[email protected] wrote:
Lee I agree some inductance is good for a controller -- but surely it's possible to have too much?
I don't think so; not as a practical matter. The controller is nothing but a switch. It connects the motor across the battery pack, and waits for the current to ramp up. Then shorts the motor, and waits for the current to ramp back down. It switches back and forth between these two states, to keep the current at the desired level.
More inductance makes the current ramp-up and ramp-down time longer. But motor speed also slows this down, exactly as if the motor had more inductance. For example, suppose the controller is 100% on, and is waiting for the current to get up to 100 amps. The voltage across the inductance is pack voltage minus motor back emf. It might take a full second for the motor current to reach 100 amps, because the present motor RPM makes its back emf very close to the pack voltage.
If you tried to ramp up or ramp down the current faster than the LR time constant
You *can't* ramp up or down faster than the LR time constant. Full pack voltage is the most you can apply, and that only occurs at stall.
might it not spike the voltage high enough to hurt something?
Inductors generate voltage spikes when you don't give them a path for the current to flow. But a controller *always* provides such a path; either through the main transistor or the freewheel diode.
Another way to ask the question is what's the fastest safe rate for changing current as a function of inductance? Would you have to slew 15x slower if you have 15x the inductance?
It's the motor that limits the current slew rate; not the controller. If you like math, it's Voltage = L x Current / Time. For a given pack voltage, more inductance automatically means a lower current slew rate (current/time).
Finally, this bigger motor might not have all that much more inductance. Inductance is a side effect of motor design. Different design choices can lead to very high, or very low inductance. My *guess* is that it will be high, because it weighs 2600 lbs for a 225kw motor (only 86 watts per pound). That implies it's got a heck of a lot of extra iron.
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