Bob,
The answer you are looking for is difficult to formulate, because people
misuse the term. A true servo motor provides drive torque, radial
positioning and position hold torque. These are analogue, multi phase motors
and they are expensive to make, inefficient and consume lots of energy. In
use, there is a transmitter that provides all the power for all the
receivers. They are effectively selsyns and they phase lock together in use,
sometimes used with a differential selsyn for small phase angle adjustment.
Today, this technology is seldom used because although accurate, they are
much more expensive and can do very limited work. They were primarily used
for compass drive and radar screen/antenna synchronization. Today that role
is being performed with stepper motors, with and without position encoders.
However, these are also inefficient and are low torque for the same reason,
using a large portion of the applied power for hold torque, but they are
less expensive. The high torque applications are usually satisfied with
permanent magnet (PM) motors, pulse width modulation (PWM), encoders,
tachometers and brakes because electronics are less expensive than copper
and they are very efficient. Unfortunately, they are all called servo
motors.
Steve

"Bob Engelhardt" wrote in message
...
Is a servo motor simply a DC motor with an encoder or tach? Or is there
something about the motor itself that makes it a "servo motor"?

Thanks,
Bob