Permanent Magnets Synchronous Motor Control Theory

One way of controlling AC motors for variable speed applications is through the open loop scalar control, which represents the most popular control strategy of squirrel cage AC motors. Nowadays it is used in applications, where information about the angular speed need not be known. It is suitable for a wide range of drives as it ensures robustness at the cost of reduced dynamic performance. Typical applications are pump and fan drives and low-cost drives. The main idea of this method is variation of the supply voltage frequency abstractedly from the shaft response (position, angular speed). The magnitude of the supply voltage is changed according to the frequency in a constant ratio. Then the motor is in the condition, where the magnetic flux represents the nominal value and the motor is neither overexcited nor underexcited. The most advantage of this simple method is running in a sensorless mode because the control algorithm does not need information about the angular speed or actual rotor position. On the contrary, the big disadvantages are the speed dependence on the external load torque, mainly for IM, and reduced dynamic performances. In the last years the dynamic performances are demanded as high as possible. AC machines have found very pretentious applications in automotive industry, where dynamic responses are important. Vector control (Field Oriented Control) of AC machines, as a novel approach in electrical drives, provides very good performance in comparison with the scalar control. Vector control eliminates almost all the disadvantages of constant V/f control. The main idea of this method is based on controlling the magnitudes and angles of the space vectors. Different kinds of vector controls cover a wide range of industrial applications. When high performance, regarding torque dynamics is required, a technique which controls the torque in a direct manner is an option that must be taken into account. This type of control is known as Direct Torque Control.