SR motor anatomy: See inside switched reluctance motors

Author : Frank J. Bartos, P.E, Control Engineering consulting editor

12 March 2010

Simple electromechanical structure of switched reluctance (SR) motors offers benefits of robustness and reliability by eliminating permanent magnets, rotor bars, and more - tutorial.

Simple electromechanical structure of switched reluctance (SR) motors offers benefits of robustness and reliability by eliminating permanent magnets, rotor bars, and more.

Origins of switched reluctance (SR) motors can be traced back to the reluctance machine design of the mid-19th century, making them one of the oldest electric motor types around. The term SR has been in use for about 40 years, where "switched" indicates sequential switching of current among stator phase windings of the motor--necessary to develop a rotating magnetic field (see main article)--and "reluctance" refers to the resistive property of the magnetic circuit.

Simple rotor, stator construction
Rotor structure of an SR motor is very simple, comprising a stack of electrical steel laminations mounted on the motor shaft. The rotor has no windings or rotor bars, no permanent magnets, and no electrical contacts.

Absence of rotor conductors eliminates a heat source, increasing bearing and lubricant life. Lower power losses improve energy efficiency. The low SR rotor losses are especially relevant during starting, when they are actually less than at normal full-load running condition, according to Emerson Motor Co. This permits prolonged operation in the stalled condition, and repeated starting under full load; starting current never exceeds full-load operating current. "Such performance is often not possible with conventional drives because of large rotor electrical losses, and subsequent rotor heating under such conditions," says Rob Boteler, director of marketing at Emerson Motor.

The "salient" pole construction-with gaps between poles rather than a solid circular cross-section-creates a relatively low-inertia rotor, which allows for rapid dynamic response and minimises risk of mechanical damage from shock loads in high-ratio, gear-driven applications, Boteler explains. "Because of its uniquely simple construction, the SR rotor is well suited to operation at high speeds. It can tolerate high levels of vibration and thermal cycling." Absence of permanent magnets brings significant benefits in terms of environmental compatibility, and permits repair of SR motors in the field or in a conventional repair shop.

An SR motor's stator is likewise a simple, robust structure. It consists of singly pitched coils placed over the salient stator poles (see diagram in main article); the coils are easily pre-wound on a bobbin. "Overall [heating] losses in an SR motor are concentrated within the stator and are relatively easily dissipated--in case of a standard totally enclosed machine by conduction to the relatively cool exterior of the motor frame," states Boteler.

Moreover, stator windings--unlike those of an induction motor--are not distributed over many slots, and the phases do not cross each other in the slot or in the end-connections. This reduces insulation stresses and largely eliminates risk of phase-to-phase insulation failure. Simplicity of the coils allows the end windings to be much shorter than those typically found in ac motors. "As a result, energy losses associated with end windings are reduced. This further improves efficiency and permits the design of different SR motor configurations--for example, flat-shaped ("pancake") motors," Boteler adds.

[Note, this section of the article is adopted from a paper, "Switched Reluctance Compressor Drive," presented by Emerson Motor Co. at EEMODS 2009 (Efficient Electric Motor Drive Systems), Nantes, France, Sept. 2009.]

Fault tolerance
An additional feature unique to SR motors is the ability to continue to operate (and also likely to start) under a phase loss condition. However, the motor would run less smooth and with less torque output. In theory, only one pole pair is required for operation. "This contrasts with ac induction machines, which will fail to operate when you lose a phase," says George Holling, technical director at Rocky Mountain Technologies.

This feature is of particular advantage in applications such as defence, aerospace, and medical, where ability to complete a mission and/or return safely overrides reduced torque output and noise generation. Holling mentions that a 4-phase SR motor can typically maintain 80% of normal starting torque with one phase inoperative.

One recent paper that addresses the issue of fault tolerance in SR systems should be of interest to automation professionals. It can be found at

Also, SPEED software, a motor design tool (developed at Univ. of Glasgow, U.K., as a valuable design tool for building SR motors) is available in the U.S. (and the Americas) through the distributor Magsoft Corp. (SPEED stands for Scottish Power Electronics and Electric Drives; it's a consortium.)

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