The increasing availability and progressive improvement of power semiconductor devices as well as intensive computer aided study of the geometry of conventional reluctance motors for optimum torque production have led to the introduction of yet another type of drive called the switched reluctance motor drive. It is becoming a serious competitor to converter supplied dc and ac variable speed drive systems.
The switched reluctance motor is being considered, nowadays, for applications ranging from low power servomotors to high power traction drives. Motors of power ratings varying from 4 to 22 kW are commercially available at present for many applications.
Construction of SRM:
The switched reluctance motor (SRM) has both salient pole stator and rotor like a variable reluctance stepper motor. Figure 3.57 shows a one phase winding of a 4-phase switched reluctance motor having 8 poles on stator and 6 poles on rotor. While the rotor has no windings, each stator pole has a concentrated winding around it and each pair of diametrically opposite coils comprise one phase of the motor. These motors are designed for applications different from those for which stepper motors are designed.
A stepper motor is designed suitable for open-loop position and speed control in small power applications, where efficiency is not significant. On the other hand a switched reluctance motor (SRM) is employed in variable speed drives and designed to operate efficiently for wide range of speed and torque and needs rotor position sensing. It is, also, quite different from a standard synchronous reluctance motor in two aspects.
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A synchronous reluctance motor has the same number of poles on stator and rotor but the rotor of a SRM has lesser poles than the stator, which is an essential feature to provide self-starting capability and bidirectional control. They are also different in stator construction—the synchronous reluctance motor has a cylindrical stator with distributed winding, while the SRM has a salient pole stator with concentrated coils like a dc motor.
Though various combinations of stator and rotor pole numbers are possible in SRMs, the commonly used are 8/6 and 6/4. The stator has concentrated coils and diametrically opposite coils are connected in series or parallel to provide one phase. Thus, motors with pole numbers 6/4 and 8/6 will have three and four phases respectively.
Operation of SRM:
Reluctance torque is produced when a stator phase is excited by means of unidirectional currents. This results in the magnetic attraction of an adjacent rotor pole as it tends to align into a position of minimum reluctance. When the numbers of stator and rotor poles differ, the sequential switching of the excitation from one set of stator poles to the next, in synchronism with the rotor position, produces an almost constant torque resulting in an uniform rotation of rotor. The synchronisation of the switching on the excitation with rotor position can be accomplished with simple rotor position feedback.
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Neglecting nonlinearity of the magnetic circuit, the instantaneous torque produced in such machines may be given as-
where i is the instantaneous current in the exciting winding and L is the self-inductance of that winding varying as a function of the angular position of the rotor. It may be noted that the torque developed is independent of the direction of current flow in the windings, so that unidirectional currents can be used to control the motor. The direction of rotation can be reversed by exciting the phases in the reverse sequence.
The motor can also provide regenerative braking. If a phase is excited after the rotor has crossed the position of minimum reluctance, the rotor will experience a torque in opposition to its motion, it will decelerate, and mechanical energy drawn from it will be converted into electrical energy and supplied to the source. In fact, the possibility of operating in all of the four quadrants of the speed-torque plane and obtaining flexible speed-torque characteristics simply by appropriate switching of current pulses makes the motor very versatile.
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Figure 3.58 (a) and (b) illustrates the ideal variation of inductance of the exciting winding with respect to the angular position of the rotor over a periphery of one rotor pole pitch and the corresponding torque developed, for an assumed value of constant current [using Eq. (3.60)].
Modes of Operation of SRM:
There are two distinct modes of operation corresponding to low or high speed. Monitoring of exciting current during low speed operation is essential because of long duration of each phase period and needs chopping of energization to restrict each phase current within the semiconductor ratings. Moreover, the developed torque is controlled by varying the average phase current.
Hence, accurate monitoring of the exciting current is required for obtaining high degree of controllability possible. During high speed operation, current control is not essential because the inductance of the winding and the motional back emf induced restrict the excitation to single pulses of current. Torque is controlled by optimal positioning of these pulses rather than the current level. Current monitoring, however, is retained for the sake of protection.
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Typical phase current waveforms of the two modes (low speed and high speed) of operation are shown in Figs. 3.59 (a) and (b).
The complete drive system, comprising a SRM coupled to a load, a power converter and a control system involving rotor position transducer and current sensor is shown in Fig. 3.60.
Power Converter for SRM:
Since the SRM needs only unidirectional currents, its operation is possible with only one switching device in service per phase, instead of two in series in each phase leg of an inverter for an ac drive. So, the power converter circuits employed for energization of SRMs have few semiconducting devices than the inverters supplying ac motors and those devices have only one forward voltage drop in series per phase so that the power losses may, in general, be lower than in conventional inverters. Because of these facts, other factors being the common, results in reduction in the physical size of converter and increase in its reliability.
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Different circuit configurations are available for the power converters for SRM drive. All of these have two essential elements—a controlled switch or switches to connect the dc voltage source to the exciting winding to build up current and an alternative path for the current to take when the switch is turned off. The alternative path is provided by a diode or diodes such that the winding experiences a reverse voltage to collapse the current.
Three alternatives for one phase of a SRM power converter are depicted in Fig. 3.61. A flexible circuit using two switches per phase is shown in Fig. 3.61 (a), whilst the use of bifilar wound motor (as in stepper motors) or a centre tapped supply permits the use of only a single switch per phase as depicted in Figs. 3.61 (b) and (c). The correct choice of power circuit configuration will depend on the power level of the drive, the supply voltage and the application.
