Re: [CAD_CAM_EDM_DRO] servo voltage

Thomas Powell wrote:

>I know a stepper can be run up to about 20 times the coil voltage. How high
>can I go safely for a servo motor. Say for a 40 volt motor.
>
>

It depends. The voltage spec can be used by the manufacturer to limit the
peak current surge when the motor is starting. A servo drive limits the
peak motor current itself, so DC bus voltage is less important. The higher
DC bus voltage increases AC ripple current at the PWM frequency, roughly
proportionally. The applied DC voltage on the motor is Kv*Speed + I*R,
for any
particular speed and torque. Kv is a fundamental constant of the motor's
design, and R (winding resistance) is too. I is the current,
proportional to
delivered torque. So, if you only need a little torque at high speed, a
servo
system can run quite close to the limit where Kv*Speed <= DC Voltage.
If you will need a lot of torque at this upper speed, you have to allow
for the resistive voltage drop, and use a higher DC voltage.

Motor inductance comes into this too, as low inductance motors will suffer
lots of eddy heating due to the AC ripple current, on drives that don't have
filter inductors built into them. In this case, external series
inductors are
sumetimes needed.

So, there is no pat answer, it depends a LOT on the motor. For
instance, the
Ametek motors that so many people are using has over 4 Ohms of resistance.
It has a peak current rating of 12 A, and a 36 V supply voltage rating.
It should be clear that 4*12 > 36, therefore, the motor cannot even REACH
its peak current rating from a 36 V supply! That is when sitting still!
When
running at 900 RPM, you can't get ANY current through it, as the Kv*900RPM
= about 36 V, leaving no margin for IR drop. So, to get rated
performance, you
HAVE to run it above the manufacturer's rating! But, then, that is a rather
poor motor for machine tool motion applications. I throw it out there as an
indication of how all these parameters interact.

Jon