Re: [CAD_CAM_EDM_DRO] Power Supply

Ron K wrote:

>Hi all
>I have a question for the electronic guys out there .
>This is something that I have disagreed with for a long time, but I
>don't open my mouth until I have some facts to go with it .
>( I'm the try it and see if it works type , not the work it out on
>paper type )
>
>If I want to build a stepper control with the following specs how many
>amps do I need out of the transformer !!!
>
>3 Motors , Slo-Syn 840 oz size 42 3.6V 6.1A. ea.
>Using 3 Gecko 201 drives with no current set resistor ( set for 7 Amp max )
>Power supply puts out 49.5 Volts at idle and 48 with all motors turning .
>
> From what I have seen on the list this should require something like 12
>to 18 Amps !
>
>WEELLLLL I'm running this system on a Bridgeport clone called a
>Wizard and it is set up as follows .
>Power supply puts out 49.5 Volts at idle and 48 with all motors turning
>. ( this is checked at the cap )
>There is a single fast blow 5 amp fuse coming out of the capacitor and
>then a 4 amp fast blow fuse to each Gecko .
>I have never had any of these fuses blow so to me that would indicate
>that the entire system is drawing less than 5 amps DC .
> How can this be wrong ???
>
>In reality the 5amp fuse was a mistake I thought I had put in a 10amp but the system has been running like this for several months .
>
>
>
>

If you ran a CNC program that had all the motors accelerating and
decelerating
rapidly, over and over again, I think you could eventually blow that 5 A
fuse.
It might take a while for it to heat up. At idle, the Gecko reduces
power to
the motors. Also, at idle, even if the current was not reduced, the
power input
to the motor is a maximum of 141% of 3.6 V x 7 A. This comes out to
35.5 W. Now, 35.5 W requires .74 A at 48 V. So, idling at full current,
each drive would draw .74 A, for a total of 2.2 A. When the motors are
spinning, and especially when driving a load, the current draw increases
dramatically, and would probably exced the 5 A fuse's nominal rating.
But, the way fuses are rated is that they must never blow at 100% of the
rated current, drawn indefinitely. And then, depending on the time
rating of the fuse, 200% of rated current should blow it in 2 seconds
(semiconductor fast blow) to 2 minutes (motor slow blow).

Jon