Regen braking: confirmed!

[teknorc]

Originally Posted by jhautz

This is cool Brian. I always wanted to do this test myself but never really had a good enough multimeter to do it. Especially after a little experiment I did myself a couple weeks ago. I have actually been meaning to start a thread on this very topic. I have heard for a long time that they did have regenerative braking in just about all ESCs but never saw any proof of the fact.



I'm hopeing someone can answer why it is that I see 10-15% better milage when I run mechanical brakes? This is on the same truck on the same day on the same track with the same setup and batteries. I did some testing the last time I went out for a practice day and wanted to see what effect the mechanical vs motor brakes had on run time. I ran 2 5 minute runs with the mechanical brakes on 2 different packs, and then took the mechanical brake off, and charged the packs back up recording the amount of energy I put back in each one. Then ran two 5 minute runs with the mechanical brakes taken off and used motor brakes. I tried to drive just as hard with both brake setups and ran 2 trials on each just to add a little extra data.

But for sure, both runs with the motor brakes used more mah from the packs than the runs with the mechanical brakes did.

A day or 2 after I actually did this I realized I could have used my eagle tree to get better data. Next time I will do it that way.



I just dont get it, the evidence would support the opposite effect being true. Longer run time with motor brakes.

Wondering if anyone has any thoughts?

The motor is still using power to stop. In other words, ESC braking is not free. The 'regenerative' part is that it can recapture small (if not tiny) amounts of power in between the pulses while the braking power is applied. After all, a force is essentially being applied in the opposite direction to bring the car to a stop, just as it would if it was starting from a standstill. This is also why motor brake setups run hotter since the motor is working in both directions. Also, braking force is usually applied more abruptly than while accelerating. AFAIK, a single mechanism (the motor) can't function as a generator and a source of power at the same time. You must use power to brake the car. I'm not an electrical engineer, but I know a few things about physics and the forces that are present and being applied in this scenario simply can't be ignored.

Please correct me if I'm wrong here because our results coincide with jhautz and others that have found mechanical brakes to be more efficient. I'm not arguing that some energy isn't being harnessed or regenerated while braking, I'm saying that it does in fact use power to brake and that the overall net of the system is negative, not positive. Mechanical brakes are obviously using power as well (servo), but compare a servo's energy requirements to a 1/8th brushless motor's requirements and I think the picture becomes clearer.

[jhautz]

Thats what I was always thinking. I can see how coasting off power would actually spin the motor and make it act like a generator. That makes sense to me. But braking to me always seemed like it would take energy to offset the energy that the truck already had in its momentum.

[Arct1k]

Well I'm not disagreeing 100% but have you ever tried shorting a brushed motor - No power input is actually applied but almost impossible to turn over - This is the easiest way to dig a clod....

You don't need to have power to provide breaks when using a motor...

Patrick is the one we need to chime in here...

[Arct1k]

PS http://www.ezonemag.com/pages/faq/a504.shtml

[jhautz]

Originally Posted by Arct1k

Patrick is the one we need to chime in here...

That would be nice. Someone who actually knows how these thigs work. lol

[tom255]

Originally Posted by teknorc

The motor is still using power to stop. In other words, ESC braking is not free. The 'regenerative' part is that it can recapture small (if not tiny) amounts of power in between the pulses while the braking power is applied. After all, a force is essentially being applied in the opposite direction to bring the car to a stop, just as it would if it was starting from a standstill. This is also why motor brake setups run hotter since the motor is working in both directions. Also, braking force is usually applied more abruptly than while accelerating. AFAIK, a single mechanism (the motor) can't function as a generator and a source of power at the same time. You must use power to brake the car. I'm not an electrical engineer, but I know a few things about physics and the forces that are present and being applied in this scenario simply can't be ignored.

Please correct me if I'm wrong here because our results coincide with jhautz and others that have found mechanical brakes to be more efficient. I'm not arguing that some energy isn't being harnessed or regenerated while braking, I'm saying that it does in fact use power to brake and that the overall net of the system is negative, not positive. Mechanical brakes are obviously using power as well (servo), but compare a servo's energy requirements to a 1/8th brushless motor's requirements and I think the picture becomes clearer.

