There are 2 seperate issues here. First, the Cuk converter may not be getting you to the required voltage. As this required a closed loop controller, it is impossible to know why this is happening without an example model of the system. The issue could be in the modelling of the converter, or the implementation of the control aglorithm, or model settings. There is also the possiblitity the converter is not designed correctly to handle this load, although I don't think that is the issue here.
2. the next iss is determining if you have the correct motor paramters. The help file for the model explains how to calculate the Voltage constant, but you can also choose a different constant to calculate if you find that easier.
