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RMS Measurements
By jimovonz, Section Homebrewed Electricity Posted on Wed May 23, 2007 at 10:03:02 AM MST
The meaning of RMS values

I've seen a few comments lately that show there is a bit of confusion out there with regards to the meaning of 'RMS' when it comes to waveform measurements. RMS stands for Root Mean Square and would more accuratley be described as the 'square root of the mean of the squares'. Rms measurements apply to varying waveforms and can be measured by dividing the periodic time interval up so that the voltage/current changes very little from division to division, taking the square of the measurment over each division, calculating the mean of the squared values then calculating the square root of that mean. This calculated value gives you the equivalent DC measurement which would dissapate the same power across a given resistance as the waveform measured. For example if Vrms of a particular waveform is 10V then 10Vdc would dissapate the same energy across a given resistance. The rms measurement applies to any waveform, not just sine waves. It just so happens that most (cheap) multimeters expect all AC waveforms to be a sine wave and estimate the rms value based on this assumption (0.707 * Vpeak for instance). This will give you unreliable measurements when the waveform varies significantly from a sine - such as the current in a phase of an alternator charging a battery through a rectifier. Meters that are 'true rms' should calculate the rms value based on the actual waveform measured and thereby give meaningful information regardless of the waveform.
I have put together a spreadsheet that (hopefully) illustrates the value of rms measurements. The spreadsheet compares a 50Hz sine wave with an arbitary (random) waveform. A new random waveform can be generated for comparison by clicking the 'Regen' button at the top of the sheet. You can also specify the peak voltage and the resistance across which the output of both waveforms will be compared. Both waveforms are divided up into 200 divisions over a 0.02S interval (1 cycle @ 50Hz). For each waveform the energy dissapated across the specified resistance is calculated along with the rms voltage. The total energy dissapated is then compared using the two methods at the bottom of the table. As you can see, the calculation using the rms value gives the same energy dissapation for both methods. The ratio of Vrms/Vpeak is always 0.707 (1/2^0.5) for the sine wave but varies for the arbitary wave.
http://www.otherpower.com/images/scimages/2210/RMSComparison.xls

RMS Measurements | 55 comments (55 topical)

Re: RMS Measurements (3.00 / 0) (#1)
by finnsawyer on Wed May 23, 2007 at 08:07:45 AM MST

O.K. Now show us how this relates to charging a battery with a windmill from cut-in to furling with the diode voltage drops included. I guess you could write a program to do that for various assumed waveforms. Sometimes these things get too complicated to be useful. And how many people have true RMS reading meters, anyway.
GeoM

Re: RMS Measurements (3.00 / 0) (#2)
by Lumberjack on Wed May 23, 2007 at 08:23:57 AM MST

"And how many people have true RMS reading meters, anyway."

Since this is the standard most meter are calibrated to.... everyone.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#3)
by jimovonz on Wed May 23, 2007 at 09:23:38 AM MST

"O.K. Now show us how this relates to charging a battery with a windmill from cut-in to furling with the diode voltage drops included."

Simple, the waveform in the your coils of your alt is certainly not a sine wave in this situation. If you were to measure a phase with a cheap meter that assumes a sine wave the reading you would get would be wrong. I assumed that this would be information that folk would want to know. It doesn't take any sort of program to work out - any true rms meter will give you readings that can accurately determine losses etc. If you were to look at the spreadsheet I made you will see that the results are indedpendant of the waveform - just hit the 'regen' button and you will see the same results for another random wave that proves the point just as well.

"I don't mean RMS or Root Mean Square voltage. It is not a very good measure for an alternator who's output is not a pure sine wave. Your RMS meters will give an erroneous reading."

This is just plain wrong. A true rms meter is the only one that will give a correct and useful reading.

I have a couple of true rms meters, so that's at least one... Mine were quite expensive but I see many cheaper meters available these days that claim to give true rms.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#4)
by finnsawyer on Wed May 23, 2007 at 09:47:17 AM MST

I don't see how you people don't get this. The battery voltage in series with the diode voltage drops cuts off the bottom of the voltage waveform. The battery never sees the entire RMS voltage anyway. The current that flows into the battery at any time is given by:
Ib = (Va - Vb - 2xVd)/(Ra +Rb).

So, where is the RMS voltage in this? You could plot Ib as a function of time, plot Vb as a function of time, and plot their product as a function of time to get the instantaneous power. From that you can find the average power by using as fine a grid as you like. I'm sorry, but I don't see the RMS voltage as being of any use. End of discussion for me.
GeoM
[ Parent ]

Re: RMS Measurements (3.00 / 0) (#10)
by jimovonz on Wed May 23, 2007 at 12:58:15 PM MST

"I don't see how you people don't get this." Hmmm... you do seem to have that problem. I thought that the whole "heating in the coils" thing - or efficiency as I like to call it, was rather topical.
Your obviously a theory man and may not even own a meter, so I can perhaps understand. For those of us that build and take measurments from real systems, I though it might be useful information.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#22)
by finnsawyer on Thu May 24, 2007 at 08:36:47 AM MST

As usual Flux, once he decided to wade in, took the time to explain things nicely. For my case, I was getting rather exasperated by your apparent thick headedness, so I set a small trap. Apparently you missed the point that to find the power into the battery all you need to do, with the low resistance batteries used, at any time, is measure the "average current" and multiply that by the voltage across the battery. No RMS voltage. So, the bottom line is that peak voltage determines whether you are below or above cut-in, and the average current determines along with the battery voltage the power into the battery. Most people here are really concerned only by the power delivered to the battery. You never stated that you were concerned with power loss in the resistance of the coils. Usually this can be ignored or reduced by adding a small resistance in series with the battery. Or another way to look at it is to design the alternator robust enough to start with (to keep to coil resistances low enough without affecting cut-in voltage). It is my understanding that the designs used here have pretty much solved that problem. That doesn't mean you can't work on it, but don't get angry if people yawn. They simply have a different perspective.

