Rotary Phase Converters - revisited

[CalG]

The 850 rpm direct drive motor on my 18 inch Oliver wood shop band saw would not start on a RPC.
Works fine driven by a VFD. Very fine indeed.

 

[dstig1]

In our upper midwest area, the power company won't even consider a 3-phase run to a residential address. Farm, possibly yes, but never for a residence, so that option is off the table unless you happen to have commercial/industrial space, and then the costs get quite insane.

 

[dadohead]

Back in '77 when my dad built his new shop building out in the country, the power company offered to run 3ph to it for free! 3ph was there at the black top. He said "no" because he didn't have any 3ph equipment then. He always regretted THAT decision!

 

[CalG]

That very aspect of running power has changed in my lifetime.
Early on, the PowCo was eager to run anything any where/

Then ...not so much

Now...Big Bucks!

 

[rScotty]

In the late 70's my dad got a deal on a 5hp 3ph Powermatic planer. We looked into making a RPC even running down a 15hp 3ph motor but dad ended up buying a new Ronk RPC.

We had an electrician install the unit. Upon testing, the planer wouldn't run instead making a loud grinding noise with the cutter head barely turning (20 rpm?) There was nothing was wrong with the planer or bearings. After much troubleshooting and calls to Ronk the answer was "not every 3ph motor 'likes' a generated 3rd leg." The Ronk was returned and dad bought a new 5hp Baldor 1ph motor and controller for similar money.

Sounds to me like someone made an error generating the phase of the 3rd leg.
rScotty

 

[WinterDeere]

Not all "generated" third legs are the same.

Static phase converters generally do a pretty rough job of keeping the third leg voltage and phasing constant with load. The trouble is that, with the running machine representing 100% of the load, any change in that machine's load (e.g. putting a board thru the planer) causes the third leg voltage and phase to change.

Rotaries can do a much better job, but even then, there's some adjustment to be made for your idler size and loading. One of the few advantages of the RPC is that, with an idler running, the variable machine load is a smaller fraction of the total. One trick RPC owners can sometimes play to get a larger machine started, assuming their contactors have sufficient capacity, is to start several smaller machines first, each of which add to the total "idler" capacity.

 

[CalG]

Not all "generated" third legs are the same.

Static phase converters generally do a pretty rough job of keeping the third leg voltage and phasing constant with load. The trouble is that, with the running machine representing 100% of the load, any change in that machine's load (e.g. putting a board thru the planer) causes the third leg voltage and phase to change.

Rotaries can do a much better job, but even then, there's some adjustment to be made for your idler size and loading. One of the few advantages of the RPC is that, with an idler running, the variable machine load is a smaller fraction of the total. One trick RPC owners can sometimes play to get a larger machine started, assuming their contactors have sufficient capacity, is to start several smaller machines first, each of which add to the total "idler" capacity.

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.

 

[the old grind]

Not all "generated" third legs are the same.

One trick RPC owners can sometimes play to get a larger machine started, assuming their contactors have sufficient capacity, is to start several smaller machines first, each of which add to the total "idler" capacity.

Thanks for that! I just deleted four attempts to say the same. These (any) motors don't use max energy input without max load, and idling another connected can help with voltage balance & stability vs say a switchable capacitor bank. (see large scale/industrial PF correction)

That said, like with any VFD that converts 1ph to 3ph we want a fairly larger RPC motor than the load, which also lets us use those other 'idlers' for balance & for starting the big one. Each will have a min power usage, but don't expect a 5hp RPC to draw full rated current when used for a 2 hp's full load.

btw, I might use a pull rope vs a pony motor to start a RPC <10 hp, surely <5. Also, often a 5-7.5 hp 3ph motor can be had as cheap as a 1 1/2 -2hp. The hint is to plan ahead, shop surplus center or ebay, and buy 2x the hp for an RPC than the largest 3ph motor you'd use it for or plan to buy. The rest has already been said.

 

[ultrarunner]

Many hours on the Bridgeport with my splice together 1 hp idler.

It turns so easy all I do is give it a spin with thumb and index finger and quickly plug in the cord…

The big DeVelbis compressor idler I use a rope and hit the switch.

 

[WinterDeere]

Thanks for that! I just deleted four attempts to say the same. These (any) motors don't use max energy input without max load, and idling another connected can help with voltage balance & stability vs say a switchable capacitor bank. (see large scale/industrial PF correction)

Yep, all good.

btw, I might use a pull rope vs a pony motor to start a RPC <10 hp, surely <5. Also, often a 5-7.5 hp 3ph motor can be had as cheap as a 1 1/2 -2hp. The hint is to plan ahead, shop surplus center or ebay, and buy 2x the hp for an RPC than the largest 3ph motor you'd use it for or plan to buy. The rest has already been said.

No need for a pony or pull cord to start the idler. Build your RPC with start cap's that get switched in momentarily by a motor starter, to spin up the idler. That's how I did mine, anyway. I have a "start" contactor and a separate "run" contactor. The "start" contactor puts additional capacitance in parallel with the fixed balancing capacitors.

The start capacitors can be regular old "cheap" start capacitors, only the run capacitors need to be the big expensive dielectric oil filled jobs. That's good, because start cap value is probably 2x-3x run cap value, in most configurations.

 

[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?