# Sepex differences



## somanywelps (Jan 25, 2012)

Yes I searched the forums, it was mostly speculation. 

The three reported differences are torque, heat, and efficiency. 

Kostov's site states that they didn't see an increase in efficiency, and their performance charts reflect that. 

They also show a massive drop in torque, to 105nm from 185nm on their on their 160V Sepex and 192V Series 11" motors respectively. 

Furthermore it was reported online that Sepex motors of this size produced approximately 1/3rd the heat at full load (which would imply higher efficiency?), but more heat at "creeping" speeds. 

We know they have regen (non-debatable). 

My questions are the following: 

Do Sepex's really produce that little torque? 

Do Sepex's really produce that little heat? 

Are Sepex's really no more efficient than a series DC motor? 

Please assume the series motor is rated for the exact same voltage and the armature is receiving the exact same voltage (and amperage) so power (KW).


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## major (Apr 4, 2008)

somanywelps said:


> Do Sepex's really produce that little torque?
> 
> Do Sepex's really produce that little heat?
> 
> ...


No, no, and yes.


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## somanywelps (Jan 25, 2012)

major said:


> No, no, and yes.


Thanks,

Then on the issue of torque, is it basically the same or just a little lower?


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## Tesseract (Sep 27, 2008)

No. Torque is proportional to flux * armature current in all DC motors. The field winding is used to provide the flux for all dc motors.

The two reasons why regen is easy with sepex are because the flux can be held constant while armature RPM and/or current varies and it's easy to flip the field polarity around (relatively low current compared to the armature).

The reasons why you don't see us, for example, making a sepex controller are because it requires a more complicated controller with much more complicated software, a map of armature voltage vs. field current vs. RPM is required for each motor, and sepex motors still have brushes so the anti-brush crowd still hates them. AC motors have all the same disadvantages except the brushes, so might as well make an inverter instead of a sepex controller.


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## major (Apr 4, 2008)

somanywelps said:


> Then on the issue of torque, is it basically the same or just a little lower?


What issue?


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## somanywelps (Jan 25, 2012)

Tesseract said:


> No. Torque is proportional to flux * armature current in all DC motors. The field winding is used to provide the flux for all dc motors.
> 
> The two reasons why regen is easy with sepex are because the flux can be held constant while armature RPM and/or current varies and it's easy to flip the field polarity around (relatively low current compared to the armature).
> 
> The reasons why you don't see us, for example, making a sepex controller are because it requires a more complicated controller with much more complicated software, a map of armature voltage vs. field current vs. RPM is required for each motor, and sepex motors still have brushes so the anti-brush crowd still hates them. AC motors have all the same disadvantages except the brushes, so might as well make an inverter instead of a sepex controller.


Thanks.

Unfortunately AC is still far more expensive than DC Brushed due to the availability and general willingness of the companies; so if you're going to make an inverter and have convinced netgain to make an AC (or Kostov to beef up theirs), you might be able to steal some of the OEM market...

Also the inverter and AC motor choices are slim and relatively weak power-wise compared to what we have now for DC.



major said:


> What issue?


"Subject" would be a synonym to the way I used it. If there's not a massive torque loss, it's not something to worry about.


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## mizlplix (May 1, 2011)

"Also the inverter and AC motor choices are slim and relatively weak power-wise compared to what we have now for DC."


I reference this: http://www.topekaelectricmotor.com/electric-vehicles/ac-project

I guess 72 VAC at 60 cycles, 50 hp and 150 ft lbs torque is puny.....lol

It pulls around a 5,945# full sized chevy pick up to 70 mph....
It runs off a Curtis 1238r controller too. 
watch out, the AC guys are catching up.....lol

Miz


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## somanywelps (Jan 25, 2012)

The curtis + AC-50 is the only combo, and a signficantly cheaper DC option is available. 

A similar priced Brushed DC setup with get you a TON more power.


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## Tesseract (Sep 27, 2008)

mizlplix said:


> ...
> I guess 72 VAC at 60 cycles, 50 hp and 150 ft lbs torque is puny.....lol
> 
> It pulls around a 5,945# full sized chevy pick up to 70 mph....


Tsk, Miz... the actual quote from the site is, "We have had the truck on the highway at 70 mph although the optimal speed is 55mph or lower." There's nothing magical about AC that makes it more powerful than DC. Power is power, no matter how the semiconductors slice it and dice it.

At any rate, I think the AC-50 motor that is typically paired with the Curtis 1238 is a bit on the small size for the controller (especially the 650A version), while the custom Baldor motor Topeka Electric had made is more than a bit over-sized (a 450lb motor in an EV?!?). 

Unfortunately, using too big an induction motor has a hidden penalty in that more of the controller's phase current rating goes towards making the equivalent of the "field" in the motor (this is called the "magnetizing current"). This is reactive current so it "theoretically" won't drain the batteries like the real, torque producing current will, but as the magnetizing current sloshes back and forth through the switches it does incur losses in them all the same.

