# Series or Parallel?



## valerun (Nov 12, 2010)

Hi Guys, 

hoping one of you motor gurus can help me out a bit. Am getting a bit confused on what the optimal way to run my Kostov 11" SFM/PFM switchable motor is. I now have a 256V nominal pack and Soliton1 controller. To date, I was running the motor in SFM mode due to higher torque constant in that mode. Before I upgraded my pack from 192V to 256V nominal, I was lacking severely in top end RPM. Now things seem to be back on track BUT here is the question:

What is the best way to run this motor with a 256V pack? After I thought a bit about the whole torque constant thing I realized that it doesn't really matter as I can always use the gearing in a standard 5 tranny to compensate... 

How would you decide which way to run such a motor given the pack voltage? How would you figure the battery sag into your decision? Is it thermally better for the motor to run in PFM or SFM? The efficiency seems to be the same...

Thanks!!!
Valery.


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## major (Apr 4, 2008)

valerun said:


> Hi Guys,
> 
> hoping one of you motor gurus can help me out a bit. Am getting a bit confused on what the optimal way to run my Kostov 11" SFM/PFM switchable motor is. I now have a 256V nominal pack and Soliton1 controller. To date, I was running the motor in SFM mode due to higher torque constant in that mode. Before I upgraded my pack from 192V to 256V nominal, I was lacking severely in top end RPM. Now things seem to be back on track BUT here is the question:
> 
> ...


Hi Valer,

I guess you mean PFM = Parallel Field Mode and SFM = Series Field Mode????

Anyway, here's my take on it. It is a 4 pole motor with 4 field coils, one on each pole. Internally the field coils are wired in pairs, each pair brought out to a set of 2 "S" terminals for a total of 4 "S" terminals. Your options are to run these pairs of coils in series (SFM) or in parallel (PFM). For example purpose let's consider the 2 coils inside the motor to be wired in series with each other. Let's also say each coil has 12 turns.

In SFM each field coil sees full armature current (Ia) and then has 12 * Ia ampere turns (AT). Effectively the field has 12Ia AT/p (ampere turns per pole). If each field coil has RΩ, then the field circuit has 4RΩ.

In PFM each field coil sees ½Ia and then 6Ia AT. Effectively the field has 6Ia AT/p. That is half the excitation of the SFM or a 50% field weakened (FW) state. The effective resistance of the field circuit will be RΩ or one fourth of the effective field resistance of the SFM.

Due to the decrease resistance in the field circuit, PFM is often slightly more efficient than SFM due to less Ohmic loss.

Ideally there is an optimum ratio of field AT to armature AT. It turns out that practically speaking, this ratio is a range, typically maybe about plus or minus 25%. With a 2 to 1 choice of field strength (50% FW), one would think the motor designer has compromised the optimum field strength for SFM or PFM or both. 

I suspect that Kostov has compromised this field strength a little for both the SFM and PFM. They can get away with doing so primarily because they use interpoles which can compensate for commutation difficulty usually associated with running a overly FW motor at overload. 

Having said all that, I'd think you can go to the PFM and run well. At overload, like in current limit, the motor is saturated so I doubt you'll see much torque loss. You will see some additional armature voltage due to the lower drop in the field resistance. Where you will notice a big difference is in the "normal" operating region, say under 400 or 500 amps motor current. Here, in the PFM, there will be less torque for a given motor current. And higher RPM for that given motor current compared to the SFM at same motor voltage.

Like you said, this difference in the running torque and RPM can be altered with the gear ratio selection. In the big scheme of things, I guess I think PFM might be the better choice due to less field watts (a bit higher efficiency and less heat). But you want to be very careful that with this higher RPM and your selected gear you don't overspeed the motor. Watch your tach  Also somehow try to watch for commutation arcing. And see if you notice any loss of torque.

Hope that helps,

major


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## Guest (Jun 6, 2011)

Do you want low end grunt or high end speed. In other words what is your typical speed range? Now you may want to switch that when you go to the track. Can you have it switch when you want from the drivers compartment? If so then it is only a matter of what do you want at the moment. Cream that kid next to you from light to light or a high speed run down the freeway or around the oval track. Or what have you.. 

