Author Topic: 3 Phase to Single Phase Motor Conversion/Rewinding  (Read 4854 times)

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CraigCarmichael

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3 Phase to Single Phase Motor Conversion/Rewinding
« on: June 19, 2006, 04:28:51 AM »
     3 Phase to Single Phase Motor Conversion/Rewinding

     --------------------------------------------------


I've been giving some more thought to the 3-faze to single-faze motor conversion. It must be remembered the motor needs rewinding one way or another anyway.


Since with a static faze converter we're talking about a 20 to 33 % power reduction (depending where you look), I don't see that holding to the original 'optimized' motor design is doing much good.


So how to change it? Since nobody seems to have any ready-made formulas or guidelines, I've been forced to resort to thinking for myself. (Ugh!)


In 3-faze operation, voltage A to B = 230 VAC-RMS.


Voltage across each coil (A or B to the Y point) = 115 V / sine 120 degrees = 133 V


For single faze, again A to B = 230 V, but the Y point isn't properly driven by the third faze, so voltage is closer to 115 V across each coil instead of 133, 14% low.


So it makes sense to eliminate the Y point and use 14% fewer turns in the coils and have coils 'A' and 'B' together in parallel (magnetically/fysically in parallel; electrically in series) instead of at angles to each other but generating pretty much an overall parallel magnetic field anyway. I don't see that the rotor design should less optimum for this than having the static faze converter though I wouldn't swear to it.


66 original turns per faze * sine 120 degrees = 57 turns. Since I don't expect "faze C" to draw as much current I reduced that just a bit more to 55 turns, 9 per slot with the shortest coil having 10. So the now "main coil" (12 slot pairs of 18, combined "A" and "B") provides 5.66 HP of the original rated 7.5 - almost the total power or more than with the faze converter.


I have no idea what is optimum for "Faze C". I intend to wind it similar to how it was (ie in 6 slot pairs) with 55 turns - same wire as the others. I'll drive it by run capacitors - 100uF for starters - and see what the oscilloscope shows. Ideally it will be 90 degrees out of faze. With the full 240 V across it (in series with the capacitor), it should be driven pretty hard. But this time, there's no Y point and so there's only one place a cap(s) is needed. It only has to draw 7 amps and supply 1.84 of the HP at rated output load.


Thus, for the same 3450 RPM, we have 12 coils going one way for the main faze across the 240 volt line with no center point connection, and 6 coils in the remaining slots at 90 degrees is the capacitor driven coil.


The current in the main faze at rated output is then: 66 original turns / 55 new turns * 18 Amps (original rating) = 21.5 Amps (* 85.5%, the original rated efficiency, gives 5.66 HP), with the second capacitor driven faze accounting hopefully for the remaining 7 amps (gives 1.84 HP) of the 28.5 A total for the rated 7.5 HP.


As a fringe benefit, 6 of the coils now have 3" shorter overhangs for a total savings of 28' of doubled #15 wire, or 3/4 of a pound of copper. That leaves more breathing space - and cooling airflow - at the ends, which were pretty crammed before.


I've wound the coils, and I put in the first string of 6 today. I have a bit of room left in the slots - maybe one of the wires should have been #14 instead of two #15's. Oh well, I plan to put in an overheat tripout - it may (or may not) trip more than hoped for. I'm putting on aluminum heatsink fins on the exterior case.


I'm not sure about starting yet (would it be too much to hope that it might start with just the run capacitors?) There does appear to be a place for a centrifugal switch in the end bell.


I'll write with the results when I know them.


BTW the motor capacitor voltage ratings say "VAC", AC Volts, so they must take the peak voltages of the sine waves into account in the rating, unlike the DC caps I'm used to.


--Craig

« Last Edit: June 19, 2006, 04:28:51 AM by (unknown) »

oztules

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Re: 3 Phase to Single Phase Motor Conversion/Rewin
« Reply #1 on: June 19, 2006, 04:55:40 PM »
Craig,


I'm not sure I understand how you have come to your solution, but best of luck with it.


I can say that  most single phase motors I have fiddled with will start no problems without the start cap and only a run cap  IF there is very little load (much slower).


You can probably increase the run cap a little to help with the start torque... too much will be a problem though. I'm thinking around the 80uf mark for 7hp motor. I'm guessing that a 16" blade will not require too much initial start up torque.


Provided the torque will spin up the blade in a reasonable time, approx 1 second or so you should get away with this setup.


Failing that, a momentry-on push switch  (series with the start cap) combined with the start switch will suffice... hold it down for 1/4 sec or so until the speed gets up.


Luck with the rewind.........oztules

« Last Edit: June 19, 2006, 04:55:40 PM by oztules »
Flinders Island Australia

Ungrounded Lightning Rod

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Re: 3 Phase to Single Phase Motor Conversion/Rewin
« Reply #2 on: June 19, 2006, 06:38:38 PM »
(Please:  PHase, not Faze.  The first is timing offset in two instances of a periodic function such as voltage or current in an AC circuit.  The second is what something that confuses you does to you.  As in "Speed-reading the documentaion on this three-phase stuff really fazed me.")


You'll do worse with single-phase rewind than with a static phase converter.


A three-phase motor driven by three phase (or any polyphase motor driven by the appropriate power supply, regardless of the number of phases - 2, 5, whatever) has a magnetic field of constant strength that rotates.  This pulls on the rotor with the same force during the entire revolution.


