hi
You must
1-determain diff. equation of your motor and load.
2-normaly it is a stable system.becuse all pole is in right hand of jW axis.
3- Add parallel capacitor so that pole of your diff. Eq goes to left hand of jw axis.
- If you apply an small input (permant magnet or initial charge in capacitor) when you drive your motor ,it gives large response.
And output of motor goes to large and large.so your magnetic core go to saturation region. And pole of eq. goes to jw axis . on jw axis you have sine response.