I could see the rotational frequency of the drive remaining stable within the quartz lock regime. That is a very narrow band of speeds.
I could have stayed the above post more clearly.
At steady state, the coils are energizing such that as the magnets pass each coil, the coils are energized such that they push/pull each magnetic pole on the rotor at the desired rate. Torque is fixed at some preset amount by the steady state current set in the drove circuit. If the rotation of the permanent magnet rotor and the coils gets out of sync, the coils will repel/attract the wrong poles on the rotor and all hell breaks loose.
If the rotor/spindle slows and the commutation frequency remains fixed, the magnetic poles on the rotor will be influenced by the wrong coil fields and rotation will break down. So, the controller will both reduce the rotational frequency and increase the current (put a DBT on a big DD and drag your finger on the platter. It'll light up) to give the rotor a bigger push each time the magnets pass the coils-speeding it up. As it speeds up, the coils are commutated faster to keep up until the desired rotational speed is achieved.
The reverse happens if the rotor spins too fast-depending on the drive function, the current can simply be cut and the rotor will slow, or the controller can time the coil energization to actively retard the rotor, then return to the steady state once a stable speed is achieved.
Thinking about it, most DD turntables have something between 6-24 poles on the field windings. That gives some range of degrees of rotation (greater than a few degrees) where the motor coil pulse would still most strongly affect the correct magnetic pole, as for some sweep of degrees the correct pole would a. Still be closest to the coil as compare to all the other poles and b. on the correct side of the coil to react properly to the magnetic field. So within some small range of
transient speed variation, the rotational frequency of the windings need not change:as long as the speed is corrected within that window with an applied torque, synchronous control has been maintained without adjusting the drive frequency. Since turntables operate within an extremely small range of rotational frequencies, outside of startup and fringe conditions like scratching, a fixed frequency on the drive as described by
@JP makes sense. As soon as an event causing large variation in speed is encountered, the drive frequency must compensate to maintain control of the platter.
Cheers
Nathan