rotary phase converter

[Tai Fu]

So I'm going to move into a place without 3 phase power. Problem is 3 of my machines are 3 phase...

So I'm just wondering how do you size a rotary phase converter? Some said it has to be like 3x the size of the motor you are powering. Thing is I have a band saw, a compressor, both are 5hp, and that means I need a 15hp motor to make a rotary phase converter?

I could just buy a single phase moor for one of the machines, and then use that leftover 3 phase motor as a rotary phase converter... as people are charging an arm and a leg even for used 3 phase motors...

[Phillip Mitchell]

RPCs are rated differently depending on manufacturer, so there’s not a clear cut answer to that question across the board, especially when comparing used converters.

I have a Kay Phasemaster RPC that is called a 10HP unit. The largest motor it can start is a 10 HP motor, and if enough amps are incoming, it can power up to 25 (or 30?) HP simultaneously. It just can’t start a single 30HP motor all at once. I only have 60 amps of single phase 240V feeding it though, so I stick to 30 FLA or less of 240V 3 phase.

Other manufacturers, like American Rotary for example rate their converters slightly differently and recommend you upsize some amount according to what your biggest single load will be. The best way to get an answer to your question is to call the different manufacturers tech support and tell them exactly what you’re doing. They will want specs from the motor plates of your 3 phase machines as it’s really all about FLA (full load amps.)

Is the single largest 3 phase motor you have 5 hp? I would consider individual VFDs for your 3 phase machines, especially if the shop move is somewhat temporary (?) You can get good quality 5 HP drives for around $300-400 each and not have to deal with the extra wiring and infrastructure that a RPC entails that can get involved and somewhat expensive.

I easily have ~ $1500 (+) invested in my 10 HP RPC system between the converter itself (used), repairing bad capacitors/cracked rotor, 125 Amp 3 phase sub panel/breakers, wire, receptacles, etc. I have (7) 3 phase motors in my shop (1 on a VFD) and 2 of then are 9 HP, so it makes sense for me to have the RPC power plant, but VFDs are a great alternative on lower HP motors, with a few exceptions.

The main downsides to an RPC in my book are noise (the motors make noise when on and converting) and the fact that you have to turn it on and off. Not a huge deal, but VFDs are pretty quiet, even with the fan whirring and often times your on/off control for that machine is integrated into the VFD itself. I don’t tend to leave my RPC on for extended amounts of time when I’m not using it because of the noise and heat that it generates.

[Tai Fu]

Price scales are different in this part of the world, but I think I can get VFD's for them fairly cheaply through Taobao. However one is an air compressor and I have to figure out how to program it correctly. At least for air compressors is I can program in a "quiet mode" where the RPM is lowered, so that it will run quieter and with less electricity usage but at the cost of CFM. The bandsaw actually has problem with belt slippage because the starting torque is so high, and that the belt will slip for a couple of seconds while starting... so by programming in a soft start I can probably eliminate that. Then I can also use brake function to help the saw stop faster as it takes quite a while for it to come to a stop... But also I could run the saw at much lower speeds to cut metal and stuff... no idea how those motor will handle reduced speed though.

The problem is the milling machine. It is a 2HP dahlander motor, with 4/8 poles. I'm not sure what to do about this, do I just stick with the 8 pole mode and raise the frequency to match the higher speed range, or use some weird arrangement of switches to maybe allow the higher speed mode as well. I have no idea how the motor would handle higher frequency than 60hz. Maybe this would allow higher speed that may benefit certain milling operations but I have no idea how the bearing would handle the higher RPM.

[Michael Schuch]

A phase converter idler motor should be sized 1.5x the largest motor you are powering. When I built my balanced rotary phase converter ~25 years ago I went with a 10hp motor to run my 7.5hp 1947 Redstar 16" radial arm saw. It works perfectly for that and all 3ph machines I have purchased since.

I still run my 7.5hp RAS and 5hp 12" table saw on the RPC everything else I have switched to VFD's. One very nice thing about having a RPC is when I do buy a new 3ph machine I can bring it home, plug it in and go without having to do any rewiring. VFD's are nice for there soft starts, breaking, variable speed and being able to plug a machine into any 240v outlet.

I am currently running my 2 speed mill off of my RPC. When I do switch it over to a VFD I will run it on the high speed winding configuration.

You are likely to run out of grunt on your band saw if you try to slow it down to metal cussing speeds.

