SWITCHED RELUCTANCE DRIVES.
The Switched Reluctance motor is more than a high-speed stepper motor. It combines many of the desirable qualities of both induction-motor drives and d.c. commutator motor drives, as well as PM brushless d.c. systems. Its performance and inherently low manufacturing cost make it a vigorous challenger to these drives. Its particular advantages may be summarized as follows:
(1) The rotor is simple and requires relatively few manufacturing steps; it also tends to have a low inertia.
(2) The stator is simple to wind; the end-turns are short and robust and have no phase-phase crossovers.
(3) In most applications the bulk of the losses appear on the stator, which is relatively easy to cool.
(4) Because there are no magnets the maximum permissible rotor temperature may be higher than in PM motors.
(5) The torque is independent of the polarity of phase current; for certain applications this permits a reduction in the number of power semiconductor switches needed in the controller.
(6) Under fault conditions the open-circuit voltage and short-circuit current are zero or very small.
(2) The stator is simple to wind; the end-turns are short and robust and have no phase-phase crossovers.
(3) In most applications the bulk of the losses appear on the stator, which is relatively easy to cool.
(4) Because there are no magnets the maximum permissible rotor temperature may be higher than in PM motors.
(5) The torque is independent of the polarity of phase current; for certain applications this permits a reduction in the number of power semiconductor switches needed in the controller.
(6) Under fault conditions the open-circuit voltage and short-circuit current are zero or very small.
(7) Most converter circuits used with SR motors are immune from shoot-through faults, unlike the inverters used with a.c. and brushless d.c. drives.
(8) Starting torque can be very high, without the problem of excessive inrush currents, as for example the large starting current of induction motors at high slip.
(9) Extremely high speeds are possible.
(10) The torque/speed characteristic can be 'tailored' to the application requirements more easily than in the case of induction motors or PM motors.
(8) Starting torque can be very high, without the problem of excessive inrush currents, as for example the large starting current of induction motors at high slip.
(9) Extremely high speeds are possible.
(10) The torque/speed characteristic can be 'tailored' to the application requirements more easily than in the case of induction motors or PM motors.
However, the absence of 'free' Permanent Magnet excitation imposes the burden of excitation on the stator windings and the controller, and increases the per-unit copper losses. Particularly in small motors this is a disadvantage that limits the efficiency and the torque per ampere.
Thst non-uniform, nature of the torque production which leads to torque ripple and may cone SR motor also has some clear disadvantages. The most important is the pulsed, or at leatribute to acoustic noise. Over a narrow speed range it is possible to reduce the torque ripple to less than 10 per cent r.m.s., which is comparable with the levels attainable in induction motors and other brushless d.c. drives, but it is practically impossible to maintain this level of smoothness over a wide speed range. Fortunately it is easier to achieve smooth torque at low speeds, where many loads are most sensitive to torque ripple effects. The acoustic noise can be severe in large machines where ultrasonic chopping frequencies are not practical. But even in small ones, when all steps have been taken to minimize chopper noise, there remains a characteristic sound similar to 'tick over' noise in internal combustion engines at light load; under heavy load this tends to become a 'growl' that may be difficult to eliminate. The noise level is sensitive to the size, being much less severe in small machines. It also depends on the mechanical construction and the precision of the firing angles.
The torque ripple is also sensitive to these factors. Although the construction is simple, electrical and mechanical precision are essential to keep it quiet and this tends to increase the cost.SPEED, FREQUENCY, WINDING- SWITCHED RELUCTANCE DRIVES
Only the two shown in Fig. are considered here.
The relationship between speed and fundamental switching frequency follows from the fact that if the poles are wound oppositely in pairs to form the phases, then each phase produces a pulse of torque on each passing rotor pole; the fundamental switching frequency in one phase is therefore,
The number of stator poles usually exceeds the number of rotor poles.
