Motor braking.
 

What are the downsides of using only the motor for braking?

I heard its harder on the motor and esc, but not sure to what extent. Especially on the bigger trucks (revo, emaxx)

You can use the motorbrake, but the heavier the vehicle, the harder it is to brake.

the motorbrake means more stress on the controller, and the motor will heat up sooner. What it does is 'shorting' the wires of your motor, and the energy is stored back into your cells. (tipical way of braking with electric motors)

Most controllers can handle it though. If you tell us the setup you want to use, we can tell you if it's wise to use a mechanical brake or not.

Since when are the motor brakes regenerative brakes? I thought they were just a reverse torque.

I do know it's quite hard on the esc, both the harder you brake,the weight of the RC, and gearing. Most of us use motor brakes, as far as I know.

I think this MAY have been a factor why my Quark capacitor blew, I turned the braking setting to double, very hard on the caps.

Rene correct, when you apply brakes ESC shorts the windings and can do that with bursts simulating ABS...

Also each FET has a built-in diode which works as an H-bridge with other FET's, thus charging battery and cap if generated voltage higher then battery voltage( accelerating going down hill or bursts after jump). Caps usually take care business here and if connection between batteries and ESC is not good then often fail...

So, when you use motor as a brake then you put motor in bad situation because now it need to absorb kinetic energy of rolling vehicle (mv^2) and do that fast. Think, you Ok with accelerating within couple seconds but you would like to stop on a dime, right? Do the math then.
For controller it is up for caps how good they are and how good is battery connection.

Really, I thought it was something a little different.

BTW, when my truck is not moving, and you apply the brakes, you can hear a buzz, like pulses. This is the Quark, i love the brakes it has, they are super smooth, and never lock up.

So I suppose these ultra caps should do an excellent job for braking.

If they are discharged! Getting idea?

This is a quark 125b esc. On a Revo, w/ a Neu 1512 2000kv.

I just took it for a test ride. My first ride with reverse, and the motor brake. It was pretty sweet. I could flip it with a hard brake. Once I got use to it, I liked it.

So its should cause some more heat, but for the most part its ok to do?

Yes, better then Ok, if motor can handle that.

Are you certain the brakes are regenerative? Surely that'd be dangerous if you're using lipo packs...

Yes, but charging will happen only when motor generate voltage higher then battery voltage + voltage drop across diode.

Why would they have to be discharged? It would be as long as the potential is greater than the battery...?

With normal caps the internal resistance is near 0 when they are discharged. you need a low internal resistance when braking.

Why do you say it's near 0 when discharged? Wouldn't it be the same impedance regardless? Can you explain why that would be?

Anyways, it would never be able to be discharged.

Sounds like a bunch of theories. So whats the real scoop? Does anyone actually know? One thing I know for sure, Breaking isn't nearly as demanding on the controller as hard acceleration. You can say it is all you want but I know from driving my brushless trucks that it isn't. Locking up the wheels on an RC just takes alot less energy then getting the sucker up to speed in the first place.

I don't think it is regenerative...

If it were, why is the braking force consistent? If it were regenerative, the braking force should be higher if your battery is flat.

Also, what stops the brakes from causing a spike that takes your lipo above 4.2V/cell?

And if it were, why do we see no hint of this on our onboard power meters?

Again......everyone spare us the theories. All anyone has done in this thread so far is confuse the hell out of everyone. I myself agree with what Aragon is saying, but then again.....wtf do I know. Doesn't anyone know something factual on the subject?

http://www.rc-monster.com/forum/show...?t=6361&page=4
Post #47 Team Teakin prez is talking about regen braking causing voltage spikes, and that it's hard on the esc.

Personally, this theory discussion is just fine. That's how the subject will eventually become clarified. However, I do agree that the topic could be confusing to someone just trying to find the answer. Maybe the thread title should be amended to something like "Motor Braking - theory discussion"?

Well, GriffinRU did say that it only spikes your batts when the potential is higher than battery voltage, and enough over the voltage drop on the FET diode. I am guessing it is just shorting the motor, and turns it into heat, but the FET's can control the switching of the 'SHORTING', hence proportional braking.

Also, I have actually tried running the Eagletree in reverse manner, but it does not read anything. I tried using it when charging, it needs to be flipped around I guess.

Yea, the shorting would be easy. You just have heat to dissipate. But if the controller uses the battery as an energy sink to provide braking force then I have to consider the numbers. If it takes, say, 500 Watts to accelerate the car to full speed as quick as possible, surely it's not going to take a large magnitude less power to deccelerate the car back to zero in the same amount of time? Let's assume the controller has to sink only 300 Watts to provide an equal braking speed. Is that 300 Watts going into the battery? If it were a lipo it'd probably puff or be severely damaged.

Is the controller sinking some of the energy to heat and some to the battery? And on what criteria does it perform the split? Are controllers aware enough of battery chemistry to know how much power to limit being generated back into the battery?

Yah, that's the thing, I don't think it would ever WANT to regenerate the batt, unless it's A123 M1 cells, which can take a $HIT load of charge in bursts (they can 5 min charge).

About the acceleration.....try 1500 watts with these XL motors. I have a 7XL, everytime I full throttle blip, it's around 1500 watts, acceleration is super car territory.

For braking, no one knows how much power is being dissipated. But the amount of power you can get from braking is more than you could ever use.

From my own experiences, Serum is absolutely correct...!!

