Field Weakening, what is it?

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spinningmagnets   100 GW

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Field Weakening, what is it?

Post by spinningmagnets » Jul 10 2018 6:52pm

So far, here's what I "think" I know. If your controller has FW, it raises the top speed with no other changes. You might not have as many amps available when the RPM's rise into the added FW range, but a broader range of RPM's is very useful.

Anyone have a link to a website that can explain it to me better?

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Re: Field Weakening, what is it?

Post by major » Jul 10 2018 7:15pm

Voltage is proportional to RPM * field flux. So RPM ~ voltage / field flux. When the voltage is maxed, you can increase RPM by decreasing or weakening the field flux.

But! Torque is proportional to field flux * current. So field weakening will cause an increase in current to maintain torque.

The load curve for the vehicle is such that an increase in speed needs an increase in motor torque. So even more current is required when field weakening is used to increase speed of the vehicle.

Battery voltage droops more with increased current tending to decrease motor RPM. So field weakening has limitations. When correctly applied it can be a nice feature.

Some argue against FW saying if the motor is capable with a weakened field, it was oversized to start with. For a chosen material, the field flux determines the motor size.

Hope that helps.

major

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Re: Field Weakening, what is it?

Post by spinningmagnets » Jul 10 2018 7:17pm

Thanks for that. Does a motor in the FW range of RPMs run hotter? (all other things being equal)

I'm also curious about the observed side-effects that I can expect between two identical controllers, but...one has FW and a higher top speed (no free lunch principle)

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Re: Field Weakening, what is it?

Post by major » Jul 10 2018 7:21pm

spinningmagnets wrote:
Jul 10 2018 7:17pm
Thanks for that. Does a motor in the FW range of RPMs run hotter? (all other things being equal)
I'd say not necessarily. But if it is taking more current, then yes, it will likely run hotter.

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Re: Field Weakening, what is it?

Post by Alan B » Jul 10 2018 9:36pm

Another way of thinking of this is to keep the motor current constant, but adjust the field angle with respect to the permanent magnets. Normally the angle is maintained perpendicular to generate maximum torque. In Field Weakening the angle is adjusted so that some of the field opposes the permanent magnets and reduces the net field, thus changing the motor's effective motor constant, increasing the RPM and reducing the peak torque. It also reduces the field component making torque which further reduces torque production. The motor heating is essentially unchanged because I squared R is the same. If torque were to be maintained it would require increasing motor current which would increase motor heating via I squared R.

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Re: Field Weakening, what is it?

Post by spinningmagnets » Jul 11 2018 10:28am

Thanks, I appreciate all the replies...

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Re: Field Weakening, what is it?

Post by Toorbough ULL-Zeveigh » Jul 31 2018 10:02pm

is this this you that posted the same question?
if not you should have.
compared to E•S da peeple over dare are way smarter more eager to help & just all around better looking.

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Re: Field Weakening, what is it?

Post by LockH » Jul 31 2018 11:40pm

^^ Hehe... Awesome thread anyway. THANKS EVerybuddy!
ES changed my life (for the waaaaay better).

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Joined yer local chapter of EA yet?
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Re: Field Weakening, what is it?

Post by jonescg » Aug 09 2018 1:42am

I love a good technical discussion on ES forum! Except when it gets shouty and personal, of course.

Field weakening is definitely the realm of permanent magnet motors. My personal experience with it is on the race bike - the base speed is about 3900 rpm - that is, for a given DC bus, the inverter will provide three phase AC with a variable frequency up to a point where the motor won't spin any faster. The torque available has been set up to this point. If you want it to spin faster, you have to trade torque for speed, so the power ends up being the same or less despite the faster rpm. This also means less power is drawn from the battery.

The inverter is effectively retarding it's timing so the field is no longer synchronous with the rotor. it does this by supplying current to the stator ahead of the rotor, and is defined by Id in the EEPROM.

