Kearney & Trecker Milwaukee Model K No 3 follow me home.(pics)

[Jim Stewart]

I had this discussion years ago with a friend who was a journeyman electrician and instrumentation tech. He kept referring to the 240 volts (center tapped, as is usual in the US) as "two phase" 120 volt.

I pointed up at the top of the pole in front of my neighbor's house at the 4.6KV three phase and said "OK, that's three phase, right? A-B, B-C and C-A, right?"
"Right".
"And you see the two leads from the three phase down to the transformer on the pole - let's say that's the A-B phase. Single phase, right?"
"Right."
"And on my side of the transformer you say it's two phase? Where did that second phase come from?"
"Ummmm....."

But he still insisted it was two phase. Oh, well.

-js

[Doozer]

Well instead of saying run-out, let's say whompey-jawed then.

-D

[Fasttrack]

Gary said, "...2 phases are supplying all the power"....

2 legs (wires) might be supplying the power, but that is 1 phase.
2 legs (wires) might be supplying the power, but that is 1 phase.
2 legs (wires) might be supplying the power, but that is 1 phase.

Gary said nothing about split phase. Center tapped, want to call it something else,
sure, fine, go ahead... But it is not 2 phase. Stop calling legs, phases. You confuse the newcomers.

-Doozer

I hear you. But you're not hearing me. From an electrical engineering or physics perspective, split-phase contains two distinct phases. In fact, in signal processing, we talk about multiple phases that have nothing to do with power generation and we talk about phase margin with regard loop stability. We also talk about phases in the contexts of lasers and photonics. "Phase" is a general term that has developed a very specific meaning to electricians, machinists, millwrights, farmers, and generally people like us who interact with electric motors in more than a "theoretical" way. But that is not the only meaning of the word.

I guess my point is that, when communicating with people from different backgrounds (which is what is so great about this forum), semantics can be challenging. I'm just giving Gary the benefit of the doubt and have chosen to understand "2 phases" in the context of a physics or EE perspective, not the practical perspective where 2 phase and split phase are (as you point out) very different.

[Doozer]

I don't want to steal the thread, but I'm just saying: Split phase sure does look like more phases than one to me.

It has ZERO rotational value. Try to start a 2 phase motor with it. Not going to do anything but hummmmm.

90° 2 phase will start a motor. Center tapping a transformer is not and has never been 2 phase.
You can offset tap a transformer (call it autotransformer if it makes you feel warm and fuzzy)
and get a phase shift, but not center tapping.

-Doozer

[Doozer]

...Garyhlucas is spot on with his description. 2 hot legs are best described as "split phase" and you're right that it is derived from just one phase of the three phases provided by the power company...

Gary said, "...2 phases are supplying all the power"....

2 legs (wires) might be supplying the power, but that is 1 phase.
2 legs (wires) might be supplying the power, but that is 1 phase.
2 legs (wires) might be supplying the power, but that is 1 phase.

Gary said nothing about split phase. Center tapped, want to call it something else,
sure, fine, go ahead... But it is not 2 phase. Stop calling legs, phases. You confuse the newcomers.

-Doozer

[Mcgyver]

It weighs about 7800 lbs. I was a bit confused at first, too, but Outlawspeedracer was saying the mill was 2000-3000 lbs. heavier than he expected

right. thanks....should have read more carefully.

[The Metal Butcher]

I don't want to steal the thread, but I'm just saying: Split phase sure does look like more phases than one to me.

[Fasttrack]

There's a mill that'll do some work. I'm surprised a 10hp 50 Cat machine is 2000-3000, I'd have thought it heavier......my Elliot, 40 taper, is Net wet 3130lbs and seems like its got endlesss torque, you'd think massive 50 taper 10hp would be double the weight. Anyway, good on you, have fun with it, and keep away from the moving arbor

It weighs about 7800 lbs. I was a bit confused at first, too, but Outlawspeedracer was saying the mill was 2000-3000 lbs. heavier than he expected because it was a No. 3 instead of the No. 2 it was listed as.

2 wires is 1 phase.
2 phase needs 4 (or 3) wires.

-Doozer

Garyhlucas is spot on with his description. 2 hot legs are best described as "split phase" and you're right that it is derived from just one phase of the three phases provided by the power company. But when you start talking about the actual electronics involved, that single phase or "split phase" service provides two hot legs, each 180 degrees apart. Those two legs could be called "phases" but separated by a different phase angle than what we're used to in normal 3-phase service. I've designed DC power supplies for both single phase and three-phase inputs (I didn't even think of this with regard to a VFD, probably because I never really thought about how a VFD works) and Garyhlucas is right: with 120* phase separation, the ripple current is less and the capacitors therefore tend to last longer, run cooler and/or can be smaller. In the case of the VFD, not only do you have a higher input current, but you are now separated by 180* so you have two effects increasing the ripple current.

[Mcgyver]

There's a mill that'll do some work. I'm surprised a 10hp 50 Cat machine is 2000-3000, I'd have thought it heavier......my Elliot, 40 taper, is Net wet 3130lbs and seems like its got endlesss torque, you'd think massive 50 taper 10hp would be double the weight. Anyway, good on you, have fun with it, and keep away from the moving arbor

[Doozer]

2 wires is 1 phase.
2 phase needs 4 (or 3) wires.

-Doozer

[garyhlucas]

The real reason you need twice the size is only partly that 2 phases are supplying all the power, that part was easy to solve. The bigger problem is that rectified DC from 3 phase has a relatively small amount of ripple because the phases overlap. Not so for single phase where the ripple is huge on to off swings that have to be filtered by much larger capacitors. You get them by using a much larger VFD.

[The Metal Butcher]

'Tis true. You must derate a VFD designed for three phase input. Something like 1.75 is the real number, but 2x is what will be the closest to real size available. Some of the cheaper ones designed for hobbyists are pre-derated, but those are mostly smaller units. I had a great PDF explaining in detail why this is, but I have lost it, so I'd recommend a google search "derating vfds for single phase input" for further reading.

[mattthemuppet]

Bolting it down I'll skip that but may put mortar around the base. (easy to clean, & stops mice)

your mice know how to use a mill? I'd personally use a keyed switch to stop them, but that's just me

[Fasttrack]

I'd also correct a few previous statements about power - RPCs and static converters need to be oversized ~1.5x, VFDs run actual size, so you need a 10 hp VFD or 15 hp RPC/static converter minimum.

That is patently not true when using a VFD to do 1-phase to 3-phase. The Kearney Trecker is clutched, so a 10 hp RPC will likely work just fine. You'll probably end up being able to pull something like 85%-90% name plate power continuously and, if your overloads are sized appropriately and the idler has a service factor of 1.2 or more, you'll be able to pull 100% intermittently.

The 85%-90% is assuming it's a home-made RPC balanced a'la the Fitch Williams design.

And in case you think I'm full of it regarding VFDs, here we go:

https://www.vfds.com/blog/vfd-buying-guide

https://www.flowcontrolnetwork.com/p...you-size-a-vfd

https://www.electriciantalk.com/f2/s...-phase-216729/

Why does it have to be a 20hp for a 10hp motor?

You can read more in the links or via a Google search, but the basic principle is that the three phase motor is pulling the full load amps (FLA) across three phases or "legs". When using a VFD to not only provide the variable frequency drive but also do phase conversion, it has to pull significantly more current through the two legs that are available in single phase power. If the VFD isn't rated for this higher input current, it will smoke itself.

[justanengineer]

I genuinely hope you've got some hours on big iron, you're looking at a sizable investment in a slow machine that has very limited use for most professionals, nvm hobbyists. I would also highly recommend taking a hard look at the machine otherwise before investing too heavily in it. Quite often broken handles are simply the tip of the ice berg, locating and fitting parts to these old beasts is a feat unto itself, and many of these have simply been worked to death. I'd also correct a few previous statements about power - RPCs and static converters need to be oversized ~1.5x, VFDs run actual size, so you need a 10 hp VFD or 15 hp RPC/static converter minimum.