[email protected] wrote:
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?
The L/R time constant will be slower, but not *that* much slower.
First, because the lower inductance of the big motor is compensated for
by a lower resistance as well. So it very well may not be much different
than a smaller motor.
Second, because the inductance of a motor is a side effect of its
design. We really don't know here if this was *designed* as a
high-inductance motor or not. I'm not an expert in the area of motor
design, so perhaps someone who is can help here.
But what I notice is that if you have two motors of the same type and
roughly equal horsepower, the heavier motor (the one with more iron in
it) tends to have more inductance. I think it's a bit like transformers;
you can make a light less-efficient one that runs hot, or a heavy
more-efficient one that runs cooler. The latter has more iron, and so
more inductance as a consequence.
Perhaps even worse it doesn't ramp down for seconds (it
would be like a stuck gas pedal).
With a lot of rotational mass, it will indeed take longer to slow down
if you just remove power. However, a sepex motor makes a great
generator. With regen, you can stop that motor so fast that it would be
like locking the brakes. In fact, if you overdo it, it endangers the
drive train.
emergency stop, would the inductance weld shorted a typical EV
safety, like a contactor?
You never want to suddenly open-circuit any inductive load while current
is flowing. If you do, you get a massive voltage spike. You *always*
want to have a path for the current to flow. The freewheel diodes in a
motor controller provide this path. When a transistor, contactor, fuse,
circuit breaker, etc. interrupts power to the motor, the current shifts
to the freewheel diode. The switching device then sees no more than
normal pack voltage, at whatever current was flowing.
When you cut power, the motor current transfers from the switch to the
freewheel diodes. The diodes have a very low voltage drop; under 1 volt.
This voltage also happens to be the *opposite* polarity from the applied
voltage for motor operation, so it is a generating voltage (produces a
negative torque, i.e. slows the motor down). Thus motor power goes from
(say) 500v x 450a = +225,000w motoring to -1v x 450a = -450w (generating).
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?
You can either let the motor freewheel, or release the clutch, or take
it out of gear, or just use the vehicle's brakes (which already have to
deal with the weight of an 18-wheeler).
Or, use regenerative braking. You can stop the motor so fast that it's
likely to damage something other than the motor with regen.
--
Anyone can make the simple complicated.
Creativity is making the complicated simple. -- Charles Mingus
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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