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Thread: Arm resistance vs Inductance and timming how do they relate?

  1. #1
    Join Date
    Oct 2004
    Lilburn Georgia

    Arm resistance vs Inductance and timming how do they relate?

    I am throwing a theory question out I know about resistance numbers on arms but want to know a few things about inductance and how it' affects performance. First thing this is in drag racing motors. Next if a series of arms have a range of inductance but similar resistance how will the arm with lower inductance vs one with higher inductance vary in performance? Will one have more bottom end and the other better top end (torque vs rpm)?
    Also how does motopr timming play into the rpm potential or an arm in relationship to inductance? I know higher timming is higher rpm so would you need to find an arm with higher or lower inductance to get peak power out of the arm? Also Lets throw in 2 more variables how does magnet strength affect the differences in inductance? Would an arm with a lower inductance number perform better with stronger or weaker magnets? Also how does hole size play into all these variables? Tight hole need weaker mags to rev up better bigger hole need stronger mags for same reason?
    I know this is alot but I really am curious how these variables affect each other. Last question does inductance really matter in drag motors and is inductance something that affects braking in road course motors?

  2. #2
    Join Date
    Jun 2002
    Victorville, CA

    I'm not ignoring you, just REAL busy for now. I will post an answer soon. The short answer: you look for low inductance, and that its even pole to pole, but its out of your hands. Balance corrections change inductance, but you need balance. One, of several, reasons that I don't check armatures for ANYTHING except dead shorts. My arms run OK, they all say.
    A clean slotcar is a happy slotcar!!

  3. #3
    Join Date
    Sep 2008
    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 allot 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 coils 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 board 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 your 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 manufactures 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 manufactures do not normally give out that information, trade secrets, there isn’t a way to calculate it before hand. This is were your skills testing and tuning comes to into play. Even having said that the exact combination your working with and the goals of your program and intended service will still 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 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 and informed choice of go with it. There really isn’t a shortcut.

    As to your magnet questions in the frame work 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 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 it’s no load speed. AKA, spool up rate.
    Gap is often misused and misunderstood. From the magnets 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, increased frictional drag. If the magnets have enough total energy placement to 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.


    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 electromagnets 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 decline, 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 enhancing 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 motors 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 it’s 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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