In videos of the bike accelerating onto a straight, you can see the voltage sag down, the current climb up, until it hits about 200 km/h. At this point the sag reverses, the battery current drops yet the bike is going faster still - up to about 255 km/h. We kind of top out there. So the last 55 km/h takes longer to achieve, and is representative of the field weakening region. There's a link in my signature.

But, ask Miles - he knows far more about this than me :)

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Re: Field Weakening, what is it?

Post by spinningmagnets » Aug 09 2018 5:38pm

Jonescg, thank you for taking the time to type out a reply. I am seeing more and more info on the web about FW. I want to understand the benefits and drawbacks so I can create an intelligent response about FW. It is possible to build an electric motorcycle with no FW that has the same top-speed as a second E-moto that has a lower-speed system but has FW to achieve the same top-speed.

Luke has made a persuasive argument for using a higher-Kv / lower turn-count motor, and using half-throttle to achieve cruise speed, as opposed to having a voltage and Kv that achieve top-speed at Wide Open Throttle (WOT). I am trying to figure out where FW fits into this...

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Re: Field Weakening, what is it?

Post by billvon » Aug 09 2018 5:51pm

jonescg wrote:
Aug 09 2018 1:42am
Field weakening is definitely the realm of permanent magnet motors.
Well, field weakening started in DC brushed motors as a way to change base speed. One of the first EV's I ever worked on used a separate excitation coil to generate the field, and the system would reduce the current to the field in order to increase RPM. (BTW this is exactly how alternators work but in reverse.)

Field weakening in a permanent magnet motor involves a slight penalty in terms of efficiency, because current you would ordinarily use to provide torque goes into opposing the field. But overall it can actually improve efficiency if it allows you to select a motor that spins at or close to base speed for its most common speed. i.e. if you have a bike that you want to run at 25mph with occasional bursts to 35mph, you can choose a motor with a base speed closer to 25mph (so your controller runs more efficiently) but still get the speed range.

One interesting thing I notice with phase advance/field weakening is that when you get to the motor's base speed and keep accelerating you don't notice any difference. But when you get off the throttle (i.e. motor current goes to zero) you get very strong braking as the inverter's diodes rectify the motor's voltage (now higher than the battery's voltage) and feeds it back to the battery. Once you go under base speed it stops regen'ing.
--bill von

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Re: Field Weakening, what is it?

Post by Alan B » Aug 09 2018 8:59pm

FW, like so many things, is part of a set of multiple tradeoffs that don't always have a simple answer, despite what we would like.

Fewer motor turns generate their own problems, and at least one EV uses FW as part of their system. You'd think they could get whatever motor they want. But when you have a vehicle that needs to have a high top speed that is rarely (if ever) used, the designer may choose to make the system more efficient where it lives, and allow the efficiency to suffer a bit at the top end that most EVs will never even operate at.

I would also point out that the strong regen deceleration mentioned above is apparent because the software chooses to implement it that way (a very simple approach), it could easily hide that behavior by maintaining enough FW current to just prevent the regen based on motor RPM.

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Re: Field Weakening, what is it?

Post by major » Aug 09 2018 9:33pm

spinningmagnets wrote:
Aug 09 2018 5:38pm
...
Luke has made a persuasive argument for using a higher-Kv / lower turn-count motor, and using half-throttle to achieve cruise speed, as opposed to having a voltage and Kv that achieve top-speed at Wide Open Throttle (WOT). I am trying to figure out where FW fits into this...
That's what I was talking about in the last paragraph of my first post.
major wrote:
Jul 10 2018 7:15pm
Some argue against FW saying if the motor is capable with a weakened field, it was oversized to start with. For a chosen material, the field flux determines the motor size.

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Re: Field Weakening, what is it?

Post by major » Aug 09 2018 9:36pm

billvon wrote:
Aug 09 2018 5:51pm
Well, field weakening started in DC brushed motors as a way to change base speed. ...
FW doesn't change the base speed of the motor. It allows operation above base speed.

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Re: Field Weakening, what is it?

Post by billvon » Aug 10 2018 1:29am

major wrote:
Aug 09 2018 9:36pm
FW doesn't change the base speed of the motor. It allows operation above base speed.
For DC motors, base speed is determined by rotor field strength (stator field strength in the case of a traditional brushed motor.) Change the field strength, change the base speed.

