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Thread: Amp draw questions

  1. #1
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    Question Amp draw questions

    Monty,
    I,ve got a fairly new WrightWay power supply and I'm beginning to wonder if my amp meter is correct.
    On stock 16-D motors I'm only drawing 1.2 to 2 amps @4volts. Even changing out the brushes and using Champ light springs, I don't seem to get much more than that. It seems mainly on the lower powered motors. 12s and up seem ok. I'm not sure whats going on. I've tried using stock brushes and springs, ProSlot brushes and springs, but it doesn't seem to make much difference. And that's after cutting+truing the coms. Any thoughts ??
    As always thank you, I value your input.

    Anyone else who would like to chime in on this, please do.
    I need to know if its something I,m doing, or ??

    Vic
    Keep your finger on the trigger ,and your eye on the slot

  2. #2
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    Vic,

    It's a simple law of physics and nature called "Ohm's Law".

    For a given voltage, the resistance of the load determines the amount of current drawn.

    16D motors have more winds of smaller wire, so they have more resistance.

    Group-12 motors have less winds of larger wire, so they have less resistance.

    Ohm's Law states that Current is determined by Voltage divided by Resistance.

    So, if you have 4 volts and your drawing 2 amps of current, the resistance is close to 2 ohms. That resistance is made up of arm, brush and inherent resistance in the endbell, brushes and such.

    Now if you put 10 volts on a motor and it has 2 ohms of resistance, then it should draw 5 amsps.

    I hope this helps.

    Let me know.
    Florida Slotter, aka Marty Stanley,
    A "Double 60's" Slot Racer
    Killer X Raceways Team Racer

  3. #3
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    That only applies to purely resistive static loads and is not the way motors work, as back emf is the limiting factor in amp draw and performance of a given motor. True the lower overall resistance is what we are seeking, but due to the way a motor works the amp draw should be similar at any reasonable voltage. Vic , while the 2amps is a little low this is probably due to air gap more than any thing else, I have had many 16d types that pulled "only two amps" and were ballistic on the track. You can always check the accuracy of the meter by comparing it to a digital VOM like an electrician uses, just hook it in series with the load.
    Improving performance one hazardous material at a time.

  4. #4
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    That's right for current on a free reving 16-D motor.
    Put a load on it and the current will go up.
    What are you trying to do, maybe we can help?

  5. #5
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    Vic, Marty,

    Steve is correct. As RPM increases, so does a reverse voltage generated by the collapsing field in each pole sequentially. This is a function of inductance. This back EMF, when equal to the input voltage, defines the RPM limit presuming the torque load is constant.

    On stocker 16d arms, the major limitation on current draw is the mild timing. A domestic arm with 40 - 45* timing can easily draw twice as many amps, even though the static resistance is only a bit lower (due to neater windings, which will be marginally shorter). You get a lot more RPM, similar torque - could go up or down just a little - and the motor runs better IF you gear accordingly.

    Some of the RPM scrubs off due to internal friction of bearings, brushes, moving air, etc. Most of these losses stabilize at higher RPM, so I routinely check current draw at 6 volts. It rarely goes higher with a few more volts than that, but there is a danger of exploding the comm or tossing a winding. The exception is with hot windings as found in a G7 or Eurosport motor. They spin so fast that 5 volts unloaded is a bit scary.
    A clean slotcar is a happy slotcar!!

  6. #6
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    Well, I was just trying to make sure these numbers seem normal for you guys. I've been reading on some post that some guys are getting much higher amp draws. I know the numbers can and will vary from different meters, but some guys are telling me they are getting close to 1 amp per volt?? That just seems a bit much to me. I do get that on some of my higher powered motors x-12s ect, and I have gotten that on big bore arms, but we're talking small arm 16-D s here.
    If you guys think my numbers sound right, than that's fine. At least I know I'm not going nuts lol.

    Thank you all for your input.
    Vic
    Keep your finger on the trigger ,and your eye on the slot

  7. #7
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    Quote Originally Posted by Monty@B.O.W. View Post
    Vic, Marty,

    Steve is correct. As RPM increases, so does a reverse voltage generated by the collapsing field in each pole sequentially. This is a function of inductance. This back EMF, when equal to the input voltage, defines the RPM limit presuming the torque load is constant.

    On stocker 16d arms, the major limitation on current draw is the mild timing. A domestic arm with 40 - 45* timing can easily draw twice as many amps, even though the static resistance is only a bit lower (due to neater windings, which will be marginally shorter). You get a lot more RPM, similar torque - could go up or down just a little - and the motor runs better IF you gear accordingly.

    Some of the RPM scrubs off due to internal friction of bearings, brushes, moving air, etc. Most of these losses stabilize at higher RPM, so I routinely check current draw at 6 volts. It rarely goes higher with a few more volts than that, but there is a danger of exploding the comm or tossing a winding. The exception is with hot windings as found in a G7 or Eurosport motor. They spin so fast that 5 volts unloaded is a bit scary.
    Sorry Monty, My post overlapped yours.

    So with that said, do you feel my numbers are correct or close to?
    Keep your finger on the trigger ,and your eye on the slot

  8. #8
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    Vic,

    Yes, your readings are OK for the typical 20 - 25* of timing found on imported 16d arms. I expect 3-5 amps for my 45* arms. Stronger magnets reduce current draw just a small amount.
    A clean slotcar is a happy slotcar!!

