Re: DC Motor Control: H-Bridge +5A, 48v

On Fri, 13 May 2005 23:40:12 -0400, R Adsett
<radsett@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>
>We did run into an issue with plugging. It turns out that under certain
>circumstances once you start plugging it will self sustain and the only
>way to get it to stop braking is to open the direction contactors. The
>motors we first tested on actually braked very nicely in that mode.


That reminds me of another one. We sort of accidently discovered
an interesting thing about our forklifts. If you got them up to
full speed, and while continuing to hold the control handle
forward, reached across with your left hand and hit the battery
disconnect, the momentum of the forklift turning the traction
motors while coasting to a stop would generate enough power to
continue to power all the electronics of the forklift. The power
would feed backwards right through the armature mosfets and feed
back into the B+ bus. It provided enough current to sustain the
field winding current and keep the generator effect going, and
even power the electric steering pump motor.

If you let go of the control handle, the armature mosfets would
turn off and cut the current flow to the B+ and everything would
shut down pretty much instantly. And on one model that had
electric brakes, the electric brakes would close and the forklift
would screech to a halt. :-)

So, of course, the next "unofficial" experiment was to push one
forklift with another, then have the one being pushed disconnect
the battery. This worked well enough that the one being pushed
could run all the functions including lifting and lowering the
mast. As long as you held the control handle forward so the
controller would keep the armature mosfets on, it would continue
to work as a generator.

Carl Smith
.