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

[Fasttrack]

When I hook my scope up to 120 volts in the shop I get a single sine wave. When I hook it up to 240 volts I get *a single sine wave* with twice the peak voltage. Sure looks like single phase.

-js

And what do you get when you hook up your scope to 208 volts 3-phase?




Come on folks. This isn't rocket science. Alternating current has a time-dependent voltage. That time dependent voltage is regular and oscillatory, meaning that it can be represented by a single sine or cosine wave at a fixed frequency and amplitude (neglecting small variations due to generation). Mathematically, one way to express that is A*sin(t+p) where t is the time and p is normally the Greek letter Phi, known as the phase angle. This is an arbitrary angle that is only useful when comparing multiple signals; it essentially defines a common starting point for the timer used to clock the signal. So three phase power consists of three time dependent voltages that is represented as A*sin(t), A*sin(t+120*), A*sin(t+240*). Split-phase power is represented as A*sin(t) and A*sin(t+180*). In mathematics, physics, and electrical engineering (and mechanical if we were talking about waves in some kind of solid or liquid medium) we would call each of these a "phase" because they are identical in amplitude and frequency - only their phase angle is different.

Just like phase, voltage is never an absolute quantity. It always needs to be referenced against something for it to have any meaning at all. If you only have a single channel on your oscilloscope, you will always only get one sine wave. The amplitude and offset change based on what you are referencing but it can never display more than one sine wave (on noisy signals, it may look like it does because of persistence on the screen), regardless of whether you are on a single phase, split phase, 90* two phase, three phase, 6 phase, 5 phase, whateverthehell number of phase system you're on.

The REASON electricians don't refer to split phase as two phase is to distinguish it from the three phases being provided by the utility company (each separated by 120*) and older 90* 2-phase systems (which evidently still exist in some places?). Mathematically, physically, scientifically, there are two phases in a split phase system, each separated by a phase angle of 180*.


Doozer, that's a great suggestion. I will use whompey-jawed from now on. If you catch me using "run-out" please correct me.

[The Metal Butcher]

Thanks fasttrack, you said that better than I could have. Basically it comes down to semantics. Whether you consider a negative wave to be the same as a 180* shifted wave, which since they are mathematically identical, I do. Lets leave it at that and let the reader decide for themselves.

[mattthemuppet]

aaaaanyway, back to the monster mill and the magnificent machining mice please

[Franz©]

241, 237 and 263 according to the digital meter right now. No 3Ø load on the service right now. The wildcat will drop according to load.

[RB211]

Serious metal remover

[outlawspeeder]

Will a 10HP motor work for a RPC for this? Or does it need to be larger. Also how many AMPs will I have to provide it?
Something like: https://www.grainger.com/product/MAR...e-Motor-40Z954

[Ohio Mike]

I wouldn't use anything less than a 15 HP idler, 20 HP would be better. If you are going to build an RPC I wouldn't by a new idler. I personally recommend American Rotary. For comparison... https://www.ebay.com/itm/American-Ro...A/370774922286

[RB211]

I like my American Rotary RPC

[justanengineer]

And in case you think I'm full of it regarding VFDs...If the VFD isn't rated for this higher input current, it will smoke itself.

Who said anything about overloading a drive? I'm suggesting the OP follow the manufacturer's recommendation, you're saying he needs to blindly double it. The limited info given on pretty much every catalog page and tiny data sticker includes input and output voltage and phase, and amperage, typically you dont even have to go into a spec sheet to see those match advertised hp suggestions nicely.

You made a series of general statements that are simply wrong. Get over yourself.

[Fasttrack]

Will a 10HP motor work for a RPC for this? Or does it need to be larger. Also how many AMPs will I have to provide it?
Something like: https://www.grainger.com/product/MAR...e-Motor-40Z954

I would look for a used 3-phase motor. Like I said, all you need it to do is spin so things that would make it unfit for regular service (e.g. bent or broken shaft, torn up keyways, etc.) would work for an RPC. Bearings are easy enough to change, so even one with bad bearings might be a candidate. For heavy industry, motors less than 10-15 hp are usually considered cheap enough to not be worth rebuilding if something happens to the shaft. I've found several 7.5 and 10 hp motors at the junkyard that worked fine but had cracked mounting bases or bent shafts.

