3 phase generator driven by single phase motor - Page 4
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  1. #61
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    Quote Originally Posted by ClappedOutBport View Post
    Thanks so much for that picture Jim. I always wondered how a RPC would look under on a scope. I do have a question though.

    Shouldn't the two real phases be 180 degrees apart when the power is supplied from a center tapped transformer? How are they shifted to be the proper 120 degrees?
    There is ONE "real" phase, meaning the voltage between two wires, that is between the two input wires to the RPC. The other two, being the voltages from each of those wires to the manufactured leg, do not exist without the manufactured leg.

    The phase relation comes about "mechanically" because the rotor of the RPC has a magnetic field induced into it (induction motor) which acts as a generator field, and creates the voltage for the manufactured leg. The position of the windings in the idler motor "mechanically set" the phase, just as the windings in the powerco generator do.

    The voltage being lower is due to the way the idler works, the voltage generated HAS to be lower or the motor would not work. The difference (drop) in voltage is more with more load as the motor slows, and is what allows more current to flow as load increases.

    The idler produces no net shaft power output, but it DOES produce an electrical output, which loads the idler and causes it to draw more current as the rotor slows and the voltage generated in all the windings drops. That drop is what you see on the generated leg under load.

    Your "180 degree" question is really a question based on what your reference is. Between two wires, there can be just ONE voltage, which may be changing, but is at some particular voltage at any moment. It does not matter if those two voltages are varying by millions of volts vs some other voltage..... so long as you measure from one to the other, you will pick up only that one voltage, which is by definition a 180 degree difference.

    If you measured separately from ground to each wire, you would read a different voltage, the "common mode" voltage. You would have to subtract that out to get the difference voltage.

    That's probably clear as mud, but you are OK if you remember that a voltmeter reads just the voltage "difference" between two points, which is always a 180 degree voltage. The generator action takes care of the phase differences around the three phase wires.

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  3. #62
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    to my recollection Jim Rozen was using an rpc with no run capacitors to boast the manufactured leg. as such.. there will be no amps under no load into the driven motor. this is normal. and why would there be? wouldn't the driven motor, also function as an rpc and produce the same voltage as the idler motor? usually this isn't the case because the rpc is larger than the driven motor. as such it has a lower resistance (per unit, larger motors are more efficient, almost by deffinition). but it has the same inductance, per unit. so, you get some increase in voltage and some amps flowing, but not much. and this doesn't actually matter as long as the driven motor does not overheat under load.


    the reason the manufactured leg doesn't produce any amps is because its open circuit voltage is lower than the mains voltage feeding it.
    This ratio is similar to the ratio of full load to locked rotor amps of the motor, which is typically 6 or 7:1 half of that ratio is inductance, the other half is resistance. this ratio can change. 3600 rpm and 1800 rpm motors are typically the same, but 6 and 8 pole count motors have more inductance than resistance (typical power factor for an 8 pole motor might be 70% compared to 90 for a 2 or 4 pole motor), and they are larger per unit hp (because motor power density is proportional to rpm). they are also less efficient usually. but because the inductance to resistance ratio is higher, they may require less capacitance to boost the generated voltage. this may be why some people say they work better as rpc idlers. because the motor's power density is lower, they will be more efficient at low loads, which is where the rpc operates for most of its life.

    i would be surprised if an 8 pole RPC of the same HP, balanced for 15% over voltage at no load with capacitors, can produce more starting torque into a driven motor than a 2 or 4 pole count motor could with the same 15% over voltage at no load.


    So, what i'm saying is at no load, the open circuit voltage is simply the voltage drop across the leakage inductance of the motor. you've got two sources. one is the air gap, the second is the leakage flux from the coil back into itself. the air gap inductance is why the motor consumes 1/3rd FLA at no load when driven from 3 phase this is also the reason it drops with larger motors because its easier to get a proportionally smaller air gap. a 1/2hp single phase motor might consume half of full load amps at no load..

    when you drive the motor from single phase power the motor pulls half to 60% of full load amps simply to energize the motor, because only 2 coils are suppling the current needed to generate the air gap flux. that current causes a resistive voltage voltage drop in the windings, which produces a voltage drop in the generated phase because the flux simply isn't there. that current also causes an inductive voltage drop in the driven coils in their leakage inductance, but not in the generated coils because there is no current flowing in the third leg to magnetically oppose it.


    basically, the open circuit generated phase is about 10% lower than the mains, and that value comes from 2/3rds FLA amps flowing through the air gap inductance of 14%.

    14% comes from the 7:1 ratio of full load to locked rotor amps.


    when you add capacitors you reduce the current flowing into the motor through the two input windings and you increase the current flowing through the 3rd leg. this boosts the voltage for two reasons. one being the current is decreased which decreases the voltage lost to the resistance of the coil, and second the current through the third leg contributes to the air gap flux (because its from a capacitor which forces it to flow at a 90 degree leading power factor) and this decreases the current flowing through the other two coils, decreasing the inductive voltage lost through the leakage inductance.


    as an experiment you could add a capacitor from the neutral of the motor to the third phase only. this would provide reactive current offset only, and it would not provide starting torque. you should see a reduction of no load amps through the other two windings, proportional to the share of the current provided by the capacitor.

