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Understanding value of resistance and inductance measures


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#1 Pontes1

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Posted 09 March 2017 - 12:42 PM

Hey,

 

I need to understand better this two measures in order to choose the better armature.

In our track we do have a GoFast unit that give us theese measures but we have doubts on the following:

 

As one example only:

 

Taking two JK Hawk 6 armatures out and try to see which one is the better one:

 

Armature 1:  Resistance:    .503   .495  .490 

                     Inductance:   199    198   199 

 

Armature 2:  Resistance:    .507   .508  .508

                     Inductance:   197    198    202

 

This means the first armature has less resistance but all three poles are different... 

Second armature has all three poles equal in resistance - higher OK but more equal.

 

What should in fact in theory be the best one???

 

If anyone can help, thanks. 

 

Regards,


Fernando Pontes




#2 Phil Hackett

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Posted 09 March 2017 - 03:03 PM

Just from the figures: #1 but the numbers are very close and even though you can measure the difference it's how the armature works *on the track* that matters.

 

Metering is part of a testing program that would involve many armatures being tested and then run to see if the numbers do matter and, if so, how much and then why. Then you could actually have a highly reliable system for picking arms.

 

BTW... back when I was running G27 locally I could pick out the fast arms because there was a *wide* difference between the resistance for the fast/OK armatures. Having a 12-16% difference in resistance between armatures *of the same manufacturer and part number* made a really big difference on the track. Measurable and repeatable.

 

The numbers you're showing above are almost identical (±3%) and could be just normal manufacturing tolerances.


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#3 Cheater

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Posted 09 March 2017 - 03:15 PM

Really getting out of my depth here, but the armature is just one part of the overall motor system or combination.

 

When you ask "which one is better," you've really not asked a complete question. Better in what set-up (can and magnet combo; magnets vary a lot as well)? On what power? On what track with what wiring arrangement? In what car at what weight? With what gearing? Etc., etc., etc.

 

The armature and its resistance and inductance numbers are really just a small part of a much more complex problem and my experience is that concentrating on one or a small number of aspects of a very complex situation where numerous other variables are in play is probably a not very useful approach.

 

Fortunately, we have one tool that can assess all the variables involved quickly and easily. In old school terms, it's called a stopwatch.


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#4 gotboostedvr6

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Posted 09 March 2017 - 03:16 PM

Agreed. Not enough difference between the two to nitpick.

They would require testing in several different set-ups with several different sets of magnets to find the combo they work best in.


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#5 gotboostedvr6

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Posted 09 March 2017 - 03:18 PM

Inductance wears more than one hat. Literally, a book could be written on the subject as it pertains to slot drag motors and one could bore and confuse you with a rather long list of formulas that wouldn't mean much to you after you were done with it and be of little practical use. Not that it isn't important, just that there are so many factors involved outside our control and/or knowledge or ability to measure them that even if one wrote them all out we wouldn't have a functional useable answer or formula due to missing bits of information that are privy only to the manufacture, if indeed they even know. One could tell you lower is better and a lot of heads would nod in agreement.

In broad strokes, one hat inductance wears is in its effect on the production of counter electromotive force, BEMF. An other, inductance is also the measure of the coil's resistance to current change, Delta I. Lastly, it is also the measure of the coils time to saturation or full charge, also known as the electrical time constant, Tc. Each is separate yet connected, interwoven if you will. There are broad generalizations that can and have been made as to how inductance is affected by coil geometry and on the properties of the pole that coil is wound on. Exactly what result it will have on final measured power and RPM though depends on the interplay of all the variables. But the answer you're after is lower is better.

For a given armature the winding pattern and neatness of the wind will offer both lower resistance and lower inductance. Small variations in either, from coil to coil, will have little practical effect as no one coil acts independently of the other two. Close is good though as it speaks to the attention the builder paid to the details, a quality thing. Among armatures from different manufacturers and assuming the same care has been taken in the winding there again will be small differences in both properties due to the geometry of the web and crown and the material of construction of the laminations, exact stack length, and armature OD. It is this that separates one armature and its manufacture from the others of like windings.

 

Because the manufacturers do not normally give out that information, trade secrets, there isn't a way to calculate it beforehand. This is where your skills testing and tuning come into play. Even having said that, the exact combination you're working with and the goals of your program and intended service will have yet further to say about the exact choices you make. Inductance is but one part of the successful program and as in all things slot car it's the combination and no one parameter as each is a trade-off for some other quality. The end of it then is, as Monty noted, as there is nothing you can do about it, you note it, check for shorts, and test it. Make your notes and compare as your library aquires enough information to make an informed choice of go with it. There really isn't a shortcut.

