[How do I?] Another Rotary Phase Converter Question

[JimDawson]

I’m thinking about building a 10hp RCP for my shop. Since I have limited power (50 amps, 240V) I need to minimize the start load, so what I’m thinking is to start the RCP motor with a small, about 1 hp, motor to spin up the RCP prior to switching it into the circuit. To minimize windage/friction losses, it would de-couple the starter motor after the spinup. I’m thinking maybe a commercial sewing machine clutch or automotive air conditioner clutch. This should eliminate the need for the static phase converter start circuit, and all I should need is the load balancing run caps.

I’m not worried about the complexity of the startup sequence; I can handle that with some control circuitry to automate the process.

The goal is to reduce the start load and maximize running efficiency, not building cost reduction. The largest motor I intend to run is 7.5 hp.

So a few questions:

Is my thinking correct in general?

How do I calculate the values for the load balancing caps?

Do I switch additional caps into the circuit as needed for different loads? If needed, I can install PLC control for this.

Any idea how to calculate the load on the single phase circuit when running a 3 phase motor?

Do I gain anything by installing a flywheel on the RCP motor?

What other questions do I need to be asking?


Or do I just buy a 3 phase generator and be done with the problem?
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[ch2co]

I would kind of suggest, mayby, possibly a solid state VFD? They are moderately inexpensive, QUIET, more efficient than an RPC and
a lot easier to setup and control.
What exactly are y0u trying to drive? Wouldn't the flywheel would only require more torque to get the thing running? These
things are quite noisy you know. I had to share a laboratory where an RPC was running next door, and it was a real nuisance.
Do you already have one, or would you be purchasing one?

Chuck the grumpy old guy

 

[Andy Rafferty]

I belt drive a fractional HP single phase motor to the three phase (RPC aka-5+ HP motor) once I energize the three phase I drop power to the starter motor and let it free wheel with the RPC . The bigger motor never knows the little motor is in the same room. don't be concerned about clutching the starter motor. I encourage a three phase breaker after after the RPC before the machine you are providing three phase for. Mine has worked like this for a long time.

 

[JimDawson]

You are correct Chuck, a VFD(s) would be a good option.

Right now I have one machine that is wired for 480V and has 3 motors on it, and I'm considering the options for running it. I could run the 240 from the RCP through a transformer and have 480, 3 phase, and not have to completely change out the controls. It does not need variable speed.

In addition, I am looking at bringing in another piece of equipment that will have 2 motors on it, one of those motors will require a VFD on it for variable speed.

Also, I have been running my lathe on a static phase converter for years, it would be nice to have it on 3 phase. If I were just running one or two motors, VFDs would be the best way to go. I have a VFD on my mill, and even if I had 3 phase power, I would still have a VFD on it. In fact, I would not change the power to the mill even with a RCP in the shop.

Sooooo.... the real question is; 7 VFDs or one RCP.

I will build a RCP from the ground up, specifically designed for my needs. The other thing I am considering is to run a 3 phase generator with a single phase motor, but I think this is the least efficient way to go. I'm just in the thinking stages right now.
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[JimDawson]

I encourage a three phase breaker after after the RPC before the machine you are providing three phase for. Mine has worked like this for a long time.

Oh absolutely, the RCP would be connected to a 3 phase load center then power would be distributed to the machines from there..

 

[tractorman44]

My experience has shown you should easily be able to run up to 7.5 hp three phase motor off of a 5 hp rotary phase converter. A lot of guys recommend matching hp to hp ratio, but so far I've not seen that as the case. My brother runs a 20 hp rotary welder off of a 10 hp converter we built 15 years ago and has experienced no problems to date in its ability to run it. The output of the welder is somewhat diminished however, and a few days of experimentation with different rods allowed him to mark the dial appropriately for the material thickness to rod size etc etc etc. BTW, the capacitor gets really really warm, so the whole control contraption is build into enclosures in case of capacitor burst.

We set that one up with paralleled 20 mfd capacitors and that has worked...is it balanced???...Probably not anywhere in the least, but its good enough.

We have a pony motor with 1:1 ratio pullys connecting the two to get the 3 ph motor up to full rpm BEFORE applying 240v single phase to it, the third leg being powered off one of the others through the capacitor bank. We use tdr's, (time delay relays) set for 30 seconds or so to allow the pony motor to run long enough to get the mass of the larger motors up to speed, then the tdr drops out the start relay of the pony motor and simultaneously pulls in the contactor supplying power to the rotary converter.

I have a couple others put together and sitting on the shelf, and managed to wire those in differently depending on the materials on hand....like transformers for low voltage control coils, or low voltage start stop relays vs line voltage and a low voltage control relay or two to make stuff work.

