Hi for all.
I read almost of the posts and I only can congratulate for a nice job in the restoration of these machines. Just a comment, may be to help understanding the work of the motor. This kind of motor is called induction motor or inductive motor. The speed is a directly proportion to the frequency of the source that supply it. In this case, 50 Hz. The tension applied to it's terminals only give the power to turn. The number of the coils of the motor fix the speed and less coils more speed. Normally in this application the motor have two or four coils (two will turn faster than four). This kind of motor was used in vinil record players because as the source frequency is very stable, the speed of the turntable will be too. With a reostat in series with the motor supply you only change the power making the motor strong or weak. O course if you have some obstacles to fight like the fluid resistence of the cleanning liquid, reducing the power, indirectly you will reduce the speed. That's why when you turn it on outside the liquid the speed raise to the top limit. You can lower the speed outside the liquid using a reostat with high resistece value, and the motor will lower the speer against the atrit between the axis and the supports, that it is not perfect. However, if you do this, when the basket is inside the liquid, the reostat will only increase the speed at the end of his cursor, and the speed will have a fast icrease with a small turn of the reostat.
One more tip. The energy that is not used by the motor (in the lower speed) will be burned by the reostat. So, when you adjust for lower speeds, it is normal to the reostat to heat.
I hope this can be helpfull in future jobs.