Switched reluctance motors (SRMs) have been gaining popularity in recent years as an alternative to traditional induction and permanent magnet motors. These motors have a unique design that allows them to operate with high efficiency and low cost, making them a suitable choice for a wide range of applications. However, as with any technology, there are advantages and disadvantages to using SRMs. In this article, we will explore the key benefits and drawbacks of using these motors in order to help you make an informed decision about whether they are the right choice for your application.
Advantages of Switched Reluctance Motors
The SRM possesses a few unique features that make it a vigorous competitor to existing AC and DC motors in various adjustable-speed drive and servo applications. The advantages of an SRM can be summarized as follows:
- Machine construction is simple and low-cost because of the absence of rotor winding and permanent magnets.
- There are no shoot-through faults between the DC buses in the SRM drive converter because each rotor winding is connected in series with converter switching elements.
- Bidirectional currents are not necessary, which facilitates the reduction of the number of power switches in certain applications.
- The bulk of the losses appears in the stator, which is relatively easier to cool.
- The torque–speed characteristics of the motor can be tailored to the application’s requirements more easily during the design stage than in the case of induction and PM machines.
- The starting torque can be very high without the problem of excessive in-rush current due to its higher self-inductance.
- The open-circuit voltage and short-circuit current at faults are zero or very small.
- The maximum permissible rotor temperature is higher since there are no permanent magnets.
- There is low rotor inertia and a high torque/inertia ratio.
- Extremely high speeds with a wide constant power region are possible.
- There are independent stator phases, which do not prevent drive operation in the case of loss of one or more phases.
Disadvantages of Switched Reluctance Motors
The SRM also comes with a few disadvantages among which torque ripple and acoustic noise are the most critical. The double saliency construction and the discrete nature of torque produced by the independent phases lead to higher torque ripple compared with other machines. The higher torque ripple also causes the ripple current in the DC supply to be quite large, necessitating a large ﬁlter capacitor. The doubly salient structure of the SRM also causes higher acoustic noise compared with other machines. The main source of acoustic noise is the radial magnetic force-induced resonant vibration with the circumferential mode shapes of the stator. The absence of permanent magnets imposes the burden of excitation on the stator windings and converter, which increases the converter kVA requirement. Compared with PM brushless machines, the per-unit stator copper losses will be higher, reducing the efﬁciency and torque per ampere. However, the maximum speed at constant power is not limited by the ﬁxed magnet ﬂux as in the PM machine, and, hence, an extended constant power region of operation is possible in SRMs. The control can be simpler than the ﬁeld-oriented control of induction machines, although for torque ripple minimization signiﬁcant computations may be required for an SRM drive.