As to your magnet questions in the framework of your query, drag motors. One needs to look at the magnet and armature separately and then together. Each winding will draw a certain amount of current and have an exact number of windings. Multiplied the product is called the amp/turns of the armature and is analogous to armature flux. In fact if one knew the hidden values we can't obtain, it can be calculated directly to flux. Measurement of gauss is not a measurement of flux and in reality it is the flux that creates the torque, the gap flux to be exact. The flux of the armature repelling the flux of the magnet in the air gap gives the torque force. The stronger this repulsion, the quicker the mechanical time constant of the motor and the quicker the car accelerates. Ergo, weak magnets are not helpful in this regard. The mechanical time constant is the length of time it takes the motor to reach 63.2% of its no-load speed. AKA, spool up rate.

Gap is often misused and misunderstood. From the magnet's contribution to gap flux it isn't how close the armature is to the magnet face but how close the magnets are to each other. Then the armature to the magnet face controls the armatures contribution. Decreases in either provide more gap flux up to a point.

If the armature to magnet gap is too small, much of the power of the motor will be spent shearing the air, which increases frictional drag. If the magnets have enough total energy, placement too close together may saturate the steel. Not normally an issue with ceramic motors unless the armature core material is prone to flux saturation. Again it comes to experimentation, testing, and tuning. Saturation (magnetic) A useful link to understanding saturation.

Timing is often thought of as a means to improve RPM and motor efficiency. In a wound stator motor this would be true in part. An electromagnet's field will distort when under the influence of an opposing field, the armature and to the degree of the strength of that field, amp/turns. Permanent magnets do not exhibit the same degree of distortion, near nil. The purpose of brush timing advancement in wound fields is to realign the fields so that they address each other at right angles providing the most torque for the implied load at a specific RPM.

 

This function is a bit different in a PMDC motor under transitional operation such as a drag motor. As the armature field strength is in a constant state of change and in fact declines, the fields can then only be aligned at a single point of load and RPM. In this case timing is, in part, used to enhance one particular area of the RPM range. Even that depends on the orientation of the magnet and/or degree of segmentation. Lower numbers enhance the lower RPM and the more timing you dial in the higher up the RPM range the enhancement is effective. It's almost an aside that increased timing also increases the motor's RPM limit by the effect of slip angle which for all practical purposes is akin to field weakening and delays the onset of BEMF. Because the RPM increase is not accompanied with an increase in applied voltage, increased peak efficiency is not guaranteed but may provide an increase in average increases over the useful RPM range and assures one at the point of perfect alignment. When the field misalignment approaches 135 degrees included angle or 45 degrees advancement, no further RPM increase is possible without a marked sacrifice in power.

 

A drag motor does not operate at peak efficiency but over the entire range from zero to about 90 to 95% of its no-load speed. Having said that, peak efficiency is but a bragging right. It is the RMS power of the motor that sends the car down the track, not the peak nor even the simple mathematical average.


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#6 havlicek

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Posted 09 March 2017 - 03:45 PM

I would guess that armature #2 *might* be slightly better than armature #1, but testing an armature outside of a motor (*other than when winding them) is more likely than not pointless. Even testing a motor outside of a car on a track can be really misleading, because how a motor performs with a load is the most important information, and even then, the gear ratio, weight of the car, and even the drag of the body will affect the "loading" of the motor.  

 

In any case, it seems clear to me that the best "predictor" of whether or not a motor will run well is consistency from pole to pole. The readings above aren't at all "bad" for thin gauge machine wound armatures. The way the wire lands on these arms is pretty haphazard, so it's not a surprise that there is some variation. I think they're surprisingly good.  
 
Then, too, none of this takes into account one of the most important aspects... commutation. Instead of getting lost in all kinds of theoretical physics that even a lot of physicists don't fully understand (let alone us mere mortals)

 

Greg's got the best and proven way to find a good motor:

 

Fortunately, we have one tool that can assess all the variables involved quickly and easily. In old school terms, it's called a stopwatch.

 
-john


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#7 zipper

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Posted 09 March 2017 - 03:59 PM

BTW... back when I was running G27 locally I could pick out the fast arms because there was a *wide* difference between the resistance for the fast/OK armatures. Having a 12-16% difference in resistance between armatures *of the same manufacturer and part number* made a really big difference on the track. Measurable and repeatable.


Yep, 10-15 years ago there were bad 27 arms, even one of three would break the wire, once for me when running in by a couple of volts and just some minutes! Probably the winding tension was far too tight causing narrow spots on the pole and giving uneven results when measuring. Mostly the wire was breaking near the comm tab but sometimes elsewhere where it couldn't be seen.