POLE ARCS-SWITCHED RELUCTANCE DRIVES
The pole arcs are determined by the essential torque-production mechanism, which is the tendency of the poles to align. If fringing is neglected there must be overlap between a pair of rotor poles and the poles of the excited stator phase; in this case torque can be produced through an angle β, which is the smaller of the stator and rotor pole arcs. To produce unidirectional torque through 360° it is obvious that β must not be smaller than the step angle, otherwise there will be 'gaps' where no torque is produced: thus,
In order to get the largest possible variation of phase inductance with rotor position, the interpolar arc of the rotor must exceed the stator pole arc. This leads to the condition,
which ensures that when the rotor is in the 'unaligned' position relative to the stator poles of one phase, there will be no overlap and therefore a very low inductance. The unaligned position is defined as the conjunction of any rotor interpolar axis with the axis of the stator poles of the phase in question.
In Fig. the phase on the vertical axis is 'unaligned' while the phase on the horizontal axis is 'aligned'.
A further constraint on the pole arcs is that usually the stator pole arc is made slightly smaller than the rotor pole arc. This permits slight increases in the slot area, the copper winding cross-section, and the aligned/unaligned inductance ratio.
A further constraint on the pole arcs is that usually the stator pole arc is made slightly smaller than the rotor pole arc. This permits slight increases in the slot area, the copper winding cross-section, and the aligned/unaligned inductance ratio.
The constraints on pole arcs can be expressed graphically as shown, in which the 'feasible triangles' define the range of combinations normally permissible. As might be expected, the variation in performance of machines defined by different points in these triangles is considerable.
Figure shows, for a three-phase motor, the cross-sections corresponding to the vertices A, B, and C on graph. Design C is likely to have too high an unaligned inductance and too little winding area. Design B has more copper area but still the unaligned inductance will be high because of fringing. Design A has a large winding area and a high inductance ratio, leading to a high efficiency and power density, but its torque ripple is higher than in the others.
The 'optimum' tooth-width/tooth-pitch ratio used in stepper motor design is not applicable to the SR motor. It is of course possible to determine a combination of pole arcs that gives the highest inductance ratio and therefore the highest 'static torque per ampere'. But too many other factors have to be considered to make this the universal choice. Among them are the torque ripple, the starting torque, and the effects of saturation. Curvature effects are also more pronounced than in steppers because of the small number of poles. As in steppers, pole taper is likely to be of benefit in reducing core losses, the m.m.f. drop in the rotor and stator steel, and the adverse effects of saturation. Stator pole taper also reduces the unaligned inductance, but it slightly decreases the winding area.
Several other detailed modifications to the simple geometry of Fig. above are permissible and advantageous, such as the use of a hexagonal stator blank which increases the winding area and can produce a mechanically stiffer core, at the same time reducing the scrap from the punching process. By reducing the scrap, the total production cost can also be minimised. Pole overhangs can be used to control the local saturation during the initial overlap period. Welding the outside of the stator stack is permissible as in a.c. motors, but this cannot be used on the rotor, which requires mechanical means for compressing the lamination together. The rotor may be skewed slightly to reduce noise.
The 'optimum' tooth-width/tooth-pitch ratio used in stepper motor design is not applicable to the SR motor. It is of course possible to determine a combination of pole arcs that gives the highest inductance ratio and therefore the highest 'static torque per ampere'. But too many other factors have to be considered to make this the universal choice. Among them are the torque ripple, the starting torque, and the effects of saturation. Curvature effects are also more pronounced than in steppers because of the small number of poles. As in steppers, pole taper is likely to be of benefit in reducing core losses, the m.m.f. drop in the rotor and stator steel, and the adverse effects of saturation. Stator pole taper also reduces the unaligned inductance, but it slightly decreases the winding area.
Several other detailed modifications to the simple geometry of Fig. above are permissible and advantageous, such as the use of a hexagonal stator blank which increases the winding area and can produce a mechanically stiffer core, at the same time reducing the scrap from the punching process. By reducing the scrap, the total production cost can also be minimised. Pole overhangs can be used to control the local saturation during the initial overlap period. Welding the outside of the stator stack is permissible as in a.c. motors, but this cannot be used on the rotor, which requires mechanical means for compressing the lamination together. The rotor may be skewed slightly to reduce noise.
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