Question for Sparkies

Brad Buchanan · Apr 3, 2014

Hey guys.

I built a rotary converter to power my newest tool, a 36" bandsaw at 550 volts 3 phase.

I am interested in fine tuning this sinusoidal voltage and correcting waveshape as is possible.

Is there an instrument which would allow viewing of the waveshapes that is reasonably inexpensive?

I see quite a few oscilloscopes for sale at auctions but don't know what to look for.

Also if the waveshape can be displayed will the harmonics also appear?

Brad

Ted in NE-OH · Apr 3, 2014

Most oscilloscopes won't handle 550 V and they want one side grounded.

buickanddeere · Apr 3, 2014

Unless the saw is used under constant load. Tuning with fixed capacitors is going to be "too much " then" too little".
Measuring phase to phase voltages and adding caps until the voltages are near equal under load. It"s going to be close and an a lot evener than 99.99% of other rotary converters.
As the motor is not subjected to 100% load for 100% of the time. Heating and power loss from imbalanced phases is of little concern.
Of course VFD would have been exactly 120/120/120 phase separation.0

Geo-TH,In · Apr 3, 2014

Brad, Like Ted said, one side of a scope is grounded. You would need a special input probe to look at 550v. You would need a dual trace scope if you want to look at the voltage phase angles.

What you are doing with a phase converter is playing a game of shifting the phase angle of the voltage and current with capacitors, which the two won't be in phase with each other.

Keep in mind, you are taking 220v, the current is 180 degrees out and you are trying to make 3 phase, which the 3 currents are 120 degrees out.

A phaser isn't going to be a perfect converter.

I would say take an amprobe, measure L1, L2 and L3. Record the data.

Let's say L2 is your wild leg. Add more run capacitors and see what happens to the current in all 3 legs. I would shoot for the minimum current in L1 and L3. Your wild leg usually has less current than the other two. Just try to balance things out the best you can.

Another thing you may want to look at the the operating temperature of the phaser and motor. The more your phase angle is off, the greater the power factor and more heat will be generated in your phaser and motor. Buy an IR thermometer, experiment with different run caps. Go for a minimum operating temp.

I would do the temperature thing first along with trying to balance the current in all 3 lets.

You have the hard part done, it's working. Lets us know how the fine tuning works.
George

MarkB_MI · Apr 3, 2014

Brad, how do you know your converter's output is a less-than-perfect sine wave? And why does it matter? The motor doesn't really care if it's getting a sine wave or not.

If you're really interested in measuring harmonic distortion, your best bet is a modern digital sampling oscilloscope (DSO), rather than a traditional analog scope. You can OBSERVE distortion with an analog scope, but you can MEASURE it with a DSO. DSOs use an algorithm known as a Fast Fourier Transform (FFT) to calculate distortion from the sampled signal.

When you view a distorted signal on an oscilloscope, you are seeing the harmonics. Even-numbered harmonics appear as "crossover distortion" (an S-curve near the zero-cross point). Odd-numbered harmonics appear as "clipping" (a squared-off sine wave). To precisely measure the frequency and amplitude of the harmonics, you need a spectrum analyzer, a once-exotic instrument that is now implemented in relatively inexpensive DSOs using the afore-mentioned FFT algorithm. There are even software oscilloscopes that use your PC's sound card to digitize signal; these can do fairly sophisticated analyses. I have not tried any of these so I can't comment on how well any of them work.

Geo-TH,In · Apr 3, 2014

Brad,
After thought, you are using a transformer to make 550V from 220V, right?

I never used a transformer before, but this may work to your advantage. Simply measure the 220v input current. When you find the right size run caps, you will have achieve the best power factor possible, which will never be 1, and you should find the input current will go to minimum, I THINK. (What say everyone else?)

I would still do what I posted below.
George

Anonymous-0 · Apr 3, 2014

You need a drawn reciprocation dingle arm, to reduce sinusoidal repleneration, Like that contained in the original
turbo encabulator

Brad Buchanan · Apr 3, 2014

Hey Dave.

Thanks for the smile. I laughed so hard that I almost bent my dingle arm!!

Brad

Geo-TH,In · Apr 3, 2014

Brad, I think you will need 3 dingle arms for your converter.

Bob · Apr 3, 2014

DANGIT! I thought you did pretty good just getting it up and running, now you are trying to get all uppity and hoity-toity with us!

