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Old 03-27-2007, 11:58 PM   #1
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Default Motor Theory Question

think I've this problem figured out, but I was hoping to get a sanity check here. Please keep in mind that even though this example includes LiPo and brushless, I believe this would apply to all batteries or motors.

We've got two guys running Orion Carbon batteries with the Novak 13.5 motor. The start and end voltage of the batteries for both racers are nearly identical, and the motors appear to be very similar. Keep in mind the 13.5 is all torque, which may make a difference.

Driver 1 gears his motor to the very edge of its limits, and is noticeably faster at the start of a race than Driver 2 who gears a bit more conservatively. As the race goes on, toward the middle, Driver 1 flattens out and has about the same speed as Driver 2. By the end of the race, Driver 2 is quite a bit faster than Driver 1. While this all seems to make pretty good sense, when you check the voltage of the packs by the end of the race, they're the same for both drivers.

The first theory is that Driver 1 is getting his motor hotter than 2, and it is therefore suffering from head-related fade, or stator fatigue. The sintered rotors are supposed to mitigate this somewhat, but it's still a reasonable assumption, and has indeed been pointed to as the likely cause of this problem.

The second theory is that when the race starts, and the batteries have more voltage, the taller gearing of Driver 1 works out well. However, as the voltage begins to fall, Driver 1's motor doesn't have the extra power available to work as hard as it did before, while Driver 2's motor is humming along happily since it's now more appropriately geared for the available voltage.

So, do motors work that way? Is it even a possibility that the motor that's geared higher will perform better at the beginning and worse at the end than the same motor/battery that's geared lower? And I mean completely exclusively of issues relating to heat, fatigue or anything else. Does a motor's ideal gearing change based on available voltage?

Hopefully that's a clear way of explaining it. You never really hear about motor gearing being targeted to be consistent throughout the race rather than fast early, slow later. Perhaps the problem is more pronounced with torquey motors, or brushless in general.
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Old 03-28-2007, 12:10 AM   #2
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Well,

How would you gear a 12th scale with a six cell pack versus a 4 cell pack.

Higher for the four cell pack. (bigger pinion)

Also, what you didn't state was whether when you checked the packs after the race if it was on a LOAD. IE, like on a discharger.

The resting voltages of the packs probably would be very similar to a nearly dead battery. NiMH are the same. If you had a pack you ran conservatively, like a 5min stock race, and a pack that was fully charged, but sitting around for 15 mins, and you just checked the voltages with a volt meter, they would probably only be about .1-.2volts different. But the fully charged pack will obviously have more power.

So, the driver with the taller gearing is probably using MORE of the available power in the battery, thus getting into the lower side of the discharge curve towards the end of the race. Not to mention heat and such in the motor.

also, driving style can affect this as well, so two drivers with the exact same set-up can end up with different results at the end of the race (one uses alot of breaks, the other doesn't....etc)
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Old 03-28-2007, 12:11 AM   #3
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Default Could it be

Could it be that driver one jumps into the powerband quicker and has to adjust his driving more so than the conservatively geared car. Seeing that #2 works to the powerband and rides it a lot better, the amp draw is constant where the guy who is geared to the teeth jumps in and out of the band. In essence, they are essentially consuming the same amount of power over time. I am sure if you check the voltage half way through the race, driver #2's voltage would register a bit higher.
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Old 03-28-2007, 12:15 AM   #4
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Quote:
Originally Posted by bxpitbull
Could it be that driver one jumps into the powerband quicker and has to adjust his driving more so than the conservatively geared car. Seeing that #2 works to the powerband and rides it a lot better, the amp draw is constant where the guy who is geared to the teeth jumps in and out of the band. In essence, they are essentially consuming the same amount of power over time. I am sure if you check the voltage half way through the race, driver #2's voltage would register a bit higher.
Possibly. Maybe one driver does better with more punch (available in the beginning) and suffers as that goes away toward the end. However, in this case, it's quite noticeable even in a straight line, and not just punching out of a turn.
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Old 03-28-2007, 02:31 AM   #5
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It does sound to me like a motor heat issue, I've never experienced or witnessed a motor setup that "comes into" it's gear ratio as the voltage drops down.
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Old 03-28-2007, 08:30 AM   #6
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I have to agree with the motor heat issue. As heat goes up, so does the resistance in a peice of wire (or any conductive material), thus making the motor feel slower. As heat saturates magnets, they tend to feel weaker. So the rotor can also be a contributing factor.

