In an earlier post, I mentioned that I bought a Webb Mill a while back. It has a three hp., 3 phase induction motor that powers it with a variable belt system. When I got it in mid-2009, I was vaguely aware that I needed some form of power conversion. I knew from talking to people I know at the local electrical utility that real three phase power would be difficult to get from the utility. Therefore, I ruled that out very quickly.
My intention here is not to instruct anyone how exactly to build a Rotary Phase Converter since many of you may already know how this is done or have done it yourselves. I mainly want to show what I did.
I know a bit about electricity, enough to have wired a home and barn, but that was all single phase equipment. I do have a 100 amp subpanel in my barn workshop that I wired in some 32 years ago when the barn was put up so there was plenty of single phase 120 and 240 VAC available.
So, not knowing exactly what was required I bought a 5 hp rotary phase converter from the Phoenix company on eBay. I hooked that up to a single phase 240 VAC line and ran it for about a year. It worked fairly well but it had some annoying characteristics. It had a 3,600 rpm idler which made an annoying whine most of the time when it was running as well as the fact that it was underpowered. The mill would bog down on a heavy cut. I finally checked the motor and discovered that it really only drew enough current for a 3 hp motor. Apparently the motor was overrated. At least it seemed that way.
With that as a background I started studying the designs available on the internet for RPC's. I knew that I wanted a real 5hp 3 ph idler with no more than 1,750 rpm. I felt that this would have sufficient power for the mill and would also be quieter. Turns out that I was right. With this design, it will actually start the mill even if, for test purposes, the mill switch is left on while the RPC is started. That's something that the Phoenix unit wouldn't do. I also wanted an RPC that was automatic startup.
I located a very nice 5 hp 3 ph motor with 1,740 rpm on eBay and had it shipped in. It was reasonable siince it was a flange mount and had no base. I had to turn a large ring and drill four holes to mount it but it worked out nicely.
This design (schematic shown below) uses dual 250 µf of start capacitors in parallel in conjunction with a Steveco 90-660 potential relay. The start capacitors will start the motor when power is turned on and when the third leg of the three phase begins to generate from the idler, the relay will remove the start caps from the circuit.
During operation, output of the phases is balanced with two 50 µf run caps in parallel across the two legs L1 to L3 , and L2 to L3. It seems to be OK since voltage on all three phases is very close to 240 VAC when the mill is under load.
I initially used one leg of the 240 VAC to obtain 120 VAC to power the contactor to switch the voltage to the circuits. After some study, I changed this to incorporate a 240 to 120 VAC control transformer. This give me what is in effect a separately-derived circuit with a neutral which powers the relay coil in the definite purpose contactor. Taking one side of 240 VAC to make 120 will work but it's unsafe since you don't want to run any power into the 240 VAC ground. The transformer also gives me 120 VAC for a red power light courtesy of Radio Shack. LOL.
Anyway, the nice thing about a rotary phase converter is that they are relatively inexpensive and they are extremely reliable. I have about $200 total in this one. The frame was built from a surpuls steel coffee table and some bed frame angle iron.
I chose the RPC over a VFD since I only have one three-phase machine and it's a variable speed with belts and it's fine for what I do, mainly small pieces. My other machine, the Enco 13 X 40 lathe has a nice Chinese 2 hp single phase motor that's been running nicely for 15 years now and I'll just keep that as is. 8 speeds is plenty for me.
I understand that VFD's are quite nice but I don't know enough about them to be able to deal with issues that might come up. With the RPC, there's literally nothing that I can't fix on it since there are no solid state or digital features.
I hope that this will encourage people to build a similar unit. An RPC is a neat way to enable them to use some of those larger old machines with three-phase power that they might otherwise not want to buy.
The unit sits to the right of the mill under the table where I can reach it. It's fairly quiet so I just leave it idling while I do setup and so forth. Thanks for listening.![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![5HPRPC2.jpg 5HPRPC2.jpg](https://www.hobby-machinist.com/data/attachments/101/101262-7370f319c75108b29a2698bfbfdbe4c5.jpg)
Here's a picture of the insides. It looks sort of intimidating but it's really quite simple.
![5HPRPC1.jpg 5HPRPC1.jpg](https://www.hobby-machinist.com/data/attachments/101/101263-7370f319c75108b29a2698bfbfdbe4c5.jpg)
This is the schematic that I found on the internet. It's a good starting point. With a little swapping of capacitor values, it's possible to balance out the three legs of the RPC output so that they are very close when under load.
