3 phase 20" Jointer Questions

[Jim Becker]

Rick...and Dev...I'm finding this discussion most enlightening and don't think it's beyond this particular woodworking forum. SMC is "different" in that respect!! There are potentially a lot of folks here who might choose big, old iron or new Euro equipment that requires 3-phase power. Understanding how 3-phase works to a certain extent and how to get there is as important as the cutting/milling features of those tools.

[Aaron Mills]

Dev & Rick,

Thanks for all the info, it really helps make purchasing older 3 phase machinery a much more feasible route. The jointer was in great shape and ran fine, but I wasn't able to recognize or find more info about the manufacturer (Mccoy-Kewland). It's a huge and beefy machine, but $1800 plus setting up a converter doesn't make it a very smart financial decision regardless of the quality of manufacture. I'm going to keep my eyes open for old iron, especially 3 phase machines since they are so much cheaper.

Rick, I've been to your site before and it actually helped out a lot when I first thought I would be buying a 3 phase General 350. Turned out it wasn't 3 phase, but because I was prepared to do the work to make 3 phase work, I was the only guy who went to go check out the saw. Worked out great for me!

Thanks,

Aaron

[Dev Emch]

So Aaron...
I assume from your last post that you bought the general 350. That would be great!

Jim, I have to agree with you. This is a nasty but critical topic. The most frequent reason given by folks for not using some of the older, industrial machines is the barrier of three phase power. The fear of going to a phase converter and the fear of its associated cost. Often it takes a small garage with a hobbyist using three phase on an old oliver or other machine to finally push a person over the fence. As for me, I am eclectic. I love hand tools and power tools. I love metalworking machines and woodworking machines. I love new stuff and old stuff. I like shaft drive and electric motor drive. I like single phase and three phase. So to get all this work in harmony, I have had to apply most of all the three phase stuff I learned in school. The only stuff I did not apply was high school geometry but the woodworking quickly filled in that excuse!

Rick. As you know, there are two main types of three phase AC motors. Syncronous and Induction. The outer portion of these two motors is called the stator and consists of a bunch of flat plates stacked up along with the windings going through slots in this stack of plates. The hole in the center of this assembly or stator is what houses the rotor and the distance between the rotor and the stator is your air gap.

The most popular motor in use today is the induction motor invented by Nickola Tesla along with the corporate support of Mr. Westinghouse. In an induction motor, the rotor is made of a stack of steel plates packed together on the rotor's main shaft. These plates also have grooves or holes in them forming "passages" from one end of the rotor to the other. This assembly is then put into a special mold in an injection molding machine and molten aluminum is injected into the mold. This aluminum forms the end caps of the rotor which includes some cooling fins. But molten aluminum also fills in the "passages" between the newly formed front and end cap thus connecting the two electrically. Because all the molded "bars" are shorted together, an induction rotor is a chameleon. That is, it can take on any number of motor poles which ultimately determine how fast your motor runs. The more poles, the slower the motor. This pole count is determined by how many sub groups each of the three stator windings has and how many slots are available in the stator plates. So as you can see, this is very simple construction. For folks restoring machines such DeWalt radial arm saws, here is a world of caution. Water can get in between the rotor plates and freeze. Although this is rare, this can cause the rotor pack to exapand and crack some of the bars. A rotor growler test can help in determining this. In the case of the DeWalt motor, this can be an issue as the motor is unique to the tool. The same caution applies to most of the direct drive woodworking machines. Often, machines that are restored from rust bucket cases sitting under trees for years are susceptable to this issue. In the case of my Dewalt, I was lucky. There were no cracked rotor bars.

In the other kind of motor, life gets much more complex. Here, the rotor is built up using electromagnets. For most motors running *LESS* than 3600 RPM, these are built using bulbous shaped magnets called "Salient Poles". In those cases where your at 3600 RPM, the rotor is made by winding the magnet coils into slots milled into a large steel rotor drum. DC power is fed to these magnets using slip rings on the rotor's shaft. This is a much more complex construction and requires you to have a unique DC supply for your rotor's magnets or rotor coils. This type of motor runs at exactly its sycnronous speed which is a function of its stator pole count. This is not the same as an induction motor which runs at slightly less than its syncronous speed with the difference between sycncronous speed and real speed being called the slip.

