phase converters

On Wed, 20 Jul 2005 18:43:02 GMT, Ignoramus27341


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
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I think of a rotary phase converter consisting of a three phase motor as a motor generator. Of course I think of a single phase motor as a motor generator too. Works for me, but apparently not for a lot of people. If you did not have the back EMF generated in a motor, the motor would draw a huge amount of current.
Dan
jim rozen wrote:

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Chuck
I have made some measurements on my dyno with various sized idlers feeding tool motors. My data shows that the idler who's HP size is about equal to the tool motor size will provide very little improvement over *no* idler. They will bioth produce chatter at high load. If a person wants to run a 3 phase surface grinder thru an idler from single phase, he has choosen a difficult task because the grinder it extra sensitive to the torque chatter. An idler for runing a 3 phase surface grinder from single phase should probably be several times the grinder motor's HP and tuned for max smoothness at the load expected.
I couldnt finish my experiment with motor heating due to load on the tool motor. I ran out of interest. But, my preliminary data showed that the 3 phase tool motor gets warm rather slowly even at near max HP when supplied single phase. The unfed winding supplies some heat sink. And, a home/hobby/money saver guy could easily provide a little extra fan to cool the motor.
I have no clear cut favorites when choosing how to use 3 phase motors when only single phase is available. VFDs can be a little more expensive, and do put some RFI in the shop.
Jerry
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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).
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    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.
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Ignoramus4745 wrote:

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
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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
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You might want to consider that a induction motor that is driven acts as a induction generator. Jim Pentagrid has posted some very accurate information about induction generators. To summarize, they are very load sensitive if they are used as a stand alone generator. But if they are connected to the power grid, they work very well.
So you can make a motor generator using a single phase motor driving a three phase motor which has one phase connected to the same single phase that drives the single phase motor. Nota Bene In order to actually be driving the three phase motor, you have to drive it faster than the rpm that it would run at when running on single phase as a idler. The way to do this is with a belt drive. One pulley should be a variable pulley. So one can measure the current drawn by the single phase motor and adjust the pulley so the single phase motor draws its nameplate current or somewhat less.
This is the way to make a phase converter for driving something as a surface grinder that is sensitive to variations in torque.
I made one like this some twenty years ago to run a centerless sander. Worked slick. The single phase motor worked as a pony motor to start the three phase motor and then drove the three phase motor to produce a better three phase than most rotary three phase converters.
Dan
Robert Swinney wrote:

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snipped-for-privacy@krl.org wrote:

Dan,
This is a really interesting suggestion. What size motors did you use? I can get a 2.5 hp three phase motor free, and a 3/4 hp single phase motor for little money. How do you think this would work? From what I've read phase convertors are pretty forgiving about the size of motors used.
Many thanks,
Chris
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The motors I used were a 7.5 hp three phase motor and a 2 hp single phase motor. My motor picks were dictated by what I had available, but seemed to work well. The centerless sander had a 5 hp main drive motor and a couple of smaller three phase motors.
I think a 3/4 hp single phase and a 2.5 hp three phase motor should also work well.
If you do a' little googling on induction generators, you will find they are not uncommon. But mostly used on things as street cars when braking.
Dan
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snipped-for-privacy@krl.org wrote:

Thanks very much for the opinion. I'll probably make a trip soon to get the 2.5 hp motor and start experimenting. Will let everyone know how it goes.
Many thanks,
Chris
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snipped-for-privacy@krl.org wrote:

Dan,
That's a really interesting suggestion. What size motors did you use? I can get a 2.5 hp three phase motor free, and a 3/4 hp single phase motor for little money. How do you think these would perform in a phase convertor? From what I've read phase convertors seem quite forgiving when it comes to the size of motors used.
Many thanks,
Chris
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Ignoramus4745 wrote:

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
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