Re: [CAD_CAM_EDM_DRO] I want to cry !!! another fried Gecko + very, very hot motor,

Dave Fisher wrote:

>Sounds like the volts to the motor are too high - the motor should not get
>excessivly warm in the idle (locked) position.
>
>
>

No, that isn't right. If the voltage was over a few volts, the motor
would have
been forced to move. Since it didn't fly off to the limits (I don't think)
that couldn't be it. There are a couple of possible causes.

1. The machine needs a lot of force to move it, and the motor is on the
weak side. But, the encoder is not right on the commanded position, so
some current flows to the motor, just not enough to break the static
friction. (Now, Mariss says the integral term of the drive's PID will
just keep increasing current until the motor does move, but I have seen
this scenario in enough cases that I know it can happen. It may have
something to do with a noisy encoder and the anti-dither feature of the
Gecko 320/340 drives resetting the integrator.) Anyway, this continuous
current, essentially a stalled motor, can cause a lot of heat in high
resistance motors - like small ones.

2. Dithering at high speed keeps the motor current moderately high as
the motor constantly overshoots the commanded position and has to
beack up. This will cause lots of heating, but is quite audible, so most
people will know if it is happening.

3. A very low inductance motor allowing a high ripple current at 25 KHz
to flow. The solution is to add a milliHenry or so of inductance in series
with the motor. This mostly applies to printed or pancake motors which
have VERY few turns per armature coil.

Jon