Re: [CAD_CAM_EDM_DRO] Basic Servo Motor Question...
Posted by
Jon Elson
on 2000-07-31 23:05:56 UTC
djr@... wrote:
provide
velocity info. Also, on the old CNC controls, the servo update rate was
so slow
(60 to 100 Hz, typically) that the servo amp needed much higher
bandwidth to
keep cutter paths smooth. The DC tach could allow the velocity loop to
have
higher bandwidth than the CNC control could provide.
High speed motors need large belt reduction ratios, but then a larger
belt
reduction gives more torque at the leadscrew.
motor.
Exceed this, even for an instant, and the magnets will be degraded,
generally
ruining the motor. Often, this info is not on the label. Another
important data
point is the continuous rating, either current or power. Armature
winding
resistance is a big player in this, as a higher resistance winding will
get hot
faster at the same current. Without additional info, it is very hard to
say.
Likely, with a 2:1 or so belt reduction, this motor should work fine.
The important parameters are Kv (voltage at 1000 RPM) and Kt (torque /
Amp).
These parameters are almost NEVER on the nameplate, but can be had from
the manufacturer. Your motor likely has a Kv of 8V/1000 RPM, but the
Kt is totally unknown with only the above info.
Jon
> I want to jump in head first in a CNC servo sherline mill conversion,Yes, but for an analog servo amp, it is a lot easier to use a DC tach to
> but I am really ignorant when it comes to DC servo motors and
> drivers.
>
> I see that some servos have slotted encoder wheels with optical
> detectors and/or tachs and/or brakes. I can see what the slotted
> wheels and brakes do... the tell position and stop the thing, but
> what
> is the need for tachs? Can't you tell position AND speed with the
> optical detectors?
provide
velocity info. Also, on the old CNC controls, the servo update rate was
so slow
(60 to 100 Hz, typically) that the servo amp needed much higher
bandwidth to
keep cutter paths smooth. The DC tach could allow the velocity loop to
have
higher bandwidth than the CNC control could provide.
> What motor stats are important for me if I amModest RPMs are most likely best, somewhere around 2000 - 3000 RPM.
> going to use a servo motor with the sherline? Does stall current x
> volts (watts) tell me something? What RPM range is good?
High speed motors need large belt reduction ratios, but then a larger
belt
reduction gives more torque at the leadscrew.
>The most critical piece of info is the max current on a permanent magnet
> I was browsing a local surplus store this past weekend ran across a
> servo motor with one of those slotted wheels and an optical detector
> circuit board attached still in the anti-static wrap. It was made by
> Japan Servo and it's rated at 24v 3000 RPM. It is about 3" in
> diameter and 5" long. I don't have any other data on it. How can I
> determine if this motor is a good candidate for the sherline? At 12
> volts it really seems to have a good bit or torque.
motor.
Exceed this, even for an instant, and the magnets will be degraded,
generally
ruining the motor. Often, this info is not on the label. Another
important data
point is the continuous rating, either current or power. Armature
winding
resistance is a big player in this, as a higher resistance winding will
get hot
faster at the same current. Without additional info, it is very hard to
say.
Likely, with a 2:1 or so belt reduction, this motor should work fine.
The important parameters are Kv (voltage at 1000 RPM) and Kt (torque /
Amp).
These parameters are almost NEVER on the nameplate, but can be had from
the manufacturer. Your motor likely has a Kv of 8V/1000 RPM, but the
Kt is totally unknown with only the above info.
Jon