Field-Oriented Control of Multiphase Synchronous reluctance Machines

C. Field-Oriented Control of Multiphase Synchronous reluctance Machines

Syn-Rel machines for high-performance variable speed drives have a salient pole rotor structure without any excitation and without the cage winding The model of such a machine is obtainable directly from 24 3 through 24 7 by setting the permanent magnet flux to zero If there are more than three phases, then stator equations 24 4 and 24 6 also exist in the model but remain the same and are hence not repeated Thus, from 24 3, 24 5, and 24 7, one has the model of the Syn-Rel machine, which is again given in the reference frame firmly attached to the rotor d- axis axis of the minimum magnetic reluc- tance or maximum inductance: It follows from 24 23 that the torque developed by the machine is entirely dependent on the difference of the inductances along d- and q-axis Hence constructional maximization of this difference, by mak- ing LdLq ratio as high as possible, is absolutely necessary in order to make the Syn-Rel a viable candidate for real- world applications For this purpose, it has been shown that, by using an axially laminated rotor rather than a radially laminated rotor structure, this ratio can be significantly increased From FOC point of view, it is however irrelevant what the actual rotor construction is for more details see [13] As the machine’s model is again given in the reference frame firmly attached to the rotor and the real axis of the reference frame again coincides with the rotor magnetic d-axis, transformation expressions that relate the actual phase variables with the stator d– q variables 24 9 through 24 11 are the same as for PMSMs Rotor position, being measured once more, is the angle required in the transformation matrix 24 9 Thus one concludes that FOC schemes for a Syn-Rel will inevitably be very similar to those of an IPMSM Since in a Syn-Rel there is no excitation on rotor, excitation flux must be provided from the stator side and this is the principal difference, when compared to the PMSM drives Here again a question arises as to how to subdivide the available stator current into corresponding d–q axis current references The same idea of MTPA control is used as with IPMSMs Using 24 19, electromagnetic torque 24 23 can be written as By differentiating 24 24 with respect to angle δ, one gets this time a straightforward solution δ = 45° as the MTPA condition This means that the MTPA results if at all times stator d-axis and q- axis cur- rent references are kept equal FOC scheme of Figure 24 4 therefore only changes with respect to the stator d-axis current reference setting and becomes as illustrated in Figure 24 10 The q- axis current limit is now set as ± is max 2 , since the MTPA algorithm sets the d- and q-axis current references to the same values The same modifications are required in Figure 24 9, where additionally now the permanent magnet flux needs to be set to zero in the decoupling voltage calculation 24 18 Otherwise the FOC scheme is identical as in Figure 24 9 and is therefore not repeated It should be noted that the simple MTPA solution, obtained above, is only valid as long as the satura- tion of the machine’s ferromagnetic material is ignored In reality, however, control is greatly improved and also made more complicated by using an appropriate modified Syn-Rel model, which accounts for the nonlinear magnetizing characteristics of the machine in the two axes As an illustration, some responses collected from a five-phase Syn-Rel experimental rig are given in what follows To enable sufficient fluxing of the machine at low load torque values, the MTPA is modified and is implemented according to Figure 24 11, with a constant d-axis reference in the initial part The upper limit on the d-axis current reference is implemented in order to avoid heavy saturation of the magnetic circuit Phase currents are measured using LEM sensors and a DSP performs closed- loop inverter phase CC in the stationary reference frame, using digital form of the ramp-comparison method Inverter switching frequency is 10 kHz The five-phase Syn-Rel is 4-pole, 60 Hz with 40 slots on stator It was obtained from a 7 5 HP, 460 V three-phase induction machine by designing new stator laminations, a five-phase stator winding, and by cutting out the original rotor unskewed, with 28 slots, giving a ratio of the magnetizing d-axis to q-axis inductances of approximately 2 85 The machine is equipped with a resolver and control operates in the speed-sensored mode at all times Response of the drive during reversing transient with step speed reference change from 800 to −800 rpm under no-load conditions is illustrated in Figure 24 12, where the traces of measured speed, stator q-axis current reference which in turn determines the stator current d-axis reference, according to Figure 24 11, and reference and measured phase current are shown It can be seen that the quality of Time s the transient speed response is practically the same as with a SPMSM Figure 24 6 and 24 7, since the same linearity of the speed change profile is observable again In final steady-state operation at −800 rpm the machine operates with q-axis current reference of more than 1 A rms, although there is no load This is again the consequence of the mechanical and iron core losses that exist in the machine but are not accounted for in the vector control scheme mechanical loss appears, according to 24 1a, as a certain nonzero load torque Measured and reference phase current are in an excellent agreement, indicating that the CC of the inverter operates very well

D. Field-Oriented Control of Multiphase Induction Machines