Actualy Its consume very little not more than couple amps! One guy in russian forum made tests about regenerative brakeng and he did not recorded any noticable power consumption its just shorting motor windings and power force applies through shorting Impulse lenght and opposite phase switching, power consume mostly for FET drivers, brains then braking. MGM, MMM, MM, Schulze, Quark B125. No one regenerated a power, all regenerated power goes for FET heating.

[teknorc]

Please don't misunderstand. I'm not saying that regenerating isn't happening while braking. There is stored energy in the vehicle to be recaptured. But unless the regeneration or conversion of the stored kinetic energy is 100% efficient, some other energy must be used to stop the car, especially if you are braking hard at high speed. If the conversion was 100% efficient we'd have a perpetual motion machine in our RC cars. Shorted motor resistance is one thing, but we all know how motor brakes can flip a car on it's lid. If shorting was that powerful you wouldn't be able to turn a brushless motor shaft by hand if all three wires were touching each other. This is why I think there is power being applied while braking.

Again, I'm not an electrical engineer, maybe at higher speeds, the shorting becomes more powerful and violent enough to flip the car, not sure. This still doesn't explain why a lot of people are experiencing better run times using mechanical brakes though. I'd like to know for sure from someone who can explain it as well. I've read up on full scale regenerative braking and they all have mechanical brakes to supplement the regenerative brakes for various reasons. One of the reasons cited was to more quickly bring the vehicle to a stop from higher speeds.

So, please, someone explain. If motor braking is indeed free, I have a couple things I want to try to get even more run time

[JERRY2KONE]

AFAIK, a single mechanism (the motor) can't function as a generator and a source of power at the same time. You must use power to brake the car. I'm not an electrical engineer, but I know a few things about physics and the forces that are present and being applied in this scenario simply can't be ignored.

During my 20 years in the Navy I have seen on a couple of occasions where I generator which normally puts out huge amounts of power was turned into a motor when the electrical power flow was reversed accidently. Due to the circuitry involved within the system it was not supposed to happen, but it did.

There is no way that electrical braking is accomplished without using some source of power. That's just a relitive impossibility. But as you stated Patric can probably explain this so we can all understand it.

[tom255]

Originally Posted by JERRY2KONE

AFAIK, a single mechanism (the motor) can't function as a generator and a source of power at the same time. You must use power to brake the car. I'm not an electrical engineer, but I know a few things about physics and the forces that are present and being applied in this scenario simply can't be ignored.

During my 20 years in the Navy I have seen on a couple of occasions where I generator which normally puts out huge amounts of power was turned into a motor when the electrical power flow was reversed accidently. Due to the circuitry involved within the system it was not supposed to happen, but it did.

There is no way that electrical braking is accomplished without using some source of power. That's just a relitive impossibility. But as you stated Patric can probably explain this so we can all understand it.

Why you cant brake without power, ofcourse you can by just shorting windings on motor or put some capacitance (capacitors) to windings thats it, maybe its not so effective. Maybe you cant change brake force without power source, somehow you just need change chorting impulse lengt and opposite phase switching, that basicaly any ESC do.

I will speak with guy who made a tests on ESC to check Ezrun 150A, does energy recuperation braking works there?

[crazyjr]

Originally Posted by BrianG

Just for the record, I would anticipate a smaller voltage spike, but larger current flow when braking hard in a vehicle.

Because an unloaded motor stops almost instantly (only the inertia of the rotor/shaft to stop), the magnetic field collapses instantly as well, which induces a large voltage. But since a field only has so much energy (think "watts"), a high voltage will yield small current.

In a vehicle, the motor will not stop as instantly (vehicle weight "drags" the motor further), so the field will collapse a little more slowly yielding a little less induced voltage, but the spike width would be a little longer and also flow a little more current. This is confirmed by othello's graphs above.

Isn't it nice when all the theory and formulas agree with real-world results?

