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WHY SYnchronouS motor's Rotor is stationary??

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gauthamtechie

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WHY SYnchronouS motor's Field is stationary??

I Read that in synchronous motors "Rotor magnetic field is stationary with respect to the rotor " and in Induction motor "Rotor magnetic field rotates with respect to the rotor"
If that is true how does a synchronous motor rotate? And why does it stop if it isn't running at synchronous Speed whereas an Induction Motor(IM) Must Never touch the synchronous speed?
 

I see, that you are basically replaying your previous post https://www.edaboard.com/threads/247152/

The questions are answered in various internet resources, and hopefully in your power engineering text books.

Presently, I want to contradict this statement:
an Induction Motor(IM) Must Never touch the synchronous speed
Click to expand...
Ideally, an induction motor will run at synchronous speed with zero load torque, in practice, friction an other losses are causing a small speed drop below it. The speed versus torque curve is however symmetrical with respect to synchronous speed, by driving the motor (generatoric mode), you easily achieve speeds above synchronous. In so far "must never touch" is simply wrong.
 

Thank you. I'll look into some other sources as you mention, but help me with this doubt: If induction motors practically need a 'slip' to create a resultant torque that causes it to rotate, then at synchronous speed slip is zero and hence no resultant torque is available for the rotor to keep it in motion. So in that I meant the motor will stop rotating when its rotor speed equals the rotating magnetic field's synchronous speed(am i wrong?).

In that above statements I've presented, I do have a confusion about the torque that is necessary to start a motor and whether torque is required to keep it in motion so I'd appreciate some clarification there. Thank you!
 

I meant the motor will stop rotating when its rotor speed equals the rotating magnetic field's synchronous speed
Click to expand...
Why it should stop rotating? It won't further increase speed, that's all. There's a balance between load + friction torque and motor torque

See a torque versus speed curve from https://www.ibiblio.org/kuphaldt/electricCircuits/AC/index.html



The "torque that is necessary to start a motor" is the load torque plus motor's inertia and friction torque. A load without static torque, e.g. a fan can be started slowly with very low torque. If a considerable static torque is present, e.g. with an elevator, the motor must be able to provide a large initial torque. This can be achieved by a special motor design, a starter circuit or a VFD.
 

Thank you, your explanations are really useful but this is challenging at the same time for me since its hugely altering my understanding of some concepts(most probably since I didn't understand them well in the first place) and I'll look at various sources.
So this is my final doubt: There are so many who sources online that keep asking this question "Induction motors cannot run at synchronous speed, Why?"
https://wiki.answers.com/Q/An_induction_motor_cannot_run_at_synchronous_speed_why
https://www.allinterview.com/showanswers/82002.html

- And as an answer I was satisfied with the explanation that the slip creates a necessary resultant torque need to run it and hence so.

But your answer seems to suggest its okay to run at synchronous speed?

---------- Post added at 23:14 ---------- Previous post was at 23:13 ----------

 
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"Cannot run" is different from "must never touch". In fact, the motor cannot run a synchronous speed if a load torque is present, including a friction torque generated internal to the motor. But the exact behaviour can be read from the speed versus torque diagram.

The "cannot run" staement is assuming a positive load torque. In some machines, the torque is oscillating and has at least short negative intervals.
 
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