trouble measuring coil output voltage

[cris74nd]

Hi ALL!

I'd appreciate any help!  I am preparing to build an air-gap PM generator, and I have a test coil and the rotor set-up.  When I spin the rotor to find-out what the coil performance will be, I am troubled by calculating actual RMS voltage on the single coil.  Since I don't have a true sinusoidal wave, I assume that I cannot read Peak Voltage with a occilloscope and multiply by .707 to obtain RMS.  I have 2 digital voltmeters, one of which is labeled "TRUE RMS", the other is not a true RMS meter.  However, both of these meters read nearly identically, and they both 'bounce' several volts for some reason, which makes it difficult.  Readings at test RPM was bouncing between 25-28 V.

I had help to build a simple full-wave rectifier circuit, in hopes to convert the coil AC voltage to a dependabale and easily-measured DC voltage.  However, this resulted in a surprisingly high value... 45 VDC at test speed!  I've read that if you use a capacitor in the rectifier circuit to smooth the DC (make it easy to measure) then the measured voltage may read high since the capactior will keep voltage at or near "peak".  So that is erroneous as well.

The occilloscope shows peak-to-peak voltage of 100 V at test speed.  

So, what can I do to measure the actual voltage output so that I can accurately predict the cut-in of my genny?

THANKS!

Moved to the proper section.

[Flux]

Life is not easy .

You don't really give enough information to answer your question but you give plenty to confuse yourself.

Is this thing going to be single phase or do you intend to make it 3 phase? that is probably the most vital question to sort out your cut in speed.

If you want to make it 3 phase then you will have to more or less assume your waveform is a sine wave.

If you are intending to use single phase then your cut in will be determined by the peak voltage Of 50v ( half your P-P of 100V)

The peak value actually determines cut in in absolute terms ( peak higher than battery) but in real life you will do better to ignore the scope and work from the voltmeter readings. You say that the true rms and the other agree near enough so you will not be much in error using either. The fact that your voltage is not completely constant may be due to several factors, it may be mechanical in the form of speed variation or an error in the magnet spacing or similar. It may also be metering problems if the frequency is low.

Taking your 28v ac will give you about 40v peak. Your capacitor smoothed dc you say is 45v and that seems reasonable if you have voltage jitter, the capacitor will charge to the highest peak.

For normal working it looks as though you probably have a peak dc of between 40 and 45v. the scope is probably picking up little spikes on the waveform and showing peaks with little energy nearly 10v higher.

If you are intending to make this 3 phase then you will have a factor of about 1.7 on top of this to take care of the star to delta conversion.

Scopes are nice to see what is going on but are not really very useful measuring devices, the waveform will change completely when the rectifiers conduct. When you try the complete machine you will almost certainly find the absolute cut in ( few mA) comes at a lower speed than that needed for a sensible cut in of a few hundred mA. Most alternator curves show a significant toe near cut in and then a more or less straight line output with speed above sensible cut in ( 500mA to 1A)

Working from the voltmeter will be near enough, none of this is exact and I strongly suspect that none of your measurements are better than a few %. You will need a perfectly constant speed drive, perfect construction, frequencies above 50 Hz and and a perfect sine wave to get everything to agree, doesn't happen in real life.

Flux

[cris74nd]

Flux:

Thank you for your comments.  To clear-up some questions, I am intending on a 3-phase grid-tied generator/inverter set-up.  In order not to have a step-up transformer, the machine will need 220V (+) per phase to cut-in.  I have 6 coils per phase, so I need 220V/6= 37 VRMS per coil (open-coil voltage) at cut-in speed.  Correct?

After investigating my test coil today, I see that I have a triangle-shaped waveform, and so far the math works-out to be 50V (peak) x .505 = 25.25 VRMS.  Now I am wondering if I can make a more sinusoidal waveform to increase the RMS voltage, but not sure the best way to do this.  I'm thinking that stronger magnets would also increase RMS voltage.  

It would be nice to find an easier 'fix' than starting-over from scratch.

Any other thoughts anyone?

[Flux]

"Thank you for your comments.  To clear-up some questions, I am intending on a 3-phase grid-tied generator/inverter set-up.  In order not to have a step-up transformer, the machine will need 220V (+) per phase to cut-in.  I have 6 coils per phase, so I need 220V/6= 37 VRMS per coil (open-coil voltage) at cut-in speed.  Correct?"