In permanent magnet DC motors you can't change the field strength easily, so there's one defined base speed. However, in traditional (field winding) motors you can set the base speed to whatever you like.
--bill von

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Re: Field Weakening, what is it?

Post by major » Aug 10 2018 9:38am

billvon wrote:
Aug 10 2018 1:29am
major wrote:
Aug 09 2018 9:36pm
FW doesn't change the base speed of the motor. It allows operation above base speed.
For DC motors, base speed is determined by rotor field strength (stator field strength in the case of a traditional brushed motor.) Change the field strength, change the base speed.

In permanent magnet DC motors you can't change the field strength easily, so there's one defined base speed. However, in traditional (field winding) motors you can set the base speed to whatever you like.
dc_regions.jpg
Base Speed
dc_regions.jpg (12.28 KiB) Viewed 7378 times
Operating Regions
The region below base speed where armature voltage control may be applied is called the constant torque region. Above base speed, the flux is reduced and the motor enters the "field weakening region".This region is also sometimes known as the constant power region.
From: http://people.ucalgary.ca/~aknigh/vsd/i ... oices.html

You can find other references which support my position. One definition of base speed for the shunt (or separately excited) DC motor is that RPM at rated armature voltage and full rated field voltage. When field voltage is less than rated, operation above base speed is possible.

Regards,

major

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Re: Field Weakening, what is it?

Post by Ianhill » Aug 10 2018 10:51am

A guy named Sam decok and his clang verbally attacked me on the ES Facebook page for explaining field weakening with similar info as stated above.

They wanted to know why a hub was not making anywhere near its no load speed and I said it needed more torque to reach its no load speed it could be achieved through not overvolting the motor in the first place, more amps or a rim reduction without rewinding etc.

But MR decok knew better and said field weakening was the way forward, its the way of getting the hot motor working at its limit to work even harder and slip further from its no load speed into the realms of melt down.

They were nice enough mind to call me a smackhead, said i should hand my qualifications back in engineering and his side kick said he would smash me face to face lmao shows their age and stupidity, little does he know to pick a fight size up your opponent first outside and in or you may end up on the floor in a position you have no control over what happens next, a well trained person has restrain and no need to prove what they know or how dangerous they are.

Left Facebook for good past 6 months and just use email and this forum with a bit of YouTube, keep my distance from being hassled.
Last edited by Ianhill on Aug 10 2018 11:34am, edited 1 time in total.

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Re: Field Weakening, what is it?

Post by billvon » Aug 10 2018 11:07am

major wrote:
Aug 10 2018 9:38am
You can find other references which support my position. One definition of base speed for the shunt (or separately excited) DC motor is that RPM at rated armature voltage and full rated field voltage. When field voltage is less than rated, operation above base speed is possible.
OK; we are just talking about nomenclature now. Whether you call it a new base speed or say "it's operating above the base speed with a weaker field" the result is the same - a new motor curve where the top speed is higher and the available torque is less.

BTW if you damage a PM BLDC motor by overheating and slightly demagnetizing the rotor you get the same effect - a new base speed (higher than it was before) and less available torque.
--bill von

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Re: Field Weakening, what is it?

Post by jonescg » Aug 10 2018 9:06pm

Ianhill wrote:
Aug 10 2018 10:51am
A guy named Sam decok and his clang verbally attacked me on the ES Facebook page for explaining field weakening with similar info as stated above.

They wanted to know why a hub was not making anywhere near its no load speed and I said it needed more torque to reach its no load speed it could be achieved through not overvolting the motor in the first place, more amps or a rim reduction without rewinding etc.

But MR decok knew better and said field weakening was the way forward, its the way of getting the hot motor working at its limit to work even harder and slip further from its no load speed into the realms of melt down.

They were nice enough mind to call me a smackhead, said i should hand my qualifications back in engineering and his side kick said he would smash me face to face lmao shows their age and stupidity, little does he know to pick a fight size up your opponent first outside and in or you may end up on the floor in a position you have no control over what happens next, a well trained person has restrain and no need to prove what they know or how dangerous they are.