At any rate, you can calculate how much magnetizing current is required at rated speed and load if you know the full load current ("FLA") and power factor ("PF" aka cosΦ):

FLA*((1-(PF)²)^0.5)

Plugging in the known FLA for the custom Baldor motor and a typical value for PF for motors of its size/pole count:

362*((1-(0.85^2))^0.5) = 190.7A 

Thus 191A of the 362A FLA will be used to create the field in this motor. [EDIT] The torque producing current is FLA * PF or 308A. (thanks, maj)

NB - Magnetizing current is proportional to applied phase voltage, but torque is created by the torque producing current (Iq) acting against the magnetizing current (Id). Decoupling these two currents so they can be individually controlled is the aim of so-called "field oriented control". Without FOC the torque from an induction motor declines with RPM - exactly opposite of what is needed in a traction application! - and the magnetizing current is too high when the motor is operating at anything less than full load and rated speed.


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## major (Apr 4, 2008)

Tesseract said:


> Thus 191A of the 362A FLA will be used to create the field in this motor while the balance of 171A will be used to produce actual torque.


Hi Tess,

I think you need to use vector arithmetic to figure Iq.

major


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## Tesseract (Sep 27, 2008)

major said:


> Hi Tess,
> 
> I think you need to use vector arithmetic to figure Iq.
> 
> major


Aye... I thought there was something wrong with that when I was typing it out. Corrected now thanks to you.


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## somanywelps (Jan 25, 2012)

Tesseract said:


> Aye... I thought there was something wrong with that when I was typing it out. Corrected now thanks to you.


So that's why the torque is so much lower?


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## Tesseract (Sep 27, 2008)

somanywelps said:


> So that's why the torque is so much lower?


Not really. The main reason is that once the stator iron fully saturates torque no longer increases in proportion to current. This is in contrast to a series DC motor torque where continues increasing even after saturation, just linearly with current, rather than to the square of current.


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## JRP3 (Mar 7, 2008)

Tesseract said:


> At any rate, I think the AC-50 motor that is typically paired with the Curtis 1238 is a bit on the small size for the controller (especially the 650A version)


In what way? Because it doesn't produce that much torque from 650A? I always thought the motor was limited by the Curtis 1238, not the other way around.


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## mizlplix (May 1, 2011)

I agree...the AC50 is a bit small for the controller.

I also agree...Kevin's motor is a bit too large for it too.

But, the truck does drive nicely. The 400# motor is ok just because it is in a full sized truck and he has a humongous battery pack.

This controller will nicely run a motor somewhere between the two above examples.

My comments on this thread seem to have some how drawn the ire of the DC proponents. I made no boasting claims as to the old AC vs Dc argument so if taken that way, it was not ment so. (or I would have said something like...one day we will view the use of DC motors like we do the use of lead acid batteries now.)

I had not ment to hi jack the sepex thread.

Miz


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## somanywelps (Jan 25, 2012)

Tesseract said:


> Not really. The main reason is that once the stator iron fully saturates torque no longer increases in proportion to current. This is in contrast to a series DC motor torque where continues increasing even after saturation, just linearly with current, rather than to the square of current.


Make an AC controller please 

Maybe netgain or kostov can also make an affordable (<$7000) 150kw+ peak AC motor.


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## Tesseract (Sep 27, 2008)

JRP3 said:


> In what way? Because it doesn't produce that much torque from 650A? I always thought the motor was limited by the Curtis 1238, not the other way around.


The *speed* of the motor is limited by the low voltage rating of the 1238; the maximum current rating of the 1238, however, seems a bit much for the AC-50. Mind you, I do not know this for sure and I only said that the AC-50 appears to be a bit small, not drastically undersized. My guess is that it is probably already close to saturation at 550A so bumping the phase current up to 650A might not get you much more torque.

It's not like I'm going to buy one and dyno test it, though, so this is all just my opinion. Worth price paid, etc...


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## JRP3 (Mar 7, 2008)

Jack already did that actually, don't remember the results off hand, other than the expected improvement going from the 550 amp to the 650 amp controller.


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## Tesseract (Sep 27, 2008)

JRP3 said:


> Jack already did that actually, don't remember the results off hand, other than the expected improvement going from the 550 amp to the 650 amp controller.


So 18% more torque at 650A? Then my *guess* was wrong. The AC-50 is perfectly matched to the 650A Curtis 1238. I don't need to futz around with an AC inverter at all and can go back to designing industrial/scientific electronics.


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## mizlplix (May 1, 2011)

somanywelps said:


> Make an AC controller please
> 
> Maybe netgain or kostov can also make an affordable (<$7000) 150kw+ peak AC motor.


Try asking Baldor..............................they seem to know a thing or two.

Miz


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## JRP3 (Mar 7, 2008)

Tesseract said:


> So 18% more torque at 650A? Then my *guess* was wrong. The AC-50 is perfectly matched to the 650A Curtis 1238. I don't need to futz around with an AC inverter at all and can go back to designing industrial/scientific electronics.


That would be true if you were designing a 130V max 650A AC inverter for the AC 50. If not, then get back to work


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## JRP3 (Mar 7, 2008)

I did a little digging and found this on Jack's blog:


> As you can see, the -7501 model offers improvements of 15 to 25% depending on what you're looking at but torque jumps as much as 25%.


