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Ohm's Law?


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Sorry about the power factor letter folks, it was only intended as a flippant throwaway. I, too, hadn't read the letter from Kevin Annells at that point but now I've had a ganders it does seem a trifle misleading.

If it is related to some sort of constant-power application it's a bit difficult to see what it is, particularly when applied to model aeroplanes, and as far as I'm concerned, unnecessary when I'm sorting out power requirements . If not, then it would certainly appear to stand Ohm's Law on it's head.

Would it not be possible to invite Kevin to expand his original letter, and answer the comments, perhaps on the forum even. Then we might be able to see where he is coming from. Or not.

Pete.

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Peter R and gentlemen - readers letters are printed with just corrections for spelling and punctuation. Printing a letter amended/re-written to correct what we believe to be factual errors would bring us an empty postbag.

As the footer says, opinions expressed are not ours, it's the chance for our readers to say what they think and believe. Letters stimulate debate, bring help and assistance where required and er....make for colourful reading. Published articles are a different matter of course.

This letter has certainly sparked a debate and the biggest response to any letter since I started here. Along with all the forum comments I've a growing pile of paper letters here on my desk too.
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peter rieden wrote (see)
Finally, his (David's, rvs) sign-off statement on why larger aircraft use higher voltages rather than higher currents is true, but the reasons for it are simply that it's more efficent. It's more efficient because using higher currents burns energy against the resistance of the cabling and motor windings, whereas using higher voltages does not, so it is more efficient to increase the voltage rather than the current to obtain more power.

PDR

From the motors point of view, system voltage does not matter, assuming we use motors with the same copper fill factor (preferably maximal of course).

Say we want to go up a factor two in voltage. To keep the same rpm (and powerconsumption), Kv has to go down by a factor 2 . In order to get that lower Kv, the number of winds must increase by factor two -> wire resistance increases by factor 2. To make rome for the extra winds, the cross-sectional area of the wires must be reduced by a factor 2 too -> wire resistance again increases by factor 2. All in all, resistance now has increased by factor four. Current is half the original current. And since P_loss = I²R, nothing has changed.

On the other hand, losses in the battery-leads are reduced by factor four. And asssuming we use batteries with half the original capacity to keep the amount of available battery energy the same, battery losses are halved because battery resistance doubles and battery current halves.

Vriendelijke groeten Ron

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Wow!  How this thread grows, may be due to the lousy weather - again!

I think Peter Rieden is right in his suggestion that technical issues in letters should be vetted.  It is in no ones interest to publish anything which is wrong or mis-leading.  There is a difference between an opinion with which I would reserve the editor's right to disagree but yet print and an incorrect technical statement.

Peter

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The problem with Gorgonzola wing ribs is getting the cyano to stick to them.....

Peter R just to pick you up on your point that "......Our motors are all DC motors, not AC motors...." I understood that a brushless motor WAS in fact an AC motor & that the job of the ESC was to suppy 3 phase AC from a DC supply in the form of a quasi-sine wave (ie chopped up DC). Admittedly the motors we use differ from full size induction motors in that they have permenant magnets rather than electro-magnets but other than that its an 3-phase AC motor.......

isn't it?

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Steve,

The trick with the cheesy ribs is to leave the rind on. (Does Gorgonzola have rind?) Then use a hot-glue gun with cheese sticks. When I use cheese the mice come in and eat the middle out, thus adding lightness. Very convenient.

I have an old HEME AC/DC clamp meter, a relic from work, a long time ago. Apart from reading amps, it also reads volts, hertz, VAs, watts and gives you the phase angle. And you can display it on a ‘scope.

If I ever get sufficiently motivated, I might just have a play around with this. I might get, in my case anyway, some sort of highly misleading clue as to what is actually going on. I have thought about this before, in relation to other slight doubts that I’ve had but as I don’t want to start any more frenetic debates I’ll say no more.

But that is a big if!  I’ve only just bought this laptop and sometimes I have great difficulty in getting to grips with this, let alone a USB oscilloscope!

Pete.
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Interesting.....so instead of the current switching between windings by means of a multi-segmented commutator as in a brushed DC motor the current is switched between windings electronically by the ESC to produce a field that effectively "drags" the permenant magnets around with it yes? & to vary the speed we vary the voltage by chopping the DC.....

As you can tell I'm at the limit of my knowledge here....a grade 5 CSE in Physics from Our Lady of the Veil Convent School only goes so far you know......the nuns always were a bit hazy on power factors, KVA calculations & Lenzs Law...... 

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Peter Rieden,

Thank you for your elegant explanations. Largely over my head I’m afraid but interesting none the less.

I, too, have a question.

I’ve always vaguely known that these motors use chopped DC (I’ve always called it pulsed) to create a revolving magnetic field which carries the rotor around with it. Also that the controller senses the transient emf’s to work out the position of the field in relation to the rotor. But I’ve also always assumed that the speed is controlled by the width of these pulses, i.e. the amplitude stays constant but the duration of the pulse varies. If, as you say, the voltage controls the speed does the amplitude vary and the width remain constant?

I’m intrigued!  

Pete.  
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Hmmm...how to explain this simply!

