Converted Motor Running on Single Phase

[CraigCarmichael]

Today I tried out the previously 3 phase Baldor 7.5 HP as rewound for single phase operation. (as described in my previous diary entries.)

With 120uF of capacitors, it wouldn't start, but seemed to run fine once going. Same with 55 uF:

uF  - Voltage across Coil/Voltage across Capacitor(s) - Starting notes

55.  - ?? - ran fine, wouldn't start.
    
120. - 134/275 - ran fine, wouldn't start.
     
150. - ?? - took about a second to start up.
     
168. - 139/279 - started up in about 1/2 second.
     
205. - 141.5/279 - started up in a bit less time.
     
     

I also had the impression that it started to slow down more quickly when the power was shut off with the larger capacitor sizes connected, but I could be imagining it. My thought was that the coil and the capacitor should each have about the same voltage across them, but that's obviously not happening with any value of caps. I'm not sure what's the best value to use, or whether it's very important as long as the motor starts up fairly quickly -- the voltage on the line drops to ~~175 V at first. (I may need to shorten the 120' cord, but I don't want the mill in my laundry room, either.)

So far, I haven't got out the oscilloscope to get a better view of the voltages and fase angles, and I've been reluctant to put my 30 amp max multimeter into a circuit that's probably drawing 45 amps during startup to check the currents.

While taking the very last (naturally, says Murphy) of the above voltage measurements, there was a big spark. My hand being on the switch, I shut it off at once. I thought maybe I'd made some serious design mistake. Then I spent 2 hours making a new coil and installing it.

Moral of this event: don't pretend you can't hear the little noise of a coil end Still rubbing slightly on the fan, even for just long enough to run a few tests.

Other than that glitch, after a minute or so of running, the air blowing out is a little warm. Immediately unplugging and disassembling the motor and touching things, the coils and stator were somewhat warm with the main coil being perhaps slightly warmer than the capacitor fase, and the rotor was quite hot. I have no idea whether this is normal, having never before thought to take a motor apart right after running it and feel around inside!

Naturally, that's just idling: no saw blade, no load. I have to finish the mill (and get the motor varnished so the wires can't move) before I can see what it does under load.

BTW the fringe expenses have been killers. It was 65$ for the burned out motor, some magnet wire, capacitors, etc -- Great! Then yesterday came along: 125$ for an on/off switch and 105$ (total 230$) for 120' of cable, a dryer plug & misc connectors!) And I did look around. I could have got a 48$ switch, but it was just a beefy "light switch" type, and I thought since the mill would be outside and might be damp at times, I'd get an enclosed switch in a box in preference to electrocuting myself. A spinning saw blade should be enough to worry about!

--Craig

[richhagen]

Sure it cost a bunch of money, but look at what fun you have.  I have a couple of three phase motors on a big machine, and had thought about switching them out for single phase as opposed to getting a converter.  I'm not sure what I will end up doing, as I haven't gotten up the energy to do much of anything yet, but I have found your posts interesting.  Rich

[oztules]

Craig,

I think your warming too fast. I'm guessing the cap is still in series with your start field after starting.

Anywhere over (I'm guessing here) 80uf in run mode, will probably make the motor "hum" a little louder than you would expect, and use more current than is useful. It will no longer be just acting as a pfc, but as a current drain as well.

To test this, make a switch so you can switch  capacitance in and out after starting, (I just  shuffle leads around while the motor is running, but TomW will shoot me if I suggest someone else do it like this) , and listen to the motor hum. Use different sizes to test it with. If this attenuates the motor hum, chances are the poor use of current will come down as well. (this pathetic system is the one I use to get the best values for the rotary phase converter/machine matchup's.... not pretty, but works..... an AC current meter would be better, or measure the Vdrop over a known resistance etc.).

If this fails, then I'm guessing the wire turns are not quite right (not enough perhaps?)

I admire your efforts getting it this far, hopefully, different cap values will solve the heating prob.

.........oztules

[Flux]

Oztules is right about the capacitor. When running, choose the value that gives smoothest running ( least vibration). This will likely be too small to start it.

Parallel this with a large one for starting and as long as it is fairly quick to start, it's not too critical.

Using the correct run value and switching out the large start capacitor should prevent the capacitor braking when you switch it off. That is not very desirable as you could have serious voltages and circulating currents at resonance.