This test was just for fun really, but it does prove that back-EMF can in fact produce a higher voltage than the supply (battery). It's all about the time the field expands vs the time it is allowed to collapse.

Now, this back-EMF spike will be dependant on several factors: vehicle weight, speed just before braking, motor kv, motor style (slotted vs slotless), charge voltage (battery), traction of surface, etc.

When i first tryed my eagletree, I used it on a MM/9xl powered G2R. I recorded a spike of over 269 amps. It to was instantaneous, and saw a voltage spike, but not a big one, maybe 3 volts or so in the same space. I thought it was a glitch or a bad reading, but this made me think about it again. That's some extreme amps, but possibly the same effect. Makes me wish i had saved that file to show, I thought it was a bad reading

[George16]

Originally Posted by JERRY2KONE

During my 20 years in the Navy I have seen on a couple of occasions where I generator which normally puts out huge amounts of power was turned into a motor when the electrical power flow was reversed accidently. Due to the circuitry involved within the system it was not supposed to happen, but it did.

The mighty A-gangers did it again .

We lost power to the whole ship during my last deployment due to this. We basically shorted the diesel generator.

Anyway, whatever happens, energy is not lost or destroyed (First Law). However, it is transformed into another form of energy which is basically heat. Now you wonder why motor braking produces a hotter motor compared to mech brakes .

[BrianG]

A looong time ago, I too thought that motor braking was achieved by the ESC partially shorting the phases via PWM. Due to the way ESC FETs are wired to the motors, this would be done by turning on all the FETs going to ground. But, doing this would generate no voltage/current. And as you can see via my original post, the ESC does generate both voltage and current. The numbers don't lie boys. The fact that I got a higher voltage than the supply (battery) and negative current (current flowing into the battery rather than from the battery) both indicate, without a doubt, that there IS regenerative braking.

However, Patrick has said the ESC sends the motor's kinetic back into the ESC and then to the battery. If this is happening (and it is), simply shorting the windings would not accomplish this. Shorting the windings would brake the vehicle, but where is all that kinetic energy going? I'll tell you; either the motor and/or ESC. Neither are sized to simply dissipate that power.

There is a graph (in a previous post) by othello which proves regen braking as well.

Even Mike agrees with the results. He has Eagletree graphs where the mAh consumed is greater than the battery capacity. As well all know, when you have any decent current flow, the mAh used will actually be LESS than battery capacity. So, even though ET doesn't show negative current (does show v spikes though), just the fact you get more mAh is a giant red flag in favor of regen braking.

jhautz: why you get less runtime with motor braking is still puzzling to me. Without a consistent and repeatable test method, results are simply objective. Not trying to shoot ya down, just my $0.02.

[Arct1k]

I also think people are getting confused between energy transfer in this thread.

The car when moving has kinetic energy : E = Mass * Velocity * Velocity

That kinetic energy can only either be transferred into heat, electrical energy or potential energy (coasting up a hill) (ok light too)...

In the case of a mechanical brake it is always transferred into heat in the brake pad whilst with motor breaks some is transferred into electrical energy.

Second consideration is Newton - i.e. equal and opposite forces required to stop the vehicle.

In a mechanical setup the force is supplied by brake disk which is pressured by a servo which is powered by the battery.

In an electrical setup the force is supplied by the motor - This is where is gets interesting! How is the force generated!?*&^!

Option 1) Through converting the motor into a generator? Which would generate electricity and through losses heat but "not take" energy from the battery
Option 2) Applying power to the motor with an opposing polarity which would "take" energy from the battery
Option 3) Or a fancy combination of both...

Guessing how smart patrick is it is 3... Thats my take...

Originally Posted by BrianG

jhautz: why you get less runtime with motor braking is still puzzling to me. Without a consistent and repeatable test method, results are simply objective. Not trying to shoot ya down, just my $0.02.

Could it be the result of the quality of the battery packs in use? If low quality packs with higher internal resistance can't take back the load they're feeded during braking, the energy is transformed into heat instead of actually recharging the pack.

[Arct1k]

Thats is one thing that wouldn't be an issue with one of Jeff's setups - He's a uber quality build guy!