Rotary Phase Converters - revisited

[WinterDeere]

When I'm working the H/V milling machine, I always leave the power table feed and the suds pump motors running. The spindle motor starts so much more readily.

Perhaps that is why "balance capacitors" are a non issue for this use.

An 3Ø motor will generally run fine on single phase, the primary penalties being:

1. Vibration
2. Efficiency
3. Horsepower

If you don't mind giving up a bit of each, and you have some way to get the thing started, then it can work. Of course, balancing capacitors can offer a huge improvement on all three of the above points. The only catch is that the ideal balance capacitor arrangement varies with how hard you load down the motor while cutting.

Think of any induction motor, whether three phase or single phase, as a bicycle that's missing one pedal or a single-cylinder engine. Once you get the thing moving, inertia can bring the pedal or piston back to top dead center, where you can push again to keep moving. Getting the thing started is the job of the start winding in a single-phase induction motor, which is usually wound with a smaller wire (more resistance) and loaded with a bit of capacitance, to put it out of phase with the run winding. It's the second piston on a 90 degree crank shaft.

Three phase motors don't need a start winding, because they already have three windings, at 120° phase relationships. A 3-piston engine will never be stuck with all pistons ahead of top dead center, and likewise, a 3-phase motor always has at least one phase developing torque in the desired direction.

 

[CalG]

An 3Ø motor will generally run fine on single phase, the primary penalties being:

1. Vibration
2. Efficiency
3. Horsepower

If you don't mind giving up a bit of each, and you have some way to get the thing started, then it can work. Of course, balancing capacitors can offer a huge improvement on all three of the above points. The only catch is that the ideal balance capacitor arrangement varies with how hard you load down the motor while cutting.

Think of any induction motor, whether three phase or single phase, as a bicycle that's missing one pedal or a single-cylinder engine. Once you get the thing moving, inertia can bring the pedal or piston back to top dead center, where you can push again to keep moving. Getting the thing started is the job of the start winding in a single-phase induction motor, which is usually wound with a smaller wire (more resistance) and loaded with a bit of capacitance, to put it out of phase with the run winding. It's the second piston on a 90 degree crank shaft.

Three phase motors don't need a start winding, because they already have three windings, at 120° phase relationships. A 3-piston engine will never be stuck with all pistons ahead of top dead center, and likewise, a 3-phase motor always has at least one phase developing torque in the desired direction.

Yes

Multiple motors represents a "stiffer third leg.

Call it inertia....

 

[the old grind]

..Build your RPC with start cap's that get switched in momentarily by a motor starter, to spin up the idler.

Yes, and the best guidelines show how to calculate values in mfd, so no pony or a pull start needed for a RPC with a specific load. (have & can share circuit/math)

btw, I just bought some 370-440v 5% NP electrolytics on AMZ for ~$10 per for PF correction on my pond's aeration pump. (.41 to .89)

Call it inertia....

While that's a physical concept the net result is indeed an electric equivalent of stabilizing like by using more capacitance (usually to be calculated) or a hydraulic/pneumatic/vacuum reservoir, so yeah.

btw, it's tough to keep up with you guys. You know too much. But then I don't feel lonesome when I can learn new stuff from y'all. Just sayin'.

 

[WinterDeere]

I'll throw it out there, as I have a really nice RPC I no longer use. If anyone is interested in it, contact me on the back channel. I don't need much for it, I'd honestly sell it for a small fraction of the cost of parts alone. I really just want it out of my shop.

I'll have to pop the cover and remember what I used in this one, but I think I had it breakered at 3x30 amps, with a 7.5 hp idler. I can palletize and ship the RPC panel + idler anywhere in the US. I might even have the 3-phase breaker panel still laying around somewhere, I know I at least have some of the QO breakers hanging around.

Pushbutton start/stop, with options for remote start/stop buttons at your various machines, if you want to get ambitious with the installation. This one was wired to a 3-phase type-QO breaker panel, where two legs (1Ø 240V) were left always hot, and third leg was powered by RPC when running. This way, I could wire all of my 1Ø and 3Ø machines off the same panel / mains.

 

[CalG]

I'll throw it out there, as I have a really nice RPC I no longer use. If anyone is interested in it, contact me on the back channel. I don't need much for it, I'd honestly sell it for a small fraction of the cost of parts alone. I really just want it out of my shop.

I'll have to pop the cover and remember what I used in this one, but I think I had it breakered at 3x30 amps, with a 7.5 hp idler. I can palletize and ship the RPC panel + idler anywhere in the US. I might even have the 3-phase breaker panel still laying around somewhere, I know I at least have some of the QO breakers hanging around.

Pushbutton start/stop, with options for remote start/stop buttons at your various machines, if you want to get ambitious with the installation. This one was wired to a 3-phase type-QO breaker panel, where two legs (1Ø 240V) were left always hot, and third leg was powered by RPC when running. This way, I could wire all of my 1Ø and 3Ø machines off the same panel / mains.

Sounds like a great set up!
20 years ago (pre VFD) I would be taking a drive to Philly!

 

[CalG]

You know, the big issue that got me off the RPC, was when using my larger lathe, every time I would start the spindle (7.5HP) Our house and the neighbors (shared transformer since the POWCO "upgrade) would "brown out" with the current inrush dip.

Both the wife and the neighbor made comments on how annoying that was.

And I was thinking "what a deal, I'm out here making bank, turning out these parts."

On the VFD, there is NOTHING like that

 

[WinterDeere]

Yeah, soft-starting thru a VFD is a great way to cut down on inrush. But inrush is really dependent on the load (machine you're running), not the RPC.

I'm surprised your lathe created such a high startup load tho... must be a monster! For us woodworkers, big bandsaws are the current hogs, on startup. Spinning up 20 feet of blade strung around a pair of 100# cast iron wheels, from a dead stop to 4000 feet per minute takes an awful lot of juice.

 

[CalG]

Yeah, soft-starting thru a VFD is a great way to cut down on inrush. But inrush is really dependent on the load (machine you're running), not the RPC.

I'm surprised your lathe created such a high startup load tho... must be a monster! For us woodworkers, big bandsaws are the current hogs, on startup. Spinning up 20 feet of blade strung around a pair of 100# cast iron wheels, from a dead stop to 4000 feet per minute takes an awful lot of juice.

I mentioned

My Oliver 850 rpm motor Direct drive band saw would just NOT START with a RPC/

But is just a sweet machine on a VFD .

It's not a high HP motor. 1 HP IIRC.

But, it's a big sucker!

 

[WinterDeere]

Nice. What size is the Oliver? I have a 1930'ish Crescent 32". Belt drive (was line shaft / flat belt), so I was able to convert to a single-phase RI motor with massive start-up torque.

 

[dstig1]

What's an "RI" motor?