Doubly Fed Induction Motors (Part 2 of 2)

[@electrowizard2000]

In all the other cases of DFIG operation, you examined a "backwards" architecture (e.g. 3ph on rotor, and shorted stator for induction). Can you discuss a "backwards" DFIG where the AC/AC is supplying the stator? It might be an illuminating comparison; I'm trying to improve my understanding why the power requirements for AC/AC are so low. It's clear why they are lower than a AC/AC converting full generator power, but more wondering why it's not more 'difficult'/power hungry to drive the rotor windings to the requisite frequencies.

[@bentBuilder]

So what would happen if you went to the extreme of increasing the variable frequency and voltage on the rotor, where p mech = < 0w. The motor stalls out? VERY GOOD JOB PUTTING THIS VIDEO TOGETHER!

[@hannespallorarson4285]

Could you then have increased speed and decreased speed if you flip the L1,L2, and L2 on the rotor or can you only use it in hypo or hyper mode only?

[@BogdanTheGeek]

Great lecture. One question: What bonds the upper rotor frequency? As the rotor frequency increases, mechanical power decreases, however the torque remains constant in your examples. Seeing how mechanical power is proportional to torque, and field current is also proportional to torque, Is it safe to assume that once rotor frequency is equal to the stator frequency, the motor is basically in held in position with a torque proportional to the field current?

[@bigbadtech]

Several folks have asked about this, yes, this is a type of "soft starter in reverse" application. Rather than ramping up stator excitation frequency one instead starts both the stator and rotor at 60Hz so there is no differential (and no physical movement). Then one ramps rotor frequency down so the differential slowly increases and the rotor slowly accelerates.

[@BogdanTheGeek]

@@bigbadtech But i assume, due to the constant torque, and synchronization, you would get both velocity and position control. I couldn't find any papers describing dual excitation "induction" motors being used as servo motors, but its an interesting topic to explore, if limited in application.