I'd say for the most part set it up for you average driving speed. It might run smoother and at high rpm where the amp draw is least. It will change the sweet spot. 

Might want to play with that if its not too much trouble then you will know what you want. 

I want to know about how that Kostov runs. I have two older versions before they went to the HV setup. Still they have interpoles which is a good thing. One is going in my VW Bus. Hopefully I can test it before the end of the summer. The MG is taking lots of time. More than I had expected. 

Pete


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## valerun (Nov 12, 2010)

Thanks Jeff & Pete!

Jeff - I had to re-read that reply at least 4 times to let it sink in LOL. 

So, to summarize, I understood that PFM is likely more efficient but the tradeoff is potential commutation problem. Given that I fully plan to run at 1000A up to as high voltage I can get away with, this is a very valid concern ;-)

I think I will follow both of your advices and install 2 contactors switchable from the driver seat to shift from SFM to PFM and back. 

Likely in the next month or so (it's a pain to take out that front battery pack...). Will report the results.

One follow up question, major - by saturation, do you mean that after some point, torque vs current function becomes lower than linear?

Thanks!
Valery.


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## valerun (Nov 12, 2010)

Pete - Kostov is great IF you give it high voltage it really wants ;-) In retrospect, I should have realized that sooner and gone for 60AH cells instead of 100AH. I have already decimated my rear seats for more battery packs LOL.

Once you start, you can't stop, huh? I want more batteries, I want more torque, the cycle of upgrades is ensuing (sway bars, performance suspension, perf clutch, stiffer tranny, etc) ;-) 

EVs are so easy to upgrade - it's like an ultimate toy for a true car enthusiast.

Anyway, getting too philosophical here ;-)

V


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## Tesseract (Sep 27, 2008)

valerun said:


> ...
> So, to summarize, I understood that PFM is likely more efficient but the tradeoff is potential commutation problem. Given that I fully plan to run at 1000A up to as high voltage I can get away with, this is a very valid concern ;-)


Qer actually bothered to calculate the difference in torque/amp for the two modes awhile back and the difference between them was, indeed, around a 20-25% shift in torque/RPM vs. amps/volts. A bit like a gear change change, in other words. Given your pack voltage is so close to the motor's rated voltage, and that you seem fond of pushing 1kA through the motor often, I'd wire the fields in parallel.

That said, Kostov motors are meant to be used at a higher voltage and lower current for a given output power than the equivalent size NetGain motors. They simply won't take 1kA for as long, in other words, and we have empirically confirmed this to be the case, fully destroying (as in requiring a full armature and field rebuild) at least 3 Kostov motors. And they weren't covered under warranty so do keep that in mind, too. It's all fun and games until the field winding catches on fire, or something like that... 



valerun said:


> One follow up question, major - by saturation, do you mean that after some point, torque vs current function becomes lower than linear?


At saturation the torque vs. amps curve changes over from exponential (square) to linear (and RPM vs. volts does, too). Ie - prior to saturation doubling the current quadruples the torque whereas after saturation doubling the current doubles the torque.

This is somewhat oversimplified, but close enough for govt. work.


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## GerhardRP (Nov 17, 2009)

You might want to look at my calculation a while back where I extracted the field maps for that motor look at post #107 here: http://www.diyelectriccar.com/forums/showthread.php/dc-motor-theory-and-model-39931p11.html
The y-axis is in SI units and can be read either as Torque/Armature Amp or Volts/(radian/sec). I am still puzzled by the fact that the parallel and series curves don't scale by a factor of two in current.
You can see the saturation set in at 120A for the series connection.
Gerhard



Tesseract said:


> Qer actually bothered to calculate the difference in torque/amp for the two modes awhile back and the difference between them was, indeed, around a 20-25% shift in torque/RPM vs. amps/volts. A bit like a gear change change, in other words. Given your pack voltage is so close to the motor's rated voltage, and that you seem fond of pushing 1kA through the motor often, I'd wire the fields in parallel.
> 
> That said, Kostov motors are meant to be used at a higher voltage and lower current for a given output power than the equivalent size NetGain motors. They simply won't take 1kA for as long, in other words, and we have empirically confirmed this to be the case, fully destroying (as in requiring a full armature and field rebuild) at least 3 Kostov motors. And they weren't covered under warranty so do keep that in mind, too. It's all fun and games until the field winding catches on fire, or something like that...
> 
> ...


