# Question on rolling backwards



## Guest (Oct 15, 2009)

it is an old trans but I would not think that would be a problem but in your case I guess it is. I assume that it runs fine in forward or reverse gears but just freewheeling in a forward gear but moving backwards causes a sudden stop at about 15 ft. Mmmm. Strange. Sure it is the transmission locking up? 

Pete


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## drdonh (Aug 8, 2008)

hi Pete (Gottdi),

I only experienced the effect twice. The first time was a surprise, and I had no idea what was happening. No grinding noise, but was just like slamming on the brakes. The second time it happened, I was rolling less fast, and the deceleration was less abrupt, but still there. As mentioned, I am not looking to recreate the conditions to study this any further, for fear of damaging the transmission if is locking up. Otherwise, rolling backward in reverse is fine, which I do every time now.

Don


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## Coley (Jul 26, 2007)

I doubt that it is the transmission.

You could try it with the motor turned off and see if that makes a difference.


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## CPLTECH (Sep 14, 2007)

I think it is the controller (Curtis 1231C), as mine did the same as you describe. Hence I, too, do not let the motor turn in reverse.


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## peggus (Feb 18, 2008)

Yeah it's the controller and do not do this as it can kill your controller. 

Basically the motor starts generating a small voltage due to residual magnetic fields, when said voltage exceeds the forward drop of the freewheel diodes they start to conduct and your motor will power forwards. 

The resulting current in the freeweel diodes can be many hundred even thousands of amperes so this is a good way to kill your controller.

Have fun


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## Dave Koller (Nov 15, 2008)

peggus said:


> Yeah it's the controller and do not do this as it can kill your controller.
> 
> Basically the motor starts generating a small voltage due to residual magnetic fields, when said voltage exceeds the forward drop of the freewheel diodes they start to conduct and your motor will power forwards.
> 
> ...


Now that is nice to know! And sets me thinking of ways to stop it ( other than the obvious ) -- Thanks !!


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## drdonh (Aug 8, 2008)

Nice to know indeed, especially if it can cause serious damage. Is this documented anywhere, and I missed it? I now know how to avoid the situation, but this would certainly be a concern if one had to idiot proof the car for use by any driver. I'd be interested in knowing what the "fix" would be to prevent this.

Don


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## major (Apr 4, 2008)

drdonh said:


> I'd be interested in knowing what the "fix" would be to prevent this.


Hi Don,

Wire up A2 on your controller to the motor 

Regards,

major


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## Dave Koller (Nov 15, 2008)

major said:


> Hi Don,
> 
> Wire up A2 on your controller to the motor
> 
> ...


Major - why DO they tell us NOT to hook up A2 on a street vehicle?


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## major (Apr 4, 2008)

Dave Koller said:


> Major - why DO they tell us NOT to hook up A2 on a street vehicle?


Who is they


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## Dave Koller (Nov 15, 2008)

major said:


> Who is they


Easy one is this from Lee Hart

"Thus the generally accepted wisdom is that if you have a heavy EV, you
don't want to connect A2, because it provides negligible braking force
and will just blow the A2 diodes. AND if you have a heavy EV, you don't
want to ever let the motor be forced to rotate backwards or it will blow
the freewheel diodes."

at http://www.repp.org/discussion/ev/200201/msg00723.html

EVA says not on a street vehicle - at least with Curtis..

I have a Curtis and tend to be cautious.....


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## EV-propulsion.com (Jun 1, 2009)

A short while back I asked a curtis engineer about that A2 terminal and here is what he said:
"_Although the controller manual shows an A2 connection between the controller and motor, this connection should not be included. Plug braking on the 1231C at high voltages is not stable. Even if it were stable, plug braking while driving on a road is a dangerous maneuver. Reverse should be handled via the vehicle’s transmission. The manual has been in need of an update for years."_
_P_lug braking was designed for forklift operation using reversing contactors to reverse the motor. If the operator were to throw the unit into reverse (again, using reversing contactor system to reverse the motor) while still powering forward, the controller would not allow power to reverse until the motor came to a stop (or close) to eliminate mechanical and electrical damage. I am guessing, but I assume it was seeing the voltage created by the motor while it was still spinning.
This plug braking explanation may need some correction.....
Mike
www.EV-propulsion.com


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## major (Apr 4, 2008)

Dave Koller said:


> Easy one is this from Lee Hart
> 
> "Thus the generally accepted wisdom is that if you have a heavy EV, you
> don't want to connect A2, because it provides negligible braking force
> ...