In my view the best all around device for measuring waveforms is the oscilloscope, as you can immediately determine peak values, see any significant distortion, and determine frequency. Just to set the record straight for one of those people who prefer to put down others rather than discuss things rationally, I own a cheap analog multimeter and a 60 MHZ triple trace rather expensive oscilloscope.
GeoM
[ Parent ]

Re: RMS Measurements (3.00 / 0) (#23)
by jimovonz on Thu May 24, 2007 at 01:00:58 PM MST

"set a small trap" What's that about?! Aparently you are the one that missed the point. At no stage did I mention anything regarding measuring power into a battery. You are repeatedly going on about this as though its the central point in my post and that I have got it wrong. I have measured power into a battery by the method you describe MANY times (in fact just yesterday I logged at least a 1000 such measurements...)If you read my post and view the attached spreadsheet you will see that I was specifically discussing resistive loads. I believe that this is a valid topic specifically related to the wind turbines we build. If you don't think so thats fine - but why comment at all? Is this just a "I've got bigger ones than you so my opinion matters" situation? Go start your own post for that stuff and see how many folk join in. If there is some error in the information I presented by all means, point it out so as to not misslead anyone. It would have been nice if you had taken the time to understand what my intent was before you started comming down on my efforts to share information.

"...but don't get angry if people yawn. They simply have a different perspective."
What - you were bored so you thought you'd through a bit of negativity in just to stir things up?

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#28)
by finnsawyer on Fri May 25, 2007 at 08:01:54 AM MST

"It would have been nice if you had taken the time to understand what my intent was before you started comming down on my efforts to share information."

That cuts both ways. I always was talking about the end result, what you can get into the batteries, which does not require knowing the RMS value of the alternator voltage. So we were talking past each other. Nothing new in that. Goes on all the time. You did get disrespectful when you questioned my equipment inventory, though. Not pertinent. What is pertinent is that most people do not have and are not likely to have meters that read true RMS. That includes myself. I don't see any use for one. Anything I need to do I can do with the scope.

GeoM
[ Parent ]

Re: RMS Measurements (3.00 / 0) (#5)
by scottsAI on Wed May 23, 2007 at 10:57:18 AM MST

Finsawyer,

I have 3 true RMS reading meters, 6 that pretend to be.
One will read Vp-p, which I find useful. Why?

If RMS reading is 10v, will it start charging a 13v battery through rectifier?
Not if square wave, sin wave most likely will, verify with Vp-p.

I say knowledge is power.
Others say ignorance is bliss.

Which fits in your wallet:-)
Thanks jimovonz.
Have fun,
Scott.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#6)
by Flux on Wed May 23, 2007 at 11:28:22 AM MST

This is something I didn't intend to get dragged into, But there are issues here seemed destined to cause more confusion than it answers.

If you are interested in heating either in the load or the machine windings then yes rms is what you want.

For battery charging the rms meter may not prove to be worth the expense. The current that matters in charging a battery as far as I know is the mean. Under certain conditions the rms current will be considerably higher than the mean but it will only be important for line loss effects.

Similarly for much of the alternator testing the rms meter may be no more use than the nominal mean calibrated rms that most use.

If the waveform of the alternator is not a sine wave the 1.73 factor for 3 phase star doesn't apply and neither does the peak to rms of a 3 phase rectifier. The factor that determines cut in speed is the peak voltage and a peak reading volt meter would work irrespective of the waveform but most meters don't read peak.

If the coil doesn't give a reasonable approximation to a sine wave, it will not help in the least to measure the coil voltage with a true rms meter to determine the cut in speed.

The case where rms does come into its own is to measure the rms current from the alternator to the bridge rectifier to determine cable and winding loss. I think most people have a very poor understanding of the rectifier input current compared with the mean dc output current.

As long as we use the right meter to measure the right thing then all is fine but the true rms meter does not correct for failure to understand what you are measuring.

Flux

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#7)
by SparWeb on Wed May 23, 2007 at 12:35:28 PM MST

"This is something I didn't intend to get dragged into"

Thanks for wading in, anyway, Flux. My meters lie to me on occasion. Perhaps more often than I thought!

So... if Vrms = Vpeak only in a perfect square wave, how does one measure Vpeak with a multi-meter when one does not know the waveform?

(I expect the answer is "you can't", unless you buy an oscilloscope.)
Steven Fahey
[ Parent ]

Try this..... (3.00 / 0) (#8)
by TomW on Wed May 23, 2007 at 12:41:48 PM MST

Spar;

how does one measure Vpeak with a multi-meter when one does not know the waveform?

Well, one method may be to simply rectify the AC with a single diode, into a cap then measure the cap voltage and the diode drop voltage then add them together. Supposedly the cap should charge to peak voltage less the diode drop. Adding them together should get you pretty close to the peak voltage in the field with common equipment.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#9)
by jimovonz on Wed May 23, 2007 at 12:46:00 PM MST

You can always use a capacitor (and a diode if your measureing AC) to find the peak voltage (don't forget to account for the diode drop if you use one).

