On Thu, 23 Jun 2011 18:14:30 -0500, "RogerN"
wrote:
I don't know exactly how the R/C brushless motors and speed controls work,
they may be something like sensorless vector ac drives.

In a sensorless system, the controller measures induced voltage on the
non-driven phase of the motor to determine where in the commutation
cycle it is. For startup, where the speed is zero and therefore no
induced voltage is present, a special startup mode is needed. There
are several different techniques for starting up reliably. The
simplest method is to just commutate blindly at low speed for a while
and see if any voltage appears. This technique will result in limited
startup torque, and may even cause the motor to turn backwards a few
degrees just as it starts.

Anyway, I'm not
sure exactly but they probably work a lot like 3 phase motors with AC
variable speed drives.

Very similar, but they don't use a sine shaped drive. At partial
power, the current is modulated, but the frequency and duty cycle
remains constant through the whole cycle. At full power, the phase
current is either full on or full off.

I think some modern speed controls sense the pulse and automatically program
themselves to work properly, the pulse they receive on power up is the OFF
and then going the other way increases speed. For example, if they are
powered up receiving 1ms pulses, that is off and 2ms pulses would be full
throttle. Likewise if they are powered up receiving 2ms pulses then 1ms
pulses would be full throttle.

I have not seen a controller like that. In fact, I think it would be
dangerous. The controllers I have seen, will emit beeping noises and
refuse to start the motor at all until the operator has lowered the
throttle to full stop first. Some controllers enter the configuration
mode if they are switched on with the throttle at max.
--
RoRo