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.