Why a synchronous motor?
A unique ability of a synchronous motor is that it will turn at a very accurate rotational speed directly related only to the frequency of the applied AC power and independent of the voltage (within it’s operational limits). Synchronous motors have track records of running at very accurate speed for decades. They are also inexpensive. Other motor types need sophisticated electronic circuitry to control the motor speed and compensate for the tendency of the motor to slow down over time. Stepper motors “step” and although the steps are small they sometimes vibrate enough to be seen in the eyepiece as a slight buzzing of the image. DC Servo motors can be made very smooth and very accurate but are expensive and do not have the endurance record of synchronous motors. They seem to last just long enough to fail when replacements are no longer available. Synchronous motors do have one inconsequential disadvantage: they produce very low torque. In actual practice all are provided with internal