O.K. They is Lee Hart. Usually a good source. I think I disagree with him here. (wouldn't be the first time ) We're talking about a situation where there are not means to reverse the motor field. This plug braking only occurs when rolling backwards. Motor (acting as a series generator) is only energized from residual field. My bet is once the armature current gets to any appreciable magnitude, the residual field would be swamped out. At this point, no further generated voltage would be seen across the diode and all will be well.

Another way to look at it is would you rather chance blowing the plugging diode (which you say not to use anyway) or breaking the transmission and/or flat spotting tires.

How long have the Curtis manuals shown A2 connected? Like 20 years? You'd think if it was a serious problem, they would have changed those.

Another thing you could do if really concerned about over current in the A2 circuit is to fuse it. Put a 200 amp fuse in there and see if it blows.

Regards,

major


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## jaspersk (Jun 26, 2008)

I have seen this same thing with my Zilla controller. I reasoned that it was because I was hitting my brake and managed to hit my gas pedal just enough for the controller to do something. Given that it was seeing a reverse speed input, it would hit the motor hard with current. The car felt exactly what was described. It came to an immediate stop.

Now I always put the controller or the transmission in reverse even if I can coast just to make sure that doesn't happen.

I don't know what A2 is on a Curtis controller. That doesn't apply to my Zilla.


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## Tesseract (Sep 27, 2008)

One interesting implication of this is that there does appear to be enough residual magnetism in the field structure to enable regenerative braking without having to "tickle" the field first.

I'm still debating, though, if turning on the boost connected IGBT will be all that preferable a path for the former freewheeling, now regenerating, current than the FWD across the buck IGBT...


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## major (Apr 4, 2008)

Tesseract said:


> One interesting implication of this is that there does appear to be enough residual magnetism in the field structure to enable regenerative braking without having to "tickle" the field first.


Hi Tesseract,

I think you would find this undependable even with the same motor, let alone from different motors. It would be interesting to see you devise a method to test it out 

Regards,

major


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## Tesseract (Sep 27, 2008)

Yeah, totally unpredictable, I know, but the odds seem pretty good that there will be _some_ residual magnetism in the back iron after running a few hundred amps through the motor.

Oh, and just to remain on topic, we've noticed the same phenomenon with the Soliton1 when a vehicle it is installed in is allowed to roll backwards with the transmission in one of the forward gears. It would be interesting to check the log files to see what the motor current actually gets up to when that happens.


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## Otmar (Dec 4, 2008)

jaspersk said:


> I have seen this same thing with my Zilla controller. I reasoned that it was because I was hitting my brake and managed to hit my gas pedal just enough for the controller to do something. Given that it was seeing a reverse speed input, it would hit the motor hard with current. The car felt exactly what was described. It came to an immediate stop.
> 
> Now I always put the controller or the transmission in reverse even if I can coast just to make sure that doesn't happen.
> 
> I don't know what A2 is on a Curtis controller. That doesn't apply to my Zilla.


The Zilla will not be damaged by this plug braking action since it has very large freewheel diode capacity. It may damage your drivetrain if it can't take the torque of breaking traction with the tires. If you are going fast enough, the torque and current will rise just enough to cause your tires to slip. It is unlikely that you will flat spot the tires since they never completely stop turning in reverse, they just slow down enough to maintain sliding current in the motor. 

In road and autocross racing it is common to spin out. We used to keep our foot in it when sliding backwards in Snowhite to lessen the torque load on the drive belts. Then when we were sliding forward again we'd let off, or just keep going. Those were fun times! We probably didn't need to worry, I don't think we ever broke a belt and the torque was probably no more than when accelerating. BTW Snowhite is not for sale anymore.

On a Curtis, it doesn't hurt to hook up A2 so long as you don't hit the throttle while traveling backwards. Hooking up A2 does prevent the abrupt stop.
________
KTM 250EXC


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## JRP3 (Mar 7, 2008)

linda said:


> its because of transmission this is happening ......this is what i guess...try again and just take care of it...


Die spam bot!


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