larger motor other than price? I'm also on a 50 amp sub panel if that matters. The current draw on the entire 3 phase system is dictated by the load not the idler correct? Is there anything else I should be looking at before I settle on a motor? There are several RPCs for sale but are all 5hp and I don't want to limit my lathe choices more than the 50 amp service already is.
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Sourcing motor for RPC
- Thread starter B88i45
- Start date
Strouty
Well-known member
For what you are describing, you could probably get away with a 5 hp idler.
matt_i
Well-known member
For what you are describing, you could probably get away with a 5 hp idler.
That would be my advice also.
Strouty
Well-known member
Even if you end up with a 3 hp lathe, a five would still start it, but you may have to start another machine first, then start the lathe, then you can shut down the other machine. It acts like a cumulative effect, but I don't think you could go full one to one ratio.
On the other hand if you are thinking of buying a 7.5 HP lathe, then maybe the 20 HP idler would be better. Are you building the RPC? If so, you can always upgrade things as needed.
On the other hand if you are thinking of buying a 7.5 HP lathe, then maybe the 20 HP idler would be better. Are you building the RPC? If so, you can always upgrade things as needed.
You will run out of power well before you need a 20hp converter. I'm wouldn't want to try and start a converter that size on a 50 service. The 7.5 will run everything you want and then some.
A converter will start a motor the same size as long as it isn't a high inertia load. If you have multiple motors the total can be well above the size of the converter. Sometimes it takes some extra capacitors to keep things balanced.
A converter will start a motor the same size as long as it isn't a high inertia load. If you have multiple motors the total can be well above the size of the converter. Sometimes it takes some extra capacitors to keep things balanced.
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wyliesdiesels
Well-known member
You will run out of power well before you need a 20hp converter. I'm wouldn't want to try and start a converter that size on a 50 service. The 7.5 will run everything you want and then some.
A converter will start a motor the same size as long as it isn't a high inertia load. If you have multiple motors the total can be well above the size of the converter. Sometimes it takes some extra capacitors to keep things balanced.
yeah i was gonna say that with the 50a service, a 20HP idler is pushing it. especially if you have other loads running at the same time.
Split the difference and use a 10 hp? Anyone looked at this company?
https://www.ebay.com/str/Croman-Converters?_trksid=p2047675.l2563
https://www.ebay.com/str/Croman-Converters?_trksid=p2047675.l2563
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Strouty
Well-known member
Anyone looking at a RPC should try and build one before spending that kind of money. They are pretty simple things. My first one cost me about $125 to build (including used 5HP idler), then I stumbled on a 40 HP one while looking for a larger idler motor. So check for used ones, I paid $500 for the used one.
X2 on building your own converter.