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## Tesseract (Sep 27, 2008)

GerhardRP said:


> You might want to look at my calculation a while back...


Not me, Gerhard, rather the OP, valerun. I have but a passing interest in what a motor may or may not do, and even that is mainly along the lines of "how long before my controllers will blow it up".



GerhardRP said:


> ...
> The y-axis is in SI units and can be read either as Torque/Armature Amp or Volts/(radian/sec)....


I missed that graph, but it is very interesting. A suggestion, if I may: describing rate of rotation in rad/s is a particularly obnoxious unit, even for an engineer - RPM is likely to be much more appreciated by the average DIYer that just wants to know, "how much voltage will I need to spin the motor at x rpm when loaded down enough to need y amps?"


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## Qer (May 7, 2008)

GerhardRP said:


> I am still puzzled by the fact that the parallel and series curves don't scale by a factor of two in current.


Probably because you only shift the field, the armature stays the same.


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## major (Apr 4, 2008)

Tesseract said:


> ....describing rate of rotation in rad/s is a particularly obnoxious unit,....


I find _*rad/s*_ a beautiful unit  It makes all those pesky numerical unit conversion factors go away. For instance, looking for power of a device rotating at 1000 RPM and providing a torque of 10 lb.ft., you need to divide by 5252 to get 1.9 hp. Using rad/s and Nm, the simple product gives you 1418 W. 104.7 rad/s * 13.55 Nm = 1418 Watts.


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## Qer (May 7, 2008)

major said:


> I find _*rad/s*_ a beautiful unit  It makes all those pesky numerical unit conversion factors go away. For instance, looking for power of a device rotating at 1000 RPM and providing a torque of 10 lb.ft., you need to divide by 5252 to get 1.9 hp. Using rad/s and Nm, the simple product gives you 1418 W. 104.7 rad/s * 13.55 Nm = 1418 Watts.


I think I'm on Majors side on this, but then I'm born in Sweden which were pretty darn fast at adopting SI-units so I've never really used those weird pound, foot, inch, fahrenheit, wheaton, smooth, potrzebie etc.That is, except for converting, of course...


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## EVfun (Mar 14, 2010)

Qer said:


> I think I'm on Majors side on this, but then I'm born in Sweden which were pretty darn fast at adopting SI-units so I've never really used those weird pound, foot, inch, fahrenheit, wheaton, smooth, potrzebie etc.That is, except for converting, of course...


I'll try to give you plenty of reasons to practice unit conversion. I've heard about this new-fangled metric thingy, but I'm not to good at it.


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## electrabishi (Mar 11, 2008)

Valery,
I would be careful that you do not make this switch under power. You might talk with the manufacturer of your controller about problems that may arise. If you are going to put contactor control in a location accessible to the driver I would design some form of interlock to ensure that if one contactor sticks and shorts out a winding, (or the whole motor for that matter) that you are physically not allowed to apply power.

Mike



valerun said:


> .......
> I think I will follow both of your advices and install 2 contactors switchable from the driver seat to shift from SFM to PFM and back.
> 
> Likely in the next month or so (it's a pain to take out that front battery pack...). Will report the results.
> ...


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## valerun (Nov 12, 2010)

electrabishi said:


> Valery,
> I would be careful that you do not make this switch under power. You might talk with the manufacturer of your controller about problems that may arise. If you are going to put contactor control in a location accessible to the driver I would design some form of interlock to ensure that if one contactor sticks and shorts out a winding, (or the whole motor for that matter) that you are physically not allowed to apply power.
> 
> Mike


Thanks Mike.

Qer & Tesseract - as manufacturers of my controller (;-), what do you guys suggest I do here?

V


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## valerun (Nov 12, 2010)

and the mod may come sooner than I expected... All the electric torque from that mighty Soliton1+Kostov11" stripped my rear diff yesterday (while I was smoking a beemer in the next line LOL)... So I am going to put in a few long-overdue mods while the car is in the shop.


