SPM VS IPM Motor - And Field weaken, is it worth it

[Emoto]

Ah ok there is a missunderstanding. What we were talking about getting efficiency in terms of using all available Torque. Liveforphsics is refering an operating point past the torque optimum and there the ipm torque is decreasing faster with with increasing FW.

I Think he is generalizing , as i am, the only twist here is comparing spm 68kv non fluxed to ipm 41kv ipm fluxed . at same speed.

So ill summarize my thoughts as of now given the information as i understand it ......I wasnt going to consider a low 41kv motor but as we agreed earlier even though the ipm is fluxed past its base speed it would have close or the same efficiency and the free lunch around 30% reluctance torque. , the nice part is that torque can be from 0-rpm if Set with a constant id FW all over the operatimg range.
Correct me if that doesnt sound right

 

[madin88]

I Think he is generalizing , as i am, the only twist here is comparing spm 68kv non fluxed to ipm 41kv ipm fluxed . at same speed.

First you need to find a controller which can apply field weaken current already at 0 RPM, and second thing is you should have enough room left for BEMF to climb. Because if BEMF ((V/RPM) x RPM) starts to get higher during field weakening as battery votlage, it could blow the controller or at least it will start to charge the battery through the diodes in the FET's.

Or with other words if the controller you are using is rated for 72V and you already run a 72V battery, you will not have much room for a gain in RPM. Not so if you would run same 72V controler with only 36V battery voltage, where you could gain twice the RPM safely.

Why do you think it's a bad geometry for the reluctance torque?
image.jpeg

Nice reference, there u have it the magnetic path through the q axis is designed to be explicitly bad to create a reluctance difference between d and q axis. Providing Reluctance torque aditional to the magnetic torque.

I don't understand your answer exactly. Does it have good or poor potential for reluctance torque (referring to QS3000W midmotor)?
IMO it is poor beacause between the magnet poles there is just this small like 1mm thick iron which will saturate quickly. The flux path looks to be similar as on SPM motors (magnets interrupt the flux path), but i might be wrong.

General question: if the potential for a gain in RPM of a given IPM motor is high, does it also mean that potential for reluctance torque is also high? Or does the one thing have nothing todo with the other?

Thanks for your teaching work here tec

 

[Emoto]

Or with other words if the controller you are using is rated for 72V and you already run a 72V battery, you will not have much room for a gain in RPM. Not so if you would run same 72V controler with only 36V battery voltage, where you could gain twice the RPM safely.

Thanks for the abbreviation Madin88

I don't understand your answer exactly. Does it have good or poor potential for reluctance torque (referring to QS3000W midmotor)?
IMO it is poor beacause between the magnet poles there is just this small like 1mm thick iron which will saturate quickly. The flux path looks to be similar as on SPM motors (magnets interrupt the flux path), but i might be wrong.

I also thought the same thats why i posted that vid @17min they talk about pinching affect and rotate for a better flux path .

 

[toolman2]

maddin88 wrote "First you need to find a controller which can apply field weaken current already at 0 RPM, and second thing is you should have enough room left for BEMF to climb. Because if BEMF ((V/RPM) x RPM) starts to get higher during field weakening as battery votlage, it could blow the controller or at least it will start to charge the battery through the diodes in the FET's."

Your right in the first point and ideally for the controller to apply field weakening variably via a lookup table or calculating the right amount needed at different loads/rpms a bit like ignition mapping for an ICE.

On the second point, when field weakening is applied you don't get those damaging or regen causing voltages out of the motor ( it reduces the back emf of the motor to be able to add further rpms :wink: ) -but that's all going well, let us ponder what might happen at 250% base speed and you get a fault condition..
-so for ultimate reliability, your right.

 

[Lebowski]

Have you guys seen my thread on fieldweakening ? That it has nothing to do with weakening fields ? That you can do 'field weakening' with inductors EXTERNAL to the motor, so where they dont impact the field in the motor ? (typing on a tablet, copying link is difficult).

