I recently purchased a 5.4hp lathe and 3hp mill. It came with a 5hp RPC which is too small. Not sure how the guy that ran the equipment (deceased) was able to effectively run it with that small of a RPC? I’m wanting to upgrade and go with either a 15 or 20hp RPC. I have access to both size motors. I’ve spent the past few days reading and researching on building or buying. There’s a lot of info out there and some of it’s conflicting. I’ve read that one should go with Fitch’s design and I’ve read that Fitch’s design has shortcomings. I’ve read that one should utilize a pony motor and I’ve read that using start capacitors is the better way to go. I’ve called American Rotary and inquired about a system. They recommended purchasing their ADX-15 system and didn’t seem interested in helping me in any other way. Since I have the motor, I basically have two options from an economical standpoint…one is to purchase components and build the panel myself (assuming I can find the ‘right’ design) and the other is to purchase a panel built for the motor I have. American Rotary would not do this, it’s all or nothing. Is there any other reputable company out there that will?
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RPC design help
- Thread starter Greyhound
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You raise lots of questions, but if you have an existing motor to use the easiest approach is to buy a pre-made panel from one of the various manufacturers. That will include the run and start capacitors and controls, you will need to wire it in and *possibly* do a bit of fine tuning.
For the size of machines you have, a 15HP RPC is plenty. General rule of thumb is two times the HP of your largest machine, so a 15HP RPC will run a 7.5HP machine. Some variations depending on the startup load of that motor. For example a lathe with a clutch does needs a bit less oversizing than a machine that spins the motor up under load, so you might get away with running a 10HP clutched lathe on a 15HP RPC. 3 phase air compressors are a classic example of something that starts under considerable load, drawing more startup current and benefiting from a larger RPC.
Building it yourself will generally not save you any money over using a pre-made panel. Panels can be shipped typical UPS/Fed-Ex vs. freight for a complete RPC. Hopefully others can contribute some names of panel manufacturers.
Most of those panels (all that I've seen) use capacitive start. I run a 20HP RPC with capacitive start. I've measured the peak startup current at 205 amps. That doesn't mean it needs a dedicated 200+ amp feed, but I do feed it with a 125amp circuit which is substantial in a typical residential electrical system. A pony motor is one way to reduce the startup load and eliminate the start capacitors. That doesn't change the needed full load run current, or run capacitor requirements.
One consideration when looking at panels, some of the panels have the two single phase legs live all the time, even when the RPC is "off". Others disconnect all legs when the RPC is shut down. I prefer all of them off which is what I use. I also have a 200a disconnect switch in front of my RPC. I prefer "clearly and completely off".
For the size of machines you have, a 15HP RPC is plenty. General rule of thumb is two times the HP of your largest machine, so a 15HP RPC will run a 7.5HP machine. Some variations depending on the startup load of that motor. For example a lathe with a clutch does needs a bit less oversizing than a machine that spins the motor up under load, so you might get away with running a 10HP clutched lathe on a 15HP RPC. 3 phase air compressors are a classic example of something that starts under considerable load, drawing more startup current and benefiting from a larger RPC.
Building it yourself will generally not save you any money over using a pre-made panel. Panels can be shipped typical UPS/Fed-Ex vs. freight for a complete RPC. Hopefully others can contribute some names of panel manufacturers.
Most of those panels (all that I've seen) use capacitive start. I run a 20HP RPC with capacitive start. I've measured the peak startup current at 205 amps. That doesn't mean it needs a dedicated 200+ amp feed, but I do feed it with a 125amp circuit which is substantial in a typical residential electrical system. A pony motor is one way to reduce the startup load and eliminate the start capacitors. That doesn't change the needed full load run current, or run capacitor requirements.
One consideration when looking at panels, some of the panels have the two single phase legs live all the time, even when the RPC is "off". Others disconnect all legs when the RPC is shut down. I prefer all of them off which is what I use. I also have a 200a disconnect switch in front of my RPC. I prefer "clearly and completely off".
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I know that this doesn’t answer your question but, you may want to consider inexpensive VFD’s instead of inefficient RPC technology.
A 3hp 3phase motor can effectively run on a 5hp RPC, a 5hp motor on a 7.5hp RPC, a 7.5 motor on a 10hp RPC etc.
It’s not rocket science
A 3hp 3phase motor can effectively run on a 5hp RPC, a 5hp motor on a 7.5hp RPC, a 7.5 motor on a 10hp RPC etc.
It’s not rocket science
- Joined
- Jan 2, 2014
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- 8,899
I would start by asking @Ulma Doctor .