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#11)
by jimovonz on Wed May 23, 2007 at 01:03:30 PM MST

Ahh... dunno what happened there - the coggs are slipping. The diode is always necessary as without it the capacitor will just follow the voltage (less drop). Done it many times myself so put it down to pre-morning coffee fuzz.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#12)
by jimovonz on Wed May 23, 2007 at 01:26:20 PM MST

Flux, my intention was to inform folk as to what RMS measurements are and where pseudo rms measurements might fall short. Foremost in my mind were losses in the coils as this is something that I have spent time measuring and where the difference is great. I didn't state this but as the whole focus of my posting was on heating in a resistive load I thought it was not necessary. Unfortunately I have not been as successful at enlightening people as I had hoped. I think everyone appreciates your input where ever you see fit to offer it.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#13)
by Flux on Wed May 23, 2007 at 02:26:38 PM MST

Yes if it refers to resistive loads then the true rms value is what is needed.

In days of old all the dynamometer and moving iron meters measured true rms and nearly all conventional ac machines deal with waveforms not far removed from a sine wave so even the rectified moving coil meter calibrated to read rms on a sine wave was good enough.

Many modern digital meters often fail to give any idea of what they actually measure on ac, though in view of their extra cost, those that measure true rms tend to be more inclined to state the fact. It's probably fair to say that if it doesn't specifically state what it measures it is likely to be ac mean but scaled to read the equivalent rms figure on a true sine wave. Stating what it measures would be more useful than how many decimal points it is accurate to assuming the perfect sine wave. Waveform error can in extreme cases be up to 30% so what use is a spec saying 2% accuracy.

Once you include rectifiers and other non linear devices then the waveform error does become serious and if instead of using wattmeters you calculate power from I^2R or V^2/R then the effects become very significant.

As long as people are aware that the rms value is used appropriately then there should be no problem. For any waveform the rms gives the equivalent heating of the smooth dc value.

Flux

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#14)
by ghurd on Wed May 23, 2007 at 03:08:44 PM MST

You did fine.
Sometimes this stuff just gets confusing.

A new RMS meter is not worth the expense to me.
My RMS meter drowned. :-(
G-
Ghurd.info
[ Parent ]

Re: RMS Measurements (3.00 / 0) (#15)
by Lumberjack on Wed May 23, 2007 at 03:46:42 PM MST

The point of all this is to realise that the voltage reading on a meter may not be the actual peak output. If you get 28 volts and figure that components rated at 30 volts are good enough you might suffer a string of failures since the real voltage out is over 30. You are dealing with wild AC and not the tame stuff sent by the power company so you cant expect normal meters to be as accurate.

Re: RMS Measurements (3.00 / 0) (#16)
by Ungrounded Lightning Rod on Wed May 23, 2007 at 08:55:40 PM MST

In a battery charging application all that matters is current into the battery (if you're measuring charge) or the battery/load combination (if you're measuring effective generation).

Not RMS current, either: Average current. You're moving ions in and out of the plates, at two electrons per ion. That's IT for changes to state-of-charge. What you want is the number of ions you moved - which is the time integral of the current, i.e. the average of the product of current times the charge time at that current.

As for output to the load: The battery will clamp the voltage to a narrow range: That voltage is a function of its state-of-charge, temperature, and a voltage drop from its current charge/discharge rate times its internal resistance (with extra tweaks from its recent history as a result of ion concentration near the plates and gas bubbles on them). But the high and low ends of its range are within about 10% of each other. So average current is still the figure of interest - both for discharging the battery in the absence of wind and for stealing potential charging by consuming it as it's generated.

If you measured RMS current it would tell you a lot about how much power you were wasting as heat in the wiring resistance of the genny and the lines from it. But it wouldn't tell you squat about how much you charged your batteries (or drove your load at the battery-clamped voltage).

RMS is of interest if your load is a pure resistance, or close to it. This happens when a genny is driving a heating load directly, without any attached battery storage. It's of SOME use if you have a good max power point controller on a charging application - but in that case the average current of the output of the controller is still the number you want.

Clear?

Re: RMS Measurements (3.00 / 0) (#17)
by jimovonz on Wed May 23, 2007 at 09:16:49 PM MST

"If you measured RMS current it would tell you a lot about how much power you were wasting as heat in the wiring resistance of the genny and the lines from it. But it wouldn't tell you squat about how much you charged your batteries (or drove your load at the battery-clamped voltage)."

The point is if you measure the loss in the genny you get an idea of how well matched to you battery charging application it is and how much you could put into your battery if it were matched better. It also gives you an idea of how hard you are pushing your genny and what its limits might be. Efficiency is something that is mentioned a lot here on the board which is why I brought this up. I thought I was quite clear on RMS and its relationship to resistive loads.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#18)
by Ungrounded Lightning Rod on Thu May 24, 2007 at 01:36:40 AM MST

Efficiency is not a significant issue for mills, once it's adequate. The wind energy is free for the building of a machine to capture it. The Betz limit is as hard a limit on efficiency as the speed of light is on velocity. It's quite easy to get within a few percent of Betz with a high TSR HWAT, designed according to a few rules of thumb and built using hand tools and ruler/compass/protractor, rather than machine shops and micrometers.

The efficiency you want to measure is watts per dollar and per hour of effort. Available power goes with the square of the diameter. So you get far more watts for a given amount of resource input by making your mill the square root of a few percent larger than you do by trying to squeeze a few more percent out of a mill of a given size.

Efficiency is important in situations where the fuel is either limited (as in water power with a lmited flow) or costs you (as in a combustion engine), so inefficiency is a continual drain on your pocketbook. Wind power is not such a situation.

With wind power you don't maximize. You satisfice.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#19)
by jimovonz on Thu May 24, 2007 at 02:20:19 AM MST

"It's quite easy to get within a few percent of Betz with a high TSR HWAT, designed according to a few rules of thumb and built using hand tools and ruler/compass/protractor, rather than machine shops and micrometers."

