I know slip in induction motor determine the current and torque but the point I want to ask, when controlling with vector control it is said stator and rotor flux is always kept with 90 degrees apart but if stator and rotor is always kept at 90 degrees their speed should be same and it means no slip there. if no slip no torque exists. So there should be a speed difference between rotor and stator but how is that possible to keep 90 degrees between stator and rotor and in the same time having slip (speed difference between stator and rotor) ?
Add a comment |
3 Answers
\$\begingroup\$ \$\endgroup\$
The slip is the difference between the mechanical speed of the rotor and the speed of the rotor's magnetic field. The speed of the rotor field and the stator field are always the same regardless of the mechanical speed of the rotor. That is true no matter how the motor is powered or controlled.
It is not true that the stator and rotor flux are always 90 degrees apart, the angle does not exceed 90 degrees, but it is less if the operating torque is less.
\$\begingroup\$ \$\endgroup\$
Rotor field frequency is not the same as rotor mechanical frequency (motor speed). The FOC algorithm has to estimate the position of the rotor's field by using stator currents, stator voltages, speed sensor (could use also encoderless algorithm to estimate speed) and the mathematical model of the induction motor.
Only at this point it can further inject/correct stator current so that torque component q-axis is 90 degrees of excitation component d-axis.
\$\begingroup\$ \$\endgroup\$
Also I will add something about torque.
It is NOT stator and rotor needs to be 90 degree to get higher torque. It is wrong when you say stator and rotor should be 90 degrees.NO. Wrong words
I am explained for field oriented control (rotor flux oriented). We divide stator currents into 2 (direct and quadrative axis, id iq) this two axis should be 90 degrees to each other. Another thing is, we make an acceptance and saying that rotor flux (for rotor flux oriented field control we only use rotor flux we forget about stator flux) is aligned with id axis. So if id and rotor flux are aligned, because iq and id are 90 degree to each other, so consequently rotor flux and iq are 90 degrees to each other, too. Torque=3/2(rotor flux*iq).
(Remember the basic physic: if you want to open the door it is better to apply force quadrative to the door rotational center. İf your force is aligned with the door center you can not produce torque. That is the same for electricity. You can only produce for with iq which is 90 degrees to rotor flux. What if current change and rotor flux angle change what if it is not 90 degree. Answer is: it is the control engineering topic. Then you can put PI controller to correct the error. To understand further you can study. I studied from “new motor old tricks Field oriented control 1,2 series” drom youtube.)
