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

On Fri, 6 May 2005 10:58:59 -0400, R Adsett
<radsett@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>In article <1115333894.686939.214260@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
>mep0716@xxxxxxxxx says...
>> >In a previous life I worked for a electric forklift manufacturer.
>> >We designed our own motor controllers. It could PWM about 700A
>> >at 36V into each of the lift and traction motors. Fun stuff,
>> >where interesting things happen when things fail. The first
>> >version used the HIP4081, but it was later redesigned to use the
>> >HIP4082 to save some money.
>>
>> What kind of FETs were you using? PWM Frequency?
>
>I've worked on the same sort of drive (IR MGDs though) 1000A peak,
>nominal battery voltage of 24-48V, switching frequencies of 10kHz
>dropping to 5kHz when 'plugging'. Originally used IRFZ44s
>
>You can get some interesting pyrotechnics when these fail under load.

I remember an incident where I leaned down to take a close look
at a board under test, right when all the armature mosfets
decided to blow like popcorn. I jerked back just quick enough
that the foot long flames flying out the side just missed burning
off my eyebrows. :-)

Another memorable one wasn't a failure, but more of a software
bug. The controller stored the configuration in the flash
memory. Early in the prototype development the software guys
were more worried about getting something working than saving
write cycles on the flash memory where the configuration data was
stored, so they erased one of the flash banks and wrote the
configuration to the beginning of that bank on each power down.
The process was to open the main power contactor, then erase the
flash bank, save the configuration, then power off the
controller. Later when they had the time they implemented a wear
leveling procedure that wrote the configurations consecutively
through that flash bank, and the software used the last one on
power up. This made it so that you only had to do the erase on
one out of several hundred power off cycles, since the size of
that flash bank was much larger than the size of the
configuration data.

The problem was that this eliminated the 1 second delay, caused
by the wait for the flash to erase, that had been there on every
power down. This is where I should point out that due to the
inductance and mechanical design of the main power contactor, it
stays closed for about 0.1 seconds after power is removed from
it's coil. The result was that the controller de-energized the
main power contactor, wrote the configuration to flash, and
powered off the controller. When an erase was not needed, the
controller powered off before the contactor had actually opened.
This resulted in all the mosfets on both traction controllers and
the lift controller turning on, and full power being applied to
all the motors momentarily. It caused a huge arc as the main
power contactor tried to open under load, and more impressive to
engineer types like me, the battery cables that dangled down the
side of the forklift to the battery would jump apart by about SIX
INCHES momentarily. Addition of a short delay to the power down
routine fixed the problem. :-)

Carl Smith
.

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