Left Facebook for good past 6 months and just use email and this forum with a bit of YouTube, keep my distance from being hassled.
Like I said in my first post - I love a good technical discussion, but the moment people start calling names and appealing to authority you know it's time to just walk away. If there's a misunderstanding of a concept, a particular nuance or a poorly defined term, it's easy for this sort of thing to get out of hand. Sorry to hear about your experience.

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Re: Field Weakening, what is it?

Post by Ianhill » Aug 11 2018 9:08am

jonescg wrote:
Aug 10 2018 9:06pm
Ianhill wrote:
Aug 10 2018 10:51am
A guy named Sam decok and his clang verbally attacked me on the ES Facebook page for explaining field weakening with similar info as stated above.

They wanted to know why a hub was not making anywhere near its no load speed and I said it needed more torque to reach its no load speed it could be achieved through not overvolting the motor in the first place, more amps or a rim reduction without rewinding etc.

But MR decok knew better and said field weakening was the way forward, its the way of getting the hot motor working at its limit to work even harder and slip further from its no load speed into the realms of melt down.

They were nice enough mind to call me a smackhead, said i should hand my qualifications back in engineering and his side kick said he would smash me face to face lmao shows their age and stupidity, little does he know to pick a fight size up your opponent first outside and in or you may end up on the floor in a position you have no control over what happens next, a well trained person has restrain and no need to prove what they know or how dangerous they are.

Left Facebook for good past 6 months and just use email and this forum with a bit of YouTube, keep my distance from being hassled.
Like I said in my first post - I love a good technical discussion, but the moment people start calling names and appealing to authority you know it's time to just walk away. If there's a misunderstanding of a concept, a particular nuance or a poorly defined term, it's easy for this sort of thing to get out of hand. Sorry to hear about your experience.
Thank you jonescg.
I did let it wind me up at the time as he was talking crap and calling me fake news, it went on for around 10 hours and loads of messages from multiple people.

Thing is they were so stupid they Google'd field weakening and posted the answer from wiki which contradicted what he was saying, my concern is for the folks that dont understand and will take a bite of his turd sandwich.

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Re: Field Weakening, what is it?

Post by arber333 » Aug 21 2018 6:34am

Here is my empirical 5c.

Anywhere in mechanical engineering we are constrained with power and performance. Mechanical power equasion here is directly proportional with equasion for electrical power, with some loss of course.

Pin = I * V cca. Pout = τ * ω

Since current is proportional to torque τ , if you want more power at reasonable current best is to increase RPM. Energy density increases that way with mechanical inertia of course...
HOWEVER lots of amps are wasted with cheating the magnetic vector offset from centerline. This keeps the rotor spinning but also increases load to winding without doing the actual work other than heating up and pushing rotor against the bearings.
I think there was a discussion of using inverter and motor to faster heat-up batteries in winter by applying FW to burn more current...

ACIM motors actually depend a lot on using FW when running in constant regime. They like to rev high, but they can run hot there also. I am using some such scheme myself. I had my motor rewound at lower turns to keep it in the constant torque region more time. In reality that made motor more sensitive to voltage shift and drew more amps. In the begginning it was very sharp to drive, but at the end of SOC it was a slug. Now i convinced a friend to program a loop in inverter code to keep FW point moving with voltage and add some parameters. You cant believe what a difference this makes in user experience. Winter driving now is totaly unaffected by cell voltage drop.
In effect i optimised my power curve.

If i wish i could also run motor at lower voltage, where system shifts FW point back and motor reaches FW sooner. But i would rather have more voltage and shift FW point to area beyond 180Hz!

EDIT: Yes even DC motors can benefit from FW. Problem starts when rotor cant handle G loading and desintegrates or comutator starts arcing.
Also with ACIM and PMSM you have to be carefull not to colapse magnetic field accidentally. The offset would kill your IGBTs. That is why rotor position is very important in FW.

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