Author Topic: Direct Torque Control Of Permanent Magnet Synchronous Motor  (Read 3269 times)

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Direct Torque Control Of Permanent Magnet Synchronous Motor
« on: November 23, 2011, 06:47:37 am »
Author : Simhadri Amarendra Babu
International Journal of Scientific & Engineering Research Volume 2, Issue 10, October-2011
ISSN 2229-5518
Download Full Paper : PDF

Abstract-Permanent Magnet synchronous motors (PMSMs) are used in many applications that require rapid torque response and high-performance operation. The PMSM is very similar to the standard wound rotor synchronous machine except that the PMSM has no damper windings and excitation is provided by a permanent magnet instead of a field winding. The elimination of field coil, dc supply and slip rings reduce the motor loss and complexity. For the same frame size, permanent magnet motors have higher pull out torque. It is mathematically proven that the increase of electromagnetic torque in a permanent magnet motor is proportional to the increase of the angle between the stator and rotor flux linkages, and, therefore, the fast torque response can be obtained by adjusting the rotating speed of the stator flux linkage as fast as possible. This is achieved by using direct torque control (DTC) technique. The Direct Torque Control (DTC) has been more and more used in industrial applications with permanent magnet synchronous motor (PMSM) using two level voltage source three-phase inverter with hysteresis controller due to some advantages like: more simplicity, low dependency on the motor parameters, good dynamic torque response. This type of drive system is named as classic PMSM DTC. However, the classic PMSM DTC has some problems like more torque and flux ripples as well as more harmonic contents in the stator current. Hence, to overcome these problems a novel DTC algorithm is proposed for three-phase induction motor which employs a three-level inverter. It is an extension of the classic DTC for two-level inverters. The basic principle of DTC is to directly select stator voltage vectors according to the differences between the references of torque and stator flux linkage and their actual values.

Index Terms- Conventional Direct Torque Control System, Conclusion, Direct Torque Control (DTC) Of PMSM Drive, Introduction, Mathematical Modeling Of Permanent Magnet Synchronous Motor, Reference, Simulation Results, Voltage Vector Selection In DTC Of PMSM Drive.
Industry automation is mainly developed around motion control systems in which controlled electric motors play a crucial role as heart of the system. Therefore, the high performance motor control systems contribute, to a great extent, to the desirable performance of automated manufacturing sector by enhancing the production rate and the quality of products. In fact the performance of modern automated systems, defined in terms of swiftness, accuracy, smoothness and efficiency, mainly depends to the motor control strategies. The recent developments of the power electronics industry resulted in a considerable increase of the power that can be manipulated by semiconductor devices. In spite of that, the maximum voltage supported by these devices remains the major obstacle in medium and high voltage applications. For such applications multilevel converters have been proposed. Multilevel inverters present lower harmonic distortion of the output voltages when compared to standard two-level inverters operating at the same switching frequency. Newly developed permanent magnet synchronous (PMS) motors with high energy permanent magnet materials particularly provide fast dynamics, efficient operation and very good compatibility with the applications if they are controlled properly.   
However, the AC motor control including control of PMS motors is a challenging task due to very fast motor dynamics and highly nonlinear models of the machines. Therefore, a major part of motor control development consists of deriving motor mathematical models in suitable forms. There are two competing control strategies for AC motors i.e. vector control (VC) and direct torque control (DTC). Almost 30 years ago, in 1971 F. Blaschke presented the first paper on field-oriented control (FOC) for induction motors. Since that time, the technique was completely developed and today is mature from the industrial point of view. Today field oriented controlled drives are an industrial reality and are available on the market by several producers and with different solutions and performance.

Permanent magnet synchronous machines have been widely used in variable speed drives for over a decade now. The most common applications are servo drives in power ranges from a few watts to some kilowatts. A permanent magnet          synchronous machine is basically an ordinary AC machine with windings distributed in the stator slots so that the flux created by stator current is approximately sinusoidal and uses permanent magnets to produce the air gap magnetic field rather than using electromagnets.
The model of PMSM without damper winding has been developed on rotor reference frame using the following assumptions,
1) Saturation is neglected.
2) The induced EMF is sinusoidal.
3) Eddy currents and hysteresis losses are negligible.
4) There are no field current dynamics.

Voltage equations are given by:

vd = Rs id ωr λq + dλd /dt 
vq = Rs iq ωr λd + dλq /dt                                                 (2.1)

Flux Linkages are given by:
 λd = Ld id + λf
 λq = Lq iq                                                                              (2.2)                                                             

Substituting equations 2.2 in 2.1,
vd = Rs id ωr Lq iq+ d/dt( Ld id+ λf)
 vq = Rs iq   ωrLqiq+ d/dt(Lq iq)                                                (2.3)

Arranging equations 2.3 in matrix form,

(█(vd@vq)) = (█(Rs+sLd    -ωLq@ωrLd          Rs+sL))(█(id@iq))+(█(0@ωrλf))                      (2.4)

The developed torque motor is being given by,

Te =  3/(2 ) (p/2) ( λd iq λq id )                                                      (2.5)

The mechanical Torque equation is,

Te = TL + B ωm + J dωm/dt                                                        (2.6)

Solving for the rotor mechanical speed form equation (2.6)

ωm = ∫▒((Te-TL- Bωm)/J)   dt                                                      (2.7)

 ωm =  ωr (p/2)

In the above equations ωr is the rotor electrical speed where as ωm is the rotor mechanical speed.

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