ACIM rotor magnetic field and stator magnetic field ?

[anotherbrick]

in an AC induction motor
does the rotor and stator magnetic field have same rotation speed ?

( but rotor is turning at slower speed than the stator rotating field )

can somebody explain why ?

 

[FvM]

Synchronous speed of an induction motor would imply a fixed rotor field in the rotor reference system. But an induction motor can't maintain a static rotor field because it has no permanent magnet or exciting current. Thus, the rotor field has to rotate at a speed, which equals the difference between synchronous and actual motor speed. An induction motor has a typical S-shaped speed versus torque characteristic exposing a tilting moment.

 

[anotherbrick]

if the rotor is turning at 1400 rpm
but the stator field is synchronous at 1500

the stator field sweep the rotor 100 times / minute

so there is an 100 / minute induced rotor magnetic field ( if we think rotor is stationary )

since the rotor itself is turning , the actual speed of rotor magnetic field
1400 + 100 = 1500 too

there is only a phase difference between the two

am I thinking wrong ?

 

[FvM]

For the analysis of motor behaviour, only the field in the rotor reference system matters. It has a rotational speed of 100 /min in your example.

 

[anotherbrick]

why I wanted to know about the rotor magnetic field ?

In FOC there is the need of knowing the position of rotor magnetic field
becouse of that an encoder is placed in the shaft of ACIM

however rotor magnetic field is rotating faster than rotor itself
as I understand , it is synchronous with inverters output frequency

my question

how can I know the position of rotors magnetic field
by only counting the shaft encoder when they have different speed ?