[smdubovsky]

He said capacitor (implying a static phase converter)
You said solid state. Solid state = VFD (since solid state by definition means semiconductors).


At any expected home use scenario, a VFD will likely be cheaper than a rotary. Rotary makes sense if you have VERY high hp needs or a zillion machines to run/retrofit. VFDs are superior performance wise to any phase converter. Modern machines will use them for the spindles even when 3-ph power is used.

[BillC]

Originally Posted by smdubovsky

He said capacitor (implying a static phase converter)
You said solid state. Solid state = VFD (since solid state by definition means semiconductors).

So I mis-remembered the name. Sue me. It's been many years since I looked into converters.

Doesn't change what I wrote about static converters, but I'll go back and fix the text.

Originally Posted by smdubovsky

At any expected home use scenario, a VFD will likely be cheaper than a rotary. Rotary makes sense if you have VERY high hp needs or a zillion machines to run/retrofit. VFDs are superior performance wise to any phase converter. Modern machines will use them for the spindles even when 3-ph power is used.

Except you really need a VFD for each motor for best results. If he gets a mill with only a single motor, then he only needs one. But, if he gets a mill with one or more powered feeds, then he'll need a VFD for each one (or a VFD for the main motor and a static/rotary converter for the others). Whereas a static or rotary converter will feed all of them just fine (assuming it's sized properly).

[jhsmith]

Any opinions on Precision Mathews out of the Pittsburgh area? As far as I can tell, they import most of their components from Taiwan with some of the cast iron parts from China. They seem to make a few mills in my weight range (400 to 800 lbs.).

Charlie recommended looking at a Clausing as well.

I'm not seeing any of these on Craigslist... Eventually, I will break down and buy a new one but the frugal part of my soul (that also prevents me from buying new cars) will take a while to give up on a used one.

Any opinions on Precision Mathews or Clausing?

[BillC]

Have you looked on eBay? That's where I found my mill -- it took a while, but I finally found a decent mill in the local area, so I didn't have to spend a fortune on shipping.

[smdubovsky]

Yes, what you said about limitations of static converters is true.

Originally Posted by BillC

Except you really need a VFD for each motor for best results. If he gets a mill with only a single motor, then he only needs one. But, if he gets a mill with one or more powered feeds, then he'll need a VFD for each one (or a VFD for the main motor and a static/rotary converter for the others). Whereas a static or rotary converter will feed all of them just fine (assuming it's sized properly).

Thats what I said. Each 3-ph motor needs its own VFD unless running in a simple open loop V/Hz mode. My radial arm drill has two motors. One for the drill and one for the column lift. 2 motors = 2 VFDs. They both fit in the existing electrical cabinet. FWIW, A 1hp vfd is $150-200 ea brand new. Ebay surplus can be cheaper esp for the bigger ones. FWIW2, you can also get a 120V/1ph input, 230V/3ph output converters up to about 1hp. Super convenient.

Powered feeds are almost always DC since they need to be variable speed. The typical Bridgeport sized "Servo" brand and knockoffs are all DC. I have never seen a 3ph powered one - it would take a pulley stack or a sheave type CVT to change the feed speeds. Even my Kitamura CNC that uses AC servo drives for the three axes just puts a single phase input drive on each of the three phases to balance the load. Easy to move those wires around in the cabinet and run everything off 1 phase.

[BillC]

Originally Posted by smdubovsky

I have never seen a 3ph powered one

Actually, you have seen one, since you've been in my garage before. However, if you've forgotten, here are a couple of pics of my x-axis power feed, complete with 3-phase motor and two-stage transmission:

[Dr K]

Steve, could you explain the differences between the things you're discussing above. I read, and decided I could just ignore this thread, or be educated but I'm a bit too far behind to understand, What is the difference between a 3-phase and 1-phase system? Are these 3 separate windings on a single motor or something? Also explain the differences between static/rotary converter and a VFD. As far as I'm concerned, you could do it in a month or two because I don't need this, just want to learn. Or blow it off (I've learned a lot from you over the years including how to weld!), all the other things were things I actually needed to know at the time.

Thanks to all on this thread for posting educational stuff--wish I understood it better.

[Vicegrip]

I recommend you simply Google it. It is not a paragraph kind of thing and there are plenty of YouTube and other places that can explain single and 3 phase with graphics too.