And what do you need a table for when doing field weakening ?

 

[larsb]

I'll check your posts!

 

[tecnologic]

I Think he is generalizing , as i am, the only twist here is comparing spm 68kv non fluxed to ipm 41kv ipm fluxed . at same speed.

First you need to find a controller which can apply field weaken current already at 0 RPM, and second thing is you should have enough room left for BEMF to climb. Because if BEMF ((V/RPM) x RPM) starts to get higher during field weakening as battery votlage, it could blow the controller or at least it will start to charge the battery through the diodes in the FET's.

Or with other words if the controller you are using is rated for 72V and you already run a 72V battery, you will not have much room for a gain in RPM. Not so if you would run same 72V controler with only 36V battery voltage, where you could gain twice the RPM safely.

Nice reference, there u have it the magnetic path through the q axis is designed to be explicitly bad to create a reluctance difference between d and q axis. Providing Reluctance torque aditional to the magnetic torque.

I don't understand your answer exactly. Does it have good or poor potential for reluctance torque (referring to QS3000W midmotor)?
IMO it is poor beacause between the magnet poles there is just this small like 1mm thick iron which will saturate quickly. The flux path looks to be similar as on SPM motors (magnets interrupt the flux path), but i might be wrong.

General question: if the potential for a gain in RPM of a given IPM motor is high, does it also mean that potential for reluctance torque is also high? Or does the one thing have nothing todo with the other?

The flux path i was discussing there was the one from the V grove rotor not the qs3000w u posted up front. For the qs3000w u are right there is not much reluctance torque possible, it will only come by the magnets saturating the stator iron (5-10% max).


What do u mean by rpm gain?

Constant fw current is no good! U need FW current proportinal to the Iq because u need a constant current angle diffrent from 90°.

 

[tecnologic]

Have you guys seen my thread on fieldweakening ? That it has nothing to do with weakening fields ? That you can do 'field weakening' with inductors EXTERNAL to the motor, so where they dont impact the field in the motor ? (typing on a tablet, copying link is difficult).

And what do you need a table for when doing field weakening ?

The Table is about MTPA and FW is just the more common term out here for current angles different from 90°.

 

[larsb]

Lebowski thread on coil FW:
https://endless-sphere.com/forums/viewtopic.php?f=30&t=72692&p=1097701#p1098494

 

[larsb]

One thing i don't get from the controller manufacturers point of doing the program: the ones i tried make you set one value for field weakening, something like 1-2000 or even worse a "field weakening current" value.

Isn't the phase angle the parameter that should be defined? The actual current delivered would normally be proportional to your throttle value and max current limit (i suppose)?

Absolute current value is limited for both motor and controller so that would already be defined by a fixed value.. If you add the so called field weakening current then the controller must do some scaling on both Id and Iq that doesn't allow you to know what the controller is actually doing with the setting.

Or am i getting it wrong?

 

[Lebowski]

In my particular controller algo it will always try to make the torque producing current (Id ?) equal to what is asked for by the throttle. It will raise controller output voltage to achieve this. When raising motor voltage is not enough to produce the wanted Id (?) it will start adding (ramping up) field weakening current, again with the goal to make Id equal to the throttle wanted current. There is a mechanism in my controller that limits the field weakening current, it can never go above the max set value, and it will automatically not go into the region where more field weakening current slows down the motor.

This last limit is dynamically calculated, I do not use a table. From the motor guys at Microchip I understand that this is not a know (in the literature) algorithm, and that it is common to use tables / graphs.

If I remember correctly Arlo1's car only went around 110kmh with no field weakening, and upto 180 with...

 

[larsb]

So let's say i am doing a drag race, throttle is always max, controller is putting out Imax until rpm&BEMF is so large that it reduces the current, then phase angle is increased by the controller until max current is achieved again? but since the new phase angle is putting current on the Id axis then the torque producing current will still be lower..?

