2) Extended Number of Voltage Vectors: One of the main drawbacks of DTC is the ripple in torque and stator flux linkage。 This ripple can be reduced by using more, different voltage vectors。 When motoring in basic DTC there is only a limited number of voltage vectors available per sector, the switching table chooses the most appropriate。 However it is very unlikely that both the radial and tangential components of the vector are aligned with the desired components。 With adding more voltage vectors and/or adding sectors, a closer match for both components can be achieved。 In [11] a DTC scheme is proposed which allows, by means of space vector modulation (SVM), to use 24 voltage vector directions at three amplitude levels。 With the quantization of torque and flux error and the availability of 72 voltage vectors a different switching table is constructed。 As a result a lower torque ripple is achieved。 In [11] SVM is used to generate more, different voltage vectors during the entire operation of the drive。 It is however also possible to use a hybrid algorithm, making more voltage vectors available during certain operating conditions。 In [12] a method is proposed to ensure a fast torque response during start-up of IPMSMs。 At start-up SVM is used to generate the optimal voltage vector, i。e。 the voltage vector allowing the fastest rise in torque。 However the calculations are dependent on the rotor position, thus the initial rotor position has to be known from an encoder and during the torque development duration the rotor position is assumed to be constant。 Once the torque reference value is reached, a regular switching table (containing only voltage vectors) is used and there is no further need for the encoder。

Multilevel converters make more voltage vectors available to control flux and torque, hence reducing ripple and achieving a less variable switching frequency。 As ad is advantage more power switches are needed, thus increasing system cost and complexity as well as switching losses。 In [13] such a DTC is proposed for induction machines, but application to PMSMs is not reported in literature。 Constant Switching Frequency DTC来:自[优E尔L论W文W网www.youerw.com +QQ752018766-

To further eliminate torque and stator flux ripples and to obtain a fixed switching frequency, it is possible to use a model of the PMSM to calculate the most appropriate voltage vector during the next switching interval。 This most appropriate voltage vector can then be realized by SVM。 Furthermore the use of SVM permits avoiding some other disadvantages of switching-table DTC, such as violating polarity consistency rules, high sampling frequency for digital implementation of the comparators and distortions due to sector changes。 However there are several ways of calculating the most appropriate voltage vector and the required motor parameters and computational complexity have to be taken into account when comparing the different schemes。

Voltage Vector Calculation1) SVM-DTC with Closed Loop Torque Control: Atypical scheme of this type is shown in Fig。3。 In [14] the difference between actual and reference torque is supplied to a PI controller resulting in a desired change of load angle 。 This signal, together with the measured currents and the actual and reference flux are used in a predictive controller to calculate, based on (3), the desired voltage vector in polar coordinates。 The stator voltage command is supplied to a space vector modulator。 However, this method uses a motion-state sensor。 A very much related scheme is proposed in [15] for IPMSMs, but without the use of a position sensor。 The estimated stator flux linkage position, the load angle correction (from a PI-controller) and the reference flux amplitude are used to calculate a reference flux vector。 The error between this reference flux vector and the actual vector, both in amplitude as angle, is then corrected by applying the required voltage vector through SVM。 A lower ripple and fixed switching frequency are reported to be obtained in [14], [15]。 However one has to notice that the use of a PI-controller may deteriorate the performance of the drive as the PI-controller is sensitive to detuning。