Regen braking: confirmed!

[Pdelcast]

Sorry to resurrect a really old thread... but I saw that people were asking me to jump in and "pay my respects" so to speak... (And I'm having trouble getting to sleep tonight!!)


Regenerative braking does occur in all Castle ESCs. And yes, the actual braking action is achieved by just shorting the windings together through the FETs.

What happens is this -- the ESC shorts the windings of the motor, and forces the motor to start acting like a generator. Remember that a turning motor generates a specific voltage -- back EMF. This voltage is actually the voltage induced in the windings by the moving magnets.

Because the motor windings are shorted, the voltage drops to a very low level (usually less than .1V) and current rises very high, very quickly (often hundreds of amps.) As energy is generated (by the drag created by the voltage difference) current rises, and energy is circulated through the windings and the FETs - -- And a large magnetic field (with a LOT of energy) is stored in the winding.

After a short time, the FETs turn off -- and this is when the regeneration occurs. The current that was flowing through the windings suddenly has nowhere to go. Inductors (like a motor winding) abhor a change in current, so the stored energy (in the motor winding magnetic field) forces the voltage to rise until the winding current can continue to flow. The current flows from the battery negative, up through the body diodes of the low side FETs, through the motor winding, back through the body diodes of the high side FETs, and into the capacitors (and battery...)

This is similar to how a boost converter works:

http://en.wikipedia.org/wiki/Boost_converter

(stolen from the Wiki:) The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When being charged it acts as a load and absorbs energy (somewhat like a resistor), when being discharged, it acts as an energy source (somewhat like a battery). The voltage it produces during the discharge phase is related to the rate of change of current, and not to the original charging voltage, thus allowing different input and output voltages.

But instead of a supply, there is a magnet passing the coil that creates the current source.


Hope that clears it up for you all!

[lutach]

Awesome post Patrick. I don't know how accurate some of the data recorders are, but they usually show some sort of upward spike in the voltage. I also noticed that sometime an amp spike can occur as well, does that mean it's actually amp going back into the pack?

[Pdelcast]

Originally Posted by lutach

Awesome post Patrick. I don't know how accurate some of the data recorders are, but they usually show some sort of upward spike in the voltage. I also noticed that sometime an amp spike can occur as well, does that mean it's actually amp going back into the pack?

Yes, that amperage is going back into the pack during braking. Many data loggers do not have the ability to measure negative currents, so some just show zero current (like the Phoenix ICE data logger -- it does not have reverse current measurement, so it shows zero current during regen.) And some will show positive current, even though the current is negative. It depends on how the current sensing is achieved.

[lutach]

Originally Posted by Pdelcast

Yes, that amperage is going back into the pack during braking. Many data loggers do not have the ability to measure negative currents, so some just show zero current (like the Phoenix ICE data logger -- it does not have reverse current measurement, so it shows zero current during regen.) And some will show positive current, even though the current is negative. It depends on how the current sensing is achieved.

Sounds good. Thank you for coming in here to explain it to all of us.

Originally Posted by Pdelcast

..................

And yes, the actual braking action is achieved by just shorting the windings together through the FETs.

What happens is this -- the ESC shorts the windings of the motor, and forces the motor to start acting like a generator. Remember that a turning motor generates a specific voltage -- back EMF. This voltage is actually the voltage induced in the windings by the moving magnets.

....................

My English isn't very good, and the translator is even worse. But doesn't shorting means connecting + and - together? But, that can't be possible, there should be a fire then, right? So what does shorting actually means?

Thank you,

Patrick

[lutach]

Originally Posted by Byte

My English isn't very good, and the translator is even worse. But doesn't shorting means connecting + and - together? But, that can't be possible, there should be a fire then, right? So what does shorting actually means?

Thank you,

Patrick

I might be wrong, but I don't think the shorting Patrick meant is the same as a + and - shorting of a battery for example. It's basicaly 2 motor phases shorting to achieve braking. I'll be waiting for the experts to answer it in more details though .