[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