How can a synchronous motor have RPM other than 3000/3600?

this motor has a reduction gear that slows it down, to 5 rpm at 50Hz or 6rpm at 60 Hz.

Actually the rotation speed of a synchronous motor in rpm is given by: $$\frac{2 \times 60 \times f} n$$ where
f = frequency (Hz)
n = number of poles.

The motor in your photo cannot have 1200 poles, so it surely has a stepdown gear.


It is said, that synchronous motor is rotating at the same speed as magnetic field rotates. But three-phase made magnetic field rotates as the frequency of current, i.e. 50 or 60 times per second, which means 3000 or 3600 RPM

Incorrect in your last sentence. A synchronous motor speed has the following formula: -

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What you were failing to consider is the number of poles inside the motor.


EDIT - What is 120 here?

Consider this 3-phase machine: -

enter image description here

There are six windings because one phase has to have two poles and poles are counted in pairs so the "2" in the "2x60=120" is for that.

The 60 converts revs per second into revs per minute.