Re: Power Supply

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Ron K <godsone@e...> 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 .

>
>
> --
> Ron Kline
>
> American Precision Ceramics

Ron what you are seeing is reality vs theory. There are a lot of
factors that may be at work here. The motors Average current which
is the Gecko 7 A limit times the duty factor is evidently a lot less
than the nameplate rating. If a motor only needs 20% of the pulse
width to move the load then the AVERAGE current over time is only
20%. Since things like transformers and fuses can withstand short
periods of 100% overloads the 5A fuse could easily handle 10A at 50%
duty cycle.

The only real way to test is put a DC ammeter in the DC line in
series with the load and do some measurements. A current probe on a
scope would give you the real picture from a dynamic perspective. You
might be surprised to see how much less the current draw is on
average than you would calculate from the motor specs.

If you run mostly two axis moves then two motors would probably only
draw 8 to 10 amps worst case. Sounds like you are not running the
motors anywhere near their max limits.

Another good indicator is that things are probably not running hot:
motors, power supply, etc are warm but not too hot to leave you hand
on I would guess.

Take it as good news and don't worry about it too much. The only
true way to design a power supply is to have the actual load in the
actual conditions it runs in to measure then do the calculations.
Taking numbers off components and adding them all up will only get
you in the ballpark parking lot. We all tend to overdesign the power
supply because of the fear of putting a lot of time and effort into
something that ends up being too small and to combat Murphy's Law.

The good news is that your power supply will last a long time.