Cleaning a path for SR drives

The switched reluctance (SR) motor is making a comeback, thanks to its high-profile use on a new US energy-efficient front-loading washing machine called the Maytag Neptune. Yet the SR motor technology itself has been around for over a century. Often first cited in Davidson’s Locomotive in Falkirk in 1838, it also appeared in a variety of clock mechanisms in the 1840s, some of which can still be seen in the Time Museum in Rockford, Illinois in the United States.

The switched reluctance motor has had many names. Aside from Switched Reluctance (SR), it’s also been called a Variable Reluctance (VR) or even a Variable Switched Reluctance (VSR) motor.

To look at, an SR motor design is very simple: it is a brushless DC motor without magnets whose motor windings must be switched on and off at specific time intervals. The motor itself comprises a stator, a number of teeth, a series of coils, and a rotor. The reluctance in the motor changes as a function of current and a function of position. In effect, the motor operates like a rotary solenoid.

Correctly used, the SR motor has a very high torque-to-inertia ratio. What’s more, the motor design boasts a high power efficiency over a wide range of loads, a high power density and a simple rugged construction. It sports a fast dynamic response and low intrinsic materials and assembly costs. In fact, the SR motor has the smallest sized rotor for its power level and size of any comparable technology.

So what has held back the acceptance of the SR drive? Possibly, because it is a hard motor to design and a complex motor to drive. In most cases an inverter cannot be used to drive the motor because an inverter drives current in two directions. In the SR drive, current only has to be driven in a single direction: it is a unipolar device in which current only flows one way through the winding.

What is more, it is impossible to design the motor independently of the drive control electronics. The techniques that are used to design an SR motor/drive combination must rely on sophisticated Finite Element Analysis (FEA). Fortunately, computer programs like those designed, developed and implemented at UK switched reluctance drive developer Switched Reluctance Drives (SRD), for example, has a high accuracy when it comes to predicting how a particular motor/drive combination will actually work, according to SRD’s David Sugden.

According to Sugden, SR machines can be built with standard tolerances and can be quiet, like the Maytag Neptune SR motor. Some SR machine designs, however, often proposed by those with little experience in designing SR machines, can produce increased acoustic noise when manufactured with poor rotor eccentricity.

Despite the difficulties, many SR drives have been built some simple, some complex. In the 1980s, a US company called Megatorque developed what Dan Jones of Incremotion Associates has called the most complex SR motor ever built an SR motor that has the potential to be used as a replacement for a harmonic drive, comprising as it does a multiple pole inner and outer rotor variable reluctance motor. In the 1980s, the company was acquired and the SR motor product is now manufactured by NSK for applications such as driving robot bases, thanks to its ability to develop a tremendous amount of torque at relatively low speed.

In the next few years, the SR motor is likely to find a home in a number of applications, ranging from automobiles to home appliances. Where ferrite type brushed DC motors are presently being used in automotive applications, for example, the SR motor could potentially present some significant cost advantages.

In a power steering system, for example, an SR motor, coupled to a ball nut, could be used to directly drive a steering system’s rack and pinion gear. Obviously, eliminating the hydraulics on steering systems can lead to a substantial saving in fuel consumption. In addition to the steering application, such drives could also potentially be used in ABS systems, water and fuel pumps, and eventually perhaps in an integrated starter/generator systems the holy grail of any designer in the automobile field.

But it’s not just in cars where SR designs have made inroads. Motorbikes too have been developed using SR drives. The Lectra Motorcycle from Electric Motorbike Inc., for example, uses a 2-phase SR motor, to drive it. Priced at under $4500 US dollars, the bike has a top speed of 45mph, and an acceleration of 0-45 mph in 5secs. Like most other electric products, the bad news is that the range is no more than 25 miles at present. One unique feature of the motorbike is its electronically assisted braking mode that extends the vehicle range and preserves brake life by turning the vehicle’s kinetic energy into stored electrical energy during braking.

Aside from transportation, the SR motor is also suited to centrifuges, vacuum cleaners and food processors. The Beckman Avanti J series of laboratory centrifuge systems employs an SR drive at its heart to accelerate and brake a rotor in half the time that it would take using a conventional drive. The SR drive generates twice the torque of conventional drives and provides smooth acceleration and deceleration rates. Smallfry Industrial Design has also used an SR drive in a food processor that is 30% smaller than a conventional model. And in a somewhat larger design for the coal mining industry, BJD 100 used SR technology in its range of Diamond Drive variable speed drives that range from 35kW to 300kW. These are employed on belt conveyors, traction systems and pumps.

Some big players have entered the market with OEM products very recently. NEC, for example, recently disclosed a range of SR motors in the 100W to 10kW range. That company is working with Advanced Motion Controls (Sun Prairie, Wisconsin) in the US to supply the control electronics.