Discussion:
Sepex vs Shunt
(too old to reply)
Mark Hanson
2008-07-24 19:43:39 UTC
Permalink
Hi John etc,

A Sepex motor has *one-third* the field voltage rating of a shunt field motor. So regen works down to 1/3rd the full speed RPM's. On the Bombardier and GEM as example (I worked at GE-EV) when you put a silly scope on the field you can see that at full speed the field on the SepEx motor is a 30% duty cycle since it's rated about 23 volts. When you hit the brakes, the field PWM goes up to 100% to 72V momentarily down to about 7mph (notice I said momentarily so the field doesn't burn up).
Then if you want regen to 0 speed (which both vehicles have) you boost convert the armature PWMing a half bridge to boost the voltage back into the battery.

Before I worked at GE-EV I thought (like John) that shunt and SepEx motors are the same but a shunt motor's field is rated for full voltage wheras a SepEx is rated for approx 1/3rd the battery voltage (to make regen easier). That's the difference. In a shunt motor you'd have to *overspeed* the motor to get regen, in a SepEx you don't.

A simple way to get regen on a Sepex (typoically 20A max field surrent) is to get a rheostat trailer electric brake master cylinder, connect to the hydraulic brake and to the field to increase the field current when braking. This will work down to 7mph about.

For an NEV put a MC4013 flip flop efter the speed sensor so it will go about 45mph instead of 25mph (the controller then thinks it's going half speed).

Have a SepEx day,
Mark
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Robin
2008-07-25 17:05:25 UTC
Permalink
Thanks Mark for chiming in on this interesting conversation.

I wonder which path would you choose: ie looking for the low hanging of fruit of regen.
In a 120 volt system starting with a series motor. (Apparently only Kelly controllers have offerings in this voltage range)

Would you use a field weakening coil and a potentiometer?
Would you rewind, for sepex, and add interpoles?
Would you convert to a permanent magnet and electronically commutate and hence not need interpoles?

I have a series forklift motor out of a nissan forklift that had 'regenerative braking' using this idea of a field weaking resistor coil (to increase top speeds (ie like a shunt)is why I was told that the coil was there not necessarily that this is how regen is achieved). I have wondered if I could incorporate this into my proposed Jetta ev conversion.

Or do you know if I have to rewind the field (change amp turns)to be sep ex, and make sure not to run high field currents for anything but very short periods of time.

Or perhaps I should convert it to a permanent magnet, get rid of the commutator and not need interpoles because the copper is wound around the stator like an ac induction motor. If I were commutate it electronically I could apparently then field weaken it.

It seems that there are many options, I am curious which path you would choose and why.
To regen with a SepEx motor when it is above base speed, increase the
field current (in the same direction as motoring). Once speed falls
below base, keep the field strong in the same polarity and start
boosting the armature voltage by means of the half bridge chopper in
the armature controller.
Jeff (Jeff Major) is correct for a sepex motor. But for the more common series motor,
Davis has it right -- you *do* switch it into reverse for regenerative
braking! The motor is still rotating forward, but the current in it
reverses, which charges the battery.
The common Curtis controllers have plug braking. In their
implementation, the armature is shorted by a diode, the field
connections are reversed (with the reversing contactor), and the
controller applies a high current at low voltage to the field alone. The
generated power does not charge the battery; it is just burned up as
heat in the motor and plug braking diode.
Cheers
Robin
Lee Hart
2008-07-25 22:00:40 UTC
Permalink
Post by Robin
I wonder which path would you choose: ie looking for the low hanging
of fruit of regen. In a 120 volt system starting with a series motor.
Regen on a series motor is hard. First, the motor itself needs to be
designed with regen in mind. This means it needs interpoles, or at the
very least, the brushes need to be timed to allow regen.

Second, you have to find a series motor controller that supports regen.
It works fine when the motor and controller are both set up correctly.
But if you use such a controller with a motor *not* set up for regen,
you may soon have a damaged motor and a dead controller.
Post by Robin
Would you use a field weakening coil and a potentiometer?
Field weakening is a technique for getting more power out of a series
*motor*. It is not something you do for regen on a series *generator*.

Normal shunt and sepex motors (a.k.a. generators) use field control. I
guess some might call field weakening, but it is really something different.
Post by Robin
Would you rewind, for sepex, and add interpoles?
Adding interpoles isn't really practical, unless you know a *lot* about
motors.