So you've been holding out on us - do let us in on the secret :)

I don't disagree with a lot of what you say however where knowing the efficiency of your newly built turbine becomes useful if you want to make sure that you're not going to damage it by running it at a given output. Setting a reasonable furling point should involve some estimate of the level of energy dissapation your stator can withstand.

I admit the I have maybe become a bit preoccupied with gaining the last nth degree of performance out of a given turbine but in my case where I am looking at a mass manufactured commercial turbine that I am considering importing and selling, where improvements developed will apply to many identical turbines, it becomes more worthwhile.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#20)
by wdyasq on Thu May 24, 2007 at 04:58:10 AM MST

Be careful Jim,

You are applying logic and reason to folks pet projects. Those few percent add up. When they are multipliers and become smaller they get near critical.

Keep the good work up. Some of us need to understand what drinking the water downunder does to folks.

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: RMS Measurements (3.00 / 0) (#21)
by TomW on Thu May 24, 2007 at 07:29:31 AM MST

Ron;

So, it is the water then? Always wondered.

T

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#24)
by JW on Thu May 24, 2007 at 08:15:17 PM MST

Are all these replys some sort of a joke?

"~ SINE WAVE MEASUREMENT-

AC voltage is usually specified at a value equal to the DC voltage capable of doing the same work.

For a sine wave this value is 0.707 times the peak voltage.

Its called the RMS (ROOT-MEAN-SQUARE) voltage. The peak voltage (OR CURRENT) is 1.41 times the RMS value. Household line voltage is specified according to its RMS value. THEREFORE, a household voltage of 120-volts corresponds to a peak voltage of 120 x 1.41 or 169.2 volts.
~"

FORREST M. MIMS 3rd. 1983

JW

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#25)
by Ungrounded Lightning Rod on Thu May 24, 2007 at 09:25:42 PM MST

AC voltage is usually specified at a value equal to the DC voltage capable of doing the same work. ... Its called the RMS (ROOT-MEAN-SQUARE) voltage

And it's specified that way because, when feeding a resistive load, it produces equivalent results.

That's great for toasters, ovens, electric ranges, and light bulbs.

It's OK for motors designed for AC - though you have to correct for power factor. (RMS POWER - not RMS voltage times current - comes out OK.) It's a bit off for AC/DC motors but close enough for horseshoes. (RMS power versus shaft horsepower plus heat loass also comes out OK. But torque will be off between DC and AC power.)

And it goes all to hell for battery charging, power supplies for electronics, arc lamps, and anything else non-linear.

[ Parent ]

Re: RMS Measurements (3.00 / 0) (#26)
by jimovonz on Fri May 25, 2007 at 12:08:19 AM MST

Those of you who are jumping up and down about how RMS measurements don't apply to a wind turbine charging a battery may want to consider that on top of the fact that my post only ever related RMS measurements to resistive loads, I made the post under the general topic of 'Homebrewed Electricity' rather than the more specific topic of 'Wind'. General information that I believe to be correct for the benefit of those who didn't already know and might find application for it. The contribution was with good intention - I don't know that all others can say the same.

No good deed shall go unpunished .... (3.00 / 0) (#27)
by TomW on Fri May 25, 2007 at 05:50:57 AM MST

Jim;

Not to worry, I seriously doubt you could get a consensus here that black is the opposite of white, water is wet or the sky is blue.

It also seems that reading comprehension, at least on this board, is pretty poor.

Myself included, it sure seems we all read in to a post what we want to read.

I pretty much discount almost everything the talkers say and set at a premium what the doers say.

This board sure has gone to the talkers lately and that is too bad but inevitable when you allow everyone to post whatever they want. When the only qualifications you need to post are a computer with internet connection and a valid E-mail address you are bound to attract the know it alls, the trolls and those that simply like to stir the pot and argue for the sake of argument.

I am not much of a builder but use RE every day so I try to stay out of build discussions simply because it is not within my experience. I wish some would just shut the fsck up unless they have hands on experience at least in one aspect of a given discussion. But, no, we get the professor types and the engineers many with misconceptions they pass out like free beer at a BBQ. You engineers can calm down, I happen to hold 2 degrees in engineering but generally don't wave the sheepskin around as proof I know what I am talking about.

Sorry, it got rantish but it seems to fit here.

Bottom Line: If it puts amps into the batteries who cares about the finer points of the waveforms few of us can see anyway? But, thanks for trying to clear the air on what the term means.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#29)
by finnsawyer on Fri May 25, 2007 at 08:23:27 AM MST

Say, Tom, I've been wondering what happened to Commanda's fourth treatise on LED theory. I mention this because I have an orphan comment from a posting called, "Voltage Controlled LED Drivers" which was obviously withdrawn or erased. The poor fellow that posted it was "wrongly" skewered by people that I felt should know better. An LED is a passive device (unlike, for instance, a battery) the light output of which can be controlled by setting either the current or the voltage. The designer can choose the method that works best for him. Apparently, I missed the final fireworks.
GeoM
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#30)
by TomW on Fri May 25, 2007 at 08:35:29 AM MST

GeoM;

>
Say, Tom, I've been wondering what happened to Commanda's fourth treatise on LED theory.

Gee, I don't recall seeing the 4th one? If I get a few extra minutes I will see if I can sort it out.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#32)
by finnsawyer on Sat May 26, 2007 at 08:08:24 AM MST

Sorry if I misled you. I hadn't seen it either. I suppose a decision was made not to follow through on it. I just wondered what might have been the reason. Of course, Commanda can answer that.
GeoM
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#31)
by Lumberjack on Fri May 25, 2007 at 06:50:01 PM MST

so you have never seen a red sky? :} HeHeHe..,..

