phase converters

I realize the extent of my confusion about phase converters. I thought
that phase converters were motors/generators coupled on one shaft. It
turns out that they are some complicated combinations of capacitors.
I am curious why phase converters are not made as motor/generators.
i
Reply to
Ignoramus4745
Loading thread data ...
There are different types of phase converters.
A static phase converter is just a couple caps and probably a relay that allows a 3 phase motor to start and run off of 2 windings. It does not develop full power (about 2/3) but is still useful.
A rotary phase converter is basically a 3 phase motor running off of single phase power. It will regenerate the third phase well enough to run a smaller 3 phase motor pretty well. Commercial products use a custom wound motor that works a little better than a standard 3 phase motor (so I'm told).
Phase converters are not made from motor generators because it is expensive and not necessary unless you have a unique application such as a need to generate 50 Hz from 60 Hz.
Reply to
Chuck Sherwood
Money. Go price a 5hp 3ph 220V alternator. Wear diapers. I have about $150 into my phase converter total, including new bearings, and megging and revarnishing the stator.
GWE
Reply to
Grant Erwin
Chuck
Although I dont mean to correct you about the 2/3 power statement, most 3 phase motors will develop its full name plate HP even when fed single phase. They will slow to a lower RPM than the name plate RPM rating, and get hotter, quicker with single phase than with 3 phase. It is interesting to me to learn that an idler of equal size of the tool motor provides very little MAX. HP performance compared to no idler at all when fed from single phase. The HP from a 3 phase motor driven from single phase without an idler will perform very nearly the same at that same tool motor driven thru an "equal size" idler up to about 60 percent of MAX. HP name plate on the tool motor.
Maybe thats where the 2/3 HP figure comes from. Idlers and tuning do help the tool motor measurably when the 3 phaes tool motor is heavily loaded.
Jerry
Reply to
Jerry Martes
I suppose they could be considered a motor/alternator, just not separate units.
Don't let the thought that they are complicated slow you down, they're not all that involved. A clamp-on ammeter is good for balancing, they can be had on ebay for around $10 the last time I looked- the basic Radio Shack unit works well.
You don't have to use any capacitors at all if you don't want- you can start it with a small single-phase motor or if you want to go all the way simple you can start it with a pull-rope wrapped around the motor shaft.. but make sure the is pulled clear of the shaft before switching it on. The advantage of the caps is that you can make it more efficient..
John
Reply to
JohnM
What Grant said . . . a single phase motor driving a generator would, in fact, be a motor/generator -- but -- the 3 phase generator portion of the combination would have to be an alternator; a machine with a stationery set of coils (stator) and a revolving magnetic field set of coils, or vice versa. The field would either have to be excited by direct current (DC) or be made up of permanent magnets. Large steam driven turbo-alternators are an example. Those are synchronous machines, whereby the speed of rotation determines the output frequency; their magnetic fields are generally excited by DC generators of 110 volts, sometimes 220, and their may be other DC voltages for excitation.
Contrast this to a rotary phase converter. The type RPC we are familiar with is basically 2 three-phase motors running on single-phase current with their 3rd legs tied together. The motors are not operated in parallel. They may run with or without capacitor augmentation which aids in supplying phased current to the 3rd leg of each motor. In a manner of speaking each motor is a sort of rotating transformer which is responsible for energizing the 3rd leg or "manufactured phase". The idler motor, viewed as a source ( not a strictly accurate analogy) has to be larger than the load motor in order to keep the 3rd leg "generated" voltage from sagging down. Capacitance from the single-phase line to the 3rd leg(s) lowers impedance in those paths and may be thought of as forming a very broadly tuned series resonance circuit. Current flow in a RPC is quite complex and does not readily yield itself to mathematical modeling..
Bob Swinney
Reply to
Robert Swinney
And these have a few capacitors and a voltage sensitive relay to make it self-starting. They may (and good ones will) have additional capacitors to tune the balance of the generated phase, and to balance out imaginary current in the draw from the breaker box which can draw more from your breakers than is convenient.
Or a need for 400 Hz 3 phase to run aircraft electronic and navigation equipment.
A maker of that style of converter is Georator (in Manassas VA).
And yes -- they are a *lot* more expensive than an idler motor used as a rotary converter.
Enjoy, DoN.
Reply to
DoN. Nichols
I think I read that on the phase a matic web site but I could be wrong. I always thought it was related to the fact that the the motor is running on only 2 out of its three windings. At 2/3 rate HP, the two working windings will be drawing full rated current. Anything over 2/3 would imply that the 2 working windings must draw more that full load current. Based on this logic, I accept the 2/3 rating
Wouldn't that imply that it might be operating outside of safe limits?
From what limited testing I have done, I agree that an idler motor does not apear to do much until the tool motor is at a higher slip than the idler motor. There was a guy on here that stated his surface grinder did not work perfectly with a rotary phase converter and worked much better with a VFD. This appears to support this conclusion.