  4. #63
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    Quote Originally Posted by JST View Post
    You have just made the case for why balancing on voltage at idle is as worthless as balancing with an ammeter at idle. That's not where you need to balance.
    Darn I've been doing it wrong for 30 years. Better put all those chips back on the stock.


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    Quote Originally Posted by ClappedOutBport View Post
    Thanks so much for that picture Jim. I always wondered how a RPC would look under on a scope. I do have a question though. Shouldn't the two real phases be 180 degrees apart when the power is supplied from a center tapped transformer? How are they shifted to be the proper 120 degrees?
    The three traces are actually line-to-line. So the utility power is just one trace on that scope screen shot. Call the two utility lines A and B. Call the manufactured leg C, for
    the sake of discussion. So the scope is diplaying the real-time mathmatical differences between two wires, for each trace.

    A - B
    B - C
    C - A

    In different colors of course. The first one ( A minus B) shows the utility power. The other two are the differences between a utility wire, and the manufactured leg wire.
    (if you looked at the two utility legs, referenced to ground, they would be 180 out of phase of course. That's not a meaningful measurement however!)

    This is a somewhat difficult measurment to do because it requires:

    A digital scope that can do the mathmatical job of displaying the difference of that many signals,
    Scope inputs that can handle the peak voltages involved (way above 240 volts)
    A scope that can screen shot and output the jpg file.

    I borrowed this one from work. I had to cheat a bit and turn the 10X probes into 100X probes. Don't try this at home. Here's what it looked like:



    The lathe being run there is a hardinge ESM-59, notice that the scope has a 3.5 inch floppy drive to get the data!
    Next is a cheat to get more attenuation on the probes:


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    Quote Originally Posted by jim rozen View Post
    Darn I've been doing it wrong for 30 years. Better put all those chips back on the stock.


    You don't need to worry, you just have a much larger idler, as I recall. That's the best approach.

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    Quote Originally Posted by jim rozen View Post
    Darn I've been doing it wrong for 30 years.
    Prolly closer to 60 years?

    No fear.

    What you been "doing wrong" has nuthin' to do with electricity.

    Your RPC won't consider you a "bad person" Too dumb to notice. It JFW.


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    Quote Originally Posted by thermite View Post
    Prolly closer to 60 years?
    The converter got installed when we bought this rambling wreck of a house. The 30 year mortgage
    was up a few years ago, so that sort of dates it. I'm dated by the fact that I met my wife in the bicentenial year.

    Before the internet. Before cell phones. Practically before computers!

    All we did was sit around the fire and break rocks to make stone axes.

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    Quote Originally Posted by jim rozen View Post
    All we did was sit around the fire and break rocks to make stone axes.
    MY backline were always over-achievers. We just ATE rock and shat them to shape.

    Almost not a joke.. one of the loveliest of the several young ladies who have graced my early life first came to my attention at the office by making and delivering a lunch for me.

    I was touched. Decided to get to know her better when I opened it.

    Bundle of Old Skewl hardened blued steel cut-nails.

    Welll.... leave it to ever-clever wimmin' to have more than one way of signaling they wouldn't mind being nailed?

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    Quote Originally Posted by thermite View Post
    MY backline were always over-achievers. We just ATE rock and shat them to shape.

    Almost not a joke.. one of the loveliest of the several young ladies who have graced my early life first came to my attention at the office by making and delivering a lunch for me.

    I was touched. Decided to get to know her better when I opened it.

    Bundle of Old Skewl hardened blued steel cut-nails.

    Welll.... leave it to ever-clever wimmin' to have more than one way of signaling they wouldn't mind being nailed?
    That's pretty cool from a woman. LOL!

    Well it looks like I'm not gonna run into any problems just running the 10hp idler without any caps to power up to 3hp max load motors, most likely 1-2hp anyway.

    However wiring the motor for wye and connecting 240 across ONE winding from line to star point so it gives 415V 3 phase...well I'm gonna have to find out for myself.

    I'll have to pull this motor apart and replace the bearings before anything though. I'm new to the world of heavy (relatively, for ME) machinery, and already feel the need for some kind of crane to move this thing around.

    Thank you all again.

    mez-motor-3.jpg
    Mez Mohelnice motor made in Czechoslovakia. Weighs 180 lb.

    mez-motor-2.jpg
    A look inside the connection box

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    Quote Originally Posted by Aryan165 View Post
    feel the need for some kind of crane to move this thing around.
    Weighs 180 lb.
    Got 5 hoss as go double that. Lotta "Dinosaur Current" goods 'ere.

    Tricks of the trade:

    - furniture dollies with plywood stiffener plates.

