Field Weakening, help me understand

[Theodore Voltaire]

I'm having a hard time trying wrap my mind around Field Weakening. So far what I know is when the motor reaches top speed, the controller does something to make the motor go faster.

What I think I'm reading is, when the motor reaches top speed the controller then increases the phase current? That sounds more like Field Increasing than Field Weakening to me, so why is it called Field Weakening? Also how does the controller know when the motor has reached top speed?

I'm not sure if it matters which controller, but the one I'm referencing is a 24F Nucular

 

[dominik h]

Field weakening does not raise phase current.
The controller generates a second "signal" on the phase wires which works against the magnetic field of the motor to make it weaker.
With a weaker magnetic field the motor makes more rpm/Volt, but tourque is reduced and losses and heat generation get higher.

 

[lnanek]

Yeah, more current is drawn from the battery, but that additional current is converted by the controller differently before being applied to the phase wires. It's negative in voltage in comparison to what's normally sent, and sent with different timing.

 

[Theodore Voltaire]

OK that's very helpful. Thank you for the replys.

So forgeting about field weakening for a minute, is it correct to say that the controller phase current generates a field that makes the motor move, but as the motor rpm increases the motor starts making a sort of a reverse field that at some point counter acts the field made by the controller phase current?

 

[fechter]

Yes. The BEMF generated by the motor will equal the pack voltage at full speed, which keeps it from going faster. Field weakening is essentially advancing the timing of the output so that you can push more current. In a brushed motor, you get the same effect by moving the brushes in the advanced direction.

 

[Theodore Voltaire]

So very basically what the controller does during field weakening is, it generates a second signal on the phase wires that acts against the reverse field, which weakens the reverse field, and allows the motor to go a little faster.

I assume the primary phase current remains about the same, but because the controller is now generating a second phase signal that's what puts a higher demand on the battery?

 

[Theodore Voltaire]

I think I understand a lot better now, but I'd like to go just a little deeper to help me understand what I'm actually doing when I make adjustments to the field weakening. So hypothetically what's the difference between 10% Fw, and 50% Fw? Is it just the second signal voltage that changes, or is it the timing that changes, or both?

 

[dominik h]

At least you should know that with to much fw you can easily kill your Controller when the wheel is in the air and the motor can do 50% more revs as on the road. In the moment the controller stops the fw the motor can generate 50% more reagen voltage and your BMS and controller can die due to overvoltage. So only make fw tests on the road.

I killed two controllers until I "learned" this.

 

[Theodore Voltaire]

At least you should know that with to much fw you can easily kill your Controller when the wheel is in the air and the motor can do 50% more revs as on the road. In the moment the controller stops the fw the motor can generate 50% more reagen voltage and your BMS and controller can die due to overvoltage. So only make fw tests on the road.

I killed two controllers until I "learned" this.

Thank you, you anticipated my next question. I was going to ask what part is in the most danger of failure from too much fw.

 

[dominik h]

You can overheat the motor and the controller.
And there are a few Post where some people mention that the magnets can loose their magnetism to, but I think that is a myth.
The only way they loose their magnetism is when they get to hot.

I run my SVMC72150 with 50A fw current and have no problems on the road.
If I lift the wheel in the air and give a slight throttle, I'm shure the third sabvoton will day. Killed a SVMC72260 and the BMS (blew the fets) and a SVMC72150 (killed the hall sensor entrances)

 

[Wattman]

Field weakening does not raise phase current.
The controller generates a second "signal" on the phase wires which works against the magnetic field of the motor to make it weaker.
With a weaker magnetic field the motor makes more rpm/Volt, but tourque is reduced and losses and heat generation get higher.

Nicely explained.

 

[stancecoke]

it generates a second signal on the phase wires

It's not a "second signal", it's just a phase shift. You just apply a bigger advance angle on the phase currents...

Vector control for dummies — Switchcraft

Who doesn't want to get a quick and dirty understanding of the concepts behind vector control?

www.switchcraft.org

Normally you want to have minimum losses, therefore you control id to zero. (or you are doing MTPA to get maximium torque per amp)
To weaken the field of the permanent magnets you have to control id to a negative value.

regards
stancecoke

 

[dominik h]

After 9 year of driving brushless I still learn things.
Very good explanation.
Maybe a liitle bit to high for me.
I always thougt it is a second Signal producing more losses in the controller.