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#33)
by finnsawyer on Sat May 26, 2007 at 08:54:48 AM MST

I tend to have an uncomfortable feeling when Posts that I have commented on get deleted. As it is, you can not go and review the thread to see what it was about. There were some strident responses to the post by Engineers that I felt did not reflect the true nature of engineering. They were, in effect, wearing blinders and were not willing to consider all aspects of the situation. The poster was saying that one can control an LED by using voltage. The responders were jumping all over him and saying that you can only control with the current. Well, you can use either method depending on the application. I presented an analysis showing that voltage control can be effective. As it is, they are now producing flashlights having up to 24 LEDs. You may be sure that current control is not used in that case. Another factor to consider is that we have available very good voltage reference devices call Zener Diodes. As far I know there is no current equivalent or analog, so voltage control is generally easier to obtain in practice. I didn't say anything at the time because I was waiting for Commanda's fourth installment. I could have appended the orphan comment or one like it to that.
GeoM
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#34)
by JW on Mon May 28, 2007 at 12:33:39 PM MST

Ya, all

are too much, all kidding aside :)

If somebody manages to bypass the RMS feature using a scope, it is found that 120vac is actually 169.2vdc.

I quickly scanned thru this posting and noticed a wisper of, if people don't realize the correction factor of 1.41 x volts, when measuring this way, they may infact think they have inadvertantly discovered perpetual motion.(your results may vary)

The fact that reluctance and capacitence values seen to throw everyone off, well, puts a smirk on my face.

Remember to determine power its volts x amps. OR I squared x resistance equals power in watts period.

Last I checked, 1 horsepower is equal to 741 watts. So, power-factor affected by not only the magnetic-circuit, capacitance and inductance is capable of being corrected for, if your sneaky enough. The voltage waveform seems to give this away. Divisions of time may be corrected for using a scope.

Cheers all.

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#35)
by wdyasq on Mon May 28, 2007 at 10:36:13 PM MST

"1 horsepower = 745.699872 watts"

According to Google and many other references. Not to get picky ....

The facts are with a sine wave (or cosine wave) you will always get set values. BUT, the cobbled together things called wind generators here do not necessarily put out a 'sine' wave but throw a chunk of excited electrons. What percent of those waves is above 'cut in' voltage is critical to battery charging.

Few here have an Oscilloscope. Of those who do, not all will understand the wave form on the screen, what it means, or why it isn't as 'pretty' as the one in the textbook. Nor, does it matter if one is only charging a battery.

On the other hand, are probably a bunch of fingers. If those fingers are attached the the brain of a person trying to figure 'WTF' a windturbine is actually doing and maybe build an MPPT for it, things such as the waveform become a lot more interesting and critical to the development of that MPPT.

I wish Jim the best in his projects. If it is the water there ... I may need to try some.

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#36)
by TomW on Tue May 29, 2007 at 06:53:54 AM MST

Ron;

At risk of being seen as supporting or agreeing with you, Good points. Rant follows.

And part of the reason I think these engineer and formula types need to combat their personal cranial rectosis by getting their lilly white hands dirty, build one of these things, do some real world personal, hands on investigation rather than steep the board in a brew of confusing B.S. trying to push their own pet ideas or projects.

Folks seem to have completely and utterly missed the fact that all the potential below battery voltage [at that point in time] have about as much influence on battery charging as I do on US policy on oil. Now here is where the know it alls can tell you in great, and perhaps,convincing detail that it matters. It doesn't. Period. End Quote. Plus, once the turbine output exceeds battery voltage it is clamped to battery voltage plus line loss and rectification losses. That sure makes it look like the waveform has very little impact on direct battery charging. Which happens to be what damned near every user of this board is doing with it.

So, while the great salvation of MPPT is promised, until you have one in use, ignore these know it alls and save yourself some unnecessary headaches.

Funny they all carry on about perfect waveforms but nobody will fess up and tell you every 3 phase connection scheme has losses and canceling waveforms. I don't have any ideas for a solution other than what Jerry does but that is another kettle of fish entirely.

I don't do much building lately but I do have a couple oscopes, glasses and leads so i can watch on the scope as I connect batteries and see with great clarity that, once the battery is connected the top of that waveform is as flat as a pancake from rising edge to falling edge. In my mind it is a square wave for all intents and purposes anywhere above battery voltage. This is evidence from my own eyes and not some theory I read in college. Anyone with a 'scope can prove this fact easily.

And thats all I have to say about that.

This is where someone with 25 years worth of degrees explains I am wrong. Then I guess I do a video...

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#37)
by wdyasq on Tue May 29, 2007 at 08:15:16 AM MST

"Folks seem to have completely and utterly missed the fact that all the potential below battery voltage [at that point in time] have about as much influence on battery charging as I do on US policy on oil. Now here is where the know it alls can tell you in great, and perhaps,convincing detail that it matters. It doesn't. Period. End Quote. Plus, once the turbine output exceeds battery voltage it is clamped to battery voltage plus line loss and rectification losses. That sure makes it look like the waveform has very little impact on direct battery charging. Which happens to be what damned near every user of this board is doing with it."

WOW!- I didn't know you had that much influence on oil policies.

As I don't have that 25 years of degrees - I'll have to agree most here are interested in 'battery charging' and little else. I'll quote Flux here:"the true rms meter does not correct for failure to understand what you are measuring." Which apparently is what jimovonz is discursing.

