phase converters

Agreed -- but he said "*not* based on motor/generators", which I think he intended to also exclude idlers used as rotary phase converters. The idler motor is serving both as a motor and a generator at the same time.

"Static" phase converters ("Phase-o-Matic" is an example) are not only limited to running one thing at a time, but are also designed for a fairly narrow range of load motor horsepower. If you need to start a 1HP motor, and a 2-1/2 HP motor, the odds are that you can't even switch the converter from one load to the other to run one at a time, because the starting capacitor value will be wrong for at least one of them.

"Rotary" phase converters, build around an idler motor, can not only run more than one load motor at a time, but can even handle more load horsepower when doing so, because each load motor already running off of it contributes to the "rotary converter" effect. So, if you have a 1HP motor on your lathe, and a 1-1/2 HP motor acting as your idler, you have a greater chance of starting another machine whose motor is perhaps a 2HP rating if the lathe motor is already running.

Yes, that will help. Note that "Phase-o-Matic" makes both static converters and rotary converters. (And a static converter can actually serve as the starting equipment on an idler for your own rotary converter.)

Enjoy, DoN.

I did not mean to exclude idlers. At the time when I asked my question, I did not understand the role of idler. Now I do understand its role, thanks to Jim Rozen's pictures.

Would it be also correct to say that after the phase converter started the 3 phase motor, the third leg can be turned off?

i

[ ... ]

Only if you want to have the lowered long-term horsepower out of the motor that running it from a "static" phase converter would give you.

There is little point to doing this, and lots of reasons to

*not* do this.

The *start* capacitors are removed from the idler motor once it is up to speed. This can be done for you by a "static" converter, if you already have one, or don't want to chase down the parts needed to build the starting circuit.

For better efficiency, you may want to use run capacitors between L1 and the generated phase, and/or L2 and the generated phase.

For lower current from the breaker (real + imaginary current), you may wish to add capacitors across L1 and L2, adjusting the value until the current from the breaker is at a minimum. On a residential system, you are not charged for the imaginary current, but it can cause nuisance trips of the circuit breaker, which can't distinguish between "real" and "imaginary" current.

But both of these are fancy tweaks, and you can run without them, just as you can run without a start capacitor if you use a single phase "pony motor" to spin up the idler.

If *I* were to build a rotary converter, it would be self-starting, and would have both run capacitors and the current-reduction capacitor.

Enjoy, DoN.

Thanks Bob. I like the idea of building a homemade RPC, because it is likely not going to cost too much (if I can find a suitable 3 phase motor, which should be possible), and it is not going to take up too much space.

i

Iggy sez: Would it be also correct to say that after the phase converter started

Static phase converters, i.e. Phase-a-Matic, etc, only energize the 3rd leg during start-up time. After that brief interval, the static phase converter effectively switches "off" (back to a standby mode) and your 3-phase motor continues to run on single-phase power. No 3-phase "power" is available from a static converter except for starting purposes. This takes advantage of the fact that a 3-phase motor will continue to run on single-phase "power" after the motor is spun-up via auxiliary means, be that a rope, start capacitors, large hamster, etc.

IMO, if you made the effort to learn a bit about RPCs before trying to jump in and build one, you'd find most of the "mystique" would fade away. You'd be left with the confidence and good warm fuzzy feeling of being in command of your project. There is a wealth of information re. RPCs on the Internet. If you can find a copy of "The Home Shop Machinist", Nov./Dec. 2000, it has a pretty comprehensive article about RPCs.

Bob Sw>> >>>>Would it be correct that all these phase converters that are not based

Correction: Make that Nov/Dec 2001 Home Shop Machinist. Damned proof reader, anyway!

I prefer VFDs. They allow soft start, reasonably fast reverse and variable speed. The only downside is cost. Some people here have figured out ways to use one VFD for multilple machines.

I have a rockwell combination horizontal/vertical mill. I use one VFD to run both motors. I put twist lock plugs on the motors so I can easily change from one to the other. I need to change one parameter (full load current) in the VFD when changing motors because they are very different size (.5 vs 1.5HP).

phase-A-matic ( -a- rather than -o- )

They're limited to *starting* one load at a time. Once you have one motor running it (and every subsequent motor) acts like a rotary converter allowing you to start another motor, up to the capacity of the branch circuit.

I think most phase-A-matics have a 2:1 range, 3/4 to 1-1/2 HP, for example. You can get around this by creative use of the motors in other machines as RPCs as mentioned above. Not convenient, but will work in a pinch to get something going that the static converter won't otherwise start. One very important thing to note is that you should never connect anything with sensitive electronics to the leg being generated by a static converter. There are likely to be spikes present on that leg that may damage solid state controls, etc.

Ned Simmons

Thanks to all. I have another question. For a phase converter motor, how important is the RPM? I am a little puzzled that 3 phase motors have different RPMs, 1740, 1750, 1800, 1760 etc. I cannot understand how it can be with 60 HZ incoming power.

In any case, with a motor that is not spinning at exactly 1800 RPM, how would the third leg agree with the incoming 60 Hz frequency? I am quite confused.

