I have an induction motor which is fed by a three phase inverter. I am trying to use vector control with d-axis aligned with the rotor flux vector. I have a load which increases its torque as the speed increases. I use a fixed value for direct component of current (Id) reference.
I start to test at low speed and everything goes fine, voltage amplitude increases with frequency and current is almost constant. But when I try greater speeds, at some point, the voltage amplitude reduces notoriously (instead of keep increasing), and quadrature component of current (Iq) also increases notoriously.
I also observed that Iq has a very low value before the problem occurs. Almost all current is contributed by Id. Is this low value of Iq a normal thing? or should it be increasing with speed? Is there some kind of relationship betbeen Id and Iq, that I should follow? I suspect this is related to saturation. How can I avoid this?
UPDATE: I tested with a larger value of field generating current (which I call Id) and the problem reduces, but still happens at higher speeds. So I believe may be is not saturation, but could it be like the motor looses its field?
is it normal that my controller tries keeps Iq at low values, even at high torque?
Thanks for your help.