General ESC Theory Discussion

[BrianG]

@Aragon: I see what you are saying. However, a coil can deliver more voltage than what was put into it if the magnetic field is allowed to collapse faster than it was built up. Take any coil, apply a 9v battery on it, and then remove the battery. Depending on the coil rating (in milli-henries), a voltage MUCH MUCH higher can be generated. This is how the cow fences I've seen operate. Applied to R/C; depending on how fast you brake, the field can collapse faster than it was built up and can generate higher voltages than the battery.

@DrKnow65: I did an experiment a while back to test braking force of a motor with shorted phases. At low rpm (turning by hand), there IS resistance, but not seemingly enough to stop a vehicle. But spin that sucker as fast as we do (~30k rpm) and the braking force is high indeed! At 0 rpm, I would think that the ESC actually locks (or "pulse locks") the phases to keep the vehicle from rolling from a standstill since there is no braking force from a motor not spinning at all or very slowly.

Using an oscilloscope to measure 3 phase motors can be a bit tricky since there is no "common" point. Too bad wye motors don't have the common connection point accessible to the outside so you could use that as the common point. With delta motors, there is no common point at all other than what the ESC determines as common at any particular time. The best you could do is use CH1 on one phase and CH2 on another phase and use the delta function on the scope. This procedure is used for floating ground circuits or to measure signal across a specific device. Otherwise, using the scope's actual ground lead could short stuff out depending on the circuit design. Probably not a big deal for R/C use since the vehicle ground is totally isolated from the mains ground. Also, it might be hard to get a trigger since pulsewidth and even frequency may vary with throttle and load. Internal triggering uses the 60Hz wall frequency so that won't work. External trigger is what must be used for something like this. That said, you could still get a decent measurement on one phase using a dual-trace scope and just assume the other phases will be similar, just shifted in phase 120*.

Originally Posted by BrianG

@Aragon: I see what you are saying. However, a coil can deliver more voltage than what was put into it if the magnetic field is allowed to collapse faster than it was built up. Take any coil, apply a 9v battery on it, and then remove the battery. Depending on the coil rating (in milli-henries), a voltage MUCH MUCH higher can be generated. This is how the cow fences I've seen operate. Applied to R/C; depending on how fast you brake, the field can collapse faster than it was built up and can generate higher voltages than the battery.

Do you have more info on this? Or a better explanation? :)

[DrKnow65]

Aragon, like a coil circuit in a car to fire the spark plugs, 12volts changed into 40,000volts.

I see your point BrianG, and knowing that you have tested this makes me feel better- ideas are good, but not good like tested ideas :) I knew there had to be some charging of the coils during braking and it makes good sence that this would only happen when the rpm drops below where the inductive charge is no longer doing the work of slowing the motor.

[BrianG]

Originally Posted by Aragon

Do you have more info on this? Or a better explanation? :)

You can do a Google search on inductor formulas, inductive kickback, back-EMF, etc to find all kinds of information about this phenomenon.

When you apply a voltage to a coil, a magnetic field will build up over time. The time it takes for the coil to magnetically saturate depends on the coil rating (henry value), core type (air vs iron), winding resistance, and voltage supplied. This built-up field is actually "storing" a charge, somewhat like a capacitor does. When you remove the battery from the coil, the field collapses instantly generating a voltage higher than the one applied because the collapse is quicker than the build-up. This voltage generated is opposite in polarity than the one applied because of the direction of the magnetic field motion cutting the coil windings. Incidentally, many times in circuits that energize a relay, you'll see a diode placed across the coil in reverse polarity, which shunts the back-EMF generated by the relay coil to prevent the V spike from damaging connected circuitry.

Ok cool, that makes sense, but how will one control the speed at which the magnetic field builds or collapses in a motor winding if the magnets are moving over it at a set speed?

[BrianG]

Well, that's what the FETs do. If you totally short the windings, it will be full brake. If you short the windings in pulses, you can control the intensity.

From what I understand of coils, the amount of voltage produced by a coil is proportional to the rate of change of magnetic flux. The magnets moving over the motor windings cause magnetic flux around the windings which creates a voltage potential between their terminals, the magnitude of which is proportional to the speed of the magnets' motion over the windings. (ie. the speed at which the motor is spinning)

I don't understand what you suggest though - How will the FETs pulsing a short between the windings change the rate of magnetic flux induced by the magnets?

[BrianG]

It will switch the coils on and off at a fast rate to regulate the amount of braking force. This will load the coils down when on and unload them when off. Ever notice in a 1:1 car how engine rpms drop when there is a large load on the alternator? Same thing here.

Yup, I understand how it would regulate braking force. What I don't understand is how it would increase the rate of magnetic flux generated by the magnets, and consequently increase the voltage potential across the motor windings enough for current to flow back into the battery pack...

[BrianG]

Well, if the vehicle were moving fast enough (high motor rpms) and then you brake hard allowing the field to collapse very quickly, the induced back EMF could be greater than the battery voltage.

[zeropointbug]

Originally Posted by Aragon

Yup, I understand how it would regulate braking force. What I don't understand is how it would increase the rate of magnetic flux generated by the magnets, and consequently increase the voltage potential across the motor windings enough for current to flow back into the battery pack...

Think average voltage, if you have a longer Pulse width when switching the short for braking, then you have a higher average voltage than a shorter pulse width. Correct me if I mistaken what you are talking about?

Originally Posted by BrianG

Well, if the vehicle were moving fast enough (high motor rpms) and then you brake hard allowing the field to collapse very quickly, the induced back EMF could be greater than the battery voltage.

Perhaps, but that possibility could only last for a split second then - the time taken for the ESC to stop supplying power to the windings. It would be ideal if as much of the car's kinetic energy transfered during braking as possible could be converted back into electrical energy that charged the pack...

Originally Posted by zeropointbug

Think average voltage, if you have a longer Pulse width when switching the short for braking, then you have a higher average voltage than a shorter pulse width. Correct me if I mistaken what you are talking about?

Mmm, what I'm ultimately talking about is converting kinetic energy into electrical energy and storing it in the battery pack. An RC car in motion is in a state of stored kinetic energy, the magnitude of which is proportional to its momentum, and the source of which ultimately came from the electrical energy that the system took from the battery to accelerate the car to whatever velocity. If the object's motion is stopped, its kinetic energy is transferred elsewhere. The key is to transfer as much of that energy back into the battery pack.

But the problem as I see it is as I outlined in post #14. The voltage generated by the motor needs to be stepped up somehow in order to charge the battery...

[lutach]

I just did a little test just to see how it went. I ran my DPR in live mode and hooked it up to my MBX5T that is now equiped with a Schulze 18.97 and a 7XL motor. I gave it some throttle and let it go so it would freewheel and the graph stayed flat. I then gave it some throttle and hit the brakes and saw a little spike on the graph. Just to make sure I hooked the DPR to my BPP truck that has a Schulze 40.160 and Neu 1521/1Y. Gave it some throttle and let it freewheel and the graph stayed flat. Then I gave it some throttle and hit the brakes, same thing happened a little spike on the graph.

[Serum]

it was matthias schulze who told me they used the battery trick for braking.