@lewmLewm, here is a nice explanation from the manufacturer Jean Verdier:
Besides other problems with that, that construction would also tend to be a cause of speed irregularity, as stylus drag waxes and wanes over the course of an LP.
"Now, let us approach another point: Dr. G.W. explains that playing a record with a stylus consumes energy and that this energy is variable on account of the changes of musical modulation, resulting in drive speed variations of the platter.
That is absolutely correct and it is one of the basic problems facing the turntable designer. The author than explains how to remedy this: the motor must renew the energy as swiftly as possible to keep constant speed, that is the reason for using a low inertia motor. I think that if this were true, no turntable could work well, because no motor is good enough. Fortunately, we can make use of more powerful tools to settle that problem.
The first tool is well known, it is the platter’s inertia. The heavier platter, the more it works to counter momentary variations in speed, but the challenge is to make a very thick and massive platter which doesn’t ring like a bell.
The second tool is less well-recognized than the first: the constant-coupled brake system. Since the player stylus consumes variable amounts of energy, we have to mask that consumption by permanently consuming much more energy by the use of a brake. The larger the ratio between the energy consumed by the brake and the energy consumed by the stylus, the better turntable work.
To be clear: only a few models of turntables have this device. To understand the concept better, have a look at a Thorens TD 124 which uses a brake with Foucault’s current excited by a permanent magnet, or EMT 927 which has a completely mechanical brake consisting in washers placed around the axle with the capability of adjusting the pressure and therefore the braking action.
On my turntable, the brake is present, but invisible to the casual eye-it is furnished by the axle and bearing. The two parts are unusual in their very large dimensions. The two facing surfaces of about 60 cm2 separated by lubricating viscous oil form the brake."
We can see this brilliant principle in 2020 - CS Port Analog Disk Player LFT1 (£48,000.00): "In LFT 1, the rotation control is not performed, and the constant speed rotation is left to the high moment of inertia of the 27 kg platter, creating a rotation free from unnecessary vibration caused by the control. Only the positive direction force is applied from the motor, but by using a coreless motor with LESS MOMENT OF INERTIA, the influence of the vibration is reduced."
There are more hidden things, but modern audiophiles are looking for exactly 33.33333 ....
Cheers