Electric Machines Lecture (was: Re: Fungus Gnats and CO2)

Brett Lymn (blymn@awadi.com.AU)
Mon, 6 Mar 1995 13:45:51 +1030 (CST)

Folks, I promise this is the last one on this subject!

According to Wim Osterholt:
>This is not quite true. These resistors aren't meant to reduce the voltage
>on the brushless DC motor.

Like someone else said, Ohm's Law: V = IR
Voltage and Current are inextricably linked.

> They're meant to reduce the _current_ through
>the motor. Anyway, the result is that the motor will get less energy and
>will turn less powerfull (slower) until the internal and external friction
>match the energy level you provide.
>Maybe the result isn't that stable as you were expecting on the long run.

Wim, you are assuming a linear system here. With a brushless DC motor
there is a load of electronics that go in to make the motor turn -
magnetic flux sensors, comparators, switches and the like. These
things may not work reliably with a reduced voltage.

>> You need to find a fan that is NOT using a brushless DC motor.
>Incorrect. (Please also consider the wearing out of the brushes in your toy
>fan motor. It's often copper on copper.)

Brush wear is the major reason for a brushless motor (gee that's
pretty deep isn't it?). The great thing about a permanent magnet
field DC motor with brushes is that the speed is directly proportional
to the current - it is a linear system and you really need a linear
system to control the motor with a simple resistor.

Anyone else clamoring for more about the joys of DC electric machines
can contact me directly (blymn@awadi.com.au) and I will try to share
with you some of my Electrical Eng training (Yes, I am a B. Electrical
Eng - doing a strange job)

-- Brett Lymn, Computer Systems Administrator, AWA Defence Industries
"Also, it takes a lot longer to get up North ..... The slow way"
- "Clever Trevor" Ian Drury