[Tesseract]

I had a customer in a state of mild panic email me today wanting to know how to reverse his dragster WITHOUT using reversing contactors (apparently there is some rule requiring the vehicle be capable of reversing?).

On the spur of the moment I came up with a quick-n-dirty kludge that I'm sure someone else has thought of: use a contactor to connect a single LFP cell to the field to excite it with current of the opposite polarity from the motor controller. You would then need to limit the output current of the motor controller to some fraction of the current supplied to the field by the LFP cell because the current from the motor controller will subtract from the field excitation, but it seemed legit on paper. Anyone actually tried this in Real Life and can report on how well it worked (or didn't)?

 

[JimDanielson]

It sounded like it would work to me. Since Im sitting in my shop next to my ev, I decided to hook up a power supply and try it. I can no longer read the labels on my series wound motor so I tried on both armature and field. But neither worked.(any quick way to tell which is which with a multimeter?)

My power supply can only max 20amps though. This was about 1.5 volts. I was testing on a forklift motor with a logisystems controller.

A potential mechanical way to do this would be to have a lever that rotated the brushes 180 degrees. But it would be pretty complicated to make.

 

[Woodsmith]

I don't suppose a big reversing switch would do for drag racing?
I just bought one of these from an old milkfloat.

The components could be 'beefed up'.

Alternatively a small motor (12v?) with a gear wheel to spin the main motor backwards on the tailshaft? The small motor could be on a sprung pivot to allow a lever to engage the gear teeth for reversing and then the spring would push the motor away when the lever was released.

Maybe just use a starter motor, that would have either an inertia pinion gear or a solonoid to engage the drive.

 

[Tesseract]

..Since Im sitting in my shop next to my ev, I decided to hook up a power supply and try it.
...
My power supply can only max 20amps though. This was about 1.5 volts. I was testing on a forklift motor with a logisystems controller.

20A probably wasn't enough to spin the motor even if unloaded. I'm thinking you'd need to be able to push at least 200A through the field for this to work. Thanks for giving it the ol' college try, though!

I can no longer read the labels on my series wound motor so I tried on both armature and field. But neither worked.(any quick way to tell which is which with a multimeter?)

They are difficult to tell apart. Usually the armature terminals are located nearest the brush end of the motor. Failing that, I'd try to poke a brush pigtail with one of the meter's probes and tap the other probe against each terminal - 2 will show continuity (the armature) while 2 will not (the field).

 

[Tesseract]

Forgot to add that I drew up a schematic of how this would work:

 

[peggus]

I'd carefully consider what happens if he's moving forward when this is applied, does the resulting re-gen spike harm your controller in any way?

 

[Tesseract]

I'd carefully consider what happens if he's moving forward when this is applied, does the resulting re-gen spike harm your controller in any way?

As in, the vehicle is rolling forward, the "reverse" switch is closed, and the controller is not supplying current? I suppose if the field current was high enough and the armature speed fast enough, then the armature voltage might exceed the battery pack voltage, but this seems like a rather remote possibility. Regardless, the controller in question is a Shiva so it is also unlikely that regeneration current could every get high enough to damage the anti-parallel diodes across the IGBTs. There's over 6kA of total ampacity, there.

 

[GerhardRP]

As in, the vehicle is rolling forward, the "reverse" switch is closed, and the controller is not supplying current? I suppose if the field current was high enough and the armature speed fast enough, then the armature voltage might exceed the battery pack voltage, but this seems like a rather remote possibility. Regardless, the controller in question is a Shiva so it is also unlikely that regeneration current could every get high enough to damage the anti-parallel diodes across the IGBTs. There's over 6kA of total ampacity, there.

Isn't this the scheme we discussed a while back as a way to get regen from a series motor?

 

[Tesseract]

Isn't this the scheme we discussed a while back as a way to get regen from a series motor?

Yes, except that it's not being used for regen. Motor current from the controller opposes the external field excitation, but as long as the external field excitation exceeds the controller-supplied current then the motor will spin in reverse. There are all kinds of problems with this scheme from a practical standpoint - the motor will be operating with a weakened field and with retarded timing, if the cell supplying the external field dies then the motor will abruptly change directions (and the cell will get hammered with reverse current if the switch connecting it to the field isn't opened), etc. - but it should work well enough to pass tech inspection.

And that's all I'm really after here. I suppose I'll have to go over to the shop and try it myself, but I thought it couldn't hurt to ask if someone else had tried it already.

 

[DIYguy]

I had a customer in a state of mild panic email me today wanting to know how to reverse his dragster WITHOUT using reversing contactors (apparently there is some rule requiring the vehicle be capable of reversing?).

On the spur of the moment I came up with a quick-n-dirty kludge that I'm sure someone else has thought of: use a contactor to connect a single LFP cell to the field to excite it with current of the opposite polarity from the motor controller. You would then need to limit the output current of the motor controller to some fraction of the current supplied to the field by the LFP cell because the current from the motor controller will subtract from the field excitation, but it seemed legit on paper. Anyone actually tried this in Real Life and can report on how well it worked (or didn't)?

Hey Jeff,

Yes, they need reversing and this scheme was originally thought up by ((IIRC) Lee Hart. I know a number of them use it. . . I'm not sure they all like it very much and it wouldn't be acceptable for a daily driver. . . but it is in use.

Edit. ; just remembered . . I think they all use a 12 volt battery though.

 

[Tesseract]

...Yes, they need reversing and this scheme was originally thought up by ((IIRC) Lee Hart. I know a number of them use it. . . I'm not sure they all like it very much and it wouldn't be acceptable for a daily driver. . . but it is in use.

Edit. ; just remembered . . I think they all use a 12 volt battery though.

I didn't think I had come up with something unique here...

I suggested he use a 3.2V LFP cell because a 12V battery would be working *hard* trying to energize a 4-5 milliohm field winding. Also, all that lead is heavy, you know...

 

[rankhornjp]

It would add some weight, but here's a reverse that uses a flywheel and starter motor: