DCServoMotor said:
What is the difference between a brush type "ordinary" DC motor and DC
"servo" motor?
Does adding a quadrature encoder to the ordinary DC motor convert it to a DC
sevo motor?
Ordinary brush type DC motors come in several flavors. Series wound
motors (also called universal wound because they can run on AC) have
the field windings wired in series with the armature, so that the
torque produced is proportional to the square of the current (the
armature current reacts against the equal field current). Since these
produce torque in the same direction, regardless of the direction of
current (hence the usability on AC). So these are not usable as servo
motors.
Shunt wound motors have the field winding either wired in parallel
with the armature, or excited by a separate current, entirely. The
separately excited shunt wound motors can be used as servo motors,
since their torque is essentially proportional to armature current and
their speed is approximately proportional to armature voltage.
Permanent magnet field motors are very similar in character to
separately excited wound field motors, since their field's magnetic
strength is not related to armature current. They are commonly used
as servo motors.
Any motor that can produce torque in either direction can, in theory,
be made into a servo motor if you can measure its speed to be used by
the servo loop controller. Being able to measure the torque is very
handy, also, so motors that have torque proportional to armature
current make this easy.
Strictly speaking, a servo is a motor application, not a type of
motor.