[Warren Lake]

Dont agree instead its what mike said. They are all different 10HP my brand you can start 10 HP all day long even hard start, And if you are in refrigeration of something heavier they had a more beefed up model. Three choices at the time and I picked the middle choice

[Tai Fu]

Well thing is, I could very possibly just buy ONE 5HP single phase motor, which will likely be 50% more than a 3 phase motor of the same power (or I could buy a lower HP one and lose out on resaw capacity). Then use that spare 3 phase motor as a RPC idler motor... I don't know much about this.

But I can buy VFD for all three machines for about 200 dollars (this is from Taobao), plus shipping. So I'm really torn.

Is there any reason why I shouldn't run a dahlander motor with the slow speed coil or at least use some switching methods to still allow them? Essentially the contactor switching would also turn off the VFD if they are disengaged for any reason. Do I get say 2HP at the 8 pole configuration and it goes from there? The amp draw is different at the 4 and 8 pole configuration, in fact 8 pole draw is higher.

I read that VFD must be matched to the motor, meaning you don't run a 5HP VFD on a 1HP motor...

And weirder is the VFD I seen claims an output amp of 14 amps for a 3kw VFD yet a 4KW VFD only outputs 15 amps. The nameplate of both 5HP motors are all around 14 amps at 220v. I'll wire it and test it before the move (I can still get split phase 220 out of the 3 phase power...)

Editing to say I just placed an order for 3 VFD's, 2 of them rated for 14 amps (says 3KW) and 1 rated for 9.5 amps (rated for 2.2kw). The 9.5 amp is for my mill. The 14 amp is for both the air compressor and the bandsaw....

[Warren Lake]

no one knew about VFD when I started. i wasnt till years later to one old guys shops he had a Roto, other one never heard of them. I had found out from a machine supplier and had one by then. Glad I didnt find out about VFD's and go down that route. My first three phase was 10 HP. I dont care about electronic braking or soft start, maybe speed control would be nice on a few machines.

Cleaver name on your guitars

[Richard Coers]

Your main issue for sizing will be duty cycle on the compressor, and starting load on the band saw. A big band saw has very high starting loads trying to spin the cast iron wheels. Talk to tech professionals before dropping money on a hunch. In the 80s my buddy made a static phase convertor for my big old band saw. An exploding capacitor looks like a volcano when it's really over loaded. It made quite an impression on both of us and filled the shop with oil and PCB vapors!

[andy bessette]

...I have a band saw, a compressor, both are 5hp, and that means I need a 15hp motor to make a rotary phase converter?...

No. Per North American Rotary you should have a 20 hp RPC for that heavily loaded equipment totaling 10 hp.

[roger wiegand]

Another consideration is machines with onboard electronics, if you have any. They can, apparently, not play well with VFDs. Also, when wiring your RPC to a machine, you want the electronics to be powered from the utility company legs of the 3 phase, not the generated leg. A woodworking electrical engineer I consulted when installing my 3ph jointer/planer with electronic controls strongly recommended against using a VFD with it. I don't know enough about it to know how well founded that advice was, but something to consider. I went with the American Rotary 20 HP RPC and it works perfectly.

[Tai Fu]

Update: I ordered 3 VFD for my machines. I figure that running them on single phase is always an eventuality and having brake and speed control is always a plus. Brake is not needed on the mill as it has a manual spindle brake (which is needed to change bits anyways). I'm thinking wiring the VFD in such a way that still uses its original relays but when you push the "start" button it does not spin the motor, it turns on the VFD. Then the VFD will be started and speed selected. Then when the selector is moved, the VFD is powered off and then it hooked up to the other speed setting of the motor...

But I do wonder for building a rotary phase converter. Let's say you got a few surplus 3 phase motors, but you need 20HP capacity, can you take 4 5HP motors and daisy chain them together and connect all of their outputs in parallel to make a 20HP rotary phase converter?

[Phillip Mitchell]

I don’t know about daisy chaining motors, but having 4 of them together in one place would practically be annoying to me in that it would take up a lot more space than a single 20 hp motor and certainly be louder with 4 motors running vs one larger one. Keep your eyes open for larger used motors, they are out there and sometimes for less than 5 hp motors because many don’t want to deal with them.

[Bobby Robbinett]

You can buy a 20HP panel from American Rotary and use your own 20HP or 25hp 3 phase motor to save some money. The panel itself cost about $800 approx and you can get a used 20HP 3ph motor for under $500.

[Tai Fu]

VFD sounds so much cheaper. I was able to buy the three my machine needs for only 200 dollars. But my mill has 2 3 phase motors, a main motor and a coolant pump. However as I machine wood quite a bit I don't really use coolant.

[Jeff Ramsey]

I just went through this with American Rotary and my 4HP 3-phase tool. They said twice the motor HP, so I bought their AR10 which is 10HP. Their tech support was very good.