Motor braking.

[Serum]

Simple and short, my opinion on braking.

BL motor braking: (most likely mechanics)

first: a shorted motor has incredible resistance from turning, and, the faster it spins, the more resistance it has.

- Using full motor shorting for braking would undoubtedly flip an R/C car over it's lid with ease.

- So, you need to control this shorting in order to control the motor shaft resistance, to create an average resistance or torque.

-Along came PWM... the controller sets up the FET's to unify all motor phases to create a short. So the controller will change PWM duty cycle depending on throttle position... so anywhere between say 1% to 95% (for ABS, guessing) duty cycle for proportional braking.

-These latest controllers (like the Quark) most likely have some good algorithms for braking.

This is just my simple theory on motor braking, not going into detail what happens in FET's.

And this is correct.

BTW, the ABS can 'easely' be programmed; if a motor turns, it's a generator, and by the amount of energy it produces the speed of the turning motor can be determined. Slowing a vehicle down usually take a certain amount of time and the controller can measure the energy that the motor is still producing.

Who ever ran a quark on a rather direct drive vehicle (short gearing ratio) and a low KV motor knows that the brakes work too hard. It's just a matter of software, this software assumes a certain brake power is needed.

Again; that guy at schulze (who developped his brushless controllers) told me the energy was stored back into the batteries. No need to think he was wrong. He seemed very interested in my runtime of my savage, when i told him i was going to use the mechanical brake instead of the motorbrake. So something must be happening with the batteries, agree?

And zero; if the energy is stored in the motor like you suggested, why does a controller get hotter if you use the motorbrake? (the motor would get hotter too, no doubt, but with your theory ONLY the motor would get hotter.)

@ artur; why does the controller need to know the possition of the rotor? all three phases can be shorted with PWM instead of going the complex route of following the stator.

[zeropointbug]

Did I say the motor is only takes energy? that's true, most of it, esc I would assume takes some, obviously a little from internal resistance (not much), and switching, and losses from whatever else goes on with the FET, like Artur said there is a voltage drop in FET?

I have said it before that braking IS hard on a ESC, but when you look at total energy , the esc probably dissipates small percentage of energy.

I can see regenerative braking, but let the question be asked whether the efficiency when motor is acting like generator is great enough to produce a voltage high enough to charge batts. It might depend of motor? Do you think?



"Yeah, if the motor stops dead immediately, but that's not the case. since you are trying to slow down the vehicle and not locking its tires.

Go figure boy..

Use the motor as a generator to slow down the vehicle, that's the principle of braking."

-What are you saying?

[GriffinRU]

Originally Posted by Serum

And this is correct.
@ artur; why does the controller need to know the possition of the rotor? all three phases can be shorted with PWM instead of going the complex route of following the stator.

To be in sync with rectification provided by diodes, otherwise there will be less energy put back to battery.

To charge the battery you need current. Higher current means higher energy density, in battery system means more energy being put back in battery in short time.
Now when you discharge battery under normal driving (acceleration...) battery voltage goes down based on voltage drop associated with internal resistance. Internal resistance related to battery chemistry and driven how fast this chemistry goes. This mean that for a short period of time their would be a nice current going to the battery via diodes (or FET's if controlled, very efficient) due to the earlier voltage drop on battery and it's time period will be driven by battery recovery time and vehicle dynamics (it can slow down faster).

As simple as charging battery with 100C for a 0.1 sec...
That's also explains why nice cap is always helps, but that is different story on parallel thread :)

Schulze's comment about loose contact business very well explained in App. Note I mentioned earlier, but as for braking I haven't check their soft yet if their controller does that or its given for free with built-in diodes.