Comparison between flux estimation methods for direct torque controlled permanent magnet synchronous motors (original) (raw)

A Review on Improved Flux Estimation Technique for Direct Torque Control of Induction Motor Drives

2015

In DTC drive stator flux is controllable variable so accurate flux estimation in a high-performance induction motor drive, is important to ensure proper drive operation and stability. Most of the flux estimation techniques proposed are based on the voltage model. The voltage model, on the other hand, is normally used in a high speed range, since at low speed, some problems arise. Practical implementation, even a small dc offset present in the back electromotive force (EMF) due to noise or measurement error inherently present in the current sensor, can cause the integrator to saturate. To overcome this, a low-pass (LP) filter is normally used in place of a pure integrator. The voltage only motor parameter used is the stator resistance, which can be measured quite accurately. These can be considered as an advantage of the voltage-mode and are also the reasons why the former is preferred over the latter in some industrial applications. However, the use of an LP filter in place of a pur...

DIRECT TORQUE CONTROL OF PERMANENT MAGNET SYNCHRONOUS MOTORS (DTC-PMSM

Direct torque control (DTC) of AC drives is considered one of the most interesting high performance control techniques nowadays. Permanent magnet synchronous motors (PMSM) are now replacing induction motors (IM) in a variety of applications due to their valuable characteristics especially the increased efficiency and compact size. The combination of DTC and PMSM in one drive system leads to high dynamic response drive system that seems to be an interesting topic to gain the increased research in the literature. DTC offers high dynamic torque, flux, and speed responses better than that of Field Oriented Control (FOC) technique with simple controller architecture that utilizes two hysteresis controllers and a switching table to select the most suitable voltage vector to meet the torque and flux requirements. No machine parameter dependences, no axes transformation required, no voltage modulation techniques, and no current controllers are the advantages of DTC over FOC. Comparisons between DTC and FOC of PMSM are made through MATLAB simulations to show their characteristics with respect to the other. It is found that DTC gives excellent dynamic responses in both torque and flux but the ripple content is high. DTC requires the estimation for both flux and torque. A lot of methods are found in the literature to estimate the flux and the most powerful methods are compared via simulations. Pure integration method of flux estimation is the basic method that has a lot of demerits like offset, resistance variation dependence, initial value problem, and low performance at low speeds. Low Pass Filter (LPF) is intended to solve the pure integration problems but its performance at transient period is not good and there is some drift in the flux beyond the actual flux. Modified integrator is also one of the interesting flux estimation methods that gives better performance at both transient and steady state conditions than that of LPF estimator but it contains PI controller and low pass filters so that several tunings are required and complexity is increased. A flux estimator uses tunable-cutoff-frequency-LPF is simple and accurate. It is used for IM and never be used for PMSM. It is also compared and shows excellent results in both transient and steady state periods which implies the effectiveness of it for PMSM as well as IM. Switching Look-up Table is a unique characteristic in a DTC drive and it has a lot of schemes based on the number of hysteresis levels of the torque and flux comparators and the inclusion / exclusion of the zero voltage vectors. Various switching tables appeared in the literature are compared to judge the merits and demerits of each one and to show their effect on the drive performance. A relatively new switching table using modified definition of the flux sectors is used for PMSM for the first time and its results are excellent because it avoids a fatal problem appeared in the table proposed by Takahashi, the inventor of the DTC system. Selection of the amplitude of the flux and torque hysteresis bands is a very important issue in implementing the DTC drive system as they affect torque ripples, flux distortion, switching losses, and current harmonics. Simulations are made to show these effects. Trade off should be made to correctly choosing of the hysteresis band amplitudes according to the performance requirements. Torque / flux ripples and variable switching frequency are the main disadvantages of DTC system. Solutions to these problems are an interesting researching aspect nowadays. Increased number of voltage vectors and/or sectors, Space Vector Modulation (SVM), Duty Ratio control, and Model Predictive Control (MPC) techniques are the most common solutions to the problems of high ripple content and variable switching frequency. The loss of the DTC simplicity is something that makes these solutions inconvenient so that compromise studies should be done to select the most suitable control technique that meets the application needs without increasing the complexity of the system. Digital Signal Processors (DSPs) are now considered the basic controllers used in the motor control area. DSP programming is often done using hand written code by C or Assembly languages which is very time consuming and very prone to errors. Computer Aided Control System Design (CACSD) is some sort of block programming can be implemented using Matlab/Simulink. The Simulink model itself can be downloaded to the DSP which makes the real time control system design easier. The complete description of the DTC-PMSM drive system is discussed showing the power, control, measurement, and signal conditioning circuits used in the system. Six step operation of the inverter as well as open loop V/F control of PMSM are implemented in order to verify the effectiveness of the CACSD using Matlab/Simulink.

Improved Performance of Direct Torque Control with PMSM compared to DTC with Induction Motor

2021

For induction motor torque control, direct torque control is becoming the industry standard. This paper proposes a switching loss minimization technique for improved Direct Torque Control (DTC) of permanent magnet synchronous motors in order to increase the drive system's steady-state and dynamic results. Direct torque control (DTC) of a voltage source inverter-supplied PMSM is a simple scheme that requires little computation time, can be implemented without speed sensors, and is unaffected by parameter variations. In theory, the motor terminal voltages and currents are used to calculate the flux and torque of the motor. A voltage vector is chosen to restrict the flux and torque errors within their flux and torque hysteresis bands based on the instantaneous torque and stator flux magnitude errors, as well as estimates of the flux position. The electromagnetic torque, rotor speed, and stator current of DTC with PMSM and DTC with IM were successfully calculated using Total Harmoni...

Direct Torque Control Induction Motor Drive with Improved Flux Response

Advances in Power Electronics, 2012

Accurate flux estimation and control of stator flux by the flux control loop is the determining factor in effective implementation of DTC algorithm. In this paper a comparison of voltage-model-based flux estimation techniques for flux response improvement is carried out. The effectiveness of these methods is judged on the basis of Root Mean Square Flux Error (RMSFE), Total Harmonic Distortion (THD) of stator current, and dynamic flux response. The theoretical aspects of these methods are discussed and a comparative analysis is provided with emphasis on digital signal processor (DSP) based controller implementation. The effectiveness of the proposed flux estimation algorithm is investigated through simulation and experimentally validated on a test drive.