Thanks for the explanations. That has helped crystallize my vague worries into a more concrete question.
So you have this 2600 lb motor with lots of inductance. If I understand things right, you are at the mercy of the inductance for how fast the current in the motor changes. Could it be enough inductance that you'd hit the gas and the current wouldn't ramp up for seconds? Perhaps even worse it doesn't ramp down for seconds (it would be like a stuck gas pedal). What would happen if you hit the emergency stop, would the inductance weld shorted a typical EV safety, like a contactor? Another issue would be the huge rotational mass. The armature must weigh as much as a small car, how would you stop that quickly and safely when the car was shut off? ----- Original Message ----- From: Lee Hart Sent: 08/14/13 11:27 AM To: Electric Vehicle Discussion List Subject: Re: [EVDL] S.E.P.X . controller for 500V , 450Amp Motor [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 curren t 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 choi ces 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. _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA) -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130817/0dd9d5c2/attachment.htm> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
