I haven't looked at brushless motors in detail, but from a couple years
of Physics in college I learned that when you increase voltage/current
in a coil, you increase the strength of the magnetic field generated by
the coil. So, I suspect that the "RPMs per volt" rating for brushless
motors tells you how much voltage needs to be applied to pull the
armature hard enough for a given speed. Presumably, that rating is for
a 100% duty cycle (i.e., max speed). Indeed, the controller pulses the
voltage for two purposes (a) to spin the motor one way or another and
(b) to regulate the speed (i.e., decrease the average voltage seen by
the coil).
With regard to torque, for a given duty cycle, if you decrease the input
voltage the torque will also decrease. But, if you maintain the same
input voltage, while decreasing the duty cycle, the torque will remain
relatively stable while the speed decreases. That's why really big
earth movers and locomotives use diesel-electric drive systems ...
maximal torque across a range of RPMs.
On 7/29/2015 12:18 PM, TyngTech wrote:
Can't say I truly understand the brushless nomenclature (i.e. KVA)
because nobody ever talks about the controller side. Brushless motors
are basically steppers and without the controller are inert chunks of
iron and copper. What I don't get is the KVA ratings (rpm's per volt)
of these motors. In my mind, the controller dictates rpm by how fast it
is energizing the phases, not by how much voltage is being applied.
Voltage would affect acceleration and torque or am I getting this wrong?
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