Switched Reluctance: How Intelligent Motor Control Improved a Technology

Jennifer Calantone, Marketing Executive • Striatech
Sabrina Latimer, Business Development Associate • Striatech

Companies that ignore innovation and avoid technological progress are the ones that fall behind. In an increasingly connected world driven by smart devices consumers seek sophisticated products that maximize efficiency and ease of use, and the electric motor industry is no exception. Those seeking to buy and use an electric motor typically seek high efficiency, wide variable speed range and compactness,  characteristics that make a product drastically better but result in a high price tag.

End users are often stifled by cost, particularly in terms of purchasing and maintaining regular induction motors. With China’s stronghold on the rare-earth metal market, a vital component of permanent magnets used in induction motors, prices are often quite volatile. As a result of its unpredictable pricing, cumbersome design and unimpressive efficiency, the motor industry has sought a simpler, cheaper and better alternative. To meet the market demand for smart features, the solution would have intelligent software design paired with a robust motor.

Fortunately, the industry won’t need to reinvent the wheel; this perfect alternative has been around since the 19th century. Switched reluctance (SR) motors are price stable, low-maintenance and high-powered, with the flexibility to cross multiple applications. When paired with the right controller system, they are the ideal replacement for traditional induction motors.

How a Woodworking Equipment Manufacturer Became a Motor Company… by Accident
            “It is not the application that we need to enhance with
            expensive ‘bolt on’ electronics. It is the most integral and
            basic part of our lathe that shows limitations and that requires
            improvement, a motor which is currently inefficient and unintelligent!”

Striatech’s intelligent 1,250 W SR Motor and Controller

Striatech’s intelligent 1,250 W SR Motor and Controller

As the owner of a small woodworking equipment manufacturing company called Teknatool, Roger Latimer saw other technologies rapidly advancing to remarkable levels of intelligence and efficiency, while wood lathes and most other woodworking machines were lagging behind. At that point, nearly two decades ago, “dumb” motors ruled the market.

The innovation-driven team at Teknatool began the search for a direct drive that could offer a combination of low RPM and high torque power. They found that traditional electric motors were unable to deliver that without a complex and inefficient drive train. Because Teknatool did not specialize in electric motors, they sought a low-cost, simply constructed, uncomplicated motor that was easily integrated into their wood lathe. It quickly became clear that switched reluctance was the obvious choice. The team’s research found the SR motor to be quite intelligent in its very operational heart, and when combined with a modern and powerful computer chip, the solution would be ideal.

With enough refinement of the electronic components, it could bring a large deal of intelligence and therefore novelty custom features to not only their woodworking machines but a wide variety of applications across industries.

The result? Teknatool developed a control system for the SR motor that would go on to power its woodworking machines. This technology, Digital Variable Reluctance, has been the muscle and the brain to its applications for now 15 years. The intelligence and the transformational features provided by this control system soon piqued the interest of Teknatool’s competitors and even beyond that, original equipment manufacturers in other industries. In 2012, Teknatool founded a solution-driven motor systems venture called Striatech, which now offers off-the-shelf motor systems as well as customized motor solutions for a vast variety of applications.

The Technology Behind Switched Reluctance
The SR Technology boasts the simplest construction and is free of permanent magnets, leading to low production costs. The motor has no brushes or electrical connection to moving parts, and no current flow in the rotor, leading to less wear and a much longer life than an induction motor. SR motors’ inherent characteristics enable a range of speeds and high torque resulting in high efficiency.

The SR motor is an electric motor in which the torque is produced by the tendency of the rotor to move to a position where the inductance of the excited winding is maximized. When power is applied, each stator phase is excited while its inductance increases and unexcited as its inductance decreases. The air gap is at a minimum at the aligned position, and the magnetic reluctance of the flux flow is at its lowest.

