Re: [CAD_CAM_EDM_DRO] Gecko G420's question

David Eldredge wrote:

>
> Does anyone know what type of feedback the G420's are expected to use?
> Hall effect or encoder?

Where did you get information about the G420? I heared roumors, too. But
there's no announcment or other info on their web page. Do you know when
they will be available.
(I'm new to this list)

I know brushless motors from Indramat (german company). They use both,
hall effect sensors for the commutation (6 pulses per rev) and optical
encoders (2500 p/rev) for the feedback loop.

Someone said, the brushless geckos will have the same voltage and
current rating as the others (80V 20A). But most brushless motors
available here in Europe are 600V-types (directly fed from 400V mains
without transformer). Are there any sources for 60-80V brushless motors?

> Will the G420s save money on servo motor cost? I understand the benefits
> of eliminating brushes and commutators, but are there many sources of
> motors for them?

Probably more than for servo motors with brushes. In new proffessional
machines, you find exceptionally brushless motors. None of the "big"
manufacturers i know still uses brushes.

> Someone mentioned that the brushless motor is kind of like a DC permanent
> motor turned inside out, I think.

Yes, a brushless motor is mainly a synchronous AC motor with the coils
in the stator and a permanent magnet in the rotor.

> Can someone take a three phase AC motor, and make a new rotor with permanent
> magnets glued to the rotor and make their own motors?

Yes this would work, although it is difficult to create the proper
"shape" (sinusodial or trapezoid) of the magnetic field like this. If
the waveform of the voltage of the motor (when used as generator)
doesn't match that of the driver/commutator you will have a lot of
torque ripple.

I tried to build something similar to the geckos for brushless motors
myself a few years ago but effectively gave up due to the torque ripple
problem. It is not sufficient to simply switch the coils in a constant
relation to the shaft angle the way you do with a stepper motor. You
either have to add speed dependant phase shift or drive the current in
some continous shaped (sinusodial or smoothened trapezoid) way which is
not that easy.

> Maybe a dumb question,
> but I thought I'd ask. I have re-glued permanent magnets back on some motor
> housings in the past. They seem simple enough.

Yes, theoretically it would work if you could get magnets that fit. You
could even mount a solid ring of permanent magnetic material on the
shaft and use the coils of the motor itself to magnetize the rotor with
a very high current pulse. But I doubt that this results in a high
quality motor.

Ciao
Bene

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