Re: [PSUBS-MAILIST] Interesting item on eBay web siteitem#275822570:X-9 "Y...

[Subscuba@aol.com]

Pat,

Your thinking would probably be better if you considered the motor 
controllers as a switch that turns off and on at a high rate, most operate at 
20KHz. If the length of time the switch is on you have more power. If the 
switch is on for about 10 microseconds with an off time of 40 microseconds 
then the duty cycle is 0.2. If your battery voltage is 36 volts the motor 
gets about 0.2X36=7.2 volts. If the on time is about 40 microseconds and an 
off time of 10 microseconds then the motor gets about 40/50X36=28.8 volts. 
Thus the average motor voltage is the duty cycle, defined as on time divided 
by the period or the reciprocal of frequency, times the battery voltage. 
Since the power switch doesn't dissipated much power in the on or off state 
the controller doesn't dissipate much power unlike a potentiometer.

The neat thing is two fold, you get speed control and through current sensing 
you get current limit. This prevents stator demagnitization or wiping out 
your permanent magnetics used as the stator. The circuitry is actually quite 
simple but somewhat difficult to construct due to the need to control 
parasitic inductances. You can buy the controllers or make your own as I did 
but the technology is not straight forward and you can throw away a lot of 
money doing it yourself. As you are the Great Captain of the Nautilus I would 
be glad to help with some parts and design skills. It would be interesting 
from my viewpoint. I tried to put as many batteries in my Sub without regard 
to motor voltage since I designed the controller myself. I can use 12 volt 
motors from the 36 volt batteries if I wanted to. The controller from Minn 
Kota was $169 and I built it for about $10 to $12 dollars since I have access 
to sample power switches.

I don't advise doing it without a background in power switching technology. 

For what its worth.

Have fun,

Ken Martindale

PS Fuzzy logic wouldn't help.