Final Word on Electric Motors for Woodworkers.

[Dev Emch]

I have just finished the article written by Forrest Addy in the current version of the Home Shop Machinist. This is one of several articles Forrest is writing on motors and the next issues will cover VFDs and converters among other things.

In this article, Forrest covered a massive amount on both single phase and three phase motors. Most of what was covered applies directly to the woodworker as well as the metalworker. For example, he explains why the woodworking environment is much harder on sealed for life bearings than the metalworking environment.

He also goes into detail why some asian motors have much shorter life spans than the US made motors and how a simple trip to a motor rebuild shop can cure this oversight. A simple trip to the varnish dip tank and oven for the stator windings.

He also goes into some detail on NEC, local codes and necessary controls for eletric motors attached to machine tools for either woodworking or metalworking.

The article is several pages long and repleat with pictures. Copies of this months Home Shop Machinist can be purchased at better book stores such as some Borders or directly from Village Press who owns the Home Shop Machinist magazine. ITs bimonthly. I highly reccommend that you either get a subscription for a year (about 26 bucks) or purchase the next three editions including the current one if you work with or desire a more in depth knowledge of induction motors, VFDs, motor control, NEC and local code issues and how to build a phase converter.

I have known Forrest for many years and he is an old Navy Machinist who has seen it all and done it all. He is one of the most knowlegeable Mill Wrights and Machinists out there and I often have to ask him a question. Fancy That!

These articles are now on my mandatory homework list for new guys getting into these problems. It beats having to retype the same information over and over again. You will enjoy this magaine even it its for the metal heads more than the wood heads.

Best of Luck...

[Dennis Peacock]

Dev,

I've known Forrest for some time now via woodworking forums that we've both been in. I hold him in very high regards and know that what he says to be law. I've never known him to be wrong on anything relating to the wide field of expertise that he holds in his vast knowledge. Now, I've gotta run out there and find that mag so I can read his article..!!!! Thanks for the heads up.

[Roger Myers]

Thanks for the heads up Dev....will make the stop this weekend to pick up the mag!
Roger

[lou sansone]

sounds interesting to me to ... thanks for the tip
lou

[Dev Emch]

Dennis...

I have been through this article very carefully and as an electrical engineer would be the first to protest any oversights or errors. The article is for the most part, clean. And that is why I am endorsing it. The only oversight I found and, PLEASE NOTE, this is an extremely minor one, is: Faraday was given credit for the induction motor. Well, Faraday come up with the electromagtic field threory of induction. I had to forgoe many dates with pretty ladies because of this guy and a few others!!!!!! Nicola Tesla took Farday's work and came up with the induction motor with the fiscal and supportive assistance of George Westinghouse. Westinghouse and Tesla basicly slammed the door on the future of Mr. Edison's electrical dream.

The current issue is November/December 2005. Volume 24, Number 6. The next installment will be the one that really earns the carrots for most of the folks on this forum. It covers AC motor control, switches, motor starters, phase converters and variable frequency drives.

[Dennis Peacock]

Most excellent Dev...!!!!!!!!!!!!
Thanks so much for the info. I've GOT to go and find that mag!!!!!

[Ken Garlock]

I agree, Dev. Forrest is an outstanding source of information on electric motors. Don't forget that he is a good source of information on grinding wheels for sharpening, and their care. His discussions over on WC convinced me to buy an industrial diamond dressing tool to deglaze my 3500 RPM grinder.

[Mike Henderson]

Okay, I’ve got a technical motor question for the electrical engineers on the board.

When we visualize the sinusoidal power used to drive induction motors, we can think of it as rotating vectors. For single phase power, we can actually visualize it as two counter rotating vectors rather than a single vector. We make motors run in either direction by using a starting circuit that “chooses” which vector to follow.
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For three phase power, we visualize it as three vectors 120 degrees apart. To make a motor turn, we invert one of the vectors so that we now have three vectors 60 degrees apart, which resolve through trigonometry into a single larger vector, which the induction motor follows.
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My question is, “How do I visualize reversing a three phase motor?” The vectors always rotate in one direction (I think the convention is counter-clockwise) but I have to find some way to get them to rotate in the opposite direction.
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I took courses in motors and power 40 years ago but spent my career in communications and just don’t remember this. Any explanation will be appreciated.
</O
Mike

[Dev Emch]

Mike...

By your descriptions, your are talking about phasers. In a normal X-Y world in which voltage is plotted against time, the voltage ossilates between a low value and high value with its voltage level being defined by a sinusoidual function. This is handy for looking at very simple things. But the math can get rightous difficult very quickly. Furthermore, phase shift between two or more voltage profiles is more important than the voltage profile itself. In a three phase system, I have three identical sine waves that differ from each other only through their phase shift. In this case, they are 120 degrees apart. When plotted on a polar diagram, these waves become vectors and only amplitude and phase angle are relavent. In the electrical world, these special vectors are called phasers. All your voltage and current relations now become amplitude and phase angle entities. No more sinusoidal cartesian BS.

