I found a movie about controlling motor speed.
It is a motor from a Vitamix Blender, controlled with a potentiometer.
What kind of motor they are using for these blenders?
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Every blender I've seen has used a universal motor. Most likely the brushes are in the far side end housing, in the cutouts you can see on this end (same part used for commonality) at right angles to the field (stationary) windings. Cheap blenders use just a chopping type controller, the higher end ones have a feedback mechanism, often a Hall sensor or pickup coil with a magnet in a disk on the shaft, feeding into a microcontroller.
Series motors like this have a fairly steep drop-off in speed as load is applied, hence the need for some speed regulation, as the load can vary substantially depending if you're frothing a shake or crushing ice (or iPhones). The cheap ones rely on the additional load from a cooling fan on the shaft to limit the maximum speed - I suspect that one was mounted on the stub of the shaft visible here.