A single-phase motor (without shaded poles, which are a whole different ballgame) has a magnetic field that flips back and forth.  It pulls on the rotor by varying amounts at different points of the rotation.  For a given maximum strength it ends up pulling the rotor half as hard, on the average, as the rotating field in a polyphase motor.


So a polyphase motor driven by single phase power with no phase conversion, has half the power of the same motor driven by polyphase power.


And if you rewound it for single phase, that's what you'd get.  (The only thing you'd improve by optimizing the wind for single phase is lowering the resistive losses in the copper wiring - which are a small fraction of the power delivered to the shaft and amount to a side issue.)


The reason:   The maximum field strength is limited, separately, by the saturation of the magnetic core material.  Motors are designed to be driven very near to saturation - because otherwise they'd need a longer stack of laminations in the stator and the rotor.  If you rewind to try to magnetize them farther, you just drive them into saturation.  At that point most of the inductance goes away (as if your coils were air-core instaead of iron core).  The input voltage only sees this tiny inductance plus the very low resistance of the wiring - nearly a dead short.  Your current skyrockets - and it's a race to see if the wiring melts before the circuit breaker blows.


Since your motor has been designed to operate as close to saturation as it's safe to do, if you want to get the whole horsepower rating of it rewound as a single-phase motor, you'd have to DOUBLE the length of the stack of laminations.  Take two motors, stack their stator laminations, tear apart the rotors and stack THOSE laminations, and fill in the slots with aluminum or welded copper to rebuild the squirrelcage.


Or settle for half the horsepower using the original rotor and the original stator cores.


Since all you'd get in single-phase is 50% of the rated horsepower, a phase converter that gives you enough crosswise magnetization to get you to 66%-80% is doing pretty well.


Want more?


Hang an "idler motor" of a horsepower rating somewhat larger (or at least equal) to that of your motor in parallel with it.  Go ahead and use a start or run capacitor or some other phase conversion approach to get it spinning.  While running your loaded motor will pull its half-power directly from the line.  Meanwhile the idler will act as a motor-generator, pulling the up to another half-power (of ITS rating) from the line to keep itself spinning, and then eating it up again to generate the missing phases and feed them to the loaded motor.  That way the loaded motor gets essentially normal three-phase and operates near full power.   (The bigger the idler, the closer it gets to running at full power - and the more power you burn in copper losses in the idler, which DO add up if you use, say, a 50 horse behemoth to phase convert to power your one-horse bandsaw.)

« Last Edit: June 19, 2006, 06:38:38 PM by Ungrounded Lightning Rod »

CraigCarmichael

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Re: 3 Phase to 1 Phase Motor Convert/Rewind
« Reply #3 on: June 20, 2006, 12:00:08 AM »
Thanks guys for the comments!


A pushbutton might be good if it won't start readily. I'm hoping it will and am encouraged to hear it might, as said just because there's no starting load except inertia. The guy at the motor shop gave me a 48 and a 65uF capacitor, so 48, 65 and 113uF are what I'll be trying out first.


I expect there should be a similar amount of flux to the original with this setup. The way I see it, there's less voltage across the double "main phase" than across the A:B coils in 3 phase (sine 120 degrees=.86 x), so I used .86 times the original windings. (and even a bit less since I expect the voltage to be down to about 220 under load with the long power cord.) Admittedly the magnetic rotation is rather lumpy with (any) single phase motor, but this also applies pretty much to the static phase converter, I should think.


I've left the capacitor coil like the 3rd phase except likewise reducing the number of turns a bit. If it works with the static phase converter, it should work here.


PS: I don't like using "ph" for "f" since I found out it was based on some medieval scholar's mistaken idea that that was how the ancient Greeks used it. I'll bow to peer pressure for now, but now if you fone me, I'll have to pronounce it "three pays"!


--Craig

« Last Edit: June 20, 2006, 12:00:08 AM by CraigCarmichael »

Ungrounded Lightning Rod

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Re: 3 Phase to 1 Phase Motor Convert/Rewind
« Reply #4 on: June 20, 2006, 09:49:28 AM »
Language evolves.  (If it didn't we wouldn't be speaking English, or Old English, or Deutsch, or Latin, or ...)  What is "right" is what is in use at the time - not what some academic says is what should be right - or was right a millenium or two in the past.


From the standpoint of the interests of someone writing it, using an oddball spelling (when you aren't obviously using it to tag a variant meaning) makes you look ignorant, causing others to misjudge your experience and assign a lower weight to your opinions.  It may be VERY unfair - especially when some provincial type from a large and powerful city looks down his nose at your regional word choice or usages or (in spoken words) regional accent.  But it's a very strong effect nontheless.  (Accent, for instance, is a tougher-to-overcome barrier to carreer advancement than skin color.)


Of course it's your right to spell it whatever way you want.  B-)  You should just be aware of the effects so you can create them deliberately when you want them, rather than creating them unintentionally when you don't.

« Last Edit: June 20, 2006, 09:49:28 AM by Ungrounded Lightning Rod »

CraigCarmichael

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Re: 3 Phase to Single Phase Motor Conversion
« Reply #5 on: June 22, 2006, 05:13:30 PM »
Very good points. I'm just stubborn. Else I wouldn't persist with the motor conversion contrary to all advice, either! :)


--Craig

« Last Edit: June 22, 2006, 05:13:30 PM by CraigCarmichael »