Another one at a lady-friends shop is set up with six 20 mfd and 40 mfd paralleled capacitors with provisions to switch one or more of the additional capacitors into the circuit to create a little more phase shift in the third leg, theoretically balancing the three legs more closely. Does it work? We like to think so, but being fresh out of knowledge of oscilloscope operation, we have to rely on a more rudimentary clamp-on amprobe to compare the amp draws on the load legs.

When you are referring to the term 'flywheel' I don't think a physical flywheel is what is meant. With these shop-built converters, the additional motors you start and run simultaneously kinda act as 'flywheels' in the electrical circuit and it seems to balance the phase shift more and the total of the motors seem to run more smoothly as a result. Is it my imagination? Might be.....

But please don't use my suggestions as gospel by any means, and use the utmost care in your experimentation. Remember, SAFETY IS NO ACCIDENT !!!!

 

[Smithdoor]

A 50 amp 240 v will only run a 5 HP motor
I have had a simple single to three phase converter for over 30 years works grated
I was only a 1725 motor 3 hp and use a 3/4 single phase motor for starting the main motor and helping on starting motors up to 10 hp

Dave

I’m thinking about building a 10hp RCP for my shop. Since I have limited power (50 amps, 240V) I need to minimize the start load, so what I’m thinking is to start the RCP motor with a small, about 1 hp, motor to spin up the RCP prior to switching it into the circuit. To minimize windage/friction losses, it would de-couple the starter motor after the spinup. I’m thinking maybe a commercial sewing machine clutch or automotive air conditioner clutch. This should eliminate the need for the static phase converter start circuit, and all I should need is the load balancing run caps.

I’m not worried about the complexity of the startup sequence; I can handle that with some control circuitry to automate the process.

The goal is to reduce the start load and maximize running efficiency, not building cost reduction. The largest motor I intend to run is 7.5 hp.

So a few questions:

Is my thinking correct in general?

How do I calculate the values for the load balancing caps?

Do I switch additional caps into the circuit as needed for different loads? If needed, I can install PLC control for this.

Any idea how to calculate the load on the single phase circuit when running a 3 phase motor?

Do I gain anything by installing a flywheel on the RCP motor?

What other questions do I need to be asking?


Or do I just buy a 3 phase generator and be done with the problem?
.
.

 

[Wireaddict]

Not an ideal setup but, being an experimenter, I'd try this, too. Rather than using a starter or spin-up motor the first thing I'd try is add a cap [not sure what size but I've seen info on this on the 'net] between one of the power leads & the third motor lead through a NC contact from a contactor to start the motor with no load. I'd connect this contactor coil to an on-delay timer set for more than enough time for the motor to get up to speed. I'd use a second contactor with 3 NO contacts between the 3-10 hp motor leads & the load-side disconnect switch &/or other protection equipment. The intent of the timer-contactor wiring is to allow the motor to start & accelerate unloaded, then apply the derived 3-phase power to your loads. To try it out I wouldn't even use a timer & probably just a NO push-button switch to the starting cap; I'd only want to check the impact of starting the motor on the lighting, AC & other equipment. You might find that you don't need an auxiliary motor & everything else to get it rolling.

One more thing I'd try is to add about a 10-20 uF, 350-450 WVAC "motor run" type cap across the 2 motor leads that you're applying the 240 VAC to; this'll improve power factor somewhat & reduce the motor current. This applies to the motors you'll be supplying, too. Hope this helps.

 

[JimDawson]

In my OP I noted above I have a DoAll grinder that is wired for 480V. The question: Has anyone actually tried to connect a 240V output VFD to a 3 phase transformer to produce 480V, 3 phase? I thought I would ask before I let all of the magic smoke out of a 5HP VFD. It seems like it might work, but............... To make it more interesting, the grinder has 3 motors: The hydraulic pump, the spindle, and the coolant pump so the motors would be switched in as needed. Sounds like a good way to fry a VFD, but I'm not sure.

Just thinking here (normally a dangerous condition ), I may not actually try it.

Opinions?
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[John Hasler]

In my OP I noted above I have a DoAll grinder that is wired for 480V. The question: Has anyone actually tried to connect a 240V output VFD to a 3 phase transformer to produce 480V, 3 phase? I thought I would ask before I let all of the magic smoke out of a 5HP VFD. It seems like it might work, but............... To make it more interesting, the grinder has 3 motors: The hydraulic pump, the spindle, and the coolant pump so the motors would be switched in as needed. Sounds like a good way to fry a VFD, but I'm not sure.

Just thinking here (normally a dangerous condition ), I may not actually try it.

Opinions?
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Should work as long as you leave the frequency at 60Hz and never switch off all the motors, but it will depend on the design of the VFD. I'd suggest consulting the VFD manufacturer.