Now some motors will cheat here. They will utilize permanent magnets on the rotor in place of the larger electromagnets. Tiny alternators also utilize this. For example, in the case of the APU alternator used to power up the A-300 Airbuss, there is a tiny back up alternator attached to the main alternator's shaft. SHould the main alternator lose field exciation, then there will be enough power from the permanent magnet alternator to get the ship back onto the ground.

For the SMC woodworker and electrical hobbyist in general, one should keep one imporant thing in mind. Regardless of which type of AC motor your talking about, the stator winding is identical. In theory, I can remove the induction rotor from a 15 or 20 horse induction motor and replace it with a homemade salient pole rotor and slip ring assembly. Attach a small DC generator to the motors shaft along with a gas engine and I now have a full bore, three phase alternator! Clearly, a practical implementation is more involved. As they say, the devil is in the details. The main point is that the stator of an induction motor is identical to the stator of a syncronous motor.

Now, getting back to the idler motor. The idler motor is best referred to as a "rotating transformer". The run capacitors job is to balance voltage imbalances between the grid supplied L1-L2 and the wild leg or created leg L3. The ilder's rotor bars are shorted together; however, since they are aluminium, they have very poor magnetic properties. The rotor's main construction however is a stack of steel, insulated plates made from a unique magnetic steel alloy specially formulated for magnetic core use. Stator packs, rotor packs, salient pole packs and transformer cores. So there is a ton of magentic field linkage going on between the stator's fields and the rotor's fields.

Do you need run capacitors? That depends. In my first phase converter, I used a super E high effiency baldor 15 HP motor as my idler. Bought this on ebay from an old warehouse ventilation fan application. According to the experts, I would need a certain level of capacitance in farads for each of the run caps. On installation of these, I found that they were wrong. Because of how baldor had made this motor, I did not need to use any run caps. I did wind up using tiny run caps to "fine tune" the voltage imbalances but these were not really needed. On other idler motors, you may need a suit case of run caps to balance your voltage imblances.

One item to note here. Not all rotor cores are identical. Baldor designed my rotor to be as frugal as possible in a motor application. This had a nasty side effect in that it made the idler harder to start. When I discussed this with an engineer from Kay Industries, he mentioned the fact that their converters use unique rotors designed for idler applications which makes starting easier. Personally, I am still researching the details on this topic so I am not the expert to ask questions to in this regard. I just have some empirical data to work from at this point. I do know that the number of run caps required and the ease by which the idler starts are inter-related.

Lastly, please keep in mind that idlers are rotating transformers. Like any transformer, they work on elctromagnetic field coupling. In the case of the idler, its mechanical rotation is how we re-establish the 120 phase shift. Had this been a true alternator (i.e. generator in street slang), the power input would be massivly higher. As it stands, I am single phasing the idler's rotor and that is all I need in terms of energy to get this thing to run. That and a little bit of core loss. This is why phase converters are as effiecient as they are.

Hope this helps in buiding your understanding of the motors that make sawdust. In the final analysis, all that matters is our ability to make chips and sawdust.

[Aaron Mills]

Yeah, I picked up the General 350. Got in touch with General and they helped ID it as a 1992 model. Saw has a little bit of surface rust and needs new fan belts, guard/splitter (would've replaced the stock one anyways), and motor cover.

Back in school right now, so it's not close by to take pics or to work on, but when I get the chance I'll post some pics.

[Dev Emch]

Aaron... that is great. You are going to love that saw after a bit of TLC.

By the way for everyone else, I apologize for the long post I just posted. I guess that is the bad news. The good news is that this post contains most of the slog and bash theory that you need to worry about. Not much more to talk about except some minor logistics stuff.

[Roger Benton]

now this old thread is a true gem!!!
tons of good info.

[Troy John]

Thanks for all the great info DEV, I was wondering if you could shed some light on a few things that I have been told and dont know if I believe.
Are all 3-phase power supplies the same (or should be), I am aware of 208, 230, and 460-480 volt. But I have been told that some 3-phase power supplies have a higher voltage on one of the legs? Or are all three legs suppose to be the same?
Example, if I were to buy a machine do I need to find out what power it was set to run on, besides the 208 - 230 - 460? Can most motors be rewied to run high voltage (460) or low voltage (230)?
Is power around the world the same, if a guy were to buy a machine from Italy and bring it to the USA is it designed to work the same way?
Last of all can you get single phase power from 3-phase? I was told to wire my single phase 230 volt stock feeder to any 2 outputs of the magnetic controls on my 3-phase 230 volt shaper. If this does work does it hurt the stock feeder motor any?
Thanks, Troy