Luckily it's over - but a little more epoxy might be better as the latest problem is tags flying off.


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#8 Pontes1

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Posted 09 March 2017 - 06:22 PM

OK understood, what you said makes sense, however we need to give a meaning to this litle testing machine...
today´s new REAL measure on two Hawk 3031 armatures brand new:
 
Arm 1:  resistance: .453  .461  .461
        inductance:  .215  .221  .217
 
Arm 2:  resistance:  .464  .461  .457
        inductance:  .215  .221  .217
 
So here a real situation... very difficult to choose very identical, my guess is Arm 1 is slightly better, OK we need track test in same setup and same car.
 
Now there is a question, seen that the inductance in the two motors are the same exactly!!! Why do we need to see the inductance? Maybe in this litle armatures inductance is probably equal in all of them and only by resistance we can see any differences???

Thanks for the inputs.
Fernando Pontes

#9 MSwiss

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Posted 09 March 2017 - 07:00 PM

Run the motor on the track.

Guessing by reading specs... is guessing.
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#10 havlicek

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Posted 09 March 2017 - 07:21 PM

Inductance numbers are probably more important in the hotter open-type winds, but even there... putting it on the track (as Mike Swiss said) is always going to be the only test that matters. I still say that consistency is more important than absolute meter measurements, but that's only when you are winding and trying to check that nothing went wrong... and even there, it's only really important after commutator connections have been made. "Off" meter readings can tell you it's time to go back and "hit" the comm connections again to try and get the readings to snap into place.

This all leads me back to something I was told when I was a kid and trying to wind arms... "patterns are king." Even if you make a mess on the first pole, make the same mess on the second and third ones!   :D


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#11 MikeC

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Posted 10 March 2017 - 12:39 AM

For me, the value of testing using one of those meters is more about weeding out "bad" arms, more so than finding the best between very similar measuring arms. That said, I will normally lean towards arms that have similar readings from pole to pole, provided they are within a similar range. If an arm has consistent, but high readings, it's probably not great. If it has low readings, but they aren't consistent, it may not be ideal. 

 

As Mike said, the track is the only true way to tell, but the meters can, at least for me, help weed out the bad ones, so that I focus on a smaller pool of arms when building new motors.

 

Also, measuring the arms gives me a baseline of what seems to work. Usually, when I find an arm that works well in a particular set-up, if I can find one that's similar in readings, it usually performs similarly to the first one, in the same set-up...


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#12 LindsayB

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Posted 10 March 2017 - 03:41 AM

Reluctance will be impacted by metal in the armature - laminations - whereas resistance should not care. On the track, with all things equal - e.g. bearings, etc., and alignments - the one that draws the most power (amps) should be the faster. 


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#13 Benno - SAC

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Posted 10 March 2017 - 06:42 PM

Arm 1:  resistance: .453  .461  .461
        inductance:  .215  .221  .217
 
Arm 2:  resistance:  .464  .461  .457
        inductance:  .215  .221  .217
 
So here a real situation... very difficult to choose very identical, my guess is Arm 1 is slightly better, OK we need track test in same setup and same car.
 
Now there is a question, seen that the inductance in the two motors are the same exactly!!! Why do we need to see the inductance? Maybe in this litle armatures inductance is probably equal in all of them and only by resistance we can see any differences???

 
Hi,
 
I'm an electrical engineer, so I know what I'm talkling about, but it is difficult to explain. Especially in English. what is not my native language. So I try to do my best and use examples, though they are not 100% precise.
 
The inductance you can compare with horsepower, the more the better. However, it must match to the strength of the magnets (what it surely does in a Hawk 7).
 
The resistance you can compare with the amount of fuel your car needs to gain the power. The lower, the better. But... our slot cars have unlimited fuel (= track power)! So forget about the resistance. It really doesn't matter.
 
OK, so with your little meter you just can prove if an armature will work. But you will not find out which one is the better one if the values between two are so equal (<3%).

If one coil is off by maybe two-thirds (66%) or more in one of the values, the armature is broken. Otherwise, all the other parameters like dynamic balancing and contact resistance between brushes and comm are way more important than the coil values.


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#14 Pontes1

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Posted 13 March 2017 - 05:52 AM

Dear Benno,

 

OK i understood, so my quest now is to look for the more inductance in those motors!!!

 

Any how i have a damn good armature in house (with several wins already) so I go to measure it  and try to find almost equal ones on the new arrivalls...

 

Thanks.


Fernando Pontes





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