Switched Reluctance Motors Go Mainstream
| By: Kristin Lewotsky, Contributing Editor
Although permanent magnet motors are the first that come to mind when we talk about motion control, the high price of rare-earth magnets has driven OEMs to consider other solutions like switched-reluctance (SR) motors. Switched-reluctance motors dispense with permanent magnets entirely. Instead, they use a specialized rotor and stator to generate a reluctance torque from a purely electromagnetic field. The result is a robust, efficient, low-inertia solution that provides a good alternative to conventional motors for applications like traction, pumping, and bulk-handling.
SR motors consist of a simple brushless design with a dedicated electronic controller. The most striking difference between an SR motor and a conventional induction or PM machine is the rotor design. The outer diameter of an SR rotor features radial poles that are salient, or separated by air gaps (see figure 1). The gaps perform a similar function as the slots stamped into the rotor laminations of interior-permanent-magnet servomotors - they localize the magnetic flux generated in the rotor by the stator.
As each stator winding is energized in succession, the field introduced in the rotor acts to align the rotor with the stator field, thereby producing torque. Because the designs feature fewer poles on the rotor than on the stator, the sequential stator energization acts to bring the two into alignment causing the rotor to turn. You can see the process in this computer animation.
The SR motor concept has been around for more than a century, but only with the development of cheap, powerful microprocessors did the approach become practical. “The level of processing power available in terms of cost and performance is much more advantageous than it has been,” says Steve Cummins, business development manager at Nidec SR Drives Limited. “The reduction in the price of processing power now makes technology more accessible to high-volume, low-cost applications such as automotive.”
SR motors appear complex but they’re actually straightforward and economical to design. They consist of stamped steel sheets stacked to form the rotor. Similarly, the stator consists of a stack of steel laminations, with a concentrated winding comprising wire coils on each stator pole (see figure 3). The concentrated winding reduces the length of the end turn on each coil by about 65%; given that the end-turn of the coil does not contribute directly to torque production, a more compact end-turn enables a smaller machine size for a given torque requirement.
In order to time the energization of the stator windings, the designs often use a simple position sensor to detect rotor position. Alternatively, sensorless control algorithms can perform the same function if supported by increased processing power in the controller.
With SR technology, designers are no longer constrained to thinking in terms of fixed pole speeds, like 1800 rpm, 3600 rpm, etc. They can dramatically reconsider their system, reducing gear stages or removing them altogether, as well as eliminating high-maintenance fluid couplings and operating the system in a more intelligent and beneficial way.
The most common design error is for users to base their specification around induction machine performance. Applications that require significant starting torque are often served by using a machine that is substantially oversized for the continuous requirement of the application, and typically results in an efficiency penalty. Potential users should think about the torque/speed & duty cycle requirements of their end application—based on that, the equivalent SR solution will often be smaller and more efficient than a conventional solution. “SR offers many possibilities that are often overlooked because of traditional thought-processes based around fixed pole speeds,” says Steve Cummins, business development manager at Nidec SR Drives Limited. His company, alone, has placed more than 10,000 industrial drive systems to operate compressors, pumps, fans, conveyors, and vehicles.
Pros and cons
Because SR motors have no brushes, windings, commutators, or permanent magnets, they are robust with respect to wear, temperature, and shock and vibration. The salient design reduces rotor mass, cutting inertia by 50 to 60% compared to a conventional motor and yielding high torque to inertia ratios. As a result, switched reluctance motors can be more responsive, making them a good fit for applications that demand fast dynamic response.
The pulse-width modulation scheme used for AC inverters tends to generate loss every time the insulated gate bipolar transistors (IGBTs) switch. Those switching losses generate heat, reducing efficiency. SR motors require switching as well, but at as much as an order of magnitude less frequently, resulting in lower switching losses within the power converter. Moreover, the absence of any conductors on the rotor eliminates conducted losses within the rotor further improving efficiencies.
For the same frame size, a switched reluctance motor can generate more torque than the equivalent induction motor. Compared to permanent magnet motors, SR motors have somewhat lower torque densities, but by eliminating the magnets, they are substantially lower-cost to produce. SR designs can also tolerate prolonged operation at temperatures that could demagnetize permanent magnets.
Sweet-spot uses for SR include low-speed, high-torque applications that involve high levels of overhead torque and/or rapid starting duty, such as conveyors, feeders, slurry pumps, crushers, and extruders. They are also a good fit for high-speed applications, in which the simplicity of the rotor construction means that there are no magnets or windings to be retained against the high centrifugal forces. Examples include turbine blowers, flywheelenergy storage systems, and machine tools. “Recent volatility in the pricing of rare-earth materials required to make high-performance PM machines has led many companies to consider the use of SR,” says Cummins. “This is most evident in automotive applications where volumes are high and cost targets very low.
SR technology is the best fit for applications that:
• need to offer high efficiency over a wide range of speed and load
• require wide constant power speed range
• require high levels of transient overhead torque
• require high speed operation
• need to allow OEMs to offer a proven technology alternative to differentiate them from their competition.
No solution is perfect, course. The main difficulty so far for SR technology has been noise and torque ripple. These effects are basically a consequence of the torque production mechanism of SR. When rotor and stator teeth align, they produce high radial attraction forces between them. When the two are unaligned, those radial attraction forces drop. The result is pulsating and uneven radial forces on the stator, which tend to distort the stator out-of-round, pushing into an oval shape, for example, if only two poles are aligned at a time. This periodic distortion produces audible noise. To compensate, designers can stiffen the stator by making it thicker than what is needed for magnetic performance, but that makes the stator larger, reducing the torque and power density of the motor.
“Historically, SR motors have been too noisy for traction motors in electric vehicles or hybrid-electric vehicle applications other than mining and construction applications, which don’t care too much about noise,” says David Fulton, director of advanced engineering for Remy International. “SR manufacturers are quick to point out that SR motors can be made quiet, and they are usually right. However, almost everything they do to reduce noise also reduces performance or increases size, so they are ‘squeezing the balloon.’ Who knows what future developments will bring in these motors, but that’s been the story so far.”
For the same amount of torque, an SR motor is typically a bit larger than a permanent magnet motor. Reluctance synchronous motors provide an alternative. In an RSM design, flux “barriers” or magnetic flux insulators produce anisotropy that triggers the formation of poles in the rotor, explains motor designer and IEEE fellow Jim Hendershot, co-author of Permanent Magnet Brushless Motors and Generator Design (see figure 4). The modifications allow these designs to deliver synchronous performance.
Unlike SR motors, reluctance synchronous motors use standard AC stator designs and can operate from a conventional inverter type controller. They use simple rotors with no conductors, and minimal noise. On the down side, it is difficult to fabricate rotors with high structural strength, which limits speed. High pole counts can reduce torque density and power factors. In addition, magnetic saturation reduces saliency, which can also drop torque density.
Permanent magnet and induction motors offer a wide range of capabilities but for some applications, magnet-free motors that operate based on reluctance torque can be the ideal solution. OEMs should investigate all of their options when it comes to finding the best solution for their application.