It has been interesting to me how folks stick in stuff JUST to argue. It is much more entertaining to watch the theory guys complain about what the folks actually taking the measurements and doing things are observing. I still can't figure out how one can 'set a small trap' when they never get out of the laboratory.

You Folks be nice,

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#38)
by jimovonz on Tue May 29, 2007 at 01:04:14 PM MST

Note to self: Less posting of 'theory', more posting of practical results - funny, I had resolved to do this quite some time ago, but somehow this got through. Must be those degrees of mine wanting to stretch their legs. All good. Working on some 'real' stuff to post now (though I can't promise not to show a waveform or two!)

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#39)
by finnsawyer on Wed May 30, 2007 at 07:11:24 AM MST

You have something against laboratory work? Hm, I wonder how the students that I had would comment today about the usefulness of the EE labs they were required to take. In fact the laboratory is where the rubber hits the road so to speak for new ideas, and it is a damn good teaching tool to get the basics into those thick student skulls. Everybody building a PM wind power system is in a sense doing laboratory work. The problem is that they are very poor at reporting their results. Of course, that is also something that is part of the laboratory curriculum. You might do better simply by joining the crowd and attacking the "theorists".
GeoM
[ Parent ]

Define "lab" (3.00 / 0) (#41)
by TomW on Wed May 30, 2007 at 09:49:29 AM MST

Sorry, Fin, you are an educator but you cannot see the difference between "doing" and "talking"? No idea what you consider to be a "lab" but I would consider the DanB workshop to be a real world lab, therefore I have nothing against a lab. If, however you think it is in a room with nerds in white lab coats talking over a technical publication with no tools in sight then i say i dislike the setting.

Old saying:

Those that can, do. Those who can't, teach.

Sorry I cannot blindly follow you and your fellow educators down well worn paths of rigid conformity but you get fresh crops yearly so my personal opinions should not bother you so much. When YOU build your breakthrough design you have carefully assembled according to your theory, I will perhaps apologize. Until that day I will listen to those with grease on their hands and machines in the air, not those with pocket protectors full of pens, trifocal glasses and a garage with nothing but a Lexus in it.

Nothing personal just my personal opinion gleaned from a mere half century living on this rock amongst folks from both sides of that fence.

Mostly, this forum is for real people building real devices for real world use. There are probably hundreds of forums for academics and their theorizations where some unknowing soul will not unknowingly mistake "an idea" from a "working design"

It is very seldom that I see someone completely and utterly confused by talk of how real units are built. It nearly always happens when the theoretical crowd gets in a "who knows more crap that does not apply" contest.

I could dig out the old books, do some brushing up on things and probably join in baffling folks but that would be just as useless as the other theory talk to a builder with no degree in electronics. Funny thing is, you need very little of that knowledge to build a workable power producing machine.

Why confuse people? My [personal opinion] take on it is from experience with the type and it shows most like being "experts". They seldom actually do much so I tend to avoid them. I could go on, but it would be pointless as they often seem to have become legendary in their own minds.

Just try to consider the average guy here likely has limited knowledge on the bits you think are so important and let the builder types help them get a system that works, rather than trying to baffle them with unnecessary information.

Personally, I find the discussions interesting at times but with a fair bit of experience in both the avionics and commercial broadcast fields it would be odd if I did not understand at least most of it.

I am probably wasting my time typing this all up because I figure we both hold opinions the other will not agree with but what I do and you do not seem to do is realize that talking way over peoples head on things is counterproductive. Pushing the envelope is fine and got humans where we are technologically but I think you will see looking back through history is those guys had grease under their nails and garages full of machines that they built to prove some theory. Not simply 8,000 notes about it. But real hardware. That, sir, is why I pay attention when DanB, Ed Lenz and other builders post and yawn when you and the other theoretical talkers do.

My sole intent on this forum is to help keep it a viable resource for average folks to use to discover that creating electrical power is not some arcane art requiring vast and perfect knowledge of subjects that only marginally relate to that end.

Take that recent story from Cameroon. I bet few, if any, of those guys could define the type of waveform they were getting or describe how electrons move through conductors. But, guess what. They have electricity today but not last month. I will listen to that man if he speaks because he did it. Plain and simple. We have nearly let the talkers take over the world, I hope to avoid that here.

Here is where you can misinterpret what I said and take it personally, which it was not meant to be.

Try seeing the point from the average readers angle and maybe it will make sense.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: Define "lab" (3.00 / 0) (#42)
by jimjjnn on Wed May 30, 2007 at 04:43:13 PM MST

Tom W,

Thank you.
Denver,CO
[ Parent ]

Re: Define "lab" (3.00 / 0) (#45)
by finnsawyer on Thu May 31, 2007 at 09:01:28 AM MST

I wasn't intending to argue with you, as I've been aware of your attitude for some time. I certainly get my hands dirty. Yesterday I had Balsam pitch on them. Well, more on the gloves really. In about a month I'll be living in grease trying to keep the old haying equipment going.

I used to be an educator. Today my efforts are more directed to trying to broaden people's minds here (remember that LED - voltage control versus current control issue - now there was a tempest in a teapot). And confusion. A lot of people pop up totally confused about how PM alternators work. Some waste their money and effort trying something that doesn't work. A few admit their mistake and come here for help. Some of these confused people are very strident in advancing their confusion and make statements that might have a grain of truth, but when taken in their entirety are plain wrong. Do you want such misconceptions to spread through the community? You certainly could suppress or eliminate users of my ilk, but will you be doing the community a favor? Maybe you figure Flux should be the only one to set everybody straight. I doubt he would have the time.

