# Is the rated speed of an induction motor the synchronous speed?

E.g, if a motor is rated at 50Hz, 400V, 22A, 8.29kW, and 1460 rpm, does this means that the motor should be operated from a source voltage of 50Hz, and this corresponds to a synchronous speed of the motor of 1460 rpm?

The reason I ask is because an example question and answer I have seen says the following:

Q: Estimate the number of poles the motor described above has

A: formula for synchronous speed is n=120f/p

closest synchronous speed for given rotor is 1500rpm

1500=120*50/p

p=4

I'm guessing the reason they rounded 1460 to 1500 is because the number of poles must be an integer.

So when you see a motor with a speed rating in rpm, what does this value actually mean? And how does this relate to the synchronous speed (like it has in my example question)?

Thanks!

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@Li-aung Yip 's answer is correct BUT there is a confusion of terms between (the terminology that you used) and (what you asked and he answered). ie a correct answer to a slightly confused question may lead to ongoing confusion :-). There are motors termed "synchronous motors" that DO operate at true synchronous speed - they rotate with the poles "locked to the rotating magnetic field" BUT they are not induction motors. ... –  Russell McMahon Apr 29 '14 at 19:52
... Induction motors rely on the difference between mechanical speed and field speed to produce an induced slip frequency voltage & current in their rotor which is what provides the energy to the rotor. | SO if the question that you were asking was related to a true synchronous motor it would give a correct answer using your formula | If it was related to an induction motor it is "naughty" as the induction motor strictly does not have a synchronous speed - only a fild speed that it approaches but never reaches. | I see now that Kurtovic covers similar material. Ah well. –  Russell McMahon Apr 29 '14 at 19:52

What you are looking here at is an asynchronous (induction) motor. The difference to a synchronous is in the fact that the rotor is usually just a (sqirrel) cage and some iron.

For a synchronous motor the rotor has either a magnet or has geometrically strongly defined(salient) poles and pole gaps. Sometimes it has both!

Now why is one synchronous and the other asynchronous?

• The synchronous motor has a constant excitation in the rotor (or has a constant difference between inductances). Therefore the field rotates in synchronicity with the rotor. $$f_{rotor}=0$$
• The asynchrnous motor on the other side has an induced field in the rotor that is dependent on the speed of rotation. $$f_{rotor}>0$$

The rotational speed is calculated from the difference of the stator and rotor flux frequencies.

Why is the speed 1460? The stable part of the torque/rotational speed charachteristic for a asynchronous motor is near the synchronous speed. If you want to find the number of poles round it to the nearest synchronous rotational speed and calculate it normally. In some special working regimes this won't be true as the asynchronous motor could operate at for example 900 rotations if you make some adjustments. In your case however it will have p=4.

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An induction motor never spins at synchronous speed.

A motor spinning at constant speed must be producing enough accelerating torque to overcome the decelerating torque of the load. Since an induction motor must slip to provide torque, the full-load speed will always be less than synchronous speed.

Even an un-loaded motor with nothing on the shaft still has a decelerating torque, due to the friction of the bearings and so on.

Therefore, we have:

• Rated speed is the speed of the motor at rated output (shaft) power.

• No-load speed is the speed of the motor with no load on the shaft. Note the motor bearings still impose a nominal load due to friction.

• Synchronous speed is the speed the motor would spin at if it had absolutely zero load (frictionless bearings and no other losses of any kind.)

For reference, here is a typical motor characteristic graph.

Note synchronous speed of 3,000 RPM, no-load speed also about 3,000 RPM, and full load speed 2,932 RPM.

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I thought the synchronous speed was the speed of rotation of the rotating magnetic field? I thought the synchronous speed has to be greater than the speed the motor spins at, so there is relative motion between the rotating magnetic field and the armature so there can be induction? (With the difference in speeds called slip) –  Blue7 Apr 29 '14 at 16:11
Yes, see edits above. –  Li-aung Yip Apr 29 '14 at 16:14