Regen braking: confirmed!

[BrianG]

I was doing a little playing around with my EZRun80 ESC the other night. I still need a motor mount, but I thought I'd get some data from an unloaded motor.

Before I go on, let me make the disclaimer that I do NOT suggest ever running a motor unloaded! Do so at your own risk!

I hooked a freshly charged 4s lipo pack and a Neu 1512/2d to the ESC. I also attached a Fluke 87A voltmeter to the battery wires (using a little adaptor I made). I then set the meter to "record mode" where it will take samples. In this mode, the meter will also record the min, max, and average readings. Nort sure what the sampling rate it though.

Remember, the motor is not attached to anything.

I gradually applied throttle until rpms were at max. Lipo voltage at this point was about 14.9v. Then, I jammed on the brake. When I looked at the meter's max voltage, it read 29.4v! Yeah, there's regenerative braking here, and quite a bit of it. Repeating the test gave me the same approximate result.

Then I asked myself "self, I wonder what the current is at that point?". I had to be careful here since my meter is only rated for 10A, so I changed the leads around so the meter was in series between the ESC and battery, and reset the recording mode. Again, I gradually increased throttle to almost max (to reduce current spikes), and then jammed on the brake. I measured -2.96A. Repeating the test got -1.82A. Yes, negative current. That's because my meter was set up to read current to the ESC, not from it.

Obviously, the spike is very brief and I am probably not getting the full reading due to my meter's sampling rate, but those are pretty hefty numbers from an unloaded motor, which means there is a LOT less momentum acting on the windings/magnet than there would be in a moving vehicle. I shudder to think what those poor caps and FETs deal with in a vehicle with a LOT more kinetic energy!

I wonder if all ESCs should have a transorb or something that would shunt any voltage no more than 10% higher than the cap ratings? These spikes, even as brief as they are, can't be good for components! IIRC, the MMM does have these devices which kick in at 26v, so that makes me feel a little better.

Anyway, just thought some would like to see real test results. If anything, this proves regenerative braking IS present, at least in the HobbyWing EZRun80, but the design is just like just about every other ESC on the market, so I think we can assume all the other ESCs act the same way.

And yes, motor and ESC are both still fine.

[trev3813]

Good info here. So the EZRun has regenerative braking. Does the Castle Creations Sidewinder have that?

[Arct1k]

Yes they have to put the power back to the batteries as the ESC can't take it all

[trev3813]

Oh, so theoretically, its "charging" your battery pack?

[othello]

You will also notice regenerative braking when hooking up an eagletree logger to a car setup.



Driving my truggy on asphalt with a Neu 1512/2Y (1400kv), Jazz 55 ESC and 10s1p A123. I marked the amp and voltage peaks where i applied brakes. The green line represents motor rpms. The eagletree logger records positive Amps (not negative amps) and voltage does rise instead of falling -> the battery is recharged. Amp Peaks around 15-20A with voltage rising by 1.3-1.4V

A similar behaviour can be seen with MMM and BK ESC.

Quote from Pdelcast about MMM Esc

The braking is regenerative because it has to be -- can't dump that much energy anywhere but back into the battery. (The battery is used as a load, but using a battery as a load mean charging the battery...)

It is not, however, optimized.

Still gets fairly good efficiency up to about 35% braking power -- above that the conversion efficiency gets lower.

[BrianG]

Yup! Neat, eh?

[pedeboi364]

sweet, just like the 1:1's do!!

[RUSTY XL-5]

does the mm do this?

[rabosi]

Lipos are supposed to be charged with constant current/constant voltage correct? If so do these short burst of varying voltage/current have any negative effects on the lipos?

[BrianG]

The duration of those spikes are pretty short, so it should be ok.

Now that's cool! I always thought the voltage spike going up as the amp draw comes down was just the battery responding to the reduced load placed on it, but it makes sense also that there should be some current flowing back into the batt.

Great work champ ;)

[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.

[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

[RUSTY XL-5]

This is cool stuff, does anybody know if the MM does this?

[BrianG]

Yes, ESCs are simply not big enough to absorb that amount of energy in such a small package, so it has to go somewhere...