T-Pain Is On A Boat: Re: [Electric Boats] Magnetic Circuits & The Prototype Motors

Monday, January 7, 2013

Re: [Electric Boats] Magnetic Circuits & The Prototype Motors

Thanks Craig, I'll have a look at your controller design.

It's an interesting idea to use a self-oscillating current-mode drive
for the motor. I hadn't thought of doing the power control this way,
although the technique is widely used in dc-dc buck regulators. I might
have used a hysteretic current mode control though, turning the current
flowing in a stator winding off when it has reached the desired
set-point, then turning it back on again when it has fallen by a certain
amount. The inherent source-drain diodes in the MOSFETs provide a
current path when all four of the MOSFETs are turned off, although I
have found huge improvements in reliability at full power in class-D
amplifiers if you use a suitable diode bypass route to prevent the
internal MOSFET diodes from conducting.
As you say, the switching frequency will be variable as the di/dt will
be a function of the motor speed (when the current is increasing
anyway).

Regards,
Chris.

In message , Craig Carmichael
craig@saers.com> writes
>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/MotorCont
>rollerManual.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
>