The speed controller will time the phase-switching to achieve a specific amount of "advance". In an "ideal" electric motor maximum torque occurs when the coils create a field which is at 90 degrees to the field of the permanent magnets. But in a "real" motor the optimum *efficiency* occurs at a lower value - usually around 30 degrees for motors with a lot of poles (six or more) and even less for motors with fewer poles. So the speed controller simply looks at the last few back-EMF pulses and uses that to time the next phase-switching for this sort of advance angle. So the timing of the switching is simply a function of the RPM, regardless of throttle setting - if the motor changes speed due to load or throttle changes the speed controller adjusts the frequency of the phase switchings accordingly. That is where it differs from a synchronous motor system, where the frequency is fixed and the motor draws as much current as it needs to remain at that speed.

Now our "ideal" motor will run at a speed which is determined by the applied voltage. This is what the "motor constant" (Kv) means - put 10 volts across an ideal 1000rpm/volt motor and it will turn at 10,000 rpm whether it has no load or a 30-foot diamter rotor on the shaft (although the current it draws to do so will differ a bit, obviously). Real motors do slow down a bit with more load due to magnetic and resistive losses, but the same basic principle applies. So to throttle them we vary the applied voltage. Ideally we'd like to do this directly, but this makes for inefficient speed controllers and very hot MOSFETs, so what we do is pulse the voltage on and off so that the "average" voltage is lower (before Ron leaps in it's actually the "RMS" voltage, but let's keep it simple). One could think of it as the inductance of the windings which smoothes out this choppiness so the motor "sees" the lower voltage for RPM purposes - this isn't what's actually happening but the effect is the same. So by varying the ratio of "on" and "off" in this chopping we effectively vary the applied voltage.

Does that help?

 PDR

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peter rieden wrote (see)
Now the question I've been having trouble is concerns the actual speed this motor would do [at 50Hz]. I'm not sure I can answer this with confidence - it's either 1/f or 1/2f where "f" is the applied frequency,

DOH! That'll teach me to dash these things off in a hurry! That should obviously have read:

"it's either 1/fn or 1/2fn where "f" is the applied frequency and "n" is the number of poles"

Apols,

 PDR

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Peter,

Many thanks for that.   I will give it some thought.  Just not quite sure when.

Seriously though, I think the speed controller must be quite a hard working little piece of kit, and clever too, at least when it was first designed.

I think that what you are saying in there somewhere is that my theory about pulse width is sort of correct. The term ratio ‘on’ to ‘off’ sounds familiar.

Perhaps not entirely convinced about the RMS values. As I remember, the RMS value is the effective DC equivalent value of a sine wave, can’t remember other wave forms. Although of course, if we are getting the equivalent DC power out it must be the same, sorry I spoke.  It just might not be 0.707Vmax though?

I will (and read that as a may!) try and find some reading matter on the subject.

Again, thanks.

Pete.
 
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'Our' brushed and brushless motors are/operate iso-synchronous.

Animation & simulations, in- or outrunner makes no difference:
http://www.rcgroups.com/forums/showthread.php?t=216928

How does a brushless motor work:
http://www.rcuniverse.com/forum/m_1558046/tm.htm

Workings of controllers for brushless motors:
http://www.torcman.de/peterslrk/index_eng.html
-> SPEEDY-BL self made brushless controller

Here you can see how motor inductance smoothes out motorcurrent (the higher inductance, the more a coil resists changes in current):
http://www.consult-g2.com/course.html
-> CHAPTER 9: ELECTRONIC CONTROLLER


http://www.torcman.de/peterslrk/Speedy-BL/Teillast_Motor.gif

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I have just found this thread, after an absence due to holidays, and also have just spotted the readers letter which was the subject of the OP. It seems fairly obvious to me that the writer was referring to a small piece I had submitted following up on a comment made by the editor Graham Ashby, where he inferred that by "simply switching to 6V batteries for his radio " he obtained complete piece of mind about the radio supply ". I do not, and never have, claimed to be an electrical engineer - as the writer of said letter apparantley is - and would not dare to enter the arena being built here by the likes of Ron and PDR who are very technically adept and qualified. My piece referred somewaht simplistically to Ohm's law, and not "power law"  and the summary was that given an indentical load, a circuit driven by 6V would generate more power than the same circuit driven by 4.8V due to both a higher current and voltage - I was not suggesting that the power would remain constant as the writer seemed to conclude.The context was to merely illustrate that current WILL increase with a higher voltage across the load, and therefore, in most cases a system driven by 6V will inevitably consume more current, and if the 2 x batteries were comprised of cells of identical capacity then the higher voltage battery would likely drain a little quicker than the lower voltage battery. Sure there are other factors involved, the load is not a simple fixed resistance, and many of the components involved are not ohmic in nature etc. The faster reaction times of the servos driven by the 6V battery may "compensate" somewhat for the increased current comsumed, by being operated for a slightly shorter period, and so on. I beleive my words ( which were fully discussed in private conversation with Graham Ashby prior to publication ) are generally correct, and most modellers would, I think, concur that generally speaking 6V packs will give slightly less duration than a 4 cell pack. As for whether the magazine choose to publish a follow up to this topic, then that is a matter for the editor, and should be decided on how much confusion and mis-information may be derived from the letter.   

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