Do I need a motor that is rated for precisely 1800 rpm, or can I get away with a 1760 RPM motor?

i

I

I suspect that you arent actually "confused". Perhaps you dont yet have enough information to show you what the induction motors do and how they work. If you want to get involved with studying the theory of electric motors, there are alot of good books and web sites that have all the information on these 3 phase motors. If you want to build a RPC so you can have decent 3 phase available in your shop, I'd suggest that you get Bob Swinney to send you a copy of his writings.

Note: The 3 phase motors commonly being referred to here in RCM are induction motors. They necessarily run at an RPM slightly lower than the synchronous RPM which is determined by the number of poles in the motor's construction. By far, the most common 3 phase motor used to power machine tools is the "1750". Actually the name plate RPM indicates the RPM to which the "1800 RPM" falls when the motor is fully loaded to its name plate HP. An unloaded 1750 3 phase motor will spin at close to 1798 RPM, depending on its static load from the fan and bearing load. The motor's RPM will gradually drop to the name plate RPM when it is loaded to its name plate HP.

Jerry

This is a little difficult to explain without diagrams, but I'll have a go. In an induction motor each phase winding encircles (i.e., is wrapped around) a number of pole pieces. The pole pieces are formed from laminated iron, and point towards each other on opposite sides of the rotor. The minimum number of pole pieces that a winding can encircle is two (one pair). A motor in which each phase winding encircles two pole pieces is known as a two pole motor. A three phase, two pole motor will have six poles in total (two for each phase). A single phase, two pole motor will have just two poles.

Two pole motors will turn at rotational frequency (i.e., rotations per second) slightly less that the frequency of the supply. This is true if they are single phase or three phase. Four pole motors will turn at slightly less than half the supply frequency, and six pole motors at slightly less than a third. The motor must turn at a lower frequency than the supply, or there would be no "flux cutting" taking place, and therefore the motor couldn't develop any torque, but this is getting into the physics of it. If you want to figure out how the windings produce a rotating field, try to find a good book on electrical engineering. The diagrams are a big help in understanding it. If you can't find one, I have a book buried on my bookshelf somewhere and could scan the relevant section for you. Let me know if you need it. You'll also find that the poles are somewhat hard to identify if you look inside a real motor because the windings are carefully formed into a cylindrical shape around the rotor.

You won't find an 1800 rpm motor, because there must always be some lag between the rotating field and the rotor. A 1760 rpm motor should be fine.

Hope this helps,

Chris

Thanks a lot Chris. I have a CD with a Navy electrical engineering course, I will try to read the relevant part of it. I also saw a very good article on metalwebnews

that details the procedure.

i

I would agree with that.

Is that the article on metalwebnews?

Oh, now I understand a little better. That makes full sense to me. I'll continue reading, for now.

i

Get the Audel;s motor book. It explains about the number of "poles" in an induction motor, and how induction motors work.

The rotor in an induction motor sets up a rotating B field inside the stator windings that is locked to the line frequency. Because the windings are installed in the stator with correct spacing to give the poles the correct phase relationships to each other, the three wires automatically give you correct 'factory' three phase at the output.

It's not magic, it's just a natural result of how polyphase induction motors work.

Jim

Iggy sez:

" ..... > Is that the article on metalwebnews? ...."

I suspect you are referring to Jim Hanrahan's excellent article, most generously contributed to the Internet several years ago. While the article is "classic" in every sense of the word, I would not recommend it as a "first read" on RPCs unless you already have some background in the subject. Hanrahan's article describes 'self-starting' rotary phase converters, although it fails to make the distinction between "self-starting" and the balanced type more commonly used in the home shop. The "self-starter" has no auxiliary, "start caps", relying instead on gross amounts of single-legged run capacitance.

The self-starting RPC is inherently an "unbalanced" device. "Self-starting" works against the object of achieving any sort of balance among voltages of the 3 phases. Self-starting is achieved through the use of excessive amounts of "run capacitance"; enough that if some means of switching out isn't employed, the RPC will be drastically out of voltage balance and incapable of delivering nearly perfect 3-phase power.

RCM's own Fitch Williams ran many posts a few years ago on voltage balanced RPCs. IMO, Fitch gets all the accolades, credit, and all kudos for waking up the home shop crowd to the advantages of the auxiliary-started, voltage balanced RPC. If I had a drink at the moment, it would be raised to Fitch.

Thanks, to Jerry Martes for recommending my writings, but I've just about given all of them out short of enlisting attention of the copyright police!

Best regards to all,

Bob Swinney

Thank you. I am reading an article at

if you have any other suggestions, I will appreciate. I found some posts by Fitch to rec.crafts.metalworking, as well, and will soon look at them.

i

Someone wants $100 for a [supposedly] nice condition used 10 horse motor... It is not exactly very cheap, but I can have it right now...

i

Its a time/money/hassel trade off. Lost creek machine in Ottawa, IL sells used 3ph motors for $10 per HP. Same price so its not out of site, but not cheap either.

Something to think about: You will need start and run caps. The bigger your idler motor, the more caps you will need. If you are buying the caps new, you might incure more cost for the caps than your motor. I recommend that you figure out what you need before you buy parts because you will spend more to build a 10HP converter than a 5HP converter.

For a good quality motor (I always prefer the cast iron frame motors as opposed to the aluminium ones) in good condition, guaranteed working, $100 is probably a fair price. A 10 hp three phase motor might be $1000 new. If you want one now, take a trip to see the motor if you can. If you can wait, you might be lucky and get something free.

Chris