Re: (ET) Electronic controller

["Bob Murcek" <rmurcek geisinger edu>]

Yep, my E12 motor is definitely compound, but I wouldn't have known that 
if I hadn't had to replace its bearings since the series field is 
internally wired is series with the armature.  (Ordinarily the leads for a 
series field would be externally accessible so that the series field could 
be reversed when reversing the motor.)

The presence of the series field somewhat reduces the danger of the shunt 
field not being excited.  I haven't tried it, but I suspect that without 
the shunt field the motor speed would be very unstable, like a sewing 
machine.  I do paranoidly have things arranged so that full power is 
applied to the shunt field as soon as the tractor is powered up, though.

Since the PWM controller provides fully variable control, and since the 
thing is plenty fast enough for my purposes in 4th, I haven't done 
anything with field weakening.

The deal with the series field not being reversed when the motor is 
reversed seems weird to me.  This implies that the shunt field is much 
stronger than the series field and can therefore overwhelm the series 
field to reverse the motor.  This probably means that the tractor doesn't 
have as much torque in reverse as it does in forward.  Another indication 
that the shunt field is stronger is that I do feel substantial 
regenerative braking; ordinarily a series field would reduce this effect.  
I'm wondering if a weak series field was added just to provide extra oomph 
when starting out under heavy load, like plowing.  Anyone know if that's a 
common motor design practice?

Overall the series field is so weak that the motor behaves similarly to a 
PM motor.

>>> "David Roden (Akron OH USA)" <roden ald net> 1/9/2002 1:03:19 AM >>>
On 9 Jan 2002 at 0:47, David Roden (Akron OH USA) wrote:

> The only cautionary note I would sound in using a controller intended for
> a PM motor is you should probably have some kind ot switching system that
> ensures that the field is always energized ~before~ voltage is applied to
> the armature, and that the field is always shut down ~after~ voltage is
> removed from the armature.  (This might not be so important with a
> compound motor, but it's still a reasonable precaution.)

And just a little while later I ran across this comment about separately 
excited motors on the EV mailing list:

= + = + = + = + = + = + = + = + = + = + = + = + 

... you have to be very careful in controlling a sep-ex motor, especially 
at high voltage.  The motor is the inductor, but it looks like a SHORT 
CIRCUIT without the field energized properly first. 

Without interpoles you have to be extra careful with field weakening.
(even with interpoles you have to be careful).

If you keep the field energized to an appropriate level, this won't be a 
problem.  But the gray area is how far you can field weaken without 
creating a fireworks show on the commutator.

I guess this is why sep-ex has never been very popular in the EV hobbyist 
community.  You really have to [know] the motors capabilities or have a 
pretty good controller.  This is not real difficult, but there are very 
few control vendors that offer a good sep-ex control. 

The other problem is that the sep-ex control should be matched with the 
motor.  This is not always the case with a series motor.  If the series 
motor can handle the amps, it can handle the control. Not the case with 
sep-ex. 


David Roden - Akron, Ohio, USA
1991 Solectria Force 144vac
1991 Ford Escort Green/EV 128vdc
1979 General Engines ElectroPed 24vdc
1970 GE Elec-trak E15 36vdc
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