You speak of "my" conformity, but when I advance new ideas about wind power you, like everybody else say, "prove it". You want the conformity. When someone says he wants to experiment I will call my ideas to his attention. I do realize most people really don't mean they want to experiment. They come to the table with certain preconceived ideas that usually have some type of flaw. But, my R(ead)
P(osters) M(ind) instruction doesn't work. You should be quite well aware how poorly people do in communicating exactly what they are doing or really want to do.

Apparently you have a low opinion of the labs you may have been required to take in college. What you would have seen was a snapshot taken at a particular time. Labs like curricula, in general, evolve over time. Some do deal with the basics. We had a lab in EE where the students determined the inductances and mutual inductances for transformers. They also plotted the B versus H curve for iron, and determined hysteresis effects when the iron was subjected to a time varying H field. Basically it was a working appreciation of Faraday's Law. How many people using this site had such an experience, do you think?

In closing, I'd like to say that if you want everybody to do the same thing in building wind power systems you should rename the site to "The Cookie Cutter Windpower Site". Make it clear what you are about. No controversy tolerated.
GeoM
[ Parent ]

Re: Define "lab" (3.00 / 0) (#48)
by Lumberjack on Thu May 31, 2007 at 08:27:34 PM MST

Fin, The problem is your attitude....

[ Parent ]

Re: Define "lab" (3.00 / 0) (#49)
by finnsawyer on Fri Jun 01, 2007 at 08:12:02 AM MST

My attitude, huh? I'd say its more your attitude. We like to help people, but when they continually spout misinformation... I've been looking at your diary. Pretty confusing. You don't say clearly what you are doing. Are your magnets rectangular or wedge shaped? Is there a space between the magnets or are they going to be touching? Be aware that if the magnets touch you will have severe leakage problems of the flux or flux density, which wants to take the most direct path from a north pole to the nearest south pole. The upshot of this is that the flux bends in passing from one pole to the next, and rapidly loses strength with distance above the magnets. The result of that is that the windings farther from the face of the magnet will produce less voltage until none is produced at all. So, you need to use a thin stator for the most effective use of the copper. If you do your test with a thin stator and then go to a thicker one in the actual build, you might be disappointed. The solution is to space the magnets to reduce the leakage flux. Of course, this reduces the amount of copper actually producing voltage without affecting the copper overhead, which is given for a single weave by one half the outer circumference plus one half the inner circumference as determined by the magnet placement. You will reduce the total amount of copper, however, as you will have reduced the number of radial legs. This gives a reduction in voltage as well as a reduction in resistance.

Since I am not supposed to spout theory and I don't want to get into another long winded pointless argument with you, I will suggest if you wish to study the theory behind my statements that you research the Hall Effect.
GeoM
[ Parent ]

Re: Define "lab" (3.00 / 0) (#50)
by TomW on Fri Jun 01, 2007 at 09:17:36 AM MST

fin;

This response to lumberjack reinforces the validity and accuracy of his attitude comment. You just don't get it, I guess. look up "arrogance" in a Websters once.

T

[ Parent ]

Re: Define "lab" (3.00 / 0) (#51)
by finnsawyer on Sat Jun 02, 2007 at 07:37:04 AM MST

Tom, I think you are missing something. Based on lumberjack's comments, I believe his attitude was, "I've got a better idea, and I'm going to show those rubes how to do it right." Of course, his design has to deal with the same limitations as any design. And that's all I was trying to convey, but as you say, "No good deed goes unpunished". I didn't put my comment in his Diary out of kindness, I suppose.

"You can lead a person to knowledge, but you can not make him think."
GeoM
[ Parent ]

Re: Define "lab" (3.00 / 0) (#52)
by finnsawyer on Sat Jun 02, 2007 at 08:58:12 AM MST

It being a slow day I thought I'd elaborate on this. Personally, I would just as soon drop the personality issues. So, beyond that, what did you find offensive about my response? Was it the reference to the Hall Effect. I could just as well have stated that the electric field induced in a wire moving through a magnetic field is given by E = V'cross'B. Or |E| = |V|x|B|x cos(alpha), where |V| is the magnitude of the wire's velocity, |B| is the magnitude of the magnetic flux density and alpha is the angle between the direction of the velocity and the flux density. E is a vector and is a point quantity. You get the voltage by doing a line integral of E over the length of wire. Lumberjack plans to use 120 1/2 by 4 inch magnets. The magnets will be essentially touching. The closeness of a north pole to a south pole means that the flux will tend to flow directly between adjacent magnets rather than making the double trip through the stator and the backings. In doing so the flux must flow part of the way parallel to the magnet faces, at which point cos(alpha) becomes zero. This will reduce the voltage induced in each turn of a coil or weave with the reduction becoming more severe for those turns farthest from the magnets. By increasing the spacing between the magnets one would actually get more voltage, since at some point most of the flux will make the double trip through the stator. This would increase the size of the rotor, which would already be in excess of twenty inches for the magnets touching. On the other hand maximizing the flux flowing through the stator and keeping alpha near ninety degrees may allow fewer magnets, less copper, and a smaller rotor.

All one can say from your emphasis on hands on building is that someone built an alternator that does such and such. An understanding of and some application of theory may allow one to either improve on what was done or produce a better result in the first place.
GeoM
[ Parent ]

Re: Define "lab" (3.00 / 0) (#53)
by finnsawyer on Sun Jun 03, 2007 at 08:16:15 AM MST

I see a correction is in order. For the angle alpha as defined, the correct expression for the magnitude of the electric field is: |E| = |V|x|B|xSine(alpha).
When the velocity of the wire and the magnetic flux density are in the same direction, the induced electric field (and its contribution to the induced voltage) is zero. Sorry for any confusion.
GeoM
[ Parent ]

Re: Define "lab" (3.00 / 0) (#54)
by Lumberjack on Sun Jun 03, 2007 at 09:08:20 AM MST

First you gripe about the cookie cutter approach some take and claim the board discourages alternate approaches. Then you turn around and bash me for using a different approach...