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## major (Apr 4, 2008)

electrabishi said:


> Valery,
> I would be careful that you do not make this switch under power. You might talk with the manufacturer of your controller about problems that may arise. If you are going to put contactor control in a location accessible to the driver I would design some form of interlock to ensure that if one contactor sticks and shorts out a winding, (or the whole motor for that matter) that you are physically not allowed to apply power.
> 
> Mike


Mike has a valid concern. You do want to check with your controller guy -)) if you plan on doing the switch from SFM to PFM on the fly. If you break Ia in the motor circuit it will draw a large arc across the opening contacts and also could present a voltage spike back to the controller. 

One way around this is not to open the circuit. In most cases, the scenario I used in post #2 about the coil pairs inside the motor is simplified. This means that the weak field mode will run perfectly fine with just a single set of 2 coils inside the motor instead of the series parallel arrangement. In other words, run 4 coils in series for full field and then short out 2 of those 4 for weak field. This way you don't have to open the armature circuit to switch field modes. Referring back to my example, you still get 12Ia AT/p with 4RΩ in full field and you get 6Ia AT/p with 2RΩ in weak field mode. You can double check with Kostov to make sure.

Regards,

major


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## Tesseract (Sep 27, 2008)

valerun said:


> Thanks Mike.
> 
> Qer & Tesseract - as manufacturers of my controller (;-), what do you guys suggest I do here?
> 
> V


I suggest you totally abandon this idea. There's little benefit to switching the fields in the first place (honestly - do you really need more off-the-line torque? Doesn't sound like it!?!  ), it requires putting contactors into the motor circuit capable of handling 300V/1000A (ie - at least two EV200 contactors per switch, for a total of six) and it risks damaging the controller if the contactors suddenly open-circuit the motor while powered. Note that shorting the motor out won't hurt the controller too much* - it will just go into desat and shut down - but open-circuiting the motor will cause a huge voltage spike, as maj pointed out,and that *will* hurt the controller.

* - not that you want to do this routinely. IGBT modules are typically good for 100 "hard" desaturation events (ie - turning on into a short circuit).


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## valerun (Nov 12, 2010)

Tesseract said:


> I suggest you totally abandon this idea. There's little benefit to switching the fields in the first place (honestly - do you really need more off-the-line torque? Doesn't sound like it!?!  ), it requires putting contactors into the motor circuit capable of handling 300V/1000A (ie - at least two EV200 contactors per switch, for a total of six) and it risks damaging the controller if the contactors suddenly open-circuit the motor while powered. Note that shorting the motor out won't hurt the controller too much* - it will just go into desat and shut down - but open-circuiting the motor will cause a huge voltage spike, as maj pointed out,and that *will* hurt the controller.
> 
> * - not that you want to do this routinely. IGBT modules are typically good for 100 "hard" desaturation events (ie - turning on into a short circuit).


oh snap - that's a second idea in one day that gets shot down! ;-)

Jeffrey - of course I need more torque - what am I replacing the diff, tranny, and clutch for then? ;-)

But it does sound complicated. I thought I'd need just 1 (or maybe 2) contactors to short out the 2 coils per major's suggestion. The interlock could be the zero-current signal from motor ampmeter output of Soliton1.

So how did the hairball interface do it?

Thanks guys!!

V


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## Tesseract (Sep 27, 2008)

valerun said:


> ...But it does sound complicated. I thought I'd need just 1 (or maybe 2) contactors to short out the 2 coils per major's suggestion.


You don't short out any of the coils, what you need to do is change their configuration from series to parallel (preferably above a nominal RPM). If you simplify the field schematic down to a pair of coils (rather than two pair) then it will take a minimum of 3 SPST switches (either NO or NC) to flip the coils from wired in series to parallel. It's not what I'd call complicated, but it is relatively expensive to implement as the "series" switch, at least, needs to be rated for the full motor current (the two "parallel" switches can be rated for half of that). You could use the inexpensive Albright SW200 for this job, but I much prefer the EV200 myself. So at a minimum you need (4) of the buggers, at either $55/ea. if you catch them on eBay, or $115 ea. if not. It just doesn't seem worth it to me, was my earlier point, but that's not the same as saying you can't do it. 



valerun said:


> The interlock could be the zero-current signal from motor ampmeter output of Soliton1.