Just kidding! Congratulations on getting this far!

dr sportster · Apr 4, 2014

Here I will be your ocilloscope so you can go cut wood. Roundy , roundy , spike ,roundy ,roundy spike. ,roundy ,roundy, spike. By the time you buy a scope you could get A VFD or just cut wood.If the motor is spinnin' ...

Brad Buchanan · Apr 4, 2014

Hey George.

The power factor is an interesting concept for judging the performance of the phaser.

You just gave me more homework!!

Actually I find out that the more that I learn I find out how much I don't know.

Brad

Brad Buchanan · Apr 4, 2014

Hey Mark.

I guess I could call the project finished as is but I am finding that I am looking for more than what happens but WHY it happens.

I looked up the Software oscilloscope and found PicoScope. Have to do more research but that product might do what I want. Thanks.

Brad

Brad Buchanan · Apr 4, 2014

Hey Doc.

I actually have been cuttin' some wood. I had a section of 12" by 24" Doug fir around and sliced it up like buddah.

I actually tried to find a VFD for this application but found the units that would handle the voltage were in the thousands of dollars.

The sun is out and I am jillin' for some biking weather!!!

Brad

Brad Buchanan · Apr 4, 2014

I am trying to get the balance as close as possible.

I am going to use it as a source for at least one more application, a dust extraction system that uses the same voltage and phase.

I have the system mounted on a skid for now but soon it will be mounted in the mechanical room and distributed through a load center in pipe to the loads.

Brad

Brad Buchanan · Apr 4, 2014

Hey Bob.

Thanks.

No Hoity toity here.

I am just a country boy who swiped my Sons College textbooks (he is an engineer) and am attempting to egimacate myself.

Brad

dr sportster · Apr 4, 2014

Yes , As I remeber the problem was the voltage. The price of all electrical supplies has gone so high. I think you have acoomplished the mission.

Brad Buchanan · Apr 4, 2014

I am actually interested in exploring the subject fuller as I had some interesting results.

I have run caps between L1 and L3 (the wild leg) and have L1 to L2 ant about 556 or so L2 to L3 and almost exactly 550 and L1 to L3 at about 640.

I introduced run caps between L2 and L3 and had voltage between L1 and L3 went up to around 940....

So I have a device that works but still needs more tweaking.

Brad

Brad Buchanan · Apr 4, 2014

Hey Ted.

That raises an interesting question.

Is the output of the phaser Delta Or Wye?

I guess I could measure between one of the legs to neutral (with my old meter) and see what I get.

Brad

MarkB_MI · Apr 4, 2014

Brad, PicoScope looks nice, but I think it's massive overkill for what you need to do. There are a number of cheap USB oscilloscopes you can use to sample your waveforms. Once you have the samples loaded onto your PC, you can use MS Excel to run the FFT. (I think Fourier Analysis is a plug-in that comes with Excel but isn't installed by default.)

I think you'll be surprised how clean the output of your converter is.

As others have said, you'll need to reduce the signal amplitude to something the scope can handle. This is easily done using a simple voltage divider (e.g use a 100K and a 1K resistor in series to make a 100:1 voltage divider). Pay attention to ground levels; if there's any voltage between the scope's ground and your converter's neutral, bad things can happen to the scope and PC.

MarkB_MI · Apr 4, 2014

Ah, but WHICH power factor? The PF measured at the input to your saw's motor? Or the PF measured at the input of your rotary converter? I'd say the converter, since it's single phase and is likely the limiting factor in how much power you get out of the saw.

Geo-TH,In · Apr 4, 2014

Brad,
I would try to get the wild leg closer to 550 v.

Like I've said many times, it's an experiment at this point. Trial and error to see what works the best.

Like B&D said, when the load changes, so will the voltages.

The phasers I've worked on the load was for the most part was constant.

You only have capacitance to work with. My dad would change the inductance by removing material from the phaser's rotor. I never really paid much attention to what he was doing. He was for the most part, self taught.

I'm sure when you are done, you can write a book on how to make a phaser.

I would still look at temperatures of motor and phaser.

I may even play a little with putting capacitors on the primary side of the transformer. If you add capacitors and your ammprobe shows current decreasing, then you are going in the right direction. At some point, the current will begin to increase, stop and back up.

You can do.
George

mmidlam · Apr 8, 2014

You could use shareware to make your laptop into a basic oscilloscope.
Will need an isolation transformer and a voltage divider network to make the voltage level compatible with the sound card.
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