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A Story by Lee Hart:

Here's how it is supposed to work :-)

The scene: Control room in the Curtis controller. Big clock on the wall says
"15 KHz"; its face has 66 microseconds per revolution, and it "chimes" every
time the hand comes back around to the top.

A droopy looking guy watches an amp gauge. Dozens of identical Midgets
Operating a big Switch For Each Thumb (MOSFETs) sit in rows at their panels.
The Controller sits at the command center, listening to the phone.

Controller: "The boss just called; he says give 'er just a little throttle.
Droopy, you watch that amp gauge. OK, men... Ready... set..."

The clock on the wall goes "bong".

The Controller shouts, "Go!"

The MOSFETS all shout, "Switch on!" Dozens of hands shove dozens of switches
closed, all in unison. Current slowly begins to build. The mighty vehicle
begins to move majestically forward as the microseconds pass.

At about quarter past, Droopy clears his throat. "Uhh, the current is getting
kinda high, sir. It's 500, no 510; no, make that 521 amps. Maybe we should..."

Commander, glancing at the ammeter: "Hey, we're at max current. Current limit,

The MOSFETs all shout, "Switch off." Acceleration stops, as the motor's
current diverts to flow through the freewheel diodes. The motor is just
coasting, now. Current slowly falls as the motor resistance causes the current
to gradually decay.

The Commander says, "Droopy, you've got to be on your toes. If that current
had risen any faster, we could have had real trouble..."

The clock goes "Bong!"

"Back to work", the commander says. "Go."

The MOSFETs all close their switches and the cycle repeats.


With a big, low inductance, low resistance motor, the controller can't switch
off fast enough to properly limit the current. Excessive current means
excessive torque, which means a jerky start. Like this :-)

Clock on the wall goes "Bong!"

Controller shouts, "Go!"

MOSFETs yell, "Switch on."

The current rises fast, inspiring a tremendous burst of acceleration. Within
microseconds, Droopy sees it climb all the way through the green, the yellow,
and into the red zone. "I, err, um, the current is over 500, err 600, no make
that 700 amps, sir!"

The Controller says, "What, already? Ohmygod; turn it off, quick! Emergency
current limit!

"Switch off", the MOSFETs yell.

Current shifts to the freewheel diodes. But due to the low resistance, it
doesn't fall; it hangs there, barely dropping as the microseconds tick by.

"Bong", goes the clock.

"Go", says the Controller.

"Switch on", sing the MOSFETs.

"B-b-b-but the current is still 700 amps", says Droopy. "No, now it's 800...

"Aarrgh", yells the Controller. "Current limit, current limit!"

This process continues, with the motor current above the desired value because
the controller can't respond fast enough to limit it.

Curtis changed their "C" models (1221C, 1231C, etc.) to reduce the clock speed
from 15 KHz to 1.5 KHz for throttle positions less than 15%. This gives the
controller 10 times more time to let the current drop before the next turn-on
cycle. You may hear an audible tone, but the motor will start smoothly with no

Lee Hart

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Last Update:05/02/02