Now isn't that special....

[ Parent ]

Re: Define "lab" (3.00 / 0) (#55)
by finnsawyer on Sun Jun 03, 2007 at 09:55:58 AM MST

I'm not bashing you. I was trying to voice some problems that I perceive and maybe save you some grief. I didn't put these comments in your diary because of the acrimony. The first question you should ask is whether you are really doing anything new. Secondly, how is this better than just doing a large conventional three phase design with coils, as most VAWT people do? Based on the numbers you gave, it looks like you will need about a 28 inch diameter rotor anyway. You should be aware that scaling a conventional design up from say 16 inches to 32 inches with the same magnet strength (but four times the area) will give you eight times the voltage per RPM with the same number of turns per coil. Note that the wire will also be twice as thick. You should be getting the flavor here that one has available various trade offs.

As far as with what you are trying to do, it is not clear, at least to me, that the tests you will do with the small unit will scale up nicely to the full size one. I also don't think that making saving copper the primary design criteria is the way to go. You really want to establish the desired cut-in voltage and match the power put out by the VAWT. A lttle more copper to do so shouldn't matter. Any design has some copper "overhead". Have you researched to see if others have done what you propose? A search could save you some grief.

You could also use your test unit to check my statements. Put a one wire thick "ribbon" for one weave on a thin piece (1/8 inch thick?) of plywood. Place that at various heights above the magnets (keep the iron backing for the proposed stator at its intended height) and measure the voltage produced. This may still not scale exactly to the final design, but it would show you where the region of diminishing returns develops or alternatively whether I don't know what I'm talking about (a little incentive for you there). You may need an oscilloscope to make the measurements, though.

By the way, I did like your "dreamcatcher" design. It has a certain style.
GeoM
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#40)
by finnsawyer on Wed May 30, 2007 at 07:18:31 AM MST

"Remember to determine power its volts x amps. OR I squared x resistance equals power in watts period."

While it is true that the instantaneous power is given by the voltage times the current at that point in time, it is not true that one can get the average power by taking the average voltage times the average current when reactive components such as capacitors are in the circuit. You introduce power factor but do not consider the effect when the voltage and current are out of time phase. Be careful. Nor can you rely on RMS values for voltage and current in general.
GeoM
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#43)
by JW on Wed May 30, 2007 at 07:43:40 PM MST

In 'my mind';

I was actually thinking, I ok weve got a 3kw mill producing 3kw from the alt, but out of the rectifier there's like 2.5kw dc. So its safe to say, that an mill approaching, producing, 3hp power in watts, at the alt, can never deliver 2.5 hp[or more] in watts/hp at the dc connection from the rectifier-blok, due to rectifier losses in terms of btu's.

bigdeal...

JW

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#44)
by finnsawyer on Thu May 31, 2007 at 07:54:03 AM MST

You write like you are on a controlled substance. Anyway, when the wind increases you seem to think that the power going into the battery will remain at 2.5kw or whatever you said. It will not. Most people will tell you it will increase. The rectifiers don't control the amount of power to the battery. They do, however, dissipate power and as such you must not exceed their limit. A similar statement applies to the coils. If you would read some of the postings here you will learn about black coils (and blue smoke).
GeoM
[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#46)
by JW on Thu May 31, 2007 at 07:44:21 PM MST

"You write like you are on a controlled substance."

SO...

uknow, I was thinking about what I had said lastnight, a 'liner' measurement equating watt/hp. I was hopeing someone whould have atleast have the common courtesy to correct my math [thanks wasdaq by the way, seriously, with regards to the 741# I referenced from a book several years old].

I figure 3000watts equals just under 4hp. So lets say the blades are converting rotational energies to 6hp. Its wise that you metioned losses simular to rectifier losses at the coils. So the blades take a 'net' 6 hp/watts and convert that to rotary motion, inwhich the coil arrangement losses 2 of those hp/watts thru sheer heat(and other inefficiencies). Then puts the remaining 4 hp/watts into the rectifier at 3kw. So if one gets 2.5kw dc output from the rectifiers at (granted peak output)(3.3hp/watts) all the margins of loss are well defined by comparing hp/watts at the mill under peak conditions to the most that can be delivered out of the rectifier. Good point on average wind conditions over time. Its relativly easy to see how battery load capability can be out of phase with wind conditions. Such as why youd want a dump load. Its been determined the best way to deal with this enevitable off-set(if you must have perfect conditions) is with a diesel gen-set. Actually I recently bought a 6kw segma unit. Good deal too, at less than a thousand bucks.

Anyhow Fin,

The controlled subsance is AE/RE, sorry if I get taken away by this totalLy exellent forum and its members. Last TomW checked, there is no intervention system in place to deal with this, so I guess your stuck here, with me. :) Besides you deserve the Mr. Mom candecy for 2007/8.

220, 221 whatever it takes...

Cheers

JW

[ Parent ]

Re: No good deed shall go unpunished .... (3.00 / 0) (#47)
by Lumberjack on Thu May 31, 2007 at 08:17:24 PM MST

woah dude, I see you scored some righteous....

[ Parent ]

RMS Measurements | 55 comments (55 topical)

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