fyrstormer

iTrader: (1)

I have a Tamiya XV-01 that I built as a FWD car. Because it has limited traction, I'm running a 30-turn crawler motor with the highest gearing that will fit in the car, plus 24 degrees of timing advance, to get the speed I want. (for the record, it still spins the front tires on takeoff.) It works fine, but I find myself wondering: What are the side-effects of running massive endbell timing *specifically* in a low-power vehicle?

As I understand it, the best power efficiency is found at near-zero timing, because each armature coil powers-up when it's in just the right position for its EM field to interact as productively as possible with the permanent magnetic field, resulting in the highest torque per watt -- but not the highest wattage, and thus not the highest top-speed. Advancing the endbell timing powers-up each armature coil before it's in ideal alignment with the permanent magnets, which causes the causes the armature's EM field to deform the permanent magnetic field, pushing it further away from the armature. This temporarily reduces the strength of the permanent magnetic field in the vicinity of the armature, making it easier for the armature to rotate through it at high speed, because a weaker permanent magnetic field can't induce as much reverse-current in the armature coils, so there is less parasitic drag. This reduced reverse-current, in turn, exerts less resistance to amperage flowing through the armature coils (similar to what happens when the motor is stalled), allowing more amperage to pass through, which increases the motor's power consumption, power output, and top speed. But higher amperage also means more heat, and running hot is the most obvious tradeoff for running massive endbell timing.

Having said all that, I'm specifically running a low-power motor in a vehicle much lighter than it was originally intended for, so overheating is not a concern in my specific circumstance. Soooo...what's to stop me from increasing the endbell timing to truly ridiculous settings? I know if I rotate the endbell more than 90 degrees, the motor will just run in reverse (a handy thing to know on the rare occasion that you buy a motor that was assembled incorrectly), but what about running 48, 60, 72, or even 84 degrees of timing? There must be some side-effect of running massive endbell timing, even if the problems of overloading and overheating are eliminated, or else it would be much more common.

Does anyone know what other disadvantages there are to running massive endbell timing, besides the risk of overheating?

gigaplex

iTrader: (2)

If you go too far, you'll get cogging issues when trying to launch. You might want to consider using boost on your ESC rather than relying on so much endbell timing.

fyrstormer

iTrader: (1)

Brushed motor, not brushless. No cogging to worry about. Sorry I forgot to mention that, I figured it was obvious when I said "crawler motor", but now that I think about it a lot of crawlers are brushless too, so...my bad.

howardcano

iTrader: (37)

An alternate plan would be to run conservatively on the motor timing, but gear up a bunch and turn the throttle endpoint down. It works well on low-traction surfaces to reduce wheelspin, while maintaining top speed.

fyrstormer

iTrader: (1)

I wasn't asking for alternate plans. I was asking what happens when you advance the endbell timing on a brushed motor too much.

ta_man

iTrader: (168)

It runs hotter and hotter and less efficiently.

Andy Koback

iTrader: (280)

And at some point, reverse rotation.

fyrstormer

iTrader: (1)

Hotter and less efficient I expected, but at 24 degrees of timing and running on 2S, the motor in my XV-01 FF is cool enough that I can keep my finger on it as long as I want. The motor really doesn't have to work hard to move this car around on pavement and flat dirt. It doesn't even have to work hard during takeoff, because the front wheels spin and therefore limit the maximum load on the motor.

I know if I turned the endbell to 90 degrees the motor would become completely non-functional, because the armature coils would be powered-up when they're between the two magnets, so there wouldn't be a magnetic bias to force the armature to rotate. At that point the armature coils would effectively become space-heaters, allowing the battery to dump power through them while doing no useful work, and the motor would (eventually) overheat and burn-out.

But what about slightly less than 90 degrees of timing? Where's the cutoff point where advancing the endbell timing stops increasing the motor's speed and power output? What happens to the motor's performance if you keep advancing the endbell timing beyond that point, but you never actually get all the way to 90 degrees?

gigaplex

iTrader: (2)

You're going to need a dyno to answer that. It'll also be some kind of curve with diminishing returns, I doubt there will be a sudden switch point. And the point where peak RPM and peak power stop climbing are likely to be at different timing settings.

howardcano

iTrader: (37)

That's easy to determine if there are no limits in rotating the endbell. On rebuildable locked-timing motors, removing the small plastic key is all that is necessary. Other motors might be more challenging, but it can still be achieved.

Remove the pinion gear, give her full-throttle (or just wire directly to the battery), and listen to the RPM as you advance the timing. When the RPM starts to drop, the available power is also decreasing.

As ta_man said, very high timing gives less torque, more RPM (until you reach the point above), and less efficiency. Also, the brushes and commutator will be quickly destroyed due to arcing and high peak current.