For an alternative response, I picked this up from Audio Asylum, and they seem to prefer AC. I find it interesting reading, though a little over the top at some points.
Allow me to try and explain a little on the different types of motors found on turntables. On the one hand I find it frustrating when a seemingly simple subject is mystified and on the other I realize just how sensitive the stylus and its environment is.
Basically I have seen two types of electrical motors used in turntables, AC and DC and of the AC type, synchronous and asynchronous motors.
With both AC and DC motors the objective is to get the shaft turning. With AC this is achieved by creating a rotating magnetic field in the stator. The stator is the outer part or the casing part of the motor. The motor is normally specified as having so many poles, for example you will see motor specs saying its a 24 pole, 16V synchronous motor. This means the motor will have 8 primary poles and 8 secondary poles build or wind onto the stator, if I may call it such. The poles are arranges such that you have a primary followed by a secondary and again followed by a primary and again a secondary and so on. The primary poles are connected to two external leads and the same for the secondary poles. The primary is connected directly to the power supply and a capacitor is used on the secondary to create a phase shift. Now picture this, when I turn on the supply each primary pole will reach a peak magnetic field and 90 degrees later the secondary poles, this pattern is referred to as a rotating magnetic field. When I insert a rotor, the shaft or turning part of the motor, into this rotating field it will follow this rotating field and the shaft will be turning. The difference between a synchronous motor and the asynchronous motor is the type of rotor used. With the synchronous motor a permanent magnet is used on the rotor and with a asynchronous motor the rotor has a type of winding so that when it cuts the rotating stator field a magnetic field is generated in the rotor that will interact with the rotating stator field. The rotor of the asynchronous motor will always turn a bit slower than the rotating stator magnetic field and the reason is to maintain the magnetic interaction. This speed difference is called slip and the amount of slip will vary depending on the load. With synchronous motors the permanent magnets, field on the rotor, will lock onto the rotating magnetic stator field and it will stay there with no slip. The number of poles on the stator and the supply frequency determines the speed of both types of AC motors. Remember with the asynchronous motor slip also play a role, as different load situations will change the speed. The voltage supplied to the stator and/or the current determines the strength, or torque, of the stator magnetic field. Limit either the voltage or the current and the torque will reduce. Torque reduction in an asynchronous motor will increase the slip and with a synchronous motor it can potentially result in the rotor breaking loose from the rotating field.
DC motors are more complex than AC motors. With DC motors opposing poles or magnets are arranged in the stator. You will have a North Pole followed by a South Pole and so on. There is no rotating field as poles are in the form of permanent magnets. The two wire DC supply is then supplied to the rotor. The contact interface between the rotating rotor and the outside world is via a commutator, the strips of copper on the rotor and the bushes. The commutator creates a fluctuating magnetic field in the rotor that will have a similar effect like the rotating stator field in AC. This will result in the rotor turning. The type of DC motors used in turntables have better speed stability than youre normal AC asynchronous motor but not as stable or constant as your AC synchronous motor. The DC motor is smoother than an AC motor but requires more maintenance as a result of the commutator and bushes. Speed is regulated via the DC Voltage level, supplied to the motor, and torque is directly proportional to the current supplied to the motor.
Lets have a look at what is important with both AC and DC motors when used in turntable applications. The following can be classified as most important:
1) Good Speed Stability Pitch stability
2) Good Starting Torque
3) Minimum Vibration generated by the motor
I will discuss each of these briefly and hope it will de-mystify the subject a little.
Good Speed Stability
With AC motors the number of stator poles and the supply frequency determines the speed. Synchronous motors has the best ability to maintain a constant speed when compared of all the different types of motors discussed.
The number of poles will not normally change so the only variable we need to be concerned with is the supply frequency. My experience is that the general electricity suppliers provide a reliable supply frequency that is normally very constant and one should not be concerned with that. For the guy that wants to ensure a really constant supply frequency there are ways to do that. Different technologies are used for recreating the AC sine wave all with their own unique disadvantages when considered for use in turntables. My concern would be that you really need expensive electronics to properly build an acceptable, constant frequency, AC supply. At the same time extensive filtering techniques will be required to get rid of the harmful effects to sound quality of this new supply.
With DC motors one need to ensure a very constant and stable DC voltage to the motor. In addition, to correct for any speed variations one will require a good feedback loop into the supply so that one can adapt the Voltage accordingly. Again like with the AC motor the moment one start to electronically control speed other factors like harmonics will effect the smoothness of the motor which in turn can degrade the sound.
Good Starting Torque
There are different thoughts on the subject as can be seen by discussions in the forum. For example AR used an under-rated motor in their turntable with good results. These smaller motors have less vibration than a bigger motor and are also smoother than its bigger brothers. The disadvantage is less starting torque and sometimes one need to help the platter to start or the platter can take a long time to run up to speed.
Others reduce the voltage supplied to the motor as was described by one inmate. Remember that torque is linked to the voltage supplied to an AC motor. I will not use this method on a normal asynchronous motor as it will make the slip more sensitive but on a synchronous motor it can work well. The reduced voltage mainly makes the motor smoother and therefor effects the sound quality less.
Vibration generated by the Motor
This is maybe the aspect of turntable motors that enjoyed the most attention during all the years. Both mechanical and electrical consideration needs to be considered when limiting harmful vibration generated by turntable motors.
I will only briefly discussed electrical considerations. Lets assume the synchronize motor is used which also is the better one regards speed stability. Rather than building and expensive sine wave generator my advice would be to use a good transformer to reduce the supply voltage as described on the asylum. The transformer also acts as a filter that will filter most harmful transients and to a lesser degree harmonics. There is no need for audiophile grade capacitors as no audio signal pass through the motor.
DC motor control is more complex and requires more expensive electronics than the above solution. The DC motor will have less speed stability than an AC synchronous motor, if used without proper control.
I hope this will help the audiophile enjoy their toys more