Direct Torque Control of Permanent Magnet Synchronous Machines

Synchronous machines with permanent magnets in the rotor, also referred to as PMSMs, have a smaller inertia, higher efficiency and a higher torque to volume ratio compared to asynchronous machines. These advantages have resulted in an increased application of the PMSM in hybrid vehicles, ships, windmills, compressors, pumps and fans. Furthermore high-performance, highly dynamic drives with PMSMs have many applications in such production processes and transport systems where a fast and accurate torque response is required.  As reliability and cost of modern PMSM drives are of importance, advanced control techniques have been developed.

 

PSMS
Fig.1: An PMSM

In PMSM drives, the electromagnetic torque is usually controlled indirectly via the stator  current components in a reference frame fixed to the rotor flux field. This field orientation creates the need for a position  sensor, which reduces the reliability and increases the cost of  the drive. For induction motors, direct torque control (DTC) was proposed  as an alternative control scheme by I.Takahashi and T.Noguchi and became very popular in the past two decades. DTC for induction machines is inherently motion-state sensorless as the calculations are executed in a stationary reference frame. Moreover DTC uses no current controller and (in its basic form) no parameters other than the stator resistance, which yields a faster torque response and a lower parameter dependence than with field oriented control.  At EELAB research on DTC of induction machines has been conducted in the past (DTDTCFSF, J. Maes 2001). Meanwhile DTC for induction machines has reached industrial maturity and is marketed by ABB.

The idea of combining the advantages of DTC and PMSMs into a highly dynamic drive appeared in the literature in the late 1990's and has attracted the attention of several researchers around the world. In the following paragraphs the operating principles of DTC are explained, the implementation for PMSMs is discussed and current research topics are highlighted.

In a DTC drive, flux linkage and electromagnetic torque are controlled directly and independently. This is achieved by controlling the stator flux linkage vector by selecting the most appropriate voltage vector at every switching instant. The voltage vectors are selected based on the consideration that the voltage component tangent to the flux linkage vector determines the change in electromagnetic torque and the component radial to the flux linkage changes the flux linkage magnitude.

 

fig2
Fig.2: Possible voltage vectors with a two-level inverter (left), control of the stator flux linkage (right)


From stator current and voltage measurements the flux linkage and torque are estimated and compared to the reference value. Based on the error in the electromagnetic torque and the flux linkage, the most appropriate voltage vectors are either selected from a look-up table or realized by space vector modulation (SVM). 

fig3fig3b
Fig.3: Two possibilities for implementing DTC: a switching table or SVM
 

Although the instantaneous electromagnetic torque is determined by the angle between the rotor flux linkage and the stator flux linkage vectors, the position of the rotor flux linkage vector (determined by the permanent magnets in the rotor) is required e.g. at the start-up of the drive. This is one of the main differences compared to DTC of induction machines where the initial rotor position is not necessary for the control.. 


Simulation and experimental results have shown that DTC is capable of controlling the flux linkage and torque in an PMSM. Very short torque and flux response times and high torque-control linearity are advantages of DTC. Disadvantages however include ripples in torque and flux linkage. This can be reduced by using fast-sampling signal processors and is also the focus of most research on DTC of PMSM.

 

fig4a

 

fig4b

 

fig4c

Fig.4: Simulation results. Electromagnetic torque and stator flux linkage magnitude are shown for a variable torque reference and constant flux reference value. Simulation conditions: constant load torque (1Nm), 20kHz sampling and 6-vector switching table.
The stator flux linkage vector trajectory is also shown.

Current research topics in DTC of PMSM include mainly:

  • switching algorithms to reduce torque and flux ripple and improve stability
  • accurate and robust flux estimators
  • sensorless initial rotor position detection
  • flux reference selection for optimal efficiency

 

Relevant Publications

T. J. Vyncke, R. K. Boel, and J. A. Melkebeek , "Direct torque control of permanent magnet synchronous motors - an overview," in Conf. Proc. 3rd IEEE Benelux Young Researchers Symposium in Electrical Power Engineering, no. 28, Ghent, Belgium, Apr. 27-28, 2006, p. 5.

T. J. Vyncke, J. A. Melkebeek, and R. K. Boel, "Direct torque control of permanent magnet synchronous motors," in Conf. Proc. 7th Faculty of Engineering Ph.D. Symposium, no. 34, Ghent, Belgium, Nov. 29, 2006, p. 2.

J. Maes and J.A. Melkebeek, "Speed-sensorless direct torque control of induction motors using an adaptive flux observer," in  IEEE Trans. Ind. Applicat., vol. 36,  no. 3, pp. 778-785, May/Jun. 2000.

External Links

ABB: a manufacturer selling inverters based on DTC
http://www.abb.com/product/ap/seitp322/c1256c84007b2e7ec12569000036ed05.aspx

Animation showing the switching states of the inverter, the stator voltage vector and the evolution of the stator flux linkage vector  (courtesy of ABB)
http://library.abb.com/GLOBAL/SCOT/scot201.nsf/VerityDisplay/ECE262642DF057F0C2256F7A004376A1/$File/DTCAnimation1.gif

Contact

For more information: Thomas.Vyncke@UGent.be, Jan.Melkebeek@UGent.be