    - foot-pumped hydraulic die cart

    - 9 dollah many-tiny-wheels rails for moving refrigerator.

    - Ell rails, polywood, and spray lube for SLIDING motors into place.

    - Engine hoist and extension bar for lifting whilst hiding back inside a machine-tool's motor-cave.

    - lever ratchet off an overhead beam

    - beam trolley & electric hoist.

    - several types of small pry-bars, shims and wedges, screw-in eye-bolts, chains & clevises, ratchet straps and ignorant polyrope yah even know how to tie a lot of different knots in. Blue-water and mud-boot Hairy-Ears knots, not "Boy Scout".

    "Old Skewl" rigger's attitude:

    If you break the goods, break the gear, break skin, or even break a sweat, you are doing it the hard way. Back-off, think again, let the tools do the work."

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    Quote Originally Posted by thermite View Post

    "Old Skewl" rigger's attitude:

    If you break the goods, break the gear, break skin, or even break a sweat, you are doing it the hard way. Back-off, think again, let the tools do the work."
    Yes, I never like to strain myself, because it's dangerous, and actually pointless! You have to have a method and tools whether it's 200 lb or 2000 lb. Anything else is a disaster waiting to happen. Which reminds me, there is a cool old Morris ''Levalift'' 1.5 ton chain hoist locally going cheap.

    morris-levalift.jpg
    That old roller chain scares me though. I wonder if you can easily get it apart to replace the chain, or if it's riveted/sealed together to be tamper proof. I'll still need something to hang it off.

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    Quote Originally Posted by Aryan165 View Post
    Yes, I never like to strain myself, because it's dangerous, and actually pointless! You have to have a method and tools whether it's 200 lb or 2000 lb. Anything else is a disaster waiting to happen. Which reminds me, there is a cool old Morris ''Levalift'' 1.5 ton chain hoist locally going cheap.

    morris-levalift.jpg
    That old roller chain scares me though. I wonder if you can easily get it apart to replace the chain, or if it's riveted/sealed together to be tamper proof. I'll still need something to hang it off.
    That tribe of chain yah just break a link, replace, repair. Same as bicycle or motorsickle. No sweat.

    Downside? Weakest "link" is the most-worn pin, and the pins are TINY!

    Ergo I prefer the ignorant round chain ones.

    Loads WILL twist, too.

    Best to have a swivel-hook and chain as handles a bit of twist better when it don't swivel cleanly.

  15. #73
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    Quote Originally Posted by thermite View Post
    That tribe of chain yah just break a link, replace, repair. Same as bicycle or motorsickle. No sweat.

    Downside? Weakest "link" is the most-worn pin, and the pins are TINY!

    Ergo I prefer the ignorant round chain ones.

    Loads WILL twist, too.

    Best to have a swivel-hook and chain as handles a bit of twist better when it don't swivel cleanly.
    You're right. I could just remove the hook from the end of the chain, attach a length of new chain to the end of the old one, pull through, and attach the hook back onto the new chain. I could even make it longer if I wanted. I didn't think of that, and I'm supposed to be a bike mechanic...
    I thought about the twist as well, hopefully the swivel hook will take it, but some WILL transfer to the chain.

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    Quote Originally Posted by digger doug View Post
    I wonder who has 460 vac available that's only single phase ?
    Ya good luck with that, my brother wanted to add another 1000 amps single phase for his corn dryer setup, rea, lineman said ya we can just put another tub in plenty of power in the line , the Boss said no way has to convert to 3 phase ( cubic dollars). The 480 volt single came up , no way again.
    Long story short he said f you guys and bought a 350 kw natural gas generator, cost less all around. Runs about 2 months a year


    Sent from my iPhone using Tapatalk

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  18. #75
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    Quote Originally Posted by thermite View Post
    If you break the goods, break the gear, break skin, or even break a sweat, you are doing it the hard way. Back-off, think again, let the tools do the work."
    Good riggers are really boring to watch.

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    Quote Originally Posted by jim rozen View Post
    Good riggers are really boring to watch.
    LOL! Aye. "No drama". We try hard to do that. Make it look EASY... and then?

    Chuckle when they discover the other part...

    "Tough act to follow!"


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    Quote Originally Posted by Turbowerks View Post
    Long story short he said f you guys and bought a 350 kw natural gas generator, cost less all around. Runs about 2 months a year


    Sent from my iPhone using Tapatalk
    Heck you could rent one for that short drying time.

    Can you get a rental set for propane ? That way the dryer propane could
    be piped over to the genset as well, no needing a skid tank of diesel.

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  22. #79
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    Here you go what?? No front bearing, no Amortisseur winding, 80% eff...Phil

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    Quote Originally Posted by Phil in Montana View Post
    Here you go what?? No front bearing, no Amortisseur winding, 80% eff...Phil
    I'm sorry but this forum is called....wait for it..."Practical Machinist"
    some of us can actually ...gasp....make that front bearing holder.

    "no Amortisseur winding" is it not complete without it ?


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