Gearing per the track in general will be a factor, as car 1 geared to the moon will have a better top speed at the end of a long straight/sweeper. If car 1's driver is more skilled and can carry his corner speed, the car will give the appearance that it is going faster at the beginning of the run (because it is), but toward the end of the run when the battery is on the down slope of the discharge curve, no longer puts out the same grunt to get the car out of the corner, regardless of how much corner speed that is carried. Meanwhile car 2 still has a good amount of voltage left and is geared to get out of the corners properly is able to take advantage of the situation.

I'd rather get ahead and block than try to make up from behind.
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Old 03-28-2007, 09:26 AM   #7
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I am going to ask some silly questions, because i am a newb. The end voltages are nearly identical leaves me wondering if the voltages were measured with loads on them? End voltages do rebound quickly, and with no load, they are almost always nearly identical.

Driver 1 is pushing his motor to the limit, so i think it is acceptable to say that he is creating more heat than the other driver. More heat = less speed.

I would check to see if the packs are nearly identical interms of their voltage characteristics, charge and discharge time first.

Once understanding both batteries behave almost identically under controlled conditions, then let the 2 drivers go at it again. If the same thing happens, then after the run. Take their batteries out and discharge both packs at the same time using the same method.

My guess is that driver 1 (motor geared to limit) is going to show that his battery has been drained more. Because i think there was more energy lost due to heat in his car.

i hope i actually made sense... even if i was wrong
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Old 03-28-2007, 09:34 AM   #8
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To rule out the heat issue, have both drivers do a couple laps on the packs after they have been run, but the cars have cooled for 10 minutes.
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Old 03-28-2007, 09:45 AM   #9
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NIMH2 Battery resting voltage varies dramatically from 8.4V plus full charge to 7.2 when nearly or fully discharged. From checking a lot of volts on packs, I can guess fairly well how much mAH are in a NIMH2 by the voltage.

However Orbital was talking LIPO so this is irrelevant to his main question.
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Old 03-28-2007, 10:05 AM   #10
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i think timmay has got it.

higher heat = higher resistance, higher resistance = higher wind motor.
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Old 03-28-2007, 10:24 AM   #11
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I know what you mean, however, I don't know that I agree with it being equivalent to a higher wind motor, as there is a ton of dynamics here. A higher wind motor could stand a taller gear (theoretically), and would also operate at a lower amp draw more efficiently... if timed correctly, and therefore could operate at a cooler temperature. Since the higher wind motors also typically operate at a lower rpm/volt (depending on timing), you'd still have to gear it up (lower ratio) to maintain the same speed. In turn gearing it up could also mean that motor operates outside its optimum, overheats and thus goes slower over time, just as in the first scenario...
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Old 03-28-2007, 11:36 AM   #12
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after the race when you check the battery voltages, are you checking that under a load? or does that even apply to lipo?
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Old 03-28-2007, 01:05 PM   #13
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This is how I should have wrote it.

higher heat = higher resistance, higher resistance = less magnetic field being produced (per given wind) which produces a slower motor.
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Old 03-28-2007, 01:09 PM   #14
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Thanks for the thoughtful replies, guys. I measured the packs after the race without load, so those numbers may not mean as much as I originally thought.

It's sounding like heat is indeed the issue. I'm going to try using a higher capacity pack next week (4800 LiPo rather than 3200) which seems to have more voltage at the end of a race. I suppose if it is heat, it's time to look into cooling the brushless motor, or (gasp) gearing down. Even though it's not getting hot enough to go into thermal shutdown, it is apparently getting hot enough to slow down significantly by the end of a 5 minute race. If so, I believe this would be a function of the stator getting hot and losing efficiency rather than the sintered magnet fading, since it's supposedly unfazed by normal brushless operating temperatures.
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Old 03-28-2007, 01:12 PM   #15
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Quote:
Originally Posted by swopemike
NIMH2 Battery resting voltage varies dramatically from 8.4V plus full charge to 7.2 when nearly or fully discharged. From checking a lot of volts on packs, I can guess fairly well how much mAH are in a NIMH2 by the voltage.

However Orbital was talking LIPO so this is irrelevant to his main question.
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