Re: [CAD_CAM_EDM_DRO] Cap at the Gecko, motor heating, fuse protection??

R Rogers wrote:

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>Hi All,
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>Just recently I got around to installing the 100volt 1000 uf caps at the Geckos. Today I was drilling .860" holes 3" deep with a pre-drilled pilot in steel and normally the Z motor will heat considerably but not excessively. Since I installed the caps at the drives the Z motor seems to run alot cooler. Is this possible? Usually in an hour of two of drilling with heavy loads the Z motor is up to temp, today it was like 4 or 5 hours continual drilling before it got that warm. The Z gecko seems to stay cooler too.
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Without the capacitor, the high frequency noise in the power seems to
upset the analog circuits
in the Gecko and produce some high frequency dithering that may increase
motor heating. it
usually doesn't matter whether it is moving or not, for the servo drive
Geckos.

>Also, I've asked this before about the fuse in between the power supply and the gecko. And the concensus was that If an axis stalled the fuse would not blow as the current would be isolated between the drive and the servo, if I understood correctly.
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Yes, that's a problem. The fuse helps protect against a total melt-down
of the Gecko, with traces burned off
the circuit boards, etc., making it totally unrepairable.

> Wouldn't this only be possible if the encoder was dead stopped and on only one count where the stall occured? Seems that if it stalled on one count and than made it to the next that the current would then be imposed on the circuitry coming to the drive and hence blow the protective fuse.
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The problem is that the gecko is a power IN = power OUT device. If a
low-resistance motor (let's assume 1 Ohm)
is stalled, and getting 20 Amps, the voltage drop is 20 V, so it is
dissipating 40 W. This will cook the motor pretty
fast, too. With a 70 V power supply, that will only draw 5.7 Amps.
But, if you put a 5 Amp fuse there, and the
motor needs to draw 15 Amps while spinning at high speed such that the
back-EMF is 50 V, then the applied
voltage at the motor will be 50 + 20 = 70 V, the current will be 15 A,
so the power demand will be 1050 Watts,
and the current will be 15 A. The fuse will blow, even though this is a
permitted condition. A fuse in the OUTPUT
of the Gecko would prevent this, but that is a hazard to the Gecko, so
you can't do this, either. One way would
be to put a big .1 Ohm resistor in series with the Gecko motor wires,
and then glue a PTC thermistor onto the
resistor. This would provide a way to thermally integrate the RMS
heating of the motor. It would take some
serious fooling around to get the time constants right, etc. so as to
have good protection without false trips.

Jon