# [EVDL] SepEx Motor Field Current Limiting



## EVDL List (Jul 27, 2007)

Does the field measure 0.3 ohms or does the nameplate say 0.3 Ohms? It does
not seem right to me, maybe it's got a short?

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf
Of Bruce
Sent: Sunday, 12 July 2009 1:24 p.m.
To: [email protected]
Subject: [EVDL] SepEx Motor Field Current Limiting

Hi All,

I purchased a GE Model #: 5BC49JB6002 SepEx motor. GE couldn't give me any
information on this motor so all I have is what is written on the nameplate:
2.4 HP,
1960 RPM, 24 Volts, Current - 104 Amps for the armature and 6.1 Amps for the
field.
These specs are for 1 hour operation with the cover band on - totally
enclosed - no
air circulation. The motor weighs 70 lbs. Armature resistance is .21 Ohms
and the
field resistance is .30 Ohms. The field windings are 14 gauge wire.

So I thought this might make a nice motor for a motorcycle. What should I
use for a controller? I called Alltrax thinking that a DCX-300 might work.
They
responded that this motor's field resistance was too low for it to work with
any of
their controllers as they have a 1/2 Ohm minimum.

How about controlling the field current with a rheostat? If I connected 
the 24 Volts directly to the field with no resistance, the current would be
80 Amps
through the .3 Ohm field and it would certainly burn out. If I added a 3.6
Ohm 
resistor in series with the .3 Ohm field, it would limit the current to the
proper 
6.1 Amps, but the voltage through the field would only be 1.8 Volts with the
rest 
of the 24 Volts being dropped by the resistor. That's not going to be
enough 
power for the field. If I connect a 6.6 Volt battery directly to the field,
the
power would be 146 watts which is the same as the nameplate rating of 24
Volts
times 6.1 Amps. But the current is now 22 Amps instead of 6.1 and it seems
like 
the field windings are going to be overstressed again.

How do I properly limit the current to the field?

My only experience is with Series and Permenent Magnet motors so I hope to
learn something here. Anyone have any advice?

Thanks!

Bruce
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
http://lists.sjsu.edu/mailman/private/ev/attachments/20090711/9fc357ec/attac
hment.html 
_______________________________________________
General EVDL support: http://evdl.org/help/
Usage guidelines: http://evdl.org/help/index.html#conv
Archives: http://evdl.org/archive/
Subscription options: http://lists.sjsu.edu/mailman/listinfo/ev


_______________________________________________
General EVDL support: http://evdl.org/help/
Usage guidelines: http://evdl.org/help/index.html#conv
Archives: http://evdl.org/archive/
Subscription options: http://lists.sjsu.edu/mailman/listinfo/ev


----------



## EVDL List (Jul 27, 2007)

> On 13 Jul 2009 at 6:27, Jeff Major wrote:
> 
> > A rheostat for field control could work in theory, but in practice
> > would be cumbersome and likely prove impractical. All in all, for your
> ...


----------



## EVDL List (Jul 27, 2007)

> Jeff Major wrote:
> >> A rheostat for field control could work in theory, but in practice
> >> would be cumbersome and likely prove impractical. All in all, for your
> >> EV build, you're probably better off getting a series motor. But if
> ...


----------



## EVDL List (Jul 27, 2007)

Robert Stephens wrote:
>That must be some weird ass motor if you cannot merely parallel the field
and
> armature connections and apply full nameplate motor voltage to the device.

The field has much finer windings than the armature. It says SepEx on the
motor so I doubt that it is suitable for a shunt setup.

George Tyler wrote:
> Does the field measure 0.3 ohms or does the nameplate say 0.3 Ohms? It
does
> not seem right to me, maybe it's got a short?

I measured the resistance of the field by putting 70 Volts DC through it in
series
with a known resistor. I measured the voltage drop across each resistance
to
come up with the .3 Ohm value. I doubt that it has a short as it is a new
motor
and it runs fine when I hook a battery charger up to the field and power the
armature with a battery.

My understanding is that this motor was built to go in a scissor lift.
Someone
said that it is not uncommon for motors like this to have very low
resistance
fields.

The low resistance of the field is what is keeping me from just hooking up a
rheostat to it. Any ideas on how to effectively limit the current short of
using
a whole seperate controller? And could a whole seperate controller have
trouble with that low field resistance?

Lee Hart wrote:
> My accelerator pedal had a set of switches as well as the field rheostat.
> As you pressed the pedal from off to full, the switches closed
> sequentially to select armature off, 36v on armature with a series
> resistor, 36v direct to armature, and 72v direct to armature.
> Simultanteously, the field rheostat was going from max field current
> with the pedal off, to minimum field current with the pedal floored.

So the field current is supposed to be at maximum when the motor is
putting out no power and the field is at minimum current when it is
putting out maximum power? I would have thought to emulate a
permanent magnet motor, you would just leave the field set to a constant
current. Or to emulate a series motor, you would increase the field
current as the armature current increased? Why do you want to
reduce the field current as the power demand goes up? And how can
I properly limit the current to the field with such a low field resistance?

Bruce



> Bruce <[email protected]> wrote:
> 
> Hi All,
> 
> ...


----------



## EVDL List (Jul 27, 2007)

> Bruce<[email protected]> wrote:
> > So the field current is supposed to be at maximum when the motor is
> > putting out no power and the field is at minimum current when it is
> > putting out maximum power?
> ...


----------



## EVDL List (Jul 27, 2007)

What about Kelly, don't they make a programmable sepex controller that you
can adjust through a PC?

John



> Jeff Major wrote:
> >
> >
> > Several companies make SepEx controllers, like Curtis and Sevcon. That
> ...


----------



## EVDL List (Jul 27, 2007)

> Bruce wrote:
> > I measured the resistance of the field by putting 70 Volts DC through
> > it in series with a known resistor. I measured the voltage drop
> > across each resistance to come up with the .3 Ohm value.
> ...


----------



## EVDL List (Jul 27, 2007)

> Jeff Major wrote:
> > The correct equation is RPM = K * Eg / Flux.
> > If you're working below the saturation point and reconfigure the motor
> > constant, you can use:
> ...


----------