It's not quite that easy. Let's assume you have a toggle switch to flip the field configuration from series to parallel. If you do this manually then there will be a brief moment when the motor circuit is either shorted or opened (depending on whether the switch contacts make-before-break or break-before-make). This is bad. Much better would be to:

1. have the toggle switch tell the controller (via an aux input) that you want to flip the field configuration from series to parallel (or vice versa).
2. the controller then terminates motor PWM.
3. when motor current hits 0 the series contactor is opened up and the two parallel contactors are closed.
4. motor current is ramped back up at the user-specified rate (some error detection can/should be done prior to this).



valerun said:


> So how did the hairball interface do it?


The Zilla decides when to switch for you based on motor current falling to half maximum allowed while duty cycle is at ~100%. We could do the same, it's just that adding that particular feature would be quite a bit of work for Qer and there really haven't been too many people over the last couple of years that wanted it. Most of our customers just want a sporty, reliable EV, not something they can pit against Wayland on the drag strip.


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## major (Apr 4, 2008)

Tesseract said:


> You don't short out any of the coils,


Hi Tesser,

If Kostov have the coils configured like I think they do, that is the pair on opposite poles, then you can simply short out one coil pair to run in weak field mode. You will be using only 2 of the 4 coils and using the poles on the unenergized coils as consequent poles. It works almost as well as the series parallel approach except you have twice the field resistance, but still half the resistance of the series mode. Magnetically it works out to be the same AT/p as the series parallel mode.

And you only need one contactor. You're just opening and closing it across 2 field coils, so the voltage on the coils from resistance drop is small. However, due to the inductance of the coils, I suspect you would want to switch the contactor at zero current. The contactor would have to carry full motor current.

Regards,

major


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## GerhardRP (Nov 17, 2009)

major said:


> Mike has a valid concern. You do want to check with your controller guy -)) if you plan on doing the switch from SFM to PFM on the fly. If you break Ia in the motor circuit it will draw a large arc across the opening contacts and also could present a voltage spike back to the controller.
> 
> One way around this is not to open the circuit. In most cases, the scenario I used in post #2 about the coil pairs inside the motor is simplified. This means that the weak field mode will run perfectly fine with just a single set of 2 coils inside the motor instead of the series parallel arrangement. In other words, run 4 coils in series for full field and then short out 2 of those 4 for weak field. This way you don't have to open the armature circuit to switch field modes. Referring back to my example, you still get 12Ia AT/p with 4RΩ in full field and you get 6Ia AT/p with 2RΩ in weak field mode. You can double check with Kostov to make sure.
> 
> ...


This is a series-parallel switch scheme I came up with last November. Actually I was thinking of a dual motor situation [White Zombie] where the field coils would remain in series and the armatures would be switched to parallel. Anyway, I think the diode in the middle here solves most of the potential problems. Switch 1 is break-before-make.

Do you think this could operate at power?

Gerhard


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## major (Apr 4, 2008)

GerhardRP said:


> This is a series-parallel switch scheme I came up with last November. Actually I was thinking of a dual motor situation [White Zombie] where the field coils would remain in series and the armatures would be switched to parallel. Anyway, I think the diode in the middle here solves most of the potential problems. Switch 1 is break-before-make.
> 
> Do you think this could operate at power?
> 
> Gerhard


Hi Ger,

Interesting circuit. Might work  You have me thinking about it. After all is done, or thought about, it is probably better to switch at zero current. I'm trying to figure where the stored energy from the coil ends up when switching from full to half current, or vice versa. Even with my method of just shorting 2 coils. If it ends up at the commutator or controller switch/diode, could be trouble. But on forklifts, they switch in (and out) the FW diverter resistor under load.

If Tesseract has one of these dual field Kostovs, maybe he could try it on the dyno in his spare time 

major


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## Batterypoweredtoad (Feb 5, 2008)

I would love for someone to figure out a method that works reliably. I think a dual Kostov 9HV setup with a motor powering the front diff and a motor powering the rear diff would be a great setup with a Soliton 1 that S/P switched.


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