# Switchable Stator Field



## major (Apr 4, 2008)

Jordysport said:


> Firstly on such a simple thing i should really know and be able to get my head around but some things and some points in the day you can't get your head around. this is one for me.
> 
> Switchable Stator Field on the Kostov 9" Series or Parallel? now the application is for racing so immediately people say Parallel, but why?
> 
> ...


It is just a method of field weakening. I just went thru a similar query last week. http://www.diyelectriccar.com/forums/showthread.php?p=331023#post331023 Your motor was set up for the dual field from the factory and has interpoles (I think) so should run either way. The series field connection would have 4 times the series field resistive loss and therefore the S-P connection is likely preferred. But check with Kostov.

A current limit statement from the motor supplier would likely apply to either field connection as it is probably due to the brush/commutator limitation.


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## Jordysport (Mar 22, 2009)

major said:


> It is just a method of field weakening. I just went thru a similar query last week. http://www.diyelectriccar.com/forums/showthread.php?p=331023#post331023 Your motor was set up for the dual field from the factory and has interpoles (I think) so should run either way. The series field connection would have 4 times the series field resistive loss and therefore the S-P connection is likely preferred. But check with Kostov.
> 
> A current limit statement from the motor supplier would likely apply to either field connection as it is probably due to the brush/commutator limitation.


Hi Major, Apologies for that, in fact i did read that thread but didn't twig the relevance as the motor was all apart.

The k9 was setup from factory for dual field and it does have interpoles. 

So basically what you are saying is that in Series field mode there will be a considerable amount more heat as you've got that 4 times more resistance? 

and although the battery voltage is lower at 170V it produces only 9.1% less torque. 

BUT i don't know if i can afford to loose that much torque, is there some advantage in running it in S-P in terms of overloading (Amps) 

just for arguments sake 170V 1000A in S-P mode. 

I will give Plamen an email see what his thoughts are on it. 

Again many thanks Major


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## Plamenator (Mar 6, 2009)

Actually parallel mode is semi-parallel so resistance is only 2x smaller than in series mode.
We designed the motor with SJR in mind so official amp limit is 600A.
In practice I would not pump more than 700A through the motor.
I know that way back, EVnetics did a test with 900-1000A where motor melted in ~10sec.

I would think that parallel is better for racing mostly from the battery's point of view. If you have a 220V battery, this will usually sag significantly at 600-700A. Appart from power loss this would mean that you will not be able to reach max motor rpm as both amps are higher and voltage lower.
In parallel you will not have this problem and will be able to stay in low gear longer until motor reaches its top rpm (remember motor reaches 7000rpm here at 170V; a 600A overload will reduce rpm but you will have the extra voltage to go to ~200V to increase rpm).
And stator does run colder in parallel...


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## Jordysport (Mar 22, 2009)

Plamenator said:


> Actually parallel mode is semi-parallel so resistance is only 2x smaller than in series mode.
> We designed the motor with SJR in mind so official amp limit is 600A.
> In practice I would not pump more than 700A through the motor.
> I know that way back, EVnetics did a test with 900-1000A where motor melted in ~10sec.
> ...


Hi Plamen, Thanks for your response. 

On the cells its from other peoples testing at only 600A they will sag from 3-5% so worst case will be 211V after sag.

I can't really change the battery boxes now, but from what you say if i put 211V through the motor in parallel and 600A and limit the RPM in the controller. would that yield more performance then 211V (after sag) 600A in series mode.


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## major (Apr 4, 2008)

Plamenator said:


> Actually parallel mode is semi-parallel so resistance is only 2x smaller than in series mode.


Hi Plam,

Good to see you here again  Field switching used to be common before the modern solid state motor controllers. The most common configuration was (is) to use the 4 coils in either a series (S) or series-parallel (S-P) connected circuit. In the S mode, the field resistance will be 4 times the resistance of each coil. In the S-P mode, 2 coils are connected in series with each other and the 2 pairs of series connected coils are connected in parallel. This S-P connection yields a total field resistance equal to the resistance of a single coil. So if the single coil resistance is R, then the S resistance = 4*R and the S-P resistance = R.

However the field strength in AT (Ampere Turns) is just double for the S vs the S-P connection.

There is an alternate method if the motor is designed with the 2 coils on opposite poles to use 4S for strong field and 2S for weak field. This gives the 2:1 ratio in field strength and a 2:1 ratio in field resistance. The advantage over the S-P method is that there is only one contactor required when field switch is to be done on the fly.

Regards,

major


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## Plamenator (Mar 6, 2009)

Jordysport said:


> Hi Plamen, Thanks for your response.
> 
> On the cells its from other peoples testing at only 600A they will sag from 3-5% so worst case will be 211V after sag.
> 
> I can't really change the battery boxes now, but from what you say if i put 211V through the motor in parallel and 600A and limit the RPM in the controller. would that yield more performance then 211V (after sag) 600A in series mode.


Well, 3-5% is not my experience with Winston 90Ah cells where sag is more like 15-20% at 550A draw.

In my opinion answer to your question is yes. Motor will give better performance at 211V/600A and parallel (Major's S-P is a better way to put it indeed) as rpm will be much higher and in racing this counts.
I must admit that I have not tested it in practice so just an opinion.

Field switching is an old idea indeed - early forklifts without controllers used it for starting.
Kostov Motors is actually called Balkancar "G. Kostov" JSC, Balkancar Holding being a communist world monopoly for producing forklifts - was in the top 3 world producers in the early 80s


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## Plamenator (Mar 6, 2009)

major said:


> Hi Plam,
> 
> Good to see you here again  Field switching used to be common before the modern solid state motor controllers. The most common configuration was (is) to use the 4 coils in either a series (S) or series-parallel (S-P) connected circuit. In the S mode, the field resistance will be 4 times the resistance of each coil. In the S-P mode, 2 coils are connected in series with each other and the 2 pairs of series connected coils are connected in parallel. This S-P connection yields a total field resistance equal to the resistance of a single coil. So if the single coil resistance is R, then the S resistance = 4*R and the S-P resistance = R.
> 
> ...


 
It was my delusion that S-P resistance is 2R but when I think about it, copper cross-section is 2x but length of coils is half so really makes 1R only 
It's a pity armature resistance stays constant...


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