To operate an SR motor, current is “switched,” among the phase coil windings in a sequential pattern to develop a rotating magnetic field. “Reluctance” refers to the resistive property of a magnetic circuit, also termed magnetic resistance. When a phase circuit, or pair of stator poles, is energized, the rotor moves to align with the stator poles. The direction of rotation is independent of the direction of the current flowing through the phase. Rather, it only depends on the sequence of the stator winding excitation. A clockwise field will produce rotation in the opposite direction.

Pure magnetic force is not related to rotational speed, so this motor can generate high magnetic force on the rotor pole’s high torque. Unlike induction motors, the magnetization of the rotor is static, which is why the motor knows the exact rotor position at all times. This allows the SR motor to be capable to stepping arbitrarily quickly or slowly. As a result, a control system is incredibly valuable in operating a switched reluctance motor, as it is responsible for shooting the sequential pulses to power the phase-activating circuitry.

SR has numerous compelling advantages over induction (and other) motors. With its salient rotor without rotor windings, SR motors have reduced material costs. Furthermore, independent windings make the fault tolerant operation possible and provide a robust structure. This robust structure decreases power consumption, as the windings are energized and de-energized only when required. It also has high torque-to-inertia ratio and high starting torque without the problem of in-rush current.

Where Mind Meets Motor
The root of the name “Striatech” is striatum. This is the anatomical part of the human brain that is responsible for directing movement of muscles. Neuroscientists call it “motor control”, a concept befitting of a smart motor company. With that, Teknatool’s new motor solutions company would be called Striatech. In this context, the controller serves as the mind and the motor as the muscle, providing the motor control for the respective application and offering a complete system-based solution. Striatech, in its most literal sense, is where mind meets motor, enhancing large varieties of applications with unprecedented intelligence.

The characteristic benefits of SR over “premium efficiency” induction motors include a superior performance such as a greater torque output and the same or slightly higher efficiencies. They were capable of accelerating to and running at high speed of more than 100,000 RPM. SR motors operate by pure magnetic attraction, and therefore have no natural speed or direction. Both can be controlled by the firing sequence of the phase coils’ magnetic fields, enabling the motor to maintain speeds from 30,000 to more than 100,000 RPM in either forward or reverse. These features have kept SR in the marketplace, despite a negative reputation surrounding torque ripple and noise. However, with Striatech’s controller technology, that torque ripple and noise virtually disappeared, plus, it extended the range of SR’s already intelligent capabilities.

Striatech’s smaller 1,000 W motor.

Striatech’s smaller 1,000 W motor.

Striatech’s vibration free and therefore very quiet motor is able to control torque and utilize electronic braking even from top speeds. This results in smart safety features in applications. In case of a sudden spindle lock or unexpected sharp increase in load the motor is able to shut off power to the spindle without any damage to the parts. By knowing the exact spindle position due to its micro-computer, the Striatech motor can apply more or less power to maintain a constant rpm despite varying load. A Striatech drive has the ability to turn off and on with no power spike, providing a low-voltage start and no wear on the motor when turning it off or on. Its electromagnetic boost technology allows the motor to produce the highest torque at the lowest speed and has a flatter torque curve over a wide range of speeds, as torque is not dependent on speed. This unique characteristic means that the motor can be mounted directly to the output shaft (direct drive), which allows it to achieve better output by eliminating the 20 percent of power lost in a traditional gears/belts and pulley drive systems.

Making SR Smarter with Smart Sensors
In contrast to induction motors, SR is able to provide controller feedback communication without the need for a shaft sensor like an encoder. The controller needs to know where the rotor poles are to fire the phases at the right time to minimize torque ripple. Taking it one step further, the latest generation Human Machine Interface that is used in the Striatech technology has ten sensors that can integrate to minimize energy use and simultaneously add maximum intelligence. Striatech motors use four quadrant operation to accelerate and decelerate. Due to the constant communication between the application and the control system real time information for adaptive control is being transmitted which prevents over magnetization of the circuit to satisfy the torque load. This is a huge advantage for applications that are torque dependent because the system can adjust its operation to their load and can read the environment to maximize efficiency.

