Re: [CAD_CAM_EDM_DRO] Gecko G201 and Drive Current ?

Jon,
The clamp-on amp meter is DC only, range FSD 0-1mA(one milliamp) through 0-10Amps
The old HP 428 is one of the few oldies (tubes) I keep around because it is so handy.
Calibration is easy to check and it is good.
Here's one for sale on eBay http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3308854547&category=4678

I did the current shunt, simply placed it in the 0V return total power supply. As the resistor is
very low in value 0.027 Ohm I figured it would not influence the overall performance that much, and the results are the same
resisitor in circuit or not.
My first measurement was wrong (posted earlier results) as the signal across the 0.027 Ohms is so low
and there is a lot of switching noise, I missed what appears to be steps in the wave form.
The new reading appears to be about 0.1V pp which would indicate a total circuit current of about 3.7 Amps.
I suspect the 3.7 Amps total should be divided by 2 (two windings being driven) which would make the per winding current
about 1.6 Amps.

I have a 15,000uF Cap. at the Power input of the Gecko (6 inches from it), total power lead length is 3 feet. Power unit is regulated 36 Volts 9Amp.

The motor specs. are Current/Phase 5.6 Amps, Phase resistance 0.39, Inductance 4.1mH

All measurements are taken with motor running. When the motor is OFF the disable feature is active and winding current is reduced.

One last thing that may shed some light on what's going on.
I have zero experience with the Gecko modules, and based upon all the massive heat sinks everybody seems to be selling for them, I assumed at 5 Amps the Gecko Module should at least be warm... well
it's cool to the touch and I only have a small 6" x 6" x 0.065" heat sink during this initial testing phase.

As my tests are under my control, I'm able to ensure there is no over heating. I wish there was some heat ...that would indicate some Amps are flowing..!

I this ever works, final heat sink will be the case of the device which is 0.375" think and has a surface area of over 4 square feet ! That should be enough to keep the Gecko cool....

Any more suggestions would be welcome. In the mean time I'll work on measuring AC current in a winding.
Best regards
Hans W


Jon Elson wrote:

> Hans Wedemeyer wrote:
>
> >Here's my setup.
> >Gecko G201 and PACSI H32 Series motor parallel wired to take 5.6 Amps.
> >Gecko has a 120K Current set resistor this should be a 5 AMP setting.
> >DC Supply 36 Volts able to supply Max 9 Amps.
> >Length of cable from Supply to Motor 3 feet. 14 Gauge Wire .
> >15000uF Cap. at the Gecko module.
> >
> >I did not get the power I expected. This motor is rate at 445oz-in and the project was calculated to
> >need about 160oz-in.
> >
> >When it failed I started to investigate. First I used a clamp on Amp meter on the Plus Output lead of the PSU
> >
> Does this clamp-on meter read DC? Most don't. I suspect it is reading
> the current peaks
> of the chopping action.
>
> >and started the motor. It read 1 Amp ! I know the reading is integrated, but I really did expect it to show at least 2-3 Amps.
> >
> >
> Not necessarily. Your motor probably has a resistance of 2 Ohms or less
> per winding.
> 5.6 A x 2 Ohms = 11.2 Watts per winding, or 22.4 W for both windings.
> neglecting
> switching and other losses in the Gecko, with a 36 V supply, you only
> need I=P/V =
> 22.4/36 = .622 Amps when standing still! Do you have the local
> capacitor that is
> supposed to be placed at the power input terminals when the main
> capacitor is several
> feet away?
>
> >This makes me suspect the Gecko.
> >
> >Does anyone have a good method to confirm the Gecko really is switching these windings at 5 Amps ?
> >I have equipemnt to test... just tell me how to test this :-)
> >
> >
> You need a low-inductance current shunt or a Hall effect clamp-on
> current probe to measure
> pulsed currents like this. a low-inductance current shunt can be made
> from 10 1 Ohm 1/2 Watt
> resistors in parallel. This will have a resistance of .1 Ohm at 5
> watts. You can use this with
> a DVM, or better with an oscilloscope. But, with the scope, you need
> isolation, as the
> motor outputs of the Gecko are not referenced to ground.
>
> Jon
>
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