Re: Power Supply & Gecko's

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "smith67au" <thdsmith@b...> wrote:

> Hi All,
>
> This is my first post and I'm hoping I can get some advice.
>
> I have a retrofit bidgeport mill runing on steppers with Gecko
> 201/210's.
>
> Current settings are two at 7.0 amps and one at 3.5 amps.
>
> I have been driving the mill with a 24V 25 Amp regulated power supply,
> for a few years.
>
> I wanted to upgrade the power supply, so I've built one similar to the
> "Simple Power Supply" in the files section, using a transformer I had
> laying about.
>
> I have 48VAC out of the transformer going through the bridge rectifier
> (35A 400V) and then paralleled with a 22,000 uf Cap.
> Result is I am getting around 70VDC as expected.
> I am also measuring quite a bit of AC (around 150V), but it might be
> because my multimeter is a cheap one (the 24 VDC power supply I have
> been using for a long time also has an AC voltage (30VAC).

It sounds like you meter is maybe picking up the switching noise of
the chopper circuits and giving you a false reading. WIth no load on
the power supply you should get pure DC. As you load the supply the
supply starts to have ripple. It's the difference of the peak voltage
at the center of the waveform and the amount of current you are
drawing out of the cap on average. The amount of ripple is expressed
as a percentage of the voltage. The factors are load current, power
factor of the load and the value of the capacitor. The bigger the cap
the lower the ripple. Is ripple bad? Only for the motor drive
electronics.

>
> My problem starts when I connect up the Gecko's.
> The 5A fuses blow instantly every time (are they big enough?)

Are they on the DC side or the AC side of the supply? The higher
secondary voltage will result in a higher inrush current to charge the
cap to the 70 V peak.

>
> I found another tap on the transformer that will give me about 30VAC
> and therefore 42VDC, and this seems to work fine.

Sounds to me like a good solution.

>
> Any ideas on why I can't run the higher voltage.
> I don't want to just try bigger fuses and risk the Gecko's unless I'm
> sure they won't get damaged....

Bigger fuses are not the answer. If you are running the Gecko's at
their full 7A rating a fuse should not blow regardless of the applied
voltage.

What are the motor ratings? There is a point of diminishing returns
on applied voltage and just saying that it should always be 25 times
the nameplate rating is a vast oversimplification. I would run the
unit with the 42 volt supply and see how much better speeds you get
AND how much hotter the motors run. You might be surprised to find
that the "sweet spot may be 42, 48, or even 56 volts rather that 70.

If you can get access to a variac (autotransformer) and run the
primary of your transformer through that and hook every thing up for
the 70 volts but "dial" the voltage slowly up in steps you can run the
numbers and see where you are just wasting power in the form of heat.

>
> What voltage should I be aiming for?

The lowest one that gives you the performance you *NEED*.

> I have just bought some new steppers which will be fitted soon. They
> are rated at 5.0A and 3.7VDC.
>
> Thanks
>
> Tim