[smdubovsky]

No problem Pete. (Its Stephen not Steve)





1ph = one 120V sine wave. Two wires. Power goes to zero at every zero crossing (so there is power ripple)


3ph = three 120V sinusoids, each shifted 120deg in time. three wires. Each pair of wires looks just like 1ph power. Power goes to zero at every zero crossing but due to the phase shift each phase doens't go to zero at the same time. There is actually constant (DC) power in the system. No power ripple.


Single phase motors really aren't. They wont start if the 'magnets' (real or virtual) are aligned. So you use a second winding w/ some sort of phase shift to get it running. This is why there are cap start/run motors. There are also motors that use a second winding and use its own inductance to create the phase shift. W/o a secondary winding (or if you don't hook up the cap in a cap start motor) the motor won't start on its own but will continue to run in the direction its started.

Example: The blower in my AC system died shortly after we bought the house. Bad start cap. It just 'hummed' and tripped the electronic protection. I could go downstairs w/ the cover open, manually flick the blower in the right direction, turn on the system and it would run through another cycle. We repeated this for a day or two until could get it replaced.
The power ripple also causes the developed torque to pulsate. Power is like an internal combustion engine and needs a flywheel to smooth out. The motor can be its own flywheel.



Since 3ph power is three single phase systems, it can deliver 3x the power w/ only one more wire. 3ph inherently has its own phase shift so motors don't need start/run caps. The are smaller for the same hp and have MUCH smoother shaft torque (Due to the no power ripple they theoretically have no torque ripple) This is why they are superior in powering tools like mills and lathes.



So you have a 3ph motor at your house but don't have 3ph to run it? You need to create a phase shift in the third wire...
1) Static phase converter. Uses a cap, quite literally the same as a 1ph run cap, to create enough phase shift to get the motor moving. *IF* you had a constant load (fixed speed fan) you could tune the cap to the motor and make a perfect 120deg shift and get full rateed hp out of the motor. W/ things like mills the motor might have no load or full load so you can only use a compromise cap value. Its never perfect so you typ have to derate the motors nameplate hp. One advantage is that they are silent.

2) rotary converter. Pretty much the same as a static converter w/ a free spinning / idling 3ph motor connected to it. The free spinning motor is basically a rotating transformer and helps even out the phase shift. Fun fact: If you have multiple 3ph equipment, you can use the other motors to help out. If you have a small static or rotary converter that isn't big enough to run your mill, you can just also turn your lathe on first and let it just spin. That motor helps increase the capacity of the rotary converter and will start bigger machines. A disadvantage is they are heavy and noisy (A multi-hp motor spinning at 3600rpm is definitely not silent)

3) Variable Frequency Drives (VFD) Though the magic of filtered square waves (just like a class-D audio amp) you synthesize directly the 3 phase sines needed. Once you have that ability though, why limit yourself to 120V or even 60Hz? An induction motor run at 60V and 30Hz will run at half speed w/ no loss in torque (V/Hz = constant). You can also overspeed a motor w/ 180V & 90Hz to go 50% faster, 240V & 120Hz for +100% (or more! up to the limit of the bearings, voltage on the windings, and the motor tearing itself apart.) Most 1200/1800rpm motors are the same as their 3600rpm wound version. So you can usually overspeed up to 3600rpm w/o problem. W/ most modern VFDs can even develop peak torque at zero speed. Peak torque can be MUCH higher than even the stall rating. VFDs also bring things like soft start and braking for free. Threading to a shoulder on a lathe is easy when you hit the stop button - it no longer coasts to a stop but can stop *RIGHT NOW* (don't do that w/ a threaded chuck as the momentum may cause it to unscrew.) Same w/ a mill: On regular 3-ph power, reversing is like putting your car from 1st to R and dropping the clutch. The motor will do it but its a hell of a shock load and no fine speed control. VFD can do as slow (or as fast) reversal as you program it to do. This helps tapping. Say you have a J-head bridgeport mill w/ the 4 pulleys up top. Its a PITA to change pulleys. W/ a VFD you typ leave it in the middle pulley and just change speeds w/ the VFD. Only need to swap pulleys if going for max HP or top speed. Super handy in that while cutting you can change speed on the fly. Getting chatter or about to enter/exit an interrupted cut? Turn the knob and speed up/slow down a little to avoid any harmonics. For both lathe and mill it also allows you to go slower than the lowest pulley setting. Helpful when swinging big or unbalanced fly cutters in the mill or threading on the lathe.

[Dr K]

Thank you Stephen!! That was EXACTLY what I was looking for.