Max erpm and field wakening

Electric Motors and Controllers
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bionicon   1 kW

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Max erpm and field wakening

Post by bionicon » Nov 27 2019 7:42am

Hope this is the right section, its about max erpm and field wakening.
If my controller can do 60000 max erpm my motor has 21polepairs and i am using 20s (72 volt nominal 84 max) and a kv of 34tr/volt.
So i become at max voltage 59976 erpm within the range of my controller, but when i use field wakening will the max erpm raise and do i have to make note of this for max erpm calculation?
Thx.

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madin88   1 GW

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Re: Max erpm and field wakening

Post by madin88 » Nov 27 2019 1:43pm

bionicon wrote:
Nov 27 2019 7:42am
Hope this is the right section, its about max erpm and field wakening.
If my controller can do 60000 max erpm my motor has 21polepairs and i am using 20s (72 volt nominal 84 max) and a kv of 34tr/volt.
So i become at max voltage 59976 erpm within the range of my controller, but when i use field wakening will the max erpm raise and do i have to make note of this for max erpm calculation?
Thx.
Hi, ERPM are always calculated with RPM x pole pairs. It makes no difference if you use field weakening or not.
May i ask which controller and motor you talk about?

bionicon   1 kW

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Re: Max erpm and field wakening

Post by bionicon » Nov 27 2019 2:11pm

Hey, Madin i was talking about a nuc 24fet and a alien 15070 motor https://alienpowersystem.com/shop/brush ... paramotor/
Planning for a light downhillbike arround 35kg :D

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amberwolf   100 GW

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Re: Max erpm and field wakening

Post by amberwolf » Nov 27 2019 2:19pm

I don't know if it's relevant, there is a thread by Lebowski where he added external field coils to change the field, which increased the eRPM of the system. Not sure if field weakening would do the same thing or not, as I don't really understand most of what's in taht thread. :oops:

viewtopic.php?f=30&t=72692&hilit=erpm+field+weakening

bionicon   1 kW

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Re: Max erpm and field wakening

Post by bionicon » Nov 27 2019 2:27pm

amberwolf wrote:
Nov 27 2019 2:19pm
I don't know if it's relevant, there is a thread by Lebowski where he added external field coils to change the field, which increased the eRPM of the system. Not sure if field weakening would do the same thing or not, as I don't really understand most of what's in taht thread. :oops:

viewtopic.php?f=30&t=72692&hilit=erpm+field+weakening
I dont undersand it also not :oops:

larsb   1 MW

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Re: Max erpm and field wakening

Post by larsb » Dec 02 2019 12:45pm

Field weakening doesn’t change the erpm limit of controller (what switching speed it can control)

it makes the motor run faster on a given voltage, that is possible until controller goes out of control somewhere above max erpm, possibly with a BOOM :D
Ride on!

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Re: Max erpm and field wakening

Post by marcos » Dec 07 2019 10:51pm

Different conceps there
* Max eRPM refers to the base frequency of the motor drive. If controller states 60kerpm then its PWM can generate 1kHz sinewaves while keeping the motor under control.

* Max switching freq refers to the transistor switching freq, it could be 15, 18, 21kHz for example. Note how different is base freq (1kHz) from sw freq (20kHz).

Both are somewhat related, but you can't exceed any of those figures. Can't go above 1kHz and can't switch faster than X kHz.
The base concept is that if you want to create a 1kHz sine with a 5kHz PWM freq, the result won't be a pretty sine as you only make 5 pwm pulses per cycle.
If you are switching at 20kHz, every sine period has 20 events so it will look more like a sine, and you read currents and voltages between switches so you have more data coming from the motor, and less guessing.

Most likely, in order to exceed 60kERPM the controller would need higher PWM freq to have a tigther control at high rpm, but more frequent switches mean higher losses, and also means more incoming analog data that needs processing and will just crush the CPU. Both losses and CPU usage are hard limits so thats why a controller has limited erpm and sw freq.

Field weakening algorithm could make it spin at 100,000eRPM, like it had a higher voltage battery, but the drive will lose control because of too few switching events per revolution.

In practice SVM comes with some tricks that improve high rpm situation, and there are tricks to sample twice per switching period, but unless you have very deep control of the motor drive you have to stick with the datasheet limits.
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Re: Max erpm and field wakening

Post by madin88 » Dec 08 2019 6:08am

marcos, thanks for the detailed description. Very well explained and easy to understand.
bionicon wrote:
Nov 27 2019 2:11pm
Hey, Madin i was talking about a nuc 24fet and a alien 15070 motor https://alienpowersystem.com/shop/brush ... paramotor/
Planning for a light downhillbike arround 35kg :D
Nucular controller does work with RC motors and it can do quite high ERPM on them, but most certainly you cannot use the rated amps.
The higher the ERPM, the less phase amps it can do.
In my case it works the following way: max amps work up to 5-10k ERPM (which is 10-20kmh) before it cuts out.
For a stable 50k ERPM (at WOT) i have to reduce amps to 70-75% or something like that.

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Re: Max erpm and field wakening

Post by bionicon » Dec 08 2019 6:42am

madin88 wrote:
Dec 08 2019 6:08am
marcos, thanks for the detailed description. Very well explained and easy to understand.
bionicon wrote:
Nov 27 2019 2:11pm
Hey, Madin i was talking about a nuc 24fet and a alien 15070 motor https://alienpowersystem.com/shop/brush ... paramotor/
Planning for a light downhillbike arround 35kg :D
Nucular controller does work with RC motors and it can do quite high ERPM on them, but most certainly you cannot use the rated amps.
The higher the ERPM, the less phase amps it can do.
In my case it works the following way: max amps work up to 5-10k ERPM (which is 10-20kmh) before it cuts out.
For a stable 50k ERPM (at WOT) i have to reduce amps to 70-75% or something like that.
What controller allow you to make this settings that it has max phase till 10-20km/u and then reduce it?

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madin88   1 GW

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Re: Max erpm and field wakening

Post by madin88 » Dec 15 2019 6:04am

bionicon wrote:
Dec 08 2019 6:42am

What controller allow you to make this settings that it has max phase till 10-20km/u and then reduce it?
I don't know but the developer knows about that and tries to solve it.
RC motors making it very hard for the controller not only due to the high ERPM but also because of the low inductance which leads to high ripple current (eg. you set 400A, but it will peak above 500A and cut out).

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Re: Max erpm and field wakening

Post by larsb » Dec 15 2019 7:43am

Isn’t one of the reason for the trouble with RC motors that they are driven like crazy above saturation as they’re optimised for weight with limited steel so inductance gets even lower than originally?
Ride on!

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madin88   1 GW

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Re: Max erpm and field wakening

Post by madin88 » Dec 15 2019 11:56am

larsb wrote:
Dec 15 2019 7:43am
Isn’t one of the reason for the trouble with RC motors that they are driven like crazy above saturation as they’re optimised for weight with limited steel so inductance gets even lower than originally?
Good question. Before i had 12F controller and it did cut out above 150A phase which should be well below saturation point so i believe that it has nothing to do with the problem.

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