Thanks Aqwut;

I don't know what the fuzz is at this moment; slowing down a vehicle gives you energy; charging the cells above 4.2V per cell? what did you discovered? a 120 percent efficient setup? The peaks MIGHT be higher (not saying they are) but the lipo won't bother too much from little spikes.

@aragon; 'it's easier to lock the wheels than to spin them' what do you want to do with the heat that is produced by that? stuff it in your esc? no sir, you store it in your batteries again; use the low internal resistance of the batteries for braking.
Why? does the internal resistance change when the battery is flat? the brakes are using the batteries internal resistance, not their capacity.

Physics. A lipo can take more than 4.2V in spikes bytheway. It's too slow, and the internal resistance is too low to be 5V immidiately when 5V is applied for 1/100th seccond.

It's plain simple, it works like that period.

In less words;

Motorbraking means more stress on the setup (motor/controller) > more heat for the motor and the controller (and even the batts). a mechanical brake will heat up too, but that heat is okay since it's in a thin metal plate that absorbs quite some energy before it melts down.

That's why i posted;
'If you tell us the setup you want to use, we can tell you if it's wise to use a mechanical brake or not.'

If a battery can be discharged at 20C for peaks, why can't it be charged at peaks of 20C? Braking delivers less energy than it's taking on acceleration, don't know what ratio it is put in, but i think it would be less than 60 percent difference

I didn't mean it would charge the cells to above 4.2V/cell, I meant it might charge them at/with more than 4.2V/cell.

Well that goes against every precaution I've read about Lipo chemistry. The whole basis of Lipo charging is to:

a. Not exceed the maximum factory rated charge current.
b. Not exceed 4.25V/cell.

Where have you seen that they can survive more than 4.2V/cell?

Yes, that would be ideal, without a doubt. I'm just not convinced our RC controllers are advanced enough to be doing that...

If a lipo (or any rechargable cell) is connected to voltage source of X volts that is higher than the lipo's voltage, the lipo will draw a current. If the lipo is flat, it will draw a higher current for the same source voltage X.

If the (flat) lipo is drawing more current for the same voltage produce by the spinning motor (ie. same wheel speed), the current flowing through the circuit will be higher, and the braking force will be higher.

I'll wait for a better explaination and/or real world test results before believing that...

Would love to know myself. :)

Schulze told me, team tekin told us, what more do you need? I won't invest any more time/effort in trying to convince you.

I do not know what is going on, but:
Batteries internal resistance does change with charge! Otherwise you should be able to drain the battery for long time with now change in voltage drop under load...

Battery will be charged only if voltage generated by motor upon braking is higher the battery voltage (which is usually NOT) so for ESC to charge battery it suppose to have additional circuitry which boosts this voltage to higher levels (assuming we have only two wire going to battery!)

All hands for theory and understanding here!

here here

do controllers have this additional circuitry?
If the voltage generated is lower than the battery voltage and we didn't have this circuit to boost the voltage then where would the energy generated go?

Well, from what I understand (and from testing), simply shorting the motor leads puts a substantial drag on it. Try it; simply take a motor and spin the shaft. Then, short the motor outputs together and short them again. So, really, it's the motor that is absorbing the energy magnetically. I think the FETs simply apply a "controlled shunt" so there is variable braking available.
Otherwise, I would think the FETs would heat up CONSIDERABLY if they had to take that energy.

Now, if this is true (the shunting theory), I don't understand how there can be a voltage at all to get to the battery...

Yeah, I agree with you Brian, there is a switching to the braking obviously. You can hear it if you apply the brakes and push the truck forward, it sounds like a metallicy sound switching. I've tried the shorting the motor leads too, there is alot of resistance there for sure, as all it is basically a generator with a short circuit.

Maybe we can all understand the regenerative part if we know how particularly the FET's in these controllers work. As FET's have different features. :030:

Motor takes the abuse.

Power goes to batteries if voltage higher then... and you can skip diode drop if utilize FET's instead. And as I said it is possible, but I haven't seen one yet from known brands, except claims by Novak on brushed ESC and mtroniks for all.

If you see direct connection between batteries,caps and FET's then such circuitry doesn't exist.

So, I am correct that the FETs simply apply a controlled short to the motor? If that is the case, how can the voltage develop across a short (aside from FET rdson) to get any higher? I'm not disagreeing or arguing, just trying to understand how the braking works. The rest seems relatively easy from a macro standpoint...

If controller doesn't open FET's then BackEMF from motor will be rectified by built-in diodes and will be charging battery, if battery voltage will be less then backEMF+voltage drop. You can optimize this process if you open FETs in sync with diodes, then you will cut off diodes voltage drop and losses.
If you noticed braking can be done with only one side, while recuperating involved both.

Quick question;

Would the ABS type of braking be more or less stressful than std. proportional braking?

I don't think so, should be no difference as long as ESC can handle it.

No, i don't think so; an ABS is just a way of braking, with the intention not to lock the tires. A gentle finger on the brakes does the same.

Yeah I was just wondering if an on-off pulse from ABS would be better.

Someone with an Eagletree and ABS will have to run tests for us on this!

All motor braking is switching, it does switch on/off to create braking and not just short the motor and flip the car. I probably uses simple PWM with all three phases in switch at same time.

Procharged: We discussed that braking is a loop circuit with batteries NOT included. what are you going to pick with Eagletree?

I guess you missed quite a bit then zero..

batteries ARE included...

for the tenth time; where would you stuff your energy needed for braking? in the esc?