Obviously he meant "7601".
http://jackrickard.blogspot.com/2011/04/this-week-we-return-to-slingblades.html


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## Tesseract (Sep 27, 2008)

Hmmm... I'm surprised that the AC-50 is not deep into saturation at 550A, much less 650A. I suspect I am missing something here. Maybe maj will swoop in here to correct me once again...


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## major (Apr 4, 2008)

Tesseract said:


> Hmmm... I'm surprised that the AC-50 is not deep into saturation at 550A, much less 650A. I suspect I am missing something here. Maybe maj will swoop in here to correct me once again...


O.K. Think of it like a shunt motor (instead of your series wounds). The shunt motor has a fixed field voltage and therefore fixed field current, mmf and field flux level (perhaps 80% of saturation). The torque is proportional to the vector product of the field flux and the armautre flux. For reasonable loads that is closely approximated to the field flux times the armature current. Field flux stays constant in the shunt motor so torque is proportional to armature current.


When the armature current becomes large compared to the reasonable levels, the vector product deviates significantly from the scalar approximation and torque is no longer simply proportional to armature current. Another way to look at it is that the high armature current distorts the main field so much that is reduces the field flux. Either way, the result is that increased load (torque opposing shaft rotation) will cause a disproportionally high armature current and will eventually cause the motor to stall sending the armature current even higher taking out the breakers or switches. This occurs without the primary magnetic circuit saturating. There would undoubtedly be quadrature areas of saturation. 


The induction motor can benefit from the shunt motor analogy. As long as the applied phase voltage and frequency remain correct, the main path will not saturate with increased armature current (call it Iq). Torque will increase. But only to a point. And that point is called breakdown torque (BDT). Here again, if the load (torque opposing rotation) exceeds the BDT, the induction motor stalls, current skyrockets and takes out the breakers or switches. But even here, I think it is incorrect to say the motor has saturated. 


So if an induction motor has a BDT which occurs at 700A, using a 1000A controller will not get you more torque from it. Going from a 550A to 650A controller will.


Hopefully not too confusing 


major


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## Tesseract (Sep 27, 2008)

major said:


> ...
> Another way to look at it is that the high armature current distorts the main field so much that is reduces the field flux.


Doh! I didn't even think of that but it makes perfect sense now. Why shouldn't there be the equivalent of armature reaction in an induction motor, after all?

And I think I get why the primary magnetic circuit doesn't necessarily need to saturate for the current to skyrocket - maximum inductance occurs when Id and Iq are in quadrature but if armature reaction distorts Id then the effective inductance of both rotor and stator will plummet. Desaturation and/or switch destruction soon follow.

At least that's what I got out of what you wrote. Either way, I didn't see this coming so I clearly have some more reading to do. Assuming that plowing more effort into this Sisyphean task is a productive use of my time, that is. 



major said:


> So if an induction motor has a BDT which occurs at 700A, using a 1000A controller will not get you more torque from it. Going from a 550A to 650A controller will...


That's kind of where I was heading with my earlier assumption - that the AC-50 was probably getting close to true saturation at 550A. But I overlooked/forgot that torque doesn't simply stop rising once breakdown is reached, it plummets back down into an unstable operating region centered around the pullup torque value.

Thanks for the explanation.


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## PStechPaul (May 1, 2012)

I don't have much experience with large DC motors, particularly series wound, but I did some reading and here is an interesting analysis of the torque, speed, and current in a series wound motor, where it shows a non-linear relationship of torque to current above the point of saturation:
http://electricporsche924.blogspot.com/2012/04/warp-9-motor-performance-predictions.html

Here is another explanation showing maximum torque:
http://www.globalspec.com/reference/10789/179909/chapter-3-ac-and-dc-motors-dc-motors-dc-motor-types

It states that torque is proportional to the square of current. That remains true for a portion of the curve, then it becomes proportional to just current, and then it almost flattens out.

AC induction motors are different because they represent an inductance so at a given frequency the magnetic material may or may not saturate, and the current will be a distorted waveform with peaks at the point of saturation. As the rotor starts to turn the back EMF essentially reduces the voltage in the system so the current will drop and shift more in phase with the applied voltage. And because the current in the rotor is limited by the magnetic coupling from the stator field, it can only rise to a certain limited amount, such as 4x rating. At this point you have locked rotor current which is high enough to trip a protection device, but not nearly as high as a series wound DC motor, which is essentially a short circuit limited only by winding and brush resistance.

I know this has already been stated, and I'm just trying to explain the concepts in a little different way. But I also wonder if a similar inductive limit occurs in a running series wound motor, since the rotor windings are essentially driven with AC due to commutator action. And I also do not know, off-hand, if DC motors have the equivalent of multiple poles. The small motors I have had experience with have had essentially a two-pole PM stator and a rotor with variable number of poles which (IIRC) may be an odd number (3?).

I think I'll stick with my 3 phase ACIMs. IMHO, series wound DC motors are more suitable for starting ICEs. But they do have awesome torque for dragsters and other vehicles where raw acceleration is king.


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