Direct Torque Control of Permanent Magnet Synchronous Motors (2011)

Control 130 electromotive force model is used to estimate the rotor position.However, this method is based on expert knowledge.Some researchers have proposed a combination of current and flux linkage estimation (Rahman & Toliyat, 1996), (Toliyat et al, 2002) or methods including Kalman filters, fuzzy logic, and neural network observers to obtain the rotor position angle (Dan et al, 2004), (Grzesiak & Kazmierkowski, 2007).Recently, researchers have tried to reduce the torque pulses and harmonics in PMSM.In addition, (Sozer et al, 2000) presented an inverter output filter system for PWM derives to reduce the harmonics of surface mounted permanent magnet synchronous motor (SPMSM), it shows some effectiveness in reducing switching harmonics, but very large circulating current between inverter output and filter elements is required to reshape the motor terminal voltage which may violate the inverter's current limit.Efficiency is important due to the energy scarcity of the world and higher performance is needed for modern motion control applications.PMSM offer efficiency advantages over induction machines when employed in variable speed drives.Since much of the excitation in the PMSM is provided by the magnets, the PMSM will have smaller losses associated with the magnetizing component of the stator current.The stator current may be almost purely torque producing in a PMSM drive while in an induction machine drive there is always a large magnetization current present.Due to the synchronous operation of the PMSM, rotor losses are greatly reduced.The application of the PMSMs for low speed operation in direct drives is an economic alternative for the induction motors with gearboxes.Since the speed of the direct drive PMSMs is lower than the speed of the induction motors with the gearboxes, the risk of torque harmonics appearing at the mechanical resonances is increased in the speed range of normal operation.Permanent magnet motors have been used for decades in low power applications such as servo drives and domestic appliances.Recently, the PMSM drives have been developed further and are used in industrial applications requiring high torque at low speed.PMSM drives are replacing standard induction motors with gearboxes in, for example, paper and textile industry, special applications for marine (Laurila, 2004).However, PM synchronous motors can not be fed directly from the mains supply and need to be driven by the AC motor drives.Similar to the induction machine, vector control method is employed for the PM synchronous motors to obtain high bandwidth torque control performance.For vector control, the rotor flux angle needs to be known by the AC motor drive.Therefore, sensors (e.g. an incremental encoder) on the motor shaft are utilized to sense the rotor flux angle and AC motor drives use this angle information for vector coordinate transformations.With vector coordinate transformations, the AC motor in the control coordinates is converted to a DC motor where torque control is a simple issue of current control to be achieved by the current regulator.Thus, the vector control method enables high bandwidth torque control of an AC machine, which brings high bandwidth of speed and position control (Geyer et al, 2010).Although machine drives with modern control techniques have brought high performance and robustness to the motion control area, research has continued for developments in the AC motors and drives technology.The motivation is to improve the technology for high efficiency and for high performance.The literature study regarding to the direct torque control of the permanent magnet synchronous machines is presented in this chapter.The theory of the direct torque control was developed by (Depenbrock, 1985) for the first time for asynchronous motors.(Takahashi and Noguchi, 1986) developed the direct torque control only using the torque routines for asynchronous motors.www.intechopen.