On Tue, 02 Sep 2008 13:50:56 -0400, Bob Engelhardt
wrote:

Now that brings up something else I've wondered about. I've heard
before how servo amps control the motor speed with voltage and motor
torque with current. Makes sense conceptually, but how does the amp
control the current independently of the voltage? My intuition is that
you set the voltage for the speed you want & as the motor is loaded
(more torque needed), it just draws more current to supply the torque to
keep the motor running at the voltage-selected speed.

Generally so, but it depends on the application. If you want to
control speed and deliver whatever torque is needed, you control
voltage to get desired speed and let current be whatever it is. If
you want to control torque regardless of speed (from stall to max
speed) you control current to get desired torque and let voltage (and
speed) be whatever they turn out to be. You can also control
voltage/speed until current reaches a preset level (torque limit), at
which point currrent limit takes over and voltage /speed becomes
whatever results in the set torque.

That does require
a low impedance source - is that the complication that brings in
separate control of the current?

Variables a speed/voltage, current/torque and the load's
speed/torque relationship. You usually have no control over the
load's speed/torque relationship, it is what it is. So, you can
control voltage/speed or current/torque but not both at the same time
unless you can control the load's speed/torque relationship.