Regarding the heating, I don't follow what you done with the run winding, I have the impression that you have reduced the turns significantly and the magnetising current is likely to be high. Check the run current ( after starting ) with your meter.

I also got the impression that you have the start winding at 120 deg instead of 90, which will reduce starting torque.

Reduce the capacitor to a modest value after the start and watch the temperature running light. If it gets above 50 degC running light, you may have trouble on load.

Flux

[Goose]

CraigCarmichael,

Thanks for posting your progress.  I was going to do the same thing you are doing, using a 10HP 3phase and convert it to single phase.

I have a 5HP baldor running my air compressor.  I called Baldor and got the rewind specs for the 5HP and was going to rewind the 10HP to those specs, using Zubblys method to get the right wire size.  That way I could keep the same number of turns as the 5HP.  I have given up for the time being, because when I started to order the copper wire, I decided it would be too much money.  Maybe in the future copper will go down and I will try my experiment.  

Anyways, just an idea for you.  Baldor is really good about giving info on rewind data.  Just pick out a comparable single phase motor made by baldor and call them for the rewind data.

[CraigCarmichael]

Thanks everybody for the comments!

Guys, looks like the capacitor and heat advise is right on. I never heard any humming at any time once it was running (though, I was doing it outside and I'm in town), but this time I tried running it with just the 65uF for a minute or so, and the exhaust air was cool the whole time, and on disassembly the coils and stator were cool and the rotor just a little warm - a considerable contrast to the substantial heating (rotor or rotisserie?) with 150-200uF.

Once I have more confidence there won't be any more displays of sparks, maybe I'll bring it inside and listen for the best hum as I try different capacitor values. But I'm more eager to get back to assembling some of the other parts of the sawmill - when it's all ready, it can be tested with a load!

It looks like I'll need a centrifugal switch or other startup control with start capacitor (250uF?) after all. Maybe I'll try one of the current sensing controls.

There are 3 coils of 55 windings taking 6 slots of the 18 each, two of which are vertical beside each other and the third horizontal, so they are 90 degrees.

The 2 vertical coils are in series across the 230 V creating a vertically aligned field. (This could just as well be called a single 110 winds coil in 12 slots.) The horizontal coil is in series with the capacitor across the 230 V creating a horizontal field.

All 3 sets have sine 120 degrees less windings than the original.

(original 66 winds * .86 = 57)

Actually I used 55 turns anticipating some voltage drop in the long cord, essentially optimizing (in a relative sense) for 220 V instead of 230.

I suspect (in my ignorance) there's probably a fair latitude for the start coil winds - different numbers of winds would just mean different optimum capacitor values? At least, what I've done seems to work.

And NOW I read that you can get the rewind specs straight from Baldor!

Finally, good luck to anyone else who wants to try converting a motor to single fase!

Cheers,

Craig

[oztules]

Craig,

You have tried so hard to make it plain to us as to how you have wired this baldor. I regret that for me it just don't add up.... Just my interpretation of your explanation i guess.

That being said, if it runs well for you then thats the main thing.

It will be useful to check your design against the Baldor figures and orientation and coil placement.

Zubbly has some diagrams in his files for coil placement also as I recall.... I can't help but think you should have four distinct coil sets of three concentric wound coils. This makes for 3 + 3 coils making the run field(diametrically opposed) and 3+3 coils for the start (diametrically opposed) and all four coil sets 90 deg apart.... 12 coils in all. This will give a phantom pole arrangement..... I'll stop this now...

Momentry switch is the simplest system to use for start.

best of luck with this

.............oztules

[CraigCarmichael]

"You have tried so hard to make it plain to us as to how you have wired this baldor. I regret that for me it just don't add up.... Just my interpretation of your explanation i guess.

"I can't help but think you should have four distinct coil sets of three concentric wound coils. This makes for 3 + 3 coils making the run field(diametrically opposed) and 3+3 coils for the start (diametrically opposed) and all four coil sets 90 deg apart.... 12 coils in all. This will give a phantom pole arrangement....."

It's "3600" RPM rather than the more common "1800", so there's just 2 poles instead of 4, and it's 36 slots so 18 coils total, that go across the whole stator (making for very long overhangs). So all the run windings (12 parallel coils accounting for the original fases "A" and "B") go up and down, and the start windings (6 parallel coils, replacing "C" and the static fase converter) go left and right.

--Craig