This topic encompasses an extensive review of synchronous motor drives that includes different converter-machine configurations, control topologies, feedback signal estimation, and discussion on sensorless control. Different types of synchronous machines were described in topic 6. Broadly, the machines are classified into wound-field, permanent magnet, and reluctance types. The switched reluctance machine (SRM) does not fall into this category, but is covered because of its similarity to the synchronous machine, wide literature coverage, and completeness of description in ac drives. The stepper motor drive is not covered (see topic 6) because it is not well-adapted for variable-speed applications. Note that line-start PM machines are not suitable for variable-frequency drives because of excessive copper loss in cage winding due to inverter harmonics. Currently, synchronous motor drives are close competitors to induction motor drives in many industrial applications, and their applications are continuously growing. Certain applications, as described in this topic, are uniquely suited for synchronous motor drives. These are generally more expensive than induction motor drives, but the advantage is that the efficiency is higher, which tends to lower the life-cycle costs. Note that PM drives are more popular in Japan because of high energy costs. Wound-field machines, which can be brush or brushless type, have the advantage that field current or flux y is controllable unlike PM machines. High-power multi-megawatt machines are normally used at leading power factor (near unity) for load-commutated thyristor converter drives and unity power factor for cycloconverter and voltage-fed PWM converter drives. The largest power drives in the industry are of these classes. A number of practical applications of these drives have been described that include icebreaker ship propulsion with cycloconverter drive, dual mining ore-crushing drive with cycloconverter, gearless cement mill drive with cycloconverter, cruise ship propulsion with load-commutated current-fed converters, rolling mill drive with voltage-fed converters, and future ship propulsion with high-temperature superconducting (HTS) motors. One interesting application is the use of the converter system as a variable-frequency starter for the machine (see Figure 8.49). The PM machines are generally classified into radial flux (or drum) and axial flux (or disk) types. Although the latter type has a higher power density, lower inertia, and smooth operation, the radial type is more commonly used because of the ease of manufacture. The gradually declining cost of high-energy magnets (NdFeB) is promoting applications of PM machines, and eventually with this trend, their volume of applications may exceed that of induction motors. The PM machines are again classified into surface magnet and interior magnet machines, or sinusoidal and trapezoidal machines. The interior PM (IPM) machines are preferred for the extended speed field-weakening range, such as for EV/HV drives. Trapezoidal SPM machines have simple construction, are easy to manufacture, and have a somewhat higher power density than sinusoidal SPM machines. They are widely used as "brushless dc drives." The wound-field and PM machine drives have one disadvantage: in the case of a converter fault, the machine counter emf feeds the fault and causes dangerous torque pulsation (which is a problem with PM machines because field flux is not controllable). The synchronous reluctance and switched reluctance motors are somewhat comparable because both of
them are magnetless, somewhat simpler in construction, and can operate at high speed. However, the latter has the disadvantage of pulsating torque that causes vibration and acoustic noise. Both of these machines are bulkier than PM machines, particularly with NdFeB magnet. The control of PM and synchronous reluctance machines can be generally classified into open-loop volts/Hz and self-control types, and the latter type is almost universally used. However, volts/Hz control is very simple, and is essential for close tracking of speed control in parallel machines, such as textile machine drives. With self-control, there is no command frequency, but the drive control is robust like dc machine drives. Although any ac machine (induction or synchronous) with self-control has dc drive analogy, a trapezoidal SPM drive is the closest, and is normally defined as a BLDM or BLDC (brushless dc). An absolute shaft position encoder is mandatory for self-control. Both scalar and vector control techniques are applicable to synchronous machine drives. Stator flux-oriented direct vector control is normally used. Like induction motor drives, various other control methods, such as sliding mode control, DTC control, MRAC control, and light-load flux-weakening efficiency optimization control, are also applicable for synchronous motor drives, but these are not separately covered in this topic. The feedback signal estimation of sinusoidal machines is somewhat similar to that of induction machines. A position sensorless drive is possible using the machine counter emf signal as long as the machine does not operate below a critical speed. At zero frequency (i.e., zero speed), position estimation is extremely difficult, as is also true of an induction motor (indeed more difficult here than induction matter because the latter may have slip frequency at zero speed). Currently, an external signal injection technique is being proposed in the literature [24], but the success of this method remains questionable. Of course, the machine can be started with open-loop volts/Hz control, and then at a critical speed, it can be transitioned to counter emf-based self-control.