So it does not generate more hest in the controller, only in the motor.

 

[Theodore Voltaire]

It's not a "second signal", it's just a phase shift. You just apply a bigger advance angle on the phase currents...

Vector control for dummies — Switchcraft

Who doesn't want to get a quick and dirty understanding of the concepts behind vector control?

www.switchcraft.org

Normally you want to have minimum losses, therefore you control id to zero. (or you are doing MTPA to get maximium torque per amp)
To weaken the field of the permanent magnets you have to control id to a negative value.

regards
stancecoke

Thank you for this link stancecoke. Very good for me to study.

 

[999zip999]

Is it the same as advancing the hall signals to the phase current ?

 

[Theodore Voltaire]

After 9 year of driving brushless I still learn things.
Very good explanation.
Maybe a liitle bit to high for me.
I always thougt it is a second Signal producing more losses in the controller.

So it does not generate more hest in the controller, only in the motor.

Now I think I can see where you might get the idea there was 2 signals, since there's reactive current, and active current. Field weakening occures by reducing that reactive current. Active current seems easier to understand than reactive current.

 

[Adrian_]

And there are a few Post where some people mention that the magnets can loose their magnetism to, but I think that is a myth.
The only way they loose their magnetism is when they get to hot.

It is a myth, I haven't seen any evidence that is directly connected between magnets losing magnetic field and usage of flux weakening/field weakening.
I think that myth comes from people pushing too much flux weakening/field weakening current, overheating the magnets causing them to lose or have less magnetic field and they came to the conslusion that flux weakening/field weakening caused their magnets to lose magnetic field when in fact it was the temperature related to the use of flux weakening/field weakening.

 

[thepronghorn]

It is a myth, I haven't seen any evidence that is directly connected between magnets losing magnetic field and usage of flux weakening/field weakening.
I think that myth comes from people pushing too much flux weakening/field weakening current, overheating the magnets causing them to lose or have less magnetic field and they came to the conslusion that flux weakening/field weakening caused their magnets to lose magnetic field when in fact it was the temperature related to the use of flux weakening/field weakening.

I don't think it is 100% a myth. At normal temperatures, field weakening is usually not strong enough to permanently affect motor magnets, but higher temperatures make the magnets more sensitive to the opposing field that field weakening generates.

 

[amberwolf]

For further reference, here is another thread on this topic

PM motor field weakening

Afternoon all, I've been struggling to understand the exact working of field weakening in order to increase motor speed. My understanding is that to speed up the motor the AC voltage is increased, upto the base speed of the motor. At this point the rotor voltage and stator voltage is equal due...

endless-sphere.com

and some more threads in this list (not all are relevant, but several are)

Search results for query: field weakening

endless-sphere.com

There are others but the search can't find them because they probalby used FW or another abbreviation (The search could not be completed because the search keywords were too short, too long, or too common. )

 

[Theodore Voltaire]

For further reference, here is another thread on this topic

PM motor field weakening

Afternoon all, I've been struggling to understand the exact working of field weakening in order to increase motor speed. My understanding is that to speed up the motor the AC voltage is increased, upto the base speed of the motor. At this point the rotor voltage and stator voltage is equal due...

endless-sphere.com

and some more threads in this list (not all are relevant, but several are)

Search results for query: field weakening

endless-sphere.com

There are others but the search can't find them because they probalby used FW or another abbreviation (The search could not be completed because the search keywords were too short, too long, or too common. )

Thanks amberwolf, that was very helpful.

 

[BareKuda]

I'm having a hard time trying wrap my mind around Field Weakening. So far what I know is when the motor reaches top speed, the controller does something to make the motor go faster.

What I think I'm reading is, when the motor reaches top speed the controller then increases the phase current? That sounds more like Field Increasing than Field Weakening to me, so why is it called Field Weakening? Also how does the controller know when the motor has reached top speed?