Mike has been there and done that (plus knowing him, he will likely offer to send you any spare parts he has laying around......).
https://www.hobby-machinist.com/threads/3hp-rotary-phase-converter.60791/
https://www.hobby-machinist.com/threads/rpc-simple-design-unbalanced.12712/
Brian
Mike has been there and done that (plus knowing him, he will likely offer to send you any spare parts he has laying around......).
https://www.hobby-machinist.com/threads/3hp-rotary-phase-converter.60791/
https://www.hobby-machinist.com/threads/rpc-simple-design-unbalanced.12712/
Brian
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- Jun 7, 2013
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- 10,406
I built a RPC by using a phase o matic phase converter to start the RPC motor, and then I used oil filled capacitors to generate the
"manufactured leg"., the capacitance chosen to achieve the best voltage balance between the 3 legs. I find that a 5 hp converter will successfully start and run a 5 HP motor on my 19" Regal lathe with direct connected motor
"manufactured leg"., the capacitance chosen to achieve the best voltage balance between the 3 legs. I find that a 5 hp converter will successfully start and run a 5 HP motor on my 19" Regal lathe with direct connected motor
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- 11,900
If you already have a motor that would be your cheapest solution- to purchase VFDs for both machines would be more costly
You only need some capacitors and possibly a relay and a start button- probably less than 150$ worth of stuff
Add a little more for an enclosure of some kind, some cable, some terminals...
You only need some capacitors and possibly a relay and a start button- probably less than 150$ worth of stuff
Add a little more for an enclosure of some kind, some cable, some terminals...
- Joined
- Feb 2, 2013
- Messages
- 3,850
For about $200 , you can get 2 inexpensive VFD’s
I can/have built rpc’s for less than $60 using reclaimed components
Something has not been mentioned yet is the cost of operating a rpc vs a vfd. A 10 or 15hp rpc will pull in excess of 28 amps on a 50 amp circuit, as long as it operates .
You can run a 3hp vfd on a 20 amp circuit and still have power to spare
I can/have built rpc’s for less than $60 using reclaimed components
Something has not been mentioned yet is the cost of operating a rpc vs a vfd. A 10 or 15hp rpc will pull in excess of 28 amps on a 50 amp circuit, as long as it operates .
You can run a 3hp vfd on a 20 amp circuit and still have power to spare
I had considered going the VFD route, but dismissed it due to the near certainty I'll be adding more 3phase equipment in the future. That's also the reason I'm looking to upsize what I currently need, hence the 15 or 20 hp idler motor.
What brand of VFD's are you recommending?
I have access to pretty much all type of industrial electrical parts and components which is why I originally planned to build the panel myself. The problem was I couldn't find a schematic with part descriptions and part numbers that the consensus seemed to decide was the 'best' for a 15 or 20 hp RPC.
I reached out to one manufacturer (North America Phase Converter) and they sell panels separate. American Rotary, according to their website, does as well (STK panels) but when I talked to them yesterday, they told me they didn't. Their on-line chatbot said the STK series panels would work well in a lathe application...which contradicts what I was told yesterday. Kind of confusing.
What brand of VFD's are you recommending?
I have access to pretty much all type of industrial electrical parts and components which is why I originally planned to build the panel myself. The problem was I couldn't find a schematic with part descriptions and part numbers that the consensus seemed to decide was the 'best' for a 15 or 20 hp RPC.
I reached out to one manufacturer (North America Phase Converter) and they sell panels separate. American Rotary, according to their website, does as well (STK panels) but when I talked to them yesterday, they told me they didn't. Their on-line chatbot said the STK series panels would work well in a lathe application...which contradicts what I was told yesterday. Kind of confusing.
I have just installed a 3 HP RPC Kit that I bought on E-Bay, the components came with clear instructions (had the 3 HP Motor) assembled and connected, runs my Hardinge Lathe, Tree Mill and Boyar Schultz Surface Grinder very well.
I'm furnishing you a link to their 15 Hp and 20 HP Kits.
Hope this helps.
I'm furnishing you a link to their 15 Hp and 20 HP Kits.
Hope this helps.
15HP Rotary Phase Converter Quick Build Kit | eBay
With this setup you will have a 15 hp rotary phase converter, your 15 hp motor should be rated at 230volt or 230/460volt and a rpm of 17XX or 35XX. -Heavy duty run capacitors rated at 370vac to balance the generated leg from L1 and L2.
www.ebay.com
20HP Rotary Phase Converter Quick Build Kit | eBay
With this setup you will have a 20 hp rotary phase converter, your 20 hp motor should be rated at 230volt or 230/460volt and a rpm of 17XX or 34XX. -Heavy duty run capacitors rated at 370vac to balance the generated leg from L1 and L2.
www.ebay.com
