observation on xmods brakes

skklown · #1 01-15-2004, 06:01 PM

while i was doin donuts i noticed that if u are going fast enough and hit the brakes the rear wheels will lock up. i also noticed that if u hold down the brakes and u push the car it doen't want to move. so i guess the breaks aren't just a cut off to the motor

AUDI-TT · #2 01-15-2004, 07:39 PM

yes thats right

i think it gives reverse voltage until the car stops, then it just cuts off the voltage, its just a thought tho im not sure!

eetoi · #3 01-15-2004, 11:02 PM

It's probably what 1/10 scale speed controls use (for those of you who have big AND tiny rc's...), because the brakes on my rustler do the same thing. I was wondering how these things worked, because I don't think it was a reverse current (doesn't go backward when it stops but still immovable, too cheap to sense inputs, or is it?), or just a cutoff. I asked the guy at the hobbyshop and he didn't know, but tried to act like he knew by saying "it locks up the motor", and I asked how and he repeated it. Maybe it's a high frequency alternating switching current.

I got this off of Teamnovak.com:
"While braking, energy generated by the motor is fed back through the ESC into the battery pack; this can only be accomplished when the power MOSFETs are turned ON and OFF at a very fast rate, such as in our high frequency ESCs. This regenerative power provides increased battery life."

Joey · #4 01-16-2004, 05:56 AM

That's more talking about how the FETs play into battery life than it is actual braking. I think it's the reversed-voltage thing, although that sounds like it would be a little painful to the motor after a while.

skklown · #5 01-16-2004, 03:44 PM

also u know that the wheels lock becuase if u are goin fast and slam on the brakes the car does a 180....that the same effect as goin fast and pulling the handbrake on a real car as it locks up the rear wheels

Btman · #6 01-16-2004, 06:58 PM

One thing to keep in mind is that an electric motor can also act as a generator of power if its shaft is turned and power is disconnected from its terminals. When an electric motor is used as a generator or dynamo, power is needed to turn the shaft. This is the opposite of a motor which runs while being powered. When you short a motors terminals, there is very little resistance to opposing the motor generating power. The speed control takes advantage of this characteristic of electric motors. The speed control rapidly disconnects power to the motor, shorts the motor terminals, and the motor acts as a generator. While its acting as a generator in this mode, it dissipates power and acts like a brake since the terminals are shorted.

To get a mental picture of this concept, run your xmods car at a reasonable speed with the car off the ground and let it coast to a stop. Then run the car at the same speed this time stop with the brakes while keeping the above concept in mind.

skklown · #7 01-16-2004, 07:53 PM

also...the bigger the motor u have in ur xmod the faster it stops. i just put in the stage 2 drag motor and it'll lock up the tires on my carpet.

eetoi · #8 01-16-2004, 09:14 PM

Look, the stage 2 motors probably have stronger magnets and lower winds (since they're upgrade motors), which make it easier for them to brake. BTman, is that a theory or do you know that for sure? Shorted terminals are enough to lock the motor? Brakes use current too, right? Otherwise, my battery wouldn't last longer when I brake less

AUDI-TT · #9 01-16-2004, 10:05 PM

Quote:

Originally posted by Btman
One thing to keep in mind is that an electric motor can also act as a generator of power if its shaft is turned and power is disconnected from its terminals. When an electric motor is used as a generator or dynamo, power is needed to turn the shaft. This is the opposite of a motor which runs while being powered. When you short a motors terminals, there is very little resistance to opposing the motor generating power. The speed control takes advantage of this characteristic of electric motors. The speed control rapidly disconnects power to the motor, shorts the motor terminals, and the motor acts as a generator. While its acting as a generator in this mode, it dissipates power and acts like a brake since the terminals are shorted.

To get a mental picture of this concept, run your xmods car at a reasonable speed with the car off the ground and let it coast to a stop. Then run the car at the same speed this time stop with the brakes while keeping the above concept in mind.

Makes sense, but wouldnt that create too much heat, thus wasted energy?

eetoi · #10 01-16-2004, 10:10 PM

TT, if it's true (I'm SO against it still, not a hater though!) then it's just the motor shorted out, not the batteries, right? The car has enough drag and weight to stop before the motor could generate anything to be reasonably hot, or at least from that reason alone.

hoopsta1423 · #11 01-16-2004, 11:45 PM

i dont know if something is wrong with my xmod, but when i am goin forwards and push the trigger forward to stop, my wheels do not lock up, it just stops the current, wheels keep spinnin\

anybody got any ideas to whats wrong?

skklown · #12 01-17-2004, 10:05 AM

i dont think the wheels are supposed to lock up.....it just a minor thing i notice while driving. i started to notice when my car would turn sideways while braking on slick surfaces.

neurokinetik · #13 01-17-2004, 12:47 PM

Quote:

Originally posted by Btman
The speed control rapidly disconnects power to the motor, shorts the motor terminals, and the motor acts as a generator. While its acting as a generator in this mode, it dissipates power and acts like a brake since the terminals are shorted.

You've got it mostly correct. The motor is being used as a generator, but it is not trying to generate electricity into a short circuit. The power is being fed back into the batteries, and depending on how fast the motor is spinning, and how much braking you are applying, the load on the motor is either increased or decreased. They call this regenerative braking.

The solution to the locking rear tires is to roll on the brakes gradually. As the motor slows, it will generate less braking force with the same trigger application.

The easier solution is to go AWD, so that the braking is distributed amoing all four wheels, like a real car, and the front wheels work better for braking anyway due to weight transfer. With AWD, the car will stop perfectly straight and quite quickly from top speed.

skklown · #14 01-17-2004, 01:50 PM

i was thinking about getting awd....but even putting in the motors is a pain in the butt...cant imagine how hard it is to put in awd. plus i dont want to lose out on top speed.

aperson · #15 01-17-2004, 04:30 PM

if you have ball bearings they dont take much away from your top speed. and all you have to do to install AWD is take out the 4 screws that hold the black plastic towers and the 2 on the bottom of the knuckels. then just poop the whole assembly into the car.

EDIT: and i read somewhere on this forum that they create some sort of static electricity that stops the motor.