[Lucien Nunes]

Speed stability is important for sound projection and I very much doubt that adequate results could be obtained without tacho feedback. Connecting this motor to a constant effective voltage (whether adjusted by PWM or otherwise) would probably result in audiible pitch variation through the length of the film due to change in friction and takeup load, wow at takeup and supply reel rotation rates and sprocket rotation rate, and odd jumps at splices etc.

The only way to obtain a passbly stable drive from a PM motor without tacho feedback, is to size it generously (to minimise the effect of load regulation) and improve the remaining load regulation with I*R compensation - effectively adding a negative dynamic resistance in series equal to the motor's apparent armature resistance, one of the major factors in PM speed regulation. This was used in certain portable devices that did not have centrifugal governors. However, the motor in this projector has to work hard and is subject to sudden load changes at splices etc, so I think this method would be barely adequate. It also requires a regulated supply to achieve correct speed at low load. Since a tacho is available, I would definitely use it.

A commercial PWM motor driver with e.g. a 0-10V input can be driven by a low-power analogue servo block reading the tacho and speed switch setting, and applying PID control to produce the output value.

FWIW, when 35mm sound projectors were first introduced in cinemas in the early 1930s, DC motor drive was often used with magnetic tonewheel speed feedback via valve servo-amplifiers. The cost and (then) complexity of such a scheme are indicative of the importance accorded to speed stability.