![RotaryPhaseConverterMiller.png RotaryPhaseConverterMiller.png](https://www.hobby-machinist.com/data/attachments/101/101264-7370f319c75108b29a2698bfbfdbe4c5.jpg)
My intention here is not to instruct anyone how exactly to build a Rotary Phase Converter since many of you may already know how this is done or have done it yourselves. I mainly want to show what I did.
I know a bit about electricity, enough to have wired a home and barn, but that was all single phase equipment. I do have a 100 amp subpanel in my barn workshop that I wired in some 32 years ago when the barn was put up so there was plenty of single phase 120 and 240 VAC available.
So, not knowing exactly what was required I bought a 5 hp rotary phase converter from the Phoenix company on eBay. I hooked that up to a single phase 240 VAC line and ran it for about a year. It worked fairly well but it had some annoying characteristics. It had a 3,600 rpm idler which made an annoying whine most of the time when it was running as well as the fact that it was underpowered. The mill would bog down on a heavy cut. I finally checked the motor and discovered that it really only drew enough current for a 3 hp motor. Apparently the motor was overrated. At least it seemed that way.
With that as a background I started studying the designs available on the internet for RPC's. I knew that I wanted a real 5hp 3 ph idler with no more than 1,750 rpm. I felt that this would have sufficient power for the mill and would also be quieter. Turns out that I was right. With this design, it will actually start the mill even if, for test purposes, the mill switch is left on while the RPC is started. That's something that the Phoenix unit wouldn't do. I also wanted an RPC that was automatic startup.
I located a very nice 5 hp 3 ph motor with 1,740 rpm on eBay and had it shipped in. It was reasonable siince it was a flange mount and had no base. I had to turn a large ring and drill four holes to mount it but it worked out nicely.
This design (schematic shown below) uses dual 250 µf of start capacitors in parallel in conjunction with a Steveco 90-660 potential relay. The start capacitors will start the motor when power is turned on and when the third leg of the three phase begins to generate from the idler, the relay will remove the start caps from the circuit.
During operation, output of the phases is balanced with two 50 µf run caps in parallel across the two legs L1 to L3 , and L2 to L3. It seems to be OK since voltage on all three phases is very close to 240 VAC when the mill is under load.
I initially used one leg of the 240 VAC to obtain 120 VAC to power the contactor to switch the voltage to the circuits. After some study, I changed this to incorporate a 240 to 120 VAC control transformer. This give me what is in effect a separately-derived circuit with a neutral which powers the relay coil in the definite purpose contactor. Taking one side of 240 VAC to make 120 will work but it's unsafe since you don't want to run any power into the 240 VAC ground. The transformer also gives me 120 VAC for a red power light courtesy of Radio Shack. LOL.
Anyway, the nice thing about a rotary phase converter is that they are relatively inexpensive and they are extremely reliable. I have about $200 total in this one. The frame was built from a surpuls steel coffee table and some bed frame angle iron.
I chose the RPC over a VFD since I only have one three-phase machine and it's a variable speed with belts and it's fine for what I do, mainly small pieces. My other machine, the Enco 13 X 40 lathe has a nice Chinese 2 hp single phase motor that's been running nicely for 15 years now and I'll just keep that as is. 8 speeds is plenty for me.
I understand that VFD's are quite nice but I don't know enough about them to be able to deal with issues that might come up. With the RPC, there's literally nothing that I can't fix on it since there are no solid state or digital features.
I hope that this will encourage people to build a similar unit. An RPC is a neat way to enable them to use some of those larger old machines with three-phase power that they might otherwise not want to buy.
The unit sits to the right of the mill under the table where I can reach it. It's fairly quiet so I just leave it idling while I do setup and so forth. Thanks for listening.
![5HPRPC2.jpg 5HPRPC2.jpg](https://www.hobby-machinist.com/data/attachments/101/101262-7370f319c75108b29a2698bfbfdbe4c5.jpg)
Here's a picture of the insides. It looks sort of intimidating but it's really quite simple.
![5HPRPC1.jpg 5HPRPC1.jpg](https://www.hobby-machinist.com/data/attachments/101/101263-7370f319c75108b29a2698bfbfdbe4c5.jpg)
This is the schematic that I found on the internet. It's a good starting point. With a little swapping of capacitor values, it's possible to balance out the three legs of the RPC output so that they are very close when under load.
![RotaryPhaseConverterMiller.png RotaryPhaseConverterMiller.png](https://www.hobby-machinist.com/data/attachments/101/101264-7370f319c75108b29a2698bfbfdbe4c5.jpg)