Re: [CAD_CAM_EDM_DRO] Re: Power supply for Servos - Sizign the resistor

turbulatordude wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Lance Hopper"
><snaggletto@c...> wrote:
>
>
>>Hey,
>> The e-stop circuit in discussion calls for a 6.8 Ohm 20 Watt
>>wirewound resistor for a ~68V supply. Looking on Ebay there are
>>several that are somewhat larger, like a 10 Ohm 75 Watt etc... Is
>>there any reason, other than overkill, why a resistor with much
>>higher Ohm/Wattage ratings can't be used? Any possibility of
>>
>>
>damage
>
>
>>to the PS or drives if there is to much resistance? Thanks.
>>
>>
>
>How did you come by the resistor value ?
>
>

A 6.8 Ohm resistor at 68 V will pull 10 Amps from the power supply.
This should be sufficient in nearly all cases. It is remotely possible
a Bridgeport Series II sized machine built for very high rapid feed rates
could deliver more than 10 Amps during very rapid deceleration from
top speed. That would be delivering over 680 W back to the power supply!
Otherwise, this is a pretty good estimate.

On a smaller machine, a higher resistor value should be fine. Perhaps
dividing voltage by 5 would yield something very roughly around 15 Ohms,
which would pull 4.5 Amps. The dump circuit will shut off as soon as
the voltage is back down to a safe level. If the resistor value is so high
that all the returning energy is not dumped faster than it is coming in,
then
the dump circuit will not help protect the Gecko drives. Without very
accurate models of the machine and the motors, the only way is to try it.
Start by accelerating and decelerating gradually, then increase the
accel. rate and watch for a surge in the supply voltage. For the most
part, the dump circuit is not even needed. The combination of heavy
machine tables, high feed rates, large motors with a lot of inductance,
and small capacitor values in the power supply, and a nominal supply
voltage very close to the Gecko's limits are where you can run into trouble.
Eliminate one or more of these things, and you won't need it.

I've never seen a surge over about 2 V on my Bridgeport Series I machine.
(I'm not using Geckos on that one, but the drive topology and modulation
scheme is the same as the Gecko 320.)

Jon