  9. #9
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    Thanks Monty
    Keep your finger on the trigger ,and your eye on the slot

  10. #10
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    As an aside, I've also found out that a high amp motor is not always a good thing. Sometimes it can mean trouble. I,ve been talking with alot of different guys about this, and gotten mixed feedback. However, most agree on the smaller motors, high amps are, or can be a problem. High amps = high heat=less performance and or short life. Makes sense to me.
    Also keep in mind that we're talking stock small arm motors here.

    Vic
    Last edited by trickyvic3; 10-26-2010 at 12:32 PM.
    Keep your finger on the trigger ,and your eye on the slot

  11. #11
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    It is my experience that when you find a motor that draws a lot more current that similar motors, then something IS wrong. Most likely a mechanical misalignment or worn bushings,or a bearing about to fail, or I've even seen partial shorts between the hood screws and the can screws if they are a little to long (hint: if the condition is intermittent or heat sensitive check the screws!) and if you 're using aluminum end bells pay special attention to partial shorts. It'll pull a ton of current, you'll think you have a missile, but you really have a pooch. Remember, we're working with fractional horsepower motors here, so any mechanical drag will cause an elevated current reading. One final thing that I've found, try tweaking spring tension, you may be surprised to find that a little more really helps, as this gives better contact between the brush and the com, and reduces bounce, too. Don't go crazy, but a little more tension may be just the thing you've been looking for. Another benefit is that with a little more tension is that the motor will not fall off as much as it builds heat.
    Improving performance one hazardous material at a time.

  12. #12
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    Thanks Steve, All good points. I always blueprint every motor, and always use GoldDust or BigFoot brushes and Champ light springs. I never really gave the spring tension a thought. I,ll have to give that a try when I get a troublesome motor. I,ll also check for shorts if I get one that seems to be pulling to many amps.

    Vic
    Keep your finger on the trigger ,and your eye on the slot

  13. #13
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    I, too am in the same quandary. I had always targeted a certain range of amp draw from a free-spinning motor at 5V. That range would be determined by wire size, armature timing, number of turns, and strength of magnets. Any reading outside of that range would be considered abnormal.

    But I have discussed this with some friends who impress that more amp draw makes for a monster motor. These friends are national champions and consistent podium winners.

    Yes, more amperage creates more heat. But we are only racing for 20-24 minutes with at least 7 breaks, not an 8 hr enduro.

    This cannot be explained by simple ohms law. Ohms law relates to one static circuit. We are dealing 3 rapidly switching coils in a magnetic field. The measurement of coils is "Inductance", and inductance relates to impedance, also measured in ohms, but can be an entirely different creature than simple CD resistance.

    So, how can one of their race-winning ceramic magnet C-can motors, loaded with a super-wasp armature draw more current on my power supply than a cobalt motor with 16 turns of 24 ga wire?
    Last edited by gr8rcfan; 10-26-2010 at 07:09 PM.

  14. #14
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    Quote Originally Posted by gr8rcfan View Post
    I, too am in the same quandary. I had always targeted a certain range of amp draw from a free-spinning motor at 5V. That range would be determined by wire size, armature timing, number of turns, and strength of magnets. Any reading outside of that range would be considered abnormal.

    But I have discussed this with some friends who impress that more amp draw makes for a monster motor. These friends are national champions and consistent podium winners.

    Yes, more amperage creates more heat. But we are only racing for 20-24 minutes with at least 7 breaks, not an 8 hr enduro.

    This cannot be explained by simple ohms law. Ohms law relates to one static circuit. We are dealing 3 rapidly switching coils in a magnetic field. The measurement of coils is "Inductance", and inductance relates to impedance, also measured in ohms, but can be an entirely different creature than simple CD resistance.

    So, how can one of their race-winning ceramic magnet C-can motors, loaded with a super-wasp armature draw more current on my power supply than a cobalt motor with 16 turns of 24 ga wire?
    That's actually a pretty simple answer. The SW arm has much higher timing allowing a higher charge to build up in the coil before the static field of the magnet generates the back EMF force. An open arm usually has no more than 20* or so advance compared to 40*+ timing, couple that with the higher flux density from the cobalt and there you go. It is, in fact, the cobalt that allows the use of those hot arms in the first place. Back in the days before cobalt, 25 or 26 turns was about all that would live. If you were to measure the actual DC resistances of the two arms, the difference is quite small, showing the great influence of inductance. Also,the difference between horizontal and vertical brushes plays a part in the current draw,or more specifically, the overlap of horizontal brushes permits three very brief dead shorts on each revolution drawing a little more juice. Despite this , and for a reason that I have yet to figure out, horizontal brushes work better on everything from 27's and smaller.
    Last edited by SteveDee; 10-26-2010 at 08:43 PM. Reason: Additional thought, really!!!
    Improving performance one hazardous material at a time.

  15. #15
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    Thats why we speak of ranges

    As I've stated many times , all things being equal, more current is normally faster. Generally, all things are NOT equal, in this case the timing. Excessive current draw is also a red flag. I trust my blueprinting skills. Therefore, I would be thrilled with a 16d that draws 5 amps, but concerned with one that draws 7.
    A clean slotcar is a happy slotcar!!

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