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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.

How can synchronous machines on Aliexpress have other RPMs, like below?

enter image description here

There multiple of these, no any mistake expected. Many of them has 3 explicit inputs.

UPDATE

Sorry I agree that in slow RPMs reduction gears are most probable inside.

But can I be sure, that if motor is "synchronous", then magnetic field is rotating at 50/60 Hz and inner rotor is rotating at the same speed?

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  • \$\begingroup\$ Just for the sake of completeness - synchronous motors can indeed rotate in various speeds when high power inverters are used. \$\endgroup\$ – Pavel Sep 23 '15 at 8:20
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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.

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    \$\begingroup\$ Are you sure? The rotor still rotates at the RPM of current in any synchronous motor? No any other features can affect the situation? \$\endgroup\$ – Dims Sep 22 '15 at 14:06
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    \$\begingroup\$ Thank you for your comment. Actually the rotation speed depends on the frequency and the number of poles, and the inner rotor rotates at 3600/3000 rpm only in case of a 2 pole motor. Anyway it is not possible for this motor to have 1000 poles, so it surely has a stepdown gear. I will edit my answer accordingly. \$\endgroup\$ – CasaMich Sep 22 '15 at 14:37
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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: -

enter image description here

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.

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  • \$\begingroup\$ What is 120 here? \$\endgroup\$ – Dims Sep 22 '15 at 14:56
  • \$\begingroup\$ 2 x 60 is what is there. \$\endgroup\$ – Andy aka Sep 22 '15 at 15:09
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    \$\begingroup\$ The origin of "2" and "60" is kind of trivial, but to make sure it is completely explained: there are 2 poles per pair and 60 seconds per minute/ \$\endgroup\$ – Charles Cowie Sep 22 '15 at 15:43
  • \$\begingroup\$ So it's "Pole minutes per second". (SCNR) \$\endgroup\$ – glglgl Sep 23 '15 at 9:09
  • \$\begingroup\$ @glglgl No it's pair-seconds per minute \$\endgroup\$ – Andy aka Sep 23 '15 at 9:38

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