Re: [CAD_CAM_EDM_DRO] Gecko G201 and Drive Current ?

Hans Wedemeyer wrote:

>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 wasn't aware of this model with a clamp-on probe!

>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.
>

Note that there is a power supply capacitor inside the Gecko drive, so
that the full
P-P AC value of the motor current will not be seen on the DC supply lines.
It will be split between the internal and external capacitors, based on the
eqivalent circuit impedance both ways. Also, unless the switching action
of both windings is synchronized, they may tend to sancel out each other's
on/off pulses. I suspect this is what you are seeing when you mention a
stepped
waveform. You would see 3 cases, I suspect: Both windings drawing current
from power supply, only one winding soing the same, and no windings drawing
current from supply. But, as I say, the internal capacitor will be
supplying some
of the current, so the peak reading you see will not equal output current.

Can you measure motor current in one of the motor wires with a DVM or other
DC ammeter? This should not cause any problem with the drive. You can
disable
the low-power mode for this check. You might be able to use an AC clamp-on
ammeter, and when running at 240 full-steps/second, the waveform should be
somewhere between a square wave and a sine wave at 60 Hz. If much like a
square wave, a clamp-on meter may give an artificially high reading.

DC current draw should be less than the combined motor winding currents
when standing still (without the current reduction mode), because the
switching
action only draws current from the power supply approximately in the ratio
of Vwinding / Vsupply.

Jon

>
>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.
>
>

As motor speed increases, the inductance prevents the current from
rising instantly, and there is a
speed (not all that high, usually) where the current never reaches the
limit value. Torque drops
rapidly at that point. with 4.1 mH / winding, and a 36 V supply, it
would take a minimum of
638 uS (neglecting any series resistance in the windings, current sense
resistors and transistors)
to charge to 5.6 A from zero. Since when running, the current always
has to go from one polarity
at 5.6 A to the opposite polarity, it takes twice as long to reverse
polarity, or 1.28 mS.
Since a full cycle is actually completed for 4 steps of the motor, it
can complete 4 steps in twice the time
above, or 2.55 mS. That is 391 steps/second. Since there IS
substantial resistance, likely twice or more
the .39 Ohm value stated above when you include the drive, the actual
point at which the torque,
and current, start to drop, will be a lot less than the 391
steps/second. I seem to vaguely remember
an earlier post where you mentioned 2500 steps/second. I doubt you will
get much torque
at all at that rate, especially with a 36 V power supply.

Jon