Field Weakening

General topics and discussions about the VESC and its development.
pf26
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Re: Field Weakening

Postby pf26 » 08 Sep 2017, 16:31

@Oskar: could you try to compare FW and simply using a higher bus voltage (i.e. 12s) ?
Also, I wonder how these SK3 motors would sound like when playing with such high RPM as 20,000 (near 2000 rad/s in your graph).. Did you already try that ? (10,000 RPM is already quite a lot and generally require a heavy down gearing).

rew
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Re: Field Weakening

Postby rew » 08 Sep 2017, 19:06


pf26
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Re: Field Weakening

Postby pf26 » 08 Sep 2017, 19:49

Difficult to tell what it means "top speed" and "top torque" for a given brushless motor, especially RC motors with so poor datasheets.
Top speed can be limited by mechanical reasons (mostly unbalance, bearings, centrifugal forces..) or magnetic losses, in the later case it depends how much extra loss you can tolerate. As for torque, it depends how you cool the motor and how long you want this torque..
Now if iron losses were as high as 30-50% of all losses at high power near continuous rating (say 50V, 40Amps for a ventilated SK6374), this would mean that at no load (low current, near 0 joule losses), there would still be as much as 30-50% of the full load losses (near 200W or so). So this motor would require as much as 80W at no load full speed ? I don't think so. My guess is 4 times less. But iron losses could rise strongly at higher voltages (and scarry RPM).

madcowswe
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Re: Field Weakening

Postby madcowswe » 08 Sep 2017, 20:22


madcowswe
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Joined: 08 Jan 2016, 15:37
Location: San Jose
Contact:

Re: Field Weakening

Postby madcowswe » 08 Sep 2017, 20:27


madcowswe
Posts: 14
Joined: 08 Jan 2016, 15:37
Location: San Jose
Contact:

Re: Field Weakening

Postby madcowswe » 08 Sep 2017, 20:47


pf26
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Re: Field Weakening

Postby pf26 » 10 Sep 2017, 18:02

I made some lab measurements on SK6374 149kV and Turnigy 4114 320kV motors:
At no load, the total power from the dc Power supply (including VESCs losses) varies nearly linearly with RPM. I just subtracted the VESC idle power with FETs gates driven at 20kHz (1.8Watts only, including 0.9W for idle VESC)
-> Around 0.015W/RPM for SK6374 up to 7500RPM (slightly more in FOC than BLDC mode), so 110Watts at 7500RPM (I was wrong !)
-> Around 0.0019W/RPM for 4114 up to 5500RPM -> slightly above 10W loss at 5500RPM no load
I also measured the motor current on one phase, using ACS758, and it is very near to what BLDCtools shows (within +/-5%) - Contrary to the battery current displayed by BLDCtools (generally much lower than reality, especially at low duty cycles).
For SK6374, 30Amps in each motor phase -> 50.5Watts losses (rotor locked), so around 18.7 mOhm per phase assuming all 3 are supplied with the nominal current at all time (BLDC tools says 28.7mOhms but I don't know if it is meant to be exactly)
Finally, this should make around 160Watts losses at 50V/30Amps, or 89% efficiency (assuming the iron losses do not increase more with RPM and current applied together than separately). Going for highest RPM is not always better: 40V/40Amps has also around 89% efficiency..
Turnigy 4114 has less iron losses. In this case, higher RPMs are recommanded (but limited here to 4800RPM from the manufacturer).
FOC has a slightly higher power consumption at no load high speeds than BLDC. This is probably due to non optimal control when playing with low motor currents that create large measurement errors on VESC shunts. BLDC uses bemf, so no such issue, especially at higher RPMs.


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