# Field control of SepEx motor?



## Clipper (Jan 30, 2011)

Howdy all,

I have a 180VDC GE sepex motor I'm trying to control.
It's in a TEVan with a failed controller. 
I have spoken extensively with Bill Reynolds, the controller's designer,
and he has helped me fix it three times, at a cost of about $700.
Each time something else blew, last time the reverse circuits fried on the main logic board. And the CRY7 logic chip I have is long-lost...CRY6 and CRY9 are available, but neither one works in my car.

I have given up on trying to find a 180VDC sepex controller.
Now I am trying to focus on controlling the field and armature separately, 
like Tim Wong did on his EVCort: http://home.earthlink.net/~evtkw/controller/TechPage.html

I suppose I can use a regular Zilla or Soliton to control the armature current.
I could use a second one to control the field, but it would be a little overkill...the specs on the field windings are 0.2 ohms resistance, 16.6A max, 5.4A minimum.
I understand I need to keep maximum field to about half the motor's max RPM, 
then weaken the field gradually up to full RPM. I figure I can manually control the field (with an ammeter on it) until I get a feel for what it wants, then try to automate the field control later.

The problem is finding a high-voltage, low-amperage PWM controller.
Don't need a 600-amp controller to control a 16-amp field.

I thought about using a lower voltage controller...my question is, if I use, say, a 60V PWM to control the field at 1/3 the voltage, does that mean I would triple the field current, to get the same field strength?

I know that where there is a fixed resistance involved, the relationship between V and I are linear: i.e. V=I*R. But in this case the field coil is not just a 0.2 ohm resistor, it is also an inductor. Is it the same linear relationship with inductance and more importantly, field strength?

Does anyone have a suggestion as to an adequate field controller for this application?

Thanks,
Clipper


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## major (Apr 4, 2008)

Clipper said:


> I have given up on trying to find a 180VDC sepex controller.
> ........
> I could use a second one to control the field, but it would be a little overkill...the specs on the field windings are 0.2 ohms resistance, 16.6A max, 5.4A minimum.
> ...........if I use, say, a 60V PWM to control the field at 1/3 the voltage, does that mean I would triple the field current, to get the same field strength?


Hi Clip,

These specs are a bit weird. 0.2Ω at, say 10A, is only 20 Watts. That might be a typical field excitation for the field current range you specified. However, the typical motor usually sees about a couple percent of rated power devoted to field excitation. So you seem to be an order of magnitude low on field power.

Also, a 180 Volt motor with a separately excited filed requiring only 1 to 3 Volts is an unusual design choice. The controller would have to reduce (or chop) 180V down to 1 It can be done, but strange.



> I know that where there is a fixed resistance involved, the relationship between V and I are linear: i.e. V=I*R. But in this case the field coil is not just a 0.2 ohm resistor, it is also an inductor. Is it the same linear relationship with inductance and more importantly, field strength?


It is the Ampere-turns (AT) which dictate the field strength. So just use Ohm's Law to figure the voltage needed for the field current. The inductance matters little or not at all in this case; just for changing current. The field strength (AT) is proportional to field current always. Now the flux will follow proportionally to the field strength (AT) until saturation where it then becomes nonlinear. 

Also note that the field resistance will increase with temperature. It can increase like up to 50% from a cold to hot motor. So you need to work with field current, not field voltage for your control.


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## Clipper (Jan 30, 2011)

Whoops...you are correct...the field coil is 3.2 ohms, the armature is 0.2.
Same principle applies, I'm sure.

3.2 * 16.6 = 53.2V max
3.2 * 5.4 = 17.2V min

On the high end, that's 882 watts.
I would need a 20W 180-60v DC-DC converter to feed it.
Pretty tall order. A second Soliton or Zilla starts to make sense.

I would convert it to a series or AC motor in a New York minute, but the final gearing in the modified a523 transmission is about 8:1, requiring 8000 RPM for 65 mph. So then I get into finding a new (or used) a523 5-speed tranny, too...oy....


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## major (Apr 4, 2008)

Clipper said:


> I would need a 20*W* 180-60v DC-DC converter to feed it.
> Pretty tall order. A second Soliton or Zilla starts to make sense.


Nah, and you mean 20A, right? See this thread: http://www.diyelectriccar.com/forums/showthread.php?t=81416&highlight=mechman


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## Clipper (Jan 30, 2011)

major said:


> Nah, and you mean 20A, right? See this thread: http://www.diyelectriccar.com/forums/showthread.php?t=81416&highlight=mechman


Hi, Major,

Correct, I meant 20A...

I could use a cheapie little Kelly such as this one:
http://kellycontroller.com/mini-brushless-controller-kbs12v-72v-c-60.html?page=2&sort=0a
but the problem still remains, how do I reduce the 180V to 72 volts to feed the Kelly?

Thanks,
--Clipper


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## major (Apr 4, 2008)

Clipper said:


> but the problem still remains, how do I reduce the 180V to 72 volts to feed the Kelly?


You could use a separate battery. If I think of a good solution, I'll let you know  180 Vdc might work with a lab type switch mode power supply intended to work from 110/220 Vac. Finding one of those for 60Vdc/20A might not be easy (cheap). 

For all that trouble, what about having the motor rewound to series? It might be less costly, faster to implement and more reliable. 

O.K. Thinking as I write this: How about a variable frequency drive? One intended for 230Vac might tolerate as low a 160Vdc on the bus. Then just rectify the 3 phase output. It'd give you variable voltage so you wouldn't need the Kelly.  A long shot.


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## Hollie Maea (Dec 9, 2009)

Clipper said:


> Whoops...you are correct...the field coil is 3.2 ohms, the armature is 0.2.
> Same principle applies, I'm sure.
> 
> 3.2 * 16.6 = 53.2V max
> ...


It seems to be that those azure dynamics Siemens motors everyone is throwing around would be just right for your 8:1 ratio...there are some challenges involved in getting it going, but hell, for the cost of two solitons like you are talking about for this thing you could almost get a Rinehart to control it.


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## Clipper (Jan 30, 2011)

major said:


> For all that trouble, what about having the motor rewound to series? It might be less costly, faster to implement and more reliable.


I would love to rewind it into a series...I am revisiting the transmission issue. I looked at the ratios in the original Chrysler service manual again; screen capture attached. Hi gear is 8.057:1, but the "final drive" (differential ring and pinion?) is 3.846:1, yielding an overall ratio of 31:1...??!! I calculated that the 26" OD tires on the TEVan turn about 840 RPM at 65mph... 840 x 31 is 26000 motor RPM...impossible....somethin' just ain't right. I actually happen to have a spare motor and transmission from a wrecked TEVan...I need to open up the transmission casing and count some gear teeth.

It would be awesome if I could just have the motor rewound...I just picked up a used but working Zilla for $500...!

And maybe I could UN-modify the modified A523 back into a regular old 5-speed...with reverse.


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## pm_dawn (Sep 14, 2009)

Hi !

I would think that the gear ratio is including the final drive.
So you have the transmission giving about 2,094:1 or some 4,xxx:1, and then the final drive giving the rest of the ratio.

How about using a solitonJR and a homebuilt field drive.
I can give you some pointers in how to build one or you can just down scale the Open Revolt controller. 

Then you can use the outputs of the Soliton to control the field stage.
You could go with a throttle and rpm output from the Soliton and use that as inputs for the Open Revolt. 

Does the original controller have full H-bridge for the field?

Regards
/Per


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