hias9 said:How many amps of flux/field weakening would you consider safe without risking to destabilize the magnets?
Motor is a MXUS 3k v3 3T
hias9 said:I think the maximum value I can set in the Sabvoton software would be 150A.
For the Lipos this would not be a problem.
Isn't there a risk to destabilize the motor's magnets with this amount of flux weakening?
thorlancaster328 said:Although you've probably found out already, I'm just going to put this for posterity:
BLDC / FOC controllers with field weakening only start to apply field weakening once the back-EMF from the motor reaches the battery voltage. This only occurs when you are going fast enough to generate enough BEMF to equal battery voltage.
So if you're using no field weakening and not pulling 50 amps at your top speed, field weakening will make you go a little bit faster.
If you're already pulling 50 amps without field weakening, the controller will never have a chance to apply field weakening even if you enable it.
thorlancaster328 said:BLDC / FOC controllers with field weakening only start to apply field weakening once the back-EMF from the motor reaches the battery voltage. This only occurs when you are going fast enough to generate enough BEMF to equal battery voltage.
There is an "ideal" wind where the speed you are most frequently at is close to the base speed of the motor. That way your controller is running with high PWM ratios (which makes it more efficient) and you don't need to use phase advance/flux weakening. In that case, using field weakening to get occasionally higher speeds makes sense.SafeDiscDancing said:Would it make more sense to simply rewind the motor with a higher kV?
JackFlorey said:You might even end up with lower overall efficiency for your route with the higher base speed.
Agreed. They are not huge. They may be significant for longer commutes.SafeDiscDancing said:If you use the Grin motor simulator you don't see huge differences though.
A 10% increase in kV will translate to maybe a half a percent efficiency reduction at the base speed.
JackFlorey said:Agreed. They are not huge. They may be significant for longer commutes.
Like I said, you have to do the math and look at the total overall efficiency for the trip. If you lose 40% (which you don't) for 1% of the trip, but pick up .5% for the rest of the trip, you still come out ahead.SafeDiscDancing said:A half a percent?
Compared to a 40% loss of efficiency using Flux Weakening?
Flux weakening is just an extension of centuries of actual motor drive experience. In the days of separately excited rotors, you just reduce the excitation current and get a higher base speed. It actually saves energy at the expense of torque. When considering induction motor control you explicitly control the two magnetic vectors separately, so the relative strengths of the fields are whatever you make them.Like I said on my first response I think the Flux Weakening was always a "hack" for people with a situation where they didn't want to rewind their motor but get a little more top speed anyway
That's true. But it's just as true to say that all induction motors create nothing but wasted heat with the induced field in the rotor. In a very narrow sense that's true - but that's also why they work, and the power they create is pretty useful.Unless I have the basic idea wrong ALL the Flux Weakening creates nothing but wasted heat and the further above your default kV the worse it gets.
BalorNG said:Flux weakening is good on IPM motors where a considerable portion of the power is made using reluctance torque anyway.
https://www.automate.org/industry-insights/interior-permanent-magnet-motors-power-traction-motor-applications
So, SOME motors paired with SOME controllers (middrives, basically) are indeed capable of quite effective flux weakening.
On a typical PM BLDC motor, flux weakening is more of a gimmick and is a huge battery drain.
SafeDiscDancing said:…
Try to imagine what Flux Weakening is ACTUALLY doing...
In order to advance the kV above it's default rotation speed you are literally trading heat for speed.
That's nuts.
That's just throwing battery power away for nothing.
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And finally... if you then try to argue:
"Oh, but I'll be good. I'll only use Flux Weakening for 1%."
...well you know that's bullshit.
Human psychoplogy means everyone pushes their bike at full power pretty much every opportunity they get it.
So in the "real world" if you add Flux Weakening the losses will be huge because the temptation to use it will be too great.
Better to just get the right kV in there from the start and abandon this "super trendy nonsense".
It's like Regen.
The topic has been analyzed forever and in reality an ebike due to it's physics (high wind resistance, low weight) means you cannot recapture more that a very low percentage to the battery.
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This is a really good (and hard) question. The answer is hard because the magnetic properties are actually not fixed for a magnetic material but are affected by the magnetic circuit which makes it impossible to state a general limit. In my experience demagnetisation rarely happens on neodymium magnet motors and if it happens the motor has been heated too much. This is based purely on my own motor cooking and the user posts here.hias9 said:How many amps of flux/field weakening would you consider safe without risking to destabilize the magnets?
Motor is a MXUS 3k v3 3T