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Bench Talk for Design Engineers | The Official Blog of Mouser Electronics


Move Over LEDs, Electric Motors Will Save the Planet Steven Keeping

LED lighting is the poster child of environmentalists. And they do have a point. According to the U.S. Department of Energy, solid-state lighting is a highly energy-efficient technology, using 75 to 90 percent less energy, and lasting 25 times longer than traditional incandescent bulbs. The department says that widespread adoption could cut U.S. annual energy consumption by the equivalent of 44 large power stations. LEDs are exciting, trendy, and integrate seamlessly into wireless technology. Consumers, who buy lots of the LED bulbs, feel good about doing their part for the environment. What’s not to like?

Perhaps little. Except that, while consuming a significant proportion—around a fifth—of U.S. electricity generation, lighting is far from the biggest energy consumer. That title goes to an indispensable, yet unexciting technology which works tirelessly behind factory shutters, hidden inside white goods, and lurking behind the floor panels of many autos and in a million other nooks and crannies, out-of-sight and out-of-mind of the public. Yet if we’re serious about saving the planet, we need to turn our collective attention to the largest power consumer of all—the electric motor.

Exact numbers are hard to compile, but U.S. Department of Energy figures from a few years ago revealed that electric motors account for about two-thirds of industrial power consumption and around 50 percent of total U.S. electricity consumption. That’s an incredible 2000TWh each year. Lighting comes in at a distant second, consuming about 19 percent. With numbers that large, even a one percent improvement in electric motor efficiency would eliminate the need for well over 200 large power stations.

While the environmentalists might have overlooked electric motors’ contribution to total energy consumption, engineers have been a little less tardy. To be fair, the techies’ motivation is not wholly altruistic—their customers constantly demand smaller, lighter, longer-lasting motors that are cheaper to run (over its lifetime, the electricity costs incurred by a motor are typically 20 times greater than the unit’s purchase cost)—but the result is the same—greater efficiency leading to lower power demand.  

Honing electric motor design

Motor efficiency is determined by how much power is supplied compared with the power the motor generates. For example, if it takes 2W of electrical power to generate 1W of motor power, the unit is 50 percent efficient. The difference (loss) is dissipated in overcoming things like mechanical friction, electrical resistance, and inductive losses. Through many iterations, engineers have honed their designs with innovations such as low-friction bearings, high-permeability magnets, and brushless (induction) firm factors. Contemporary motors boast efficiencies as high as 80 or 90 percent. But a few percent further improvement would have a significant impact on future electricity generating capacity.

Electronic power supplies have also played a major part in the motor revolution. A modern switch-mode power unit produces a three-phase sinusoidal input which in turn produces a rotating magnetic field pulling the device’s rotor around without the use of loss-inducing brushes. In addition, the pulse-width modulation (PWM) superimposed on the base operating frequency enables precise control of parameters such as start-up current, torque, and slip. This precise control of parameters helps to further limit electrical losses.

Now engineers are taking things further:

First, they are favoring high-voltage over traditional high-current designs. This is because nominal motor power is the product of supply voltage and current (V x A). Higher current pushes up the power but also demands the use of larger coils, increasing motor costs and size. High voltages (of the order of 10kV) have the same effect on power but don’t need expensive and heavy copper coils.

Second, engineers are spinning motors faster. Primarily this is because it allows a more compact motor to do the same work as a larger, slower rotating machine, but it also has a small effect on efficiency. For example, increasing the operating frequency limits current ripple—an artifact of the initial rectified mains input and a source of loss—and electromagnetic interference (EMI). High-frequency operation also reduces torque ripple which can cause motor vibration, increased friction, and premature wear.

WBG Semiconductors to the Rescue

A challenge remains; the silicon MOSFETs and IGBTs used as the switching elements in electric motor power supplies are reaching their limits. The problem is fourfold:

 

  • The components’ are unable to handle the higher temperatures that come with more stressful operating conditions.

 

  • Their relatively low breakdown voltage limits how high engineers can push up input voltages.

 

  • Switching losses—caused by residual resistance and capacitance every time a transistor flips from “ON” to “OFF”—increase as the operating frequency climbs (negating efficiency gains elsewhere).

 

  • Due to a long switching time, the devices have a relatively low maximum switching frequency.

 

A savior comes in the form of wide bandgap (WBG) semiconductors. Materials such as gallium nitride (GaN) have a bandgap of 2eV to 4eV compared with silicon’s 1eV to 1.5eV. A band gap is the measure of the energy required to free an electron for conduction in a semiconductor.

Because the electrons of GaN require more energy to escape from an atom and contribute to conduction than those of silicon, the semiconductor is much less prone to unscheduled switching caused by heat build-up rather than the deliberate application of a controlled voltage. GaN also exhibits a higher breakdown voltage than silicon, can switch in about one quarter of the time, and switching losses are around 10 to 30 percent those of a silicon transistor for a given switching frequency and motor current. Finally, because the electrons in GaN are able to move much more freely through the transistor’s crystal lattice than those of silicon, GaN devices can flip much faster. 

Commercial GaN solutions are now dropping in price, making them a viable option for cost-sensitive electric motor power supplies—particularly when the end-customer accounts for the motor’s lifetime energy costs as well as its initial purchase price. But part of the take-up of this energy efficient technology will be driven by customer demand. LEDs are more expensive than conventional lighting, but when running costs and longevity are considered, they work out much cheaper than other forms of lighting. That’s why consumers have adopted the technology. Now, it needs appliance manufacturers to sell the same advantages of the GaN-based electric motors in their washing machines and freezers. That way both the consumers and the environmentalists will really have a positive impact on the future environment of the planet.



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Steven Keeping gained a BEng (Hons.) degree at Brighton University, U.K., before working in the electronics divisions of Eurotherm and BOC for seven years. He then joined Electronic Production magazine and subsequently spent 13 years in senior editorial and publishing roles on electronics manufacturing, test, and design titles including What’s New in Electronics and Australian Electronics Engineering for Trinity Mirror, CMP and RBI in the U.K. and Australia. In 2006, Steven became a freelance journalist specializing in electronics. He is based in Sydney.




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