As far as sizing goes, the bigger the better but you will be able to start and run that mill with a 10 hp idler. The higher the service factor, the better. An SF of 1.15 is pretty typical for heavy duty motors above about 10 hp. If you can find one with a SF of 1.2 or 1.25, even better (example link below). I can tell you that I have succesfully started and run a 7.5 hp motor from a 7.5 hp idler because that's what I found at the junkyard. That was to power my Pacemaker and it voiced no complaints when hogging off some serious amounts of metal.

The trick to making this work is twofold:
1) My Pacemaker and your No. 3 are both clutched machines, meaning the motor starts with virtually no load on it. Starting is the hardest part when you don't have true 3-phase power.
2) In home-shop setting, it is extremely unlikely we'll be running these things at full load. Even if you're only using 6 of the 10 horses that thing has, you'll be munching huge amounts of metal.


Forgot the link:
https://www.worldwideelectric.net/pr...rs-rigid-base/

Edit: Also, a trick someone on here passed along to me - if you have other three phase equipment that you are powering from your RPC, you can turn it on before starting a large motor. If you look at the link posted by Ohio Mike, it says "single motor start: 10 hp" and "total motor run: 20 hp". If you have a 10 hp idler and you put some other three phase motors on it (e.g. maybe a drill press with a 2 hp motor and a mill with a 5 hp motor or something), then you can bump up the effective size of your idler because each additional 3-phase motor is essentially added in parallel. As long as their is no significant load on them, they will help you get a 10 hp motor started. Like I said, I didn't have to do this because my lathe is clutched but it's another trick you can use if you can't find a large enough idler motor.

Also, take a look here on instructions for building your own RPC: https://www.practicalmachinist.com/FitchWConverter.pdf

[outlawspeeder]

Fasttrack, I never thought about one motor aiding in starting another, but I understand how it could works. Last night, I almost pulled the trigger on a new 10HP for about 130 dollars. I have to go by that shop to pick up some other tooling I won at auction. I'll see if they have another 10HP going up in auction soon.

I am still trying to get my head around the math of: A*sin(t), A*sin(t+120*), A*sin(t+240*). From the way I see it, the phases are 120 degrees out from each other. The static Phase Converter is 180 degrees out from each other till it gets to "Magic Blue Smoke" to swing 180 to @120 and add a third leg? Thus the banging I hear on a very heavy cuts with my Phase-A-Matic is the Caps kicking in on the third leg?

Thanks for the links, every day I play with this stuff I learn something new and useful. I can never understand how someone can just sit around and not want to put the gray matter between the ears to work.

Doozer & Mcgyver Thanks for pointing out the CAT50 vs NMTB taper. I've only seen Cat50 in photos and didn't think about it till now. My Leblond No2 tool and die grinder uses the NMTB and I have been calling it a Cat50.

RB211 Man they are proud of their RPC. Joking aside I think that is what I have for in a 5HP RPC.

mattthemuppet I'll try to get a photo of the mice running the mill, every time I go out to the shop they have always left a mess for me.

Thanks for all the help

[PStechPaul]

It might help to add a significant inertial load (flywheel) to the idler motor. This may provide a stiffer supply for start-up and load variations of the machine tools.

[reggie_obe]

It might help to add a significant inertial load (flywheel) to the idler motor. This may provide a stiffer supply for start-up and load variations of the machine tools.

It doesn't work like that. Even big shops that might be running a 100hp RPC won't have a flywheel on the idler. Not true for motor-generators or rotary converters either.

[Doozer]

It doesn't work like that. Even big shops that might be running a 100hp RPC won't have a flywheel on the idler. Not true for motor-generators or rotary converters either.

Explain your physics to me please.
-D

[reggie_obe]

Explain your physics to me please.
-D

As soon as the EE explains his. Never have seen one with a flywheel, Often the idler motor shaft is cur off, even on commercially made units.
How large in diameter, how much mass would the flywheel have to be to add a measurable effect. Must be a valid reason why it isn't done.
Play devil's advocate elsewhere.