..no that's not what your figure in the other post shows..hrm. I am missing some basic pieces to this question.

 

[tecnologic]

Ok lets claryfy, the general field weakening is used when the supply voltage is insufficent to reach the current command.

What is special about IPMs is the additional reluctance Torque component. To utilize this torque component to reach max motor efficiency u need to run a d current lower than zero regardless of the rpm and needed voltage to reach the current command.

 

[larsb]

I'm all with you on this for the IPM motors, it's just that i don't understand lebowskis field weakening post fully. I'll try to read up on this and the basics.

Another thing that the reluctance torque on IPMs does is that the correct phase angle is so much more undefined..

It would be so interesting to see the optimisation of the magnet strength for an IPM motor.

How does all this affect kV for an IPM by the way? Is it actually reaching higher torque density / lower kV than a comparable SPM by the Id current?

So:
SPM with FW gets a "higher" kV
IPM with FW gets a "lower" kV

But IPM with FW above a certain limit gets "higher" kV - i guess?

 

[tecnologic]

For IPMs and SPMs its the same id <0 lowers the Kv so reduces magnetic torque. But reluctance torque is added for IPMs. Have a look at the curves in the ti talk emoto linked to the thread. At 17min there is a slide with the torque curves over current angle.

 

[madin88]

The flux path i was discussing there was the one from the V grove rotor not the qs3000w u posted up front. For the qs3000w u are right there is not much reluctance torque possible, it will only come by the magnets saturating the stator iron (5-10% max).

The pictured rotor with the V-groove magnets is QS3000W midmotor!
So unfortunately it really doesn't have much potential for additional reluctance torque due to the high magnet coverage, or with other words those very thin steel sections between the magnet poles don't allow it.

For IPMs and SPMs its the same id <0 lowers the Kv so reduces magnetic torque. But reluctance torque is added for IPMs.

if magnetic torque will be reduced it will lower kT and kV will be increased. typo?

On the second point, when field weakening is applied you don't get those damaging or regen causing voltages out of the motor ( it reduces the back emf of the motor to be able to add further rpms :wink: ) -but that's all going well, let us ponder what might happen at 250% base speed and you get a fault condition..
-so for ultimate reliability, your right.

Oh you will get those damaging regen because of the too high voltage from the motor!
It is right that as long as throttle or field weakening current is applied to the motor, the kV and effective BEMF voltage will be lower, but imagine what happens if you now release the throttle at this moment.
Due to the body diodes in the FET's, any voltage on motor side which is higher than battery voltage will lead to a current flow into the battery, and if you look at the circuit diagram of a Mosfet power stage, you will see the diodes do the same as a 3-phase rectifier.
I have seen regen current on Adaptto if (1/kV)x RPM starts getting higher as battery voltage, and also on Nucular controller which does even things like -1000W of regen each time a release the throttle (RPM was about 20-30% higher as base speed, or as kV x battery voltage would allow).

So you won't be able to get 200% more base speed as long as the controller has Mosfet with body diodes. Thats whishful thinking, but not possible or better call it not that practical to have regenerative braking each time you release the throttle.

On bigger controllers which have IGBT's instead of FET's, things are probably different because IGBT's don't have body diodes, but also FET's can come without...

 

[larsb]

Madin, your post would indirect mean that field weakening cannot be used? But it can?
If the throttle is released at high speed and you get 1000w regen then that's a showstopper.. Very nasty if the circuit is always closed and regen cannot be controlled.. it would also mean that you'd always have regen in a downhill run and controller couldn't stop overcharging/overvolting etc if you didn't have a freewheel.

 

[tecnologic]

The flux path i was discussing there was the one from the V grove rotor not the qs3000w u posted up front. For the qs3000w u are right there is not much reluctance torque possible, it will only come by the magnets saturating the stator iron (5-10% max).