Replacing a series field with a shunt or sepex field is fairly
straightforward. But without interpoles, the range of currents you can
get will be rather limited. It will also be a challenge to get a motor
controller for it.
Post by Robin
Would you convert to a permanent magnet and electronically commutate
and hence not need interpoles?
That would be a major redesign of the motor. It's not really practical.
Post by Robin
I have a series forklift motor out of a nissan forklift that had
'regenerative braking' using this idea of a field weaking resistor
coil (to increase top speeds (ie like a shunt)is why I was told that
the coil was there not necessarily that this is how regen is
achieved). I have wondered if I could incorporate this into my
proposed Jetta ev conversion.
Forklift controllers sometime use *plug* braking, which is simpler than
true regenerative braking. Plug braking does not use braking energy to
charge the batteries; it simply burns it up as heat in the motor and
controller. It is effective for limiting the speed of a vehicle, say,
when going downhill. But it is not strong enough for serious braking.
Post by Robin
It seems that there are many options, I am curious which path you would choose and why.
Perhaps the most straightforward way to add regen to an EV with a series
motor and controller is to add a separate generator or alternator.
Connect it to the tailshaft of the motor, and enable it whenever you
want regenerative braking.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
Bill Dennis
2008-07-26 02:07:55 UTC
Permalink
Lee,
When you had your contactor sepex controller, did you just open the
armature contactor while current was still flowing? I think you said
that you had a two-step contactor controller for the armature. So when
either stopping or switching to another armature voltage, you had to
open the contactor, right?

Bill Dennis
Lee Hart
2008-07-26 03:06:15 UTC
Permalink
Post by Bill Dennis
Lee,
When you had your contactor sepex controller, did you just open the
armature contactor while current was still flowing? I think you said
that you had a two-step contactor controller for the armature. So when
either stopping or switching to another armature voltage, you had to
open the contactor, right?
Yes. Rude and crude, lots of arc-n-spark! The armature controller had 4
steps; off, 36v with series resistor, 36v direct, and 72v direct.

Today, I would put a big diode in parallel with the armature as a
freewheel diode.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net
Jeff Major
2008-07-26 18:05:51 UTC
Permalink
Post by Lee Hart
Forklift controllers sometime use *plug* braking, which is
simpler than
true regenerative braking. Plug braking does not use
braking energy to
charge the batteries; it simply burns it up as heat in the
motor and
controller. It is effective for limiting the speed of a
vehicle, say,
when going downhill. But it is not strong enough for
serious braking.
Hi Lee,

Back in the old days, like 25 years ago, most forklifts used plug braking. Most with the old SCR controllers. These systems were quite effective at stopping the truck. Even when fully loaded with a couple of tons, the operator would simply go from full speed forward to reverse, or visa versa. It was quite serious braking and worked great. A lot of them would never use the friction brakes except to park it. The downside was the motor heat and brush wear. That was what really kept guys like Jim H busy.

Regenerative braking on forklifts was tried back then, but proved costly and unreliable with the series motors. This is a big reason lift trucks went (or are changing) to SepEx or AC systems. They can get the electric braking and not wear out the motor so quickly. It puts the kinetic energy into the battery instead of sparks and heat in the motor.

Regards,

Jeff M

Robin
2008-07-26 00:10:50 UTC
Permalink
Thank you Lee Hart for your response. 0
"Regen on a series motor is hard. First, the motor itself needs to be
designed with regen in mind. This means it needs interpoles, or at the
very least, the brushes need to be timed to allow regen."
Normal shunt and sepex motors (a.k.a. generators) use field control. I
guess some might call field weakening, but it is really something different.
That clarification will save me some grief!

I will take your word on adding interpoles or going commutatorless.
I have a series forklift motor out of a nissan forklift that had
'regenerative braking' using this idea of a field weaking resistor
coil.
Forklift controllers sometime use *plug* braking, which is simpler than
true regenerative braking. Plug braking does not use braking energy to
charge the batteries; it simply burns it up as heat in the motor and
controller. It is effective for limiting the speed of a vehicle, say,
when going downhill. But it is not strong enough for serious braking.
I will need to establish whether the 'regenerative braking' was true
braking or 'plug braking' as you describe. Again thanks for the clarification


Perhaps the most straightforward way to add regen to an EV with a series
motor and controller is to add a separate generator or alternator.
Connect it to the tailshaft of the motor, and enable it whenever you
want regenerative braking.

I understand that this is what Roland does though the alternator could be only 12 or 24 volt.
Perhaps finding a 120 volt d.c. generator to charge the pack would be in order. Or settle with a separate
battery pack for heating or lights etc.

I have learned much from you Lee, thankyou for your generous contributions to the e.v.d.l.
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