The alternator that creates these three waveforms does so because I have three windings, A-B-C, within the alternator's stator core. Each winding is broken down into groups. The number of groups and the number of coils in each group is determined by the stator slot count and the number of poles the motor/alternator is to have. But the 120 degree relationship is mechanical. The alternator stator is more or less identical to the stator found in an normal induction motor. Variations due to slot count, pole count, use of multiple windings (for dual voltage use for example) are all minor differences. The major difference is in the rotor. The rotor is a DC wound rotor most often with salient poles driven by a mechanically coupled DC generator called an exciter. My work has been in hydroelectric so we do things slow. Hydroelectrics use salient poles on the rotor. Steam turbine guys need to have cut slot poles because these alternators run much faster than those with salients.

So when all the dust clears, you have three rotating phasers on your phaser diagram. The direction of rotation represents the alternating magnetic field. This alternating magnetic field rotates about the phaser's central axis. But the phase shift of your phasers stays consistent. So in phaser analysis, your always working off snapshots as your three phasers are whirlling about. Three phase power is about relationships and mutual dependency. For it to work, I need all three phases, A, B & C. Now I can change this rotation by merely swaping out the order of say A and B or B and C. So if I am running counter clockwise with A-B-C, then my magnetic field is running clockwise with B-A-C or A-C-B.

So if plot out phaser A on the horizontal zero degree axis followed by 120 degrees CCW, I then label that one B followed by 120 more CCW degrees to establish phase C. In moving from A to B to C, I can see that my field is counter clock wise. If I were to swap out say A and C, then the only way to go would be to go from A to B to C but that entails moving CLOCKWISE. Now, if swap out B and C, and continue moving from A to B to C, then I am once again moving counter clockwise but my A phaser is now in the THIRD quadrant instead of the original FIRST quadrant. The existance of this alternating magnetic field and its direction is why three phase motors exist and why they are superior to single phase motors.

In some machine tools, there is a concept of plug reversing the motor. Plug reversing a motor is when you apply reverse power to a running motor. Three phase motors can do this, albeit each one is rated to a certain number of plug reversals per minute to prevent overheating. What your doing is throwing the magnetic field into hard reverse. You will see this more often with metalworking machines than woodworking machines although some planers like the heavy duty powermatic 25 inchers have a reversing drum switch on the feed motor. Should you find it necessary, you can instantly plug reverse the feed motor when something bad happens.

Most single phase motors will not do this. These motors run on single phasing and were designed to output their nameplate power on single phasing. The direction of rotation is established by the starting circuit. Since a capacitor is used to phase shift the incomming single phase line by 90 degrees, you now have two phasers with a 90 degree phase shift. But you can only obtain one direction of shift. The other direction of shift is reserved for inductors. Inductors and capacitors both shift 90 degrees; however, in opposite directions. The relationship between these two phasers will establish a rotating or alternating magnetic field and hence start the motor. The direction of field rotation determines the direction in which the motor will eventually run even when the start up phaser is removed from the equation and the actual circuit.

So you can plot one phaser on a phaser diagram. But it will not tell you much more than you already know. And there is no phase shift because phase shift is an angular variation between two or more phasers.

I have reversed many three phase motors. I have delt with *NO* single phase motors that had to be reversed. What we know is that the direction of the capacitive start gear will determine the direction in which the motor runs once the start gear is removed. If you need to reverse a single phase motor, most likely you need to replace it with one that can be electrically reversed. Its not in the nature of the beast to do this.

[Mike Henderson]

Dev, Thanks for the explanation - I need to think about this for a while and draw some diagrams before it will sink in. My difficulty is just visualizing how you get a single resulting phaser rotating in the CW direction when the component phasers rotate in the CCW direction.

I think what you're saying is that by wiring a motor from ABC to ACB (and inverting one phase) the resulting magnetic field will rotate in the opposite direction, even if we continue to view the individual phasers as rotating in the CCW direction. Is that correct?

Thanks also for the part about reversing a running motor. I was asking only about how to get the motor to start in different directions but your discussion of reversing was interesting - I hadn't really thought about that.

Mike

[Mike Henderson]

Okay, I figured it out. It's all about the rotating magnetic vector and has nothing really to do with the rotation of the individual phasers. By wiring a motor ABC you get a magnetic vector rotating in one direction. By wiring it ACB, you get a magnetic vector rotating in the opposite direction. Simple as that.

Thanks for taking the time to reply.

Mike

[Barry O'Mahony]

Hello, Mike; small world.

I think you got it that its all about the rotating magnetic field, although I'm no motor expert either, and the textbooks I studied them in now have a very heavy coating of dust.

