# Field switching as a transmission?



## drgrieve (Apr 14, 2011)

Hi haven't been able to find much on this so I guess it's a bust but just in case ....

Looking at Kostov 11" 250v and they mention :



> The motor's field swithcing option allows driving mods optimal for both racing and city driving. Series field mode gives lower rpm and higher torque both at a reduced amp draw making it ideal for the typical city conditions with frequent starts/stops and low average speed. Switching to parallel field mode increases rpm and nominal amp draw while decreasing torque a bit.


http://kostov-motors.com/tractionmo...c)/seriesdcmotorsforelectricvehicles/k11250v/

Not sure what that means but can that be done while driving or only can be changed in the garage?

And thus if can be done while driving could it be used as an electric transmission?


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## major (Apr 4, 2008)

drgrieve said:


> Hi haven't been able to find much on this so I guess it's a bust but just in case ....
> 
> Looking at Kostov 11" 250v and they mention :
> 
> ...


Hi grieve,

Field switching from fields in series (S) to fields in series parallel (S/P) is just a method of field weakening. S is the strong field and S/P is the weak field mode. When you have a motor built this way (4 field terminals) you can in fact switch from S to S/P on the fly if you install the proper contactor arrangement and required logic circuitry to coordinate the switch with the motor controller. These contactors must carry full motor current and are therefore large and expensive.

What does S to S/P get you in terms of performance? Maybe 10 to 15% more torque per amp in the mid-high load range in S mode. Is it worth it? Your call, but I would think that it would be less expensive, less mass and more reliable to use a 10 to 15% higher current limit and keep it in S/P mode.

Regards,

major


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## DavidDymaxion (Dec 1, 2008)

Just one data point, but I recently tried field parallel vs. series on a GE series motor with 4 field terminals.

Fields in parallel, 12V, gave 3500 rpm and 25 A drawn.

Fields in series, 12V, gave 2800 rpm and 15 A draw.

Unless you are chasing higher rpm it looks like running fields in series is the better deal.


drgrieve said:


> Hi haven't been able to find much on this so I guess it's a bust but just in case ....
> 
> Looking at Kostov 11" 250v and they mention :
> 
> ...


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## major (Apr 4, 2008)

DavidDymaxion said:


> Just one data point, but I recently tried field parallel vs. series on a GE series motor with 4 field terminals.
> 
> Fields in parallel, 12V, gave 3500 rpm and 25 A drawn.
> 
> ...


Hi David,

This was obviously a no load test. From such a test, it is not possible to determine the better way to connect the fields, S or S/P.

For the dual field series motor having both S and S/P connections available, you have a 2 to 1 difference in excitation, or field excitation to armature ratio. For the armature there exists an optimum level of field mmf with respect to its own mmf. The series wound motor is particularly unique in that this ratio remains constant with respect to load. However some adverse effects will occur at certain loads when that field to armature mmf ratio is too far off from its optimum. Typically a range of 1.5 to 1 won't hamper performance much. But a 2 to 1 range is pushing it.

So I suspect that motors like the Kostov dual field designs take a compromise approach and suffer a little in each the S and S/P modes but will run satisfactorily in either. Other motors you may find, like your GE, may not have been designed this way. I don't know what the intended application for that GE motor was. But for example, before modern controllers, some forklift motors would have dual fields where the S connection was used for only starting and were intended to run in S mode just for a short period. When in the S mode, it will have 4 times the field resistance. This was convenient to limit starting current. But also would have 4 times the loss (watts) in the field and result in excessive field heat. So running such a motor in the S mode continually would likely overheat the field coils as well as cause an inefficient power conversion in the armature.

On the other hand, that GE motor may have been designed to run in S mode most of the time and to use the S/P mode for a top speed increase at full throttle after the vehicle had accelerated past the high current draw. You then would be better off running in the S mode permanently. I doubt this was the case, but you never know unless you could find the original application information. You might be able to make an educated guess if you could count the turns in the armature and field, or accurately measure the armature and field resistance.

Without getting into all that, I say it would be better to run in S/P if you have a high frequency controller. If you experience commutation arcing which you cannot correct with advance, consider the S mode. But install field temperature sensors.

Regards,

major


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## gor (Nov 25, 2009)

i expected more s/p difference
looks like on k11 it doesn't worth to bother with series


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## DavidDymaxion (Dec 1, 2008)

Thanks for the info, I hadn't known about the forklift low speed thing. I imagine it would turn quite slowly on just 12V in that mode? I'm also surprised a series motor would overheat its fields, even with the fields in series.


major said:


> ... before modern controllers, some forklift motors would have dual fields where the S connection was used for only starting and were intended to run in S mode just for a short period. When in the S mode, it will have 4 times the field resistance. This was convenient to limit starting current. But also would have 4 times the loss (watts) in the field and result in excessive field heat. So running such a motor in the S mode continually would likely overheat the field coils as well as cause an inefficient power conversion in the armature. ...


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