Um, er...
I wasn't thinking of the PWM frequency, which is of course a good
point. But I'll surprise you and say it's around 300 Hz. Now I have
to explain this...
My controllers actually use a dual system. First there's "direct
torque control", or as I call it "Current Ramp Modulation", "CRM".
(Torque being directly proportional to current.) When you apply
voltage to the motor coils, as inductors, the current starts at zero
and rises. If the motor is stopped with no back EMF, it rises very
quickly, but much more slowly with the motor at full speed. The CRM
turns off the coils when the current has ramped up to the maximum
level (or alternatively to the control setting below maximum). It
turns it off for a fixed period like 50uS, then turns it back on to
begin again. So the modulation will be fast at low motor speeds with
high torque, and slow at high motor speeds.
This minimizes switching losses. It variably reduces switching
frequency to no more than necessary for conditions. The problem with
it as the main control is that without external feedback, unless
torque load rises and falls with RPM (as with, eg, a boat prop), the
motor will keep speeding up until it hits max or slowing until it
stops for small control adjustments.
So in an irregular load like an EV car, PWM (external in present
version V2) modulates the CRM (set to max). But since the CRM is
controlling the current, the PWM doesn't worry about it, and can
switch at any low frequency above where the on-off pulses would
become noticeable.
So it's actually my controllers that keep the switching frequencies low.
http://www.TurquoiseEnergy.com/hybridize/MotorControllerManual/MotorControllerManual.html
(BTW If the circuit is of interest - There's some component value
changes to the schematic C10 is double+ for longer 'fixed off time',
and on the layout a wire touches pin 10 of the header strip instead
of going around it.)
Craig
======
>In message , Craig Carmichael
>craig@saers.com> writes
>
>>For my axial flux "Electric Hubcap" motors (36V, 4.6KW 0-2000 RPM...)
>>I use a separate 2" O.D. x 1" thick iron powder toroidal core for
>>each coil. (Unlike typical usage, my coil is wound as a doughnut
>>around the outside, the wires epoxied with heat conductive epoxy.)
>>These donut coils are individually attached in a ring to non-metallic
>>(molded PP-epoxy) body parts with no electromagnetic drag. At 2000
>>RPM the frequency is just 100Hz, so high frequency losses don't come
>>into play. It's almost lossless. Iron losses at 1200 RPM (60Hz)
>>calculate as 1W per coil, total 9 watts. No eddy currents because
> >every powder particle is insulated from all the others.
> >http://www.micrometals.com/ makes them.
> >
>Hi Craig.
>Admittedly the basic commutation frequency is only 100Hz, but do you not
>have PWM switching at a much higher frequency to control the motor
>power? The class-D power amps I design switch the output H-bridge at
>around 300kHz, but that is of course to pass audio signals up to 20kHz.
>For a 100Hz output I guess you still have a PWM frequency above the
>audio range. (Perhaps at around 25kHz?)
>
>I would have thought this might still give excessive skin-effect and
>proximity-effect copper losses in the windings if solid core wire is
>used for the coils.
>
>Or are you driving the H-Bridge with a non-PWM signal and doing your
>speed control with a buck regulator in the dc feed supply to the
>H-bridges?
>
>Thanks,
>--
>Chris Morriss
>
>
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