Journal of ELECTRICAL ENGINEERING, VOL. 58, NO. 2, 2007, 79–84 PERMANENT MAGNETS SYNCHRONOUS MOTOR CONTROL THEORY Marek ˇ Stulrajter * — Val´ eria Hrabovcov´ a * — Marek Franko ** The paper presents different approaches of PMSM control strategies. Scalar control and vector control were chosen as possible methods for AC motors control. Theoretical background briefly describes the properties of these control techniques. Several advantages and disadvantages are highlighted and confirmed by simulations. Keywords: AC motor, PMSM, vector control, scalar control 1 INTRODUCTION 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 appli- cations, 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 per- formance. Typical applications are pump and fan drives and low-cost drives. The main idea of this method is vari- ation of the supply voltage frequency abstractedly from the shaft response (position, angular speed). The mag- nitude 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 underex- cited. The most advantage of this simple method is run- ning in a sensorless mode because the control algorithm does not need information about the angular speed or ac- tual 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 de- manded 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 com- parison 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 mag- nitudes and angles of the space vectors. Different kinds of vector controls cover a wide range of industrial applica- tions. When high performance, regarding torque dynam- ics is required, a technique which controls the torque in a direct manner is an option that must be taken into ac- count. This type of control is known as Direct Torque Control. 2 SCALAR CONTROL V/F OF PMSM Constant volt per hertz control in an open loop is used more often in the squirrel cage IM applications. Using this technique for synchronous motors with permanent mag- nets offers a big advantage of sensorless control. Informa- tion about the angular speed can be estimated indirectly from the frequency of the supply voltage. The angular speed calculated from the supply voltage frequency ac- cording to (1) can be considered as the value of the rotor angular speed if the external load torque is not higher than the breakdown torque. The mechanical synchronous angular speed ω s is pro- portional to the frequency f s of the supply voltage ω s = 2πf s p , (1) where p is the number of pole pairs. The RMS value of the induced voltage of AC motors is given as E f = √ 2πf s N s k w φ. (2) By neglecting the stator resistive voltage drop and assum- ing steady state conditions, the stator voltage is identical to the induced one and the expression of magnetic flux can be written as φ = V sph √ 2πf s N s k w = c V sph f s . (3) To maintain the stator flux constant at its nominal value in the base speed range, the voltage-to-frequency ratio is ∗ University of ˇ Zilina, Faculty of Electrical Engineering, Department of Power Electrical Systems, Vel’k´ y diel, 010 26 ˇ Zilina, Slovakia. E-mail: Valeria.Hrabovcova@fel.utc.sk, Marek.Stulrajter@kves.utc.sk ∗∗ EVP ´ U a.s., Trenˇ cianska 19, 018 51 Nov´ a Dubnica. E-mail: franko@evpu.sk ISSN 1335-3632 c  2007 FEI STU