Permanent magnet synchronous motors (PMSM) fed by PWM inverters are widely used for industrial applications, especially servo drive applications because of high power density, low moment of inertia, and simplicity of maintenance. The adjustable speed drive system of the vector controlled PMSM has two control loops: the current loop and the speed loop. To design a good controller for the PMSM drive system, it is necessary to have a precise model for each control loop. However, there are some difficult conditions in practical applications, such as variations of the inertia moment, nonlinear characteristics at different speed levels and at different loads.
In this thesis, we deal with the input-output modeling of a vector controlled PMSM drive system and design of direct digital controller for speed control. Most plant models are mainly represented in the equivalent circuit model. This model may be not sufficient for precision control design. Even if we use any conventional open-loop identification
approaches, it is not possible to consider some operating conditions and the reaction of switching device.
We present a modeling technique that allows one to model all the elements from the controller output to the controller input while the system to be identified is operating in closed loop. In this approach, we first select a simple stabilizing current controller arbitrarily and then a discrete-time plant model in the current loop including the controller is identified by using the closed loop identification algorithm, which is the so called “closed loop output error (CLOE)” identification method. It requires to add a PRBS type test input to the loop.
Once we obtain a discrete-time plant model, the digital current controllers are directly redesigned using the discrete-time characteristic ratio assignment (DCRA) so that it satisfies the given time response specifications.
Next all the elements that include in from the output to input of the speed controller along the speed control loop, that naturally include the current loop, can be regarded as a plant. Similar to the closed loop identification of the current loop, a discrete-time plant model in the speed control loop is identified using the CLOE. Based on this model, a digital speed controller is redesigned using the DCRA, similar to the current loop.
The proposed approach has been demonstrated by simulations and experiments. The experimental set up consists of a surface mounted PMSM (5 Kw, 220V, 8 poles) equipped with a flywheel load of 220Kg, an intelligent power module (PS21A7A, 600V, 75A, Mitsubishi) as a PWM inverter, a micro controller (TMS320F28335, 32bit, 150Mhz/150MMAC, TI). As a result, it is shown that all the performances coincide closely with those of simulation. This means that the proposed method for PMSM drive system is useful for the precision control.