Servo questions.

Wally asks;


So are you saying the torque curves for servos with a linear or
chopper drive look the same. Assuming they both can put enough
current and voltage out.

>>Wally wrote
> > 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.
>

Jon replied;

> 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