I'm not sure if it matters which controller, but the one I'm referencing is a 24F Nucular

Nucular Field Weakening

Maybe you already found this information but it shows Nucular takes steps to ensure you don’t overvoltage your controller. It will only allow the motor to reach a speed where the back EMF cannot destroy the controller in the event you somehow end field weakening while over rated rpm.

I think there are various ways to implement FOC, the basic concept is, if you accelerate and peak phase amps and before reaching any rpm limit you set, if phase amps begins to decrease the controller knows its BACK EMF has decreased the voltage difference and phase current slowly decreases.

in the settings you can specify the maximum amps that can go towards weakening the d-axis flux, and they give the example of if phase amp limit is 100a and FW amp limit is 30a, then basically as you accelerate all 100a of phase current goes to torque until back emp starts decreasing the phase amps. Perhaps when phase amps drop to 97a, field weakening amps will begin to increase.

As you go faster you might have 80a making torque and 20a weakening the field, reducing back EMF, but also reducing torque. Then at max speed, 70a is going towards torque and 30a for field weakening.

The important thing is it’s still 100a adding heat to the motor. So these settings should be adjusted to the motor temperature. You definitely don’t want 500 phase amps going continuously to a motor that cant handle it for more than 30s.

This controller makes it easy to limit FW depth. 0a FW if you want it off, increase the amps slowly until the steady state current drops within maximum continuous current (based on how hot you want to allow your motor to get).

Be aware that there may be other settings that help the controller know what to do. Most notably the motor parameters section. By giving it the correct data, the controller does need to entirely wait for feedback to get it in the ballpark. It knows what correction factors to add to the algorithm trying to inject field weakening current at the same time it’s controlling the torque current.

The other thing about field weakening is its very expensive in terms of efficiency. In the case where 30% was wasted to weaken the field, and that means the 70a can only make as much torque as 50a, you might see it takes 100% more current to go 20% faster if you are already going fast. Field weakening works best where the rated rpm steady current draw is below rated current of the motor.

Good luck with your controller. It seems nice. Wish I could afford one. Lol.

 

[Theodore Voltaire]

View attachment 343588Nucular Field Weakening

Maybe you already found this information but it shows Nucular takes steps to ensure you don’t overvoltage your controller. It will only allow the motor to reach a speed where the back EMF cannot destroy the controller in the event you somehow end field weakening while over rated rpm.

I think there are various ways to implement FOC, the basic concept is, if you accelerate and peak phase amps and before reaching any rpm limit you set, if phase amps begins to decrease the controller knows its BACK EMF has decreased the voltage difference and phase current slowly decreases.

in the settings you can specify the maximum amps that can go towards weakening the d-axis flux, and they give the example of if phase amp limit is 100a and FW amp limit is 30a, then basically as you accelerate all 100a of phase current goes to torque until back emp starts decreasing the phase amps. Perhaps when phase amps drop to 97a, field weakening amps will begin to increase.

As you go faster you might have 80a making torque and 20a weakening the field, reducing back EMF, but also reducing torque. Then at max speed, 70a is going towards torque and 30a for field weakening.

The important thing is it’s still 100a adding heat to the motor. So these settings should be adjusted to the motor temperature. You definitely don’t want 500 phase amps going continuously to a motor that cant handle it for more than 30s.

This controller makes it easy to limit FW depth. 0a FW if you want it off, increase the amps slowly until the steady state current drops within maximum continuous current (based on how hot you want to allow your motor to get).

Be aware that there may be other settings that help the controller know what to do. Most notably the motor parameters section. By giving it the correct data, the controller does need to entirely wait for feedback to get it in the ballpark. It knows what correction factors to add to the algorithm trying to inject field weakening current at the same time it’s controlling the torque current.

The other thing about field weakening is its very expensive in terms of efficiency. In the case where 30% was wasted to weaken the field, and that means the 70a can only make as much torque as 50a, you might see it takes 100% more current to go 20% faster if you are already going fast. Field weakening works best where the rated rpm steady current draw is below rated current of the motor.

Good luck with your controller. It seems nice. Wish I could afford one. Lol.

Thank you.