Powered by an STM32 Microcontroller, the control board offers a well suited programming environment for development and prototyping with the Striatech motor drive. With the help of its 32-bit microcontroller, the HMI allows for fast processing while offering control and monitoring of the motor. The microcontroller has a 72 MHz max CPU and 128 kb flash memory, 20 kb RAM, three 16-bit timers and two ADC conversion units. It’s compatible with several developing environments, and has one reset output, 12 programmable I/O and nine expanded I/O. With the expanded I/O port, you can add sensors or other I/O to the HMI.

The micro-processor control board is capable of interacting with the operator, across systems and via the internet. The interface also offers built in wireless and standard protocols via USB.

Transforming Applications: A Lathe and Drill Press Re-Invented
Teknatool’s wood lathes are tools mainly used for hobbyist and artistic woodturning projects, including the creation of bowls, pens, spindles and a wide variety of art work. In the past wood turners were burdened by inaccurate speed selection, excessive vibration and noise from old AC motors that were used to power their lathes. A large number of safety risks were often the result, turning the art of woodturning into a hazardous task. When Teknatool, under the NOVA brand, started integrating Striatech drives into their lathes, they did not only formulate a method for a better turning experience, they also helped the woodworking industry to advance from the technological dark ages.

NOVA’s 1624 Lathe with Bolt on Striatech Motor

NOVA’s 1624 Lathe with Bolt on Striatech Motor

The Striatech motor and controller allows users to choose a specific speed (and save eight different “favorite” speeds), and utilize the machine’s recommendations based on the workpiece’s size and shape. The lathe is fully equipped with safety features, made possible by the motor’s ability to sense load changes and apply electronic braking. Not only that, but the lathe can be password protected to keep children’s hands away from it unless supervised. The most heralded change to the woodturning world, in regards to the Striatech smart motor, is the quiet, vibration-free experience, which enables turners to work a lot more accurate, resulting in flawless artwork. NOVA started offering the motor as plug-and-play replacement motor, which was so simple to use, customers are able to swap their old lathe motor out for the Striatech motor without professional help.

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Nova’s Voyager DVR Offers Numerous Smart Features Due to its Integrated Striatech Motor

The transformation that the Striatech drive managed to perform on NOVA’s wood lathes drew Teknatool out of their comfort zone, designing a wood drill press as well as a more robust metal drill press. Before the use of Striatech drives, the traditional drill press technology required a great deal of work in order to ascertain enough power to complete a job. This has resulted in major issues with safety, reliability and efficiency. Society is fairly well-equipped with technology that keeps them safer and tools that help them save time. However, the world of power tools lags dramatically behind other industries in terms of technological breakthroughs.

Apart from their common safety issues, traditional drill press systems require management of belts that help the operation of the device in terms of speed modification. This is a major time-loss for those who are trying to find the right speed for their drilling. These belt adjustments, even for the most experienced operator, can slow a project dramatically.

By implementing the Striatech motor into a standing drill press, Teknatool has created a tool that can revolutionize workshops across the world. This tool is branded as the NOVA Voyager DVR. When applied to the drill press, a Striatech motor gives the user built-in safety features, including a highly visible emergency stop button, load sensing capabilities that allow the motor to adjust appropriately, slow start speeds to help the operator ease in to the workpiece, and more. These key features aid in limiting and preventing injuries that are common in a more traditional drill press.

Along with these safety features, numerous convenience features complete this next-generation drill press. With the variable speed motor, the user can easily adjust speed through the digital interface. The user can also take advantage of speed settings, and utilize the speed recommendations offered based on the bit and the material being drilled. There are no belts to change, speed adjustments are instantaneous which improves the user experience.

This powerful drill enabled Striatech to venture into the market of metal working but applications this intelligently controlled switched reluctance drive can transform are vast and from very diverse industries. The Striatech drive is a perfect fit for a variety of applications in manufacturing, building maintenance and agriculture, namely in the field of fluid handling, motion control and the appliance industries.

For more information, contact Striatech at www.striatech.com or info@striatech.com.

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