I don't think we are speaking the same language at present. Does your inverter need DC or 3 phase AC?. When you say 220 per phase do you really mean that or do you mean 220v between each pair of line terminals. I assume it will be star connected so if you have 220v phase, the line voltage will be 220 x 1.7 or about 380v line. The DC will be 380 x 1.4 or about 533v DC.

Don't worry about the waveform, if that is one coil then the phase voltage will also be sort of triangular, but the 3rd harmonic will get lost when you star connect and the line voltage will be a fairly good sine wave. There will be little difference on a meter between a sine and an approximate triangle nothing compared with the mess the rectifier and inverter will make of things.

Yes of course stronger magnets will increase rms volts as will a reduction in air gap.

If this is a test coil then surely if you want different voltage it is easier to change the number of turns than change the magnets.

It would help a lot if you told me the whole story, what the inverter needs, intended cut in speed and also the alternator construction ( number of magnets, coils method of construction and connection etc, then I may be able to help.

Flux

[cris74nd]

Flux:

The "whole story" is a long and quite involved one.  Of which I'm sure many don't care to read about.  However, I would like to share with you and I'm sure you would find it at least interesting.  Of course, I'm grateful for any help I can get with this project.  And I appreciate your patience with my "language at present".  

We intend to take the 'wild' ac power from the 3-phase genny and rectify it to DC.  From there, the inverter will convert back to grid quality single-phase AC.  The problem that we fear right now is not having enough voltage produced by the generator to match the grid voltage, which should be somewhere between 220-240 V.

Is there another way I can communicate to you?  I would like to supply drawings, pictures and specifications that I'm not sure how to get on here.  If I could email to you directly would be easiest for me if you are willing to take a look.  My address is cris@plmfg.com if you'd like to contact me there.

Thanks.

[finnsawyer]

The 'fix' might be to rewind the coils with heavier wire.  At least as I'm reading this you don't specify your test RPM and what you are shooting for as a cut-in RPM.

At cut-in, when the alternator just starts to provide enough current to effect grid tie (I guess), it's going to be the peak voltage that is going to make it happen.  The RMS value isn't going to make the current flow as it is bucked or balanced by the grid RMS voltage.  Now, if the RMS voltages balance at cut-in, using your 25.25 volts RMS per coil we would get:

    Vrms = 1.73*6*25.25 = 262 volts, which is 2.18 times a large as you need.

Now this is not necessarily bad, as it simply means the RPM you made your measurement at is about twice as high as the true cut-in RPM.  Or to put it another way, at your measured RPM the alternator should be providing considerable output.  Which brings me to this question.  What is the resistance of the test coil?  

     

[cris74nd]

If we are trying to back-feed a 240V AC line, then we won't have 2.18 times the voltage we need, right?

Also, I don't understand why "peak" voltage is what determines cut-in.  Peak voltage doesn't represent the ability to perform work at that value, does it?  I was under the impression that we need to be using the term RMS voltage to calculate cut in.  Besides, the peak voltage goes away when it is rectified.  Maybe I'm confused.  

The test coil is .3 ohms.

[finnsawyer]

You've changed the rules.  First you were feeding 120, now it's 240.  It should be clear to you that the alternator will in no way be matched to the grid.  Its voltage and frequency will vary all over.  When you rectify, the peak voltage doesn't go away, it gives a spike in the current.  Since your battery pack must have some minimal voltage for the whole thing to work, you will find that some current starts to flow into the battery when the alternator peak voltage exceeds battery voltage plus the diode forward voltage drops.  This current will rise quickly, but notice that it will be determined by the instantaneous difference between the alternator voltage and the battery voltage plus diode voltage drops.  The average current will be determined by the average value of that difference, not the RMS value of voltage.  The actual current at any time will be determined by that voltage difference and two winding resistances in series, which in your case will be 3.6 ohms (six coils per winding?).  I'm not going to guess anymore what you are trying to do, except to say that once you pick a battery voltage you can assume total diode voltage drops of about 1.4 volts, and using your scope measured waveform, and 3.6 ohms resistance you can determine a graph for the current into the battery as a function of time and see if that would satisfy your needs.  Note that you need to scale the output up to the star connection with six coils per winding (remember the 1.73 factor).  Actual results may differ when you build the thing, but this should tell you were you're headed.