The pictured rotor with the V-groove magnets is QS3000W midmotor!
So unfortunately it really doesn't have much potential for additional reluctance torque due to the high magnet coverage, or with other words those very thin steel sections between the magnet poles don't allow it.

I was referring to the pic from emoto that he posted after u. For your motor there u are right.

And yes was a typo it increases Kv.

Edit:

0 Torque does not mean 0 current in the fieldweakening area. For 0 torque u have to provide a minimal d current or use the automotive solution and 0 Torque just equals slightly below 0 Torque. These controllers should change their FW handling.

 

[J0nathan]

Madin, your post would indirect mean that field weakening cannot be used? But it can?
If the throttle is released at high speed and you get 1000w regen then that's a showstopper..

For no torque at zero throttle the controller just needs to apply some value of Id (for an SPM) or Id,Iq (for an IPM) which clamps the stator voltage below the battery.

If the controller cuts out entirely, then you're on your own.

 

[larsb]

If a motor is freewheeling at speeds above kv*V and circuit is always closed then the generated voltage will be higher than battery voltage (which i think is what madin is referring to) so controller cannot put out any current.. This is a paradox to me..

Mustn't circuit be open?

 

[madin88]

Madin, your post would indirect mean that field weakening cannot be used? But it can?

It can be used, but i am quite sure there is a limit because of the body diodes in the Mosfets. The theory does it tell us (circuit diagram of a Mosfet power stage), and in practice i noticed that it is around 20-30% gain in RPM above what Vbatt x kV would allow.

If the throttle is released at high speed and you get 1000w regen then that's a showstopper..

Yes, at excessive field weakening the motor brakes a little bit until int's generated voltage is "safe" or less than battery voltage.

Imagine, as long as the controller is pushing field weaken current into the motor, the effective BEMF will be lower as battery voltage. But as soon as you stop to apply this FW current (release throttle at high speed), the generated voltage jumps up to (1/kV) x RPM. Now if the generated voltage is more than a few volts higher than battery voltage, current will flow from the motor into the battery.

 

[tecnologic]

U are right maddin, but this is due to false FW handling by those controllers. For rpm/Kv > V battery id must always be != 0. regardless of torque command. Every industry frequency converter i know is handling it like this.

 

[J0nathan]

If a motor is freewheeling at speeds above kv*V and circuit is always closed then the generated voltage will be higher than battery voltage (which i think is what madin is referring to) so controller cannot put out any current.. This is a paradox to me..

Mustn't circuit be open?

The back-emf voltage from the motor still exists (i.e. appears in the equations) during field weakening, the trick is that if properly controlled the voltage you are applying from the controller is no longer in phase with the back-emf. Therefore, the voltage "seen" at the controller is lower than kv*V.

 

[tecnologic]

If a motor is freewheeling at speeds above kv*V and circuit is always closed then the generated voltage will be higher than battery voltage (which i think is what madin is referring to) so controller cannot put out any current.. This is a paradox to me..

Mustn't circuit be open?

The back-emf voltage from the motor still exists (i.e. appears in the equations) during field weakening, the trick is that if properly controlled the voltage you are applying from the controller is no longer in phase with the back-emf. Therefore, the voltage "seen" at the controller is lower than kv*V.

With just appling a voltage not in phase with the bemf u are not lowering the bemf peak to peak voltage and there for u still have the Problem of uncontrollable current flow.

U NEED id to activly lower the magnetic flow seen from the stator windings this will keep the bemf under the battery voltage

 

[J0nathan]

With just appling a voltage not in phase with the bemf u are not lowering the bemf peak to peak voltage and there for u still have the Problem of uncontrollable current flow.
U NEED id to activly lower the magnetic flow seen from the stator windings this will keep the bemf under the battery voltage

Agreed - we don't "just apply" any voltage, the voltages are controlled accordingly with the desired values of Id,Iq. However I was suggesting a different perspective, to think of these voltages as phasors to aid understanding.