Speed variation using a DC motor
It's possible to see speed variation at times using a dc motor. The issue here is drag variation, not lack of power, as a dc motor is regulated to provide constant Torque. Power is never an issue as the motor we use can comfortably turn 30kg platters. The load compensation circuit in our supply is highly sensitive and quick to respond but is limited to fairly small load swings which is normaly sufficient to keep speed stable through dynamic passages. This is why it's important to set the speed at normal operating conditions i.e normal temperature and with the needle on the record.
Ac motors operate completely differently as they are speed locked. This would appear to be the ultimate solution but as with all things audio, it's not that simple. The killer drawback with AC motors is the vibration they feed into the audio chain. No matter how much you smooth their electrical supply, these motors never come close to the subjective performance of a well designed dc motor and power supply.
You can have the most speed stable motor on the planet and it can sound terrible - why? Vibration is far more important to the stylus and hence the ear, than slight variations in speed. We have checked out dozens of motors and controllers and the result is always the same. These include the top encoded motors costing well over £1000.
So before becoming fixated on speed it's wise to consider the unseen stuff which nobody measures - vibration. This is far more audible than slight speed variation.