In the current loop debugging of FOC. The relationship between speed and Iq_ref is strange. The increase of speed is followed the increase of Iq_ref when Iq_ref is lower than a certain value. But the motor speed will reach the maximum speed immediately when the Iq_ref increase a little more than that certain value. For example, the motor speed is followed the increase Iq_Ref when Iq_ref is set from 1 to 100. But the motor speed will reach the maximum speed immediately if Iq_Ref is changed from 100 to 102. This issue cause the control problem in speed loop. because the Iq_Ref is the output of speed loop. The PID in speed loop don't have enough time to adjust speed back when Iq_Ref is more than a certain value and motor speed to reach maximum in very short time. The results is the motor speed is oscillation when speed is little high. However, The speed control is good when the setting value of speed is lower.
I appreciate if you can give me some hints to solve this problem. Thanks.