One thing that a RPC does do is provide instant reverse that you cannot get with it.
Frankly I decided not to get two wrapped up in phase converters because I prefer VFDs.
Reply to
Chuck Sherwood
Would it be correct that all these phase converters that are not based on motor/generators, are unable to run more than one 3 phase device at a time. The reason for this suspicion is that they seem to do something wholly different when a motor starts, than what they do when it runs.
So, with such a 1 phase converter, I cannot have a compressor and a mill.
i
Reply to
Ignoramus27341
That is not correct. Rotary phase converters are often used to run multiple machines. One at a time or even more that one at a time.
Do a web search for "phase a matic" and read all about it.
Reply to
Chuck Sherwood
Permit me to turn your statement inside out a bit, but retain the original meaning: "All these phase converters based on motor-generators (sic) can run more than one device at a time."
As others have suggested, motor-generator setups are not typically found in this application. The rotary phase converer which consists of a large three-phase motor, acting as a sort of rotary transformer, has rotating elements and will provide nearly factory-fresh 3~ power, and can run many machines at once. In fact, the more load machines running, the larger the apparent capacity of the system.
My idler motor setup powers a milling machine and two lathes:
Jim
Reply to
jim rozen
Thanks. I feel quite lost. Is what you have not the same as motor/generator (with the 3 phase motor being the generator)?
I can get a 3 phase motor basically for free, this or next week. A very pleasant guy from whom I already got a bunch of stuff from his liquidating factory, needs to dispose of his stuff. I posted what I bought in the "My latest catch" thread. I am VERY interested in making my own phase converter.
i
Reply to
Ignoramus27341
I am beginning to "get it".
You spin up your 3 phase motor by using the little 1 phase motor.
Then you apply single phase power to the 3 phase motor.
The third leg of the three phase motor starts producing an out of phase thrd phase.
At this point, you turn off the little 1 phase motor and start using the 3 phase motor. Is that right?
igor
Reply to
Ignoramus27341
Are you the guy that lives in downers grove IL? If so, you are close to me and I would also like to find a motor to make a phase converter. A 5HP would be about right. A 7.5 would probalby work too. chuck
Reply to
Chuck Sherwood
Hmm. Yoda-speak you do.
I have a rotary phase converter. It isn't a motor-generator because the idler motor is only a simple three phase induction motor.
Jim
Reply to
jim rozen
Exactly correct. If you read the .txt file it sorta goes through the drill on using it.
Close small switch to energize pony motor (it's a 1/8 hp repulsion-induction motor), and wait for the 5 hp motor to come up to speed. That takes all of one second or so.
Then I close the fused knife switch to apply single phase 240 volt excitation to two leads of the 5 hp idler motor. It then picks up and locks to the line frequency.
Then I shut off the pony motor, and lift up the hinged platform so the belt flips off. I deftly catch it and tuck it under the platform so the thing doesn't rattle.
Now the three wires (two of which go to incoming 240 volt line) from the idler motor give pretty good three phase power, which is routed via my distribution buss, to each load machine that I would want to run.
The converter is quiet enough so that I've been known to accidentally leave it running when I quit working.
Jim
Reply to
jim rozen
I live near downers grove.
Unfortunately, I called the seller and he said that he only has motors up to 1 HP. Which is a little strange, as I saw some that were a little big for 1 HP. He also had a 7.5 HP compressor that he sold for $200. A very nice looking lub-dub unit.
He is selling a 2HP 3 phase compressor for $100. You can put a harbor freight 1 phase motor there, I suppose.
i
Reply to
Ignoramus27341
Thanks. I finally understood what you were doing, after re-reading your .txt file.
i
Reply to
Ignoramus27341
Too small to make a useful rotary phase converter. cs
Reply to
Chuck Sherwood
With a motor-generator the generator generates all three phases. With the typical Rotary Phase Converter only 1 of the three phases is generated. The other two come from the two legs of the 220 single phase that power the RPC. So when you connect your three phase motor to the RPC two of the wires connect to the single phase wires that are also connected to the RPC. The other wire connects to the wire from the RPC that is not connected to the single phase line. Capacitors are used to balance the RPC and to provide a self starting feature so the small single phase motor can be disposed with. But a quick and dirty one can even be built using just a three phase motor and a rope. Wind the rope around the motor shaft and pull it to spin the motor up. When the rope comes off turn on the single phase power and if the motor is spinning fast enough it will speed up to normal running speed. I have a well balanced RPC in my shop that is made from a 15 HP motor. At idle it draws about 1500 watts. It can start a 15 HP motor but can run 60 HP when the motors are connected in steps. Currently it is used to run two CNC lathes with 10 HP spindles, 1 CNC mill with a 5 HP spindle, and various other manual manual machines. ERS
Reply to
Eric R Snow

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.