# New Stable Non-Vector Control Structure for Induction Motor Drive
## 6 Comparison of the Proposed Control Structure with the Vector Control Structure  
The vector drive control of the IM presents one of the most common control methods in technical practice today. Its specific dynamic properties depend on the used modification and are described by a large amount of the literature, e.g., [6; 10]. At first, the proposed control properties will be compared with those of the vector control on the basis of a comparison of their structures. Secondly, the performance comparison of these two controlled structures will be given. The basic structures of both control methods are shown in Figures 8-10. In both structures, the upper control level controls the desired mechanical speed \(\omega_{m}\) of the drive, and eventually it sets the excitation magnetic flux magnitude of the motor through its current components in the rectangular rotating reference frame \(\{x,y\}\) rotating at the angular speed \(\omega_{1}\). At the upper control level (Figure 8a,b), there are two basic differences:  
1. The first difference relates to the control structures: both structures are linear. They differ in their internal interconnection and by the control structure order;
2. The second difference consists of the type of the output action variable. In the vector control, they are variable setpoints of the stator current components \(i_{1x}\), \(i_{1y}\). In the proposed control structure the angular speed of the stator current vector presents the action variable and, in principle, the components \(i_{1x}\), \(i_{1y}\) are kept constant. The lower control level (Figure 9) contains the controllers of the stator current components \(i_{1x}\), \(i_{1y}\) and the transformations between the stator system \(\{a,\,b,\,c\}\) and the reference  
Figure 8: The upper control level: (**a**) of the designed control structure; (**b**) of the vector control structure. Figure 9: The lower control level of the proposed control structure, where \(\gamma_{1}\) is the instantaneous position of the stator voltage vector and \(u_{1x}\), \(u_{1y}\) are the components of the stator voltage space vector \(\mathbf{u}_{x}\). frame \(\{x,y\}\). At the lower control level, the two structures being compared are practically identical. A significant difference consists of the stator currents transformations into the reference frame \(\{x,y\}\). While in the vector control, this system most often tied to the position of the rotor flux vector \(\gamma\), in the proposed control structure it is firmly tied to the position of the stator current vector \(\mathbf{i_{1}}\) (which in the fact presents an integral from the angular speed \(\omega_{1}\)). In the case of the vector control (Figure 10), the problems arise when using the Park transformation. It requires the measurability of the rotor flux position, the dependence of this position estimation on the unknown and variable value of the rotor resistance, problems at low mechanical angular speeds, etc. These uncertainties can significantly affect the quality of the control. In the proposed control structure, the value of the stator angular velocity \(\omega_{1}\) inputs from the higher control level. Its value is always precisely known which guarantees the accuracy of this transformation and its independence from the current states or other parameters of the IM. A basic comparison of the drive's behavior with closed control loops for exactly known IM parameters is shown in Figure 11. The first two figures from the left compare the response of the closed control loop to a step of the controlled system setpoint and to a disturbance. Here, the motor is starting, running to the angular speed \(\omega_{m}=150\) rad/s, then in time \(t=3\) s it is loaded by the nominal load torque \(T_{L}=T_{N}=20\) Nm and finally, it is stopping.