Re: [CAD_CAM_EDM_DRO] Re: Best source for Servo's ??

Hi Dave,

>I guess I am not sure what makes a proper servo motor.
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Proper servo motors have very little torque ripple and low inertia. They
are also usually designed for high torque at low speeds. I have some
nice 36V servos. They will only do 1000RPM but they get to that speed in
less than half a turn.

Torque ripple is the cogging effect you feel when you turn a DC motor by
hand. The control electronics have to compensate for this so it
introduces a very slight variation in speed, especially at low speed. If
you turn a servo motor by hand there is virtually none of this cogging.
Using a higher resolution encoder helps compensate because the drive has
more information about what the motor is doing at any given moment.

>There are a lot of PM motors of different voltages and different amps.
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>Also, it would appear that for many motors, one could drill and tap
>the opposite end and then fit in a shaft for an encoder.
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Yes you can. Here <http://www.truetex.com/servomod.htm> is an article
by someone who did that. Sometimes it is easier to mount the encoder on
whatever is being driven (ballscrew etc).

>Looking at my steppers, a NEMA34 with 1.7v and 4.7A are only about 8
>watts. most of the PM DC motors are much higher.
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Steppers are complicated beasts. When they are stationary your steppers
will probably dissipate about 16 watts (8*2) as heat. When they are
moving the voltage goes up so the power goes up. The true output power
can be complicated to work out.

>Is there a speed that is desirable ? again, 1700 and 3700 and even
>20,000 RPM and many others seem to be common.
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Slow is good. You need less reduction gearing and you will waste less
power overcoming the rotor inertia. A high speed motor will take a while
to get up to speed. Long thin motors are better than short fat ones
because the rotor inertia is generally lower which gives better
acceleration. As an exception to that rule pancake type motors can work
well. They often have very low inertia due to the completely different
rotor design. The big down side is that they have very low inductance so
you often need to add an external inductor to control the switching
current spikes.

Les

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