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Switching Frequency and current loop speed

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bobfrank
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Joined: 2019-04-23 09:43
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Switching Frequency and current loop speed

Hi,

I am looking for a motor controller or to modify a motor controller to operate a very low inductance motor at high voltage.
The motor is a 5-10uh motor, and I intend to operate it at around 180-200v~
With such a low inductance, with most controllers, current decays to 0 before the next PWM cycle, leading to massive eddy current losses, hysteresis losses, skin effect losses and the controllers have issues regulating/controlling the current.

On common solution to this is to add inductors in series with the motor windings to increase the inductance, but these are big, heavy, cost money and saturate at high currents so they aren't an "elegant" solution.

I was hoping to instead solve this issue by drastically increasing the PWM frequency. In the order of 500khz.
You might think increasing PWM frequency would make all of the above losses worse, however it doesn't, because the current will not drop significantly between switching cycles, this will effectively lower the frequency of the current - instead of it being say, +/-50A at 10khz because the low frequency lets the current drop, it will be +/-1A at 500khz because the current doesn't drop, + 49A at the motors electrical frequency for normal commutation.

The current control loop shouldn't need to be run much faster/any faster because the current control loop speed only really depends on the ERPM/electrical frequency of the motor, max ERPM is approx 30k.

I can see that by default the switching frequency is 20-30khz depending on configuration, Is changing this and deadtime (to match hardware), as well as adding custom hardware (FETs, drivers, current sensors, probably isolation, lots of low ESR/ESL capacitance etc) all that is needed to achieve 500khz operation from a motor control stand point? I know i would have a lot of work to do with EMI, ringing, transients etc, as well as general stuff to get the board working at 500khz switching but I would be happy to have the current loop run at 20khz and the switching to run at 500khz.

A brief overview of hardware ideas involve GaN fets, high speed fet drivers, lots of ceramic and film caps, low side shunts, some SiC diodes, many layer board etc...

bobfrank
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Last seen: 5 years 1 month ago
Joined: 2019-04-23 09:43
Posts: 6
jerome
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Joined: 2019-05-08 14:55
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bonjour a tous 

j'ai acheté un vesc 6.6 je le connecte a mon pc il ne veux pas ,j'ai besoin de pouvoir dialoguer avec pour activer la fonction 

uart ...quelqu'un a t il une solution ....

merci