I'd also like to add to the sentiments that the article in question is very good. I don't normally pick up that mag, but my mechanical engineer BIL was over for Thanksgiving (he has 2 CNC machines in his garage), and he picked up the issue on our pre-Thanksgiving trip to Barnes and Noble.

[Dev Emch]

Yup. The fact is that the phasers will always rotate and this is in part due to the alternator being a roating mehanical machine. In fact, alternators are often referred to by their generic name of Poly-Phase-Syncronous-Machines.

So for us to ply our baggie of electrical tricks, there is not much your going to do to change the AC line frequency of 60 hz. In fact, very large and expensive mechanical-hyraulic governors are used to set the drive speed for these alternators. In my case, we had many smaller guide vanes in the franics type turbines that effectively throttle water into the runner. This controls speed or shaft RPM. Each guide vane has a tiny mechanical linkage and all these linkages, arranged in a large circle, are controlled by a single large push rod. This push rod is driven by a hyraulic system which in turn is connected to a mechanical governor. The ultimate speed controller is the electrical grid frequency. So when we put an alternator onto the grid, we had to first drop the exiciter out, set the rough governor speed for the francis turbine, engage the penstock feed valves, bring the alternator up to aproximate speed, switch in the excitor, check grid frequency and fine tune the governor speed to match this AC drive frequency, then monitor out alternator phase and the grid phase. At just the right moment, these are in line and then we slam a hunkin, 80 year old knife switch to the marble wall. If you dont do it right, the alternator begins a hunting movement. This is bad! If everything was done right, the line load begins to go up with demand and your on semi auto control. Oh ya, dont bother trying to talk to your buddy. Even yelling at the top of your voice will not help. The old hydroelectric stuff is just too cool and you will be amazed by how much is still running. There is tons of it still running in the high country of northern California, Idaho, Colorado, Utah, etc. Much of it is being changed over to or has been changed over to much more modern control systems which can now be run from a central control center in a nice city. It beats having to slog through snow to some tiny high country hole in the wall although playing on the ice was always fun. I think I got the order right on this as its been 20 years since I even looked at one of these.

My power professor used to work this circuit years before some of us did. He used to carry a Ronco Pocket Fisherman in his truck before he had to quit (medical). One day, they were changing out the cups on an old pelton wheel. Would you believe there was the nicest fattest 12 inch german brown stuck in the bottom of this thing. The bottom of a twin nozzle pelton is the start of the raceway and the fish must have swam up there and got trapped when the nozzels were bypassed. So here are two techs and an engineer trying to catch this fish with a Roco pocket fisherman hanging the line down the open pelton turbine chamber. When the cat's away the mice will play. For the record, they never got the fish. When the turbine was brought back online, the water level in the bottom rose again and the fish eventually swam out with the down stream raceway current.

So Mike, you can see that we are interested in the phase and magnitude of the phasers and, secondarily, the direction of field rotation. But the fact that these phasers are rotating really does not bother us because they only rotate from our vantage point. If I am standing on one of the phasers, then it should be clear that nothing is moving. Nothing is moving from that new vantage point. If I look at you, then I get dizzy real fast.

But now I am courious. How does one reverse a single phase motor? What changes in hardware need to be added to get this to work? I think that if you used a type of VFD in place of the capacitor start system, you could get a single phase motor to reverse. This is because I can use one of the other phases, one that has a leading phase shift, to drive the aux winding. Not sure, I need to so some research and maybe give the guys at baldor a call and ask them this. Given three phases A, B, and C out, B would be my line drive voltage. A would lead B and C would lag B. Even if this works, is it worth the effort? I dont think so. Get a three phase motor and be done with it.

[Keith Christopher]

The thing I find most outstand here (besides the information in the magazine) is FINALLY something Dev likes and agrees with ! Just rivvin you bro.

[Dev Emch]

Reversing Single Phase Motors:

O.K. Kiddies, I have been lucky in some light research tonight. Here is how to reverse single phase motors.

Most single phase motors that can run on both 110 volts or 220 volts usually have two run windings and two start windings. For 220 volts, the run windings are directly connected to the service lines and tied together at the other end. They are in series. For 110 volts, these windings are wired in parallel.

For 220 volts, both start windings are wired in series using the same service connections as the run windings and this will produce a clockwise rotation. If you were to reverse this connection, then the rotation is counter clockwise. The composite start winding is in parallel to either the first or second run winding even though the two run windings are themselves in series. You need to think about this.

A similar connection will work for 110 volts except that now the composite start winding is in parallel to both the first and second run windings which are themselves in parallel. Everbody is in parallel here.

The main point is this. For clockwise rotation, the start winding and run windings have the same current flow direction. For counter clockwise rotation, the start winding current flow is opposite to the run winding current flow.

This can be achieved by using a reversing drum switch. So things are not that hard after all. Glad I looked into this although I have no idea of where this triva will be of use. I am a three phase guy.