Re: [CAD_CAM_EDM_DRO] wrong guy, but have servo question.

Wally K wrote:

> Jon, in you last post you said "Wally K wrote:" the following, but it
> was Ron that wrote it. I am using steppers on my system.

OK, sometimes when I chop up a quoted message when replying
more to the original question than the most recent answer, I chop
it wrong. Sorry.

> Ron is doing a bridgeport type retrofit and has been trying to
> determine how much HP he will need to move his table and if his
> servos are big enough. And i have been trying to gather information
> about his system so we could help.

First, I think his motors would be adequate, but not for direct drive.
There was also something in another reply about 14 RPM at xx V.
If that was really a motor spec of 14,000 RPM at xx V, then he is
going to need REAL speed reduction, 2:1 won't be anywhere near
enough.

> I have a few questions on servos that you could clear up for me.
> First i thought servos were chopped drives. But it seems that when
> people here talk about a servo rated at a certain voltage, that is
> the voltage they run it at. Example, the servo is rated at 24vdc.
> So they run it at 24vdc.

Well, either with linear or PWM servo amps, you need to allow some
drop in the amplifier. Also, the voltage ratings are not as absolute as
the current ratings. You can't exceed the peak current rating on a PM
motor, even for an instant, or it can demagnetize the magnets, ruining
the motor. You can exceed the voltage, at least for short intervals, as
long as the motor isn't exceeding safe RPMs. But, it all comes back
to desired table IPM (or mm/Min). If you want 120 IPM rapid feed, and the
leadscrew is 5 TPI, you need 600 RPM on the leadscrew. With a 2:1
reduction ratio, you need 1200 RPM on the motor. If the motor was
rated at 1000 RPM at 24 V no load, you'd need a little more to get
1200 RPM, say 28.8 V. Plus, if the motor has a 1.5 Ohm resistance
and the lesser peak rating of the servo amp and motor is 5 A, then the
IR drop in the armature is 7.5 V. Plus, you probably should allow
20% extra voltage for drops in the amp, so you need 28.8 + 7.5 x 1.2 =
about 44 V supplied to the servo amp. There's no harm in going higher
with a switching-type amp, due to the current limiting, it is not likely
that it will overvolt the motor. (One possible scenario that has to be
thought about is a broken belt. Running a 24 V motor on a 100 V
supply could cause the motor to turn at extremely high speeds and
throw the windings. Proper calibration of the servo amp and CNC
control should arrange it such that the motor would not go more than
20% above normal speeds under these conditions.)

> Second what do the torque curves of a servo look like.

Given sufficient resources (current and voltage as described above)
a good PWM servo amp should provide totally linear torque curves
out to the full rating points of the motor. The torque is proportional
to current within a couple of percent, independant of voltage or speed.
The speed should also be linear within a few percent, under no-load
conditions. Armature resistance comes into play when load is applied,
but a velocity servo amp compensates for this.

The ability to deliver peak torque all the way up the RPM curve
is what distinguishes a servo system from a stepper system.

Jon