Performance improvement of direct torque controlled permanent magnet synchronous motor drives

Abstract

Permanent Magnet Synchronous Motors (PMSM) have been gaining ever-increasing popularity in many applications thanks to their advantages such as long life, high torque and power density, excellent efficiency, etc. The applications of PMSM could be found in almost every industry such as home appliances, robotics, flywheels, industrial automation, electric vehicles, so on and so forth. Various control methods have been proposed for PMSM to attain satisfying performance with different application needs. For example, V/f control with stabilization loops could be used to control fan-type applications where dynamic response is not a must but efficiency and quietness matter. Field Oriented Control (FOC) is the dominant PMSM control method that achieve good dynamic response, smoothness and energy efficiency.Direct Torque Control (DTC) excels FOC in simpler control structure and supreme dynamic response but suffers from variable switching frequency and severe torque ripples. These defects generally lead to poor torque and flux regulation, high current harmonics and acoustic noise.

The Implementation of Permanent Magnet Synchronous Motor Direct Torque Control

“…In permanent magnet synchronous motor, stator winding connections includes star connection and delta connection [1], [2], [3] . Regardless of whether the motor is star or a delta connection, there are eight space voltage vectors according to the state of the three-phase inverter bridge arm switches, as shown in Table 1.The eventually permanent magnet synchronous motor direct torque control block diagram is as shown in Figure 2.…”

“…Figure 2 shows that the actual torque and the referenced torque from the speed regulator input the torque regulator and they determine the torque output of the regulator after comparison; that motor flux amplitude and given stator flux magnitude input the flux regulator, and they determine the output of the stator flux regulator after comparison, along with the stator flux sector number, the proper space voltage vector can be determined according to table 2, and it means the direct control of motor flux and torque. The control process does not require complex coordinate transformation, the rotor position is not strictly required and the control process is simple [1], [2], [3] .…”

“…Voltage and flux equations of permanent magnet synchronous motor are a set of differential equations with periodic variable coefficients [1][2] [3] . The internal electromagnetic relationship in permanent magnet synchronous motor is very complex and it is difficult to establish a precise mathematical model.…”

“…Being different from classic vector control, direct torque control can achieve high-performance motor torque control by considering the inverter and the motor as a whole and considering the motor torque as the direct control target [1][2] . This control method eliminates the current loop that is necessary in vector control, avoids the complex rotation coordinate transformation, is with no need for accurate rotor position information, and is easy to run without speed sensor [3] [4] . It can be seen from the torque and the torque-angle relationship that due to the different torque generating mechanism the permanent magnet synchronous motor does not exist the so-called slip of the asynchronous motor, that fast torque response can be achieved rapidly by changing torque angle by keeping the stator flux amplitude constant and due to torque being directly related to the torque angle.…”