By Paul Sharke, Associate Editor

Like a balky child pushing against other kids on a playground merry-go-round, an induction motor's fifth harmonic works against its fundamental frequency.

According to Chris Bourne of Chorus Motors plc, 120 electrical degrees separate each leg of a three-phase machine. Because the three legs wind around the stator, there are actually six distinct regions, or pole/phase groups, making up the revolving field. Sixty electrical degrees separate one pole/phase group from the next.

For the fifth harmonic of the incoming current, 300 electrical degrees separate the pole/phase groups, Bourne said. That's five times the separation of the fundamental phases. But 300 degrees is really minus 60 degrees for a periodic function, Bourne explained.

So adjacent phases of the fundamental and the harmonic frequencies are exactly opposite. For a three-phase machine, that means that the fifth harmonic creates a field of the same number of poles as the fundamental, but with a negative phase sequence, Bourne said. And that field rotates counter to the fundamental, at five times the speed.

Similar arguments apply to other harmonics, Bourne said. In a three-phase machine, multiples of the third harmonic are neutral, producing no negative effect, but making no contribution either. Harmonics 5, 11, 17, and so on produce reverse sequence currents. Harmonics 7, 13, 19, etc., produce nonsynchronous forward sequence currents, he said.

"Ask any motor person in the world what happens when you run a motor on the fifth harmonic," said Isaiah Cox, CEO of Chorus. "They will tell you it runs in reverse at five times the speed. It is gospel."

A prototype motor from Chorus blasphemes, then. When powered by the fifth harmonic alone, it rotates in the same direction that it does under the fundamental. Cox, along with motor inventor and Chorus's research director, Jonathan Edelson, demonstrated this recently at an industrial warehouse on the outskirts of Boston.

The demonstrator machine winds a standard 36-slot stator with 18 coils to create a high-phase order, two-pole motor, Edelson said. "Every harmonic that enters the machine up to the phase count will produce a rotating field that is synchronous with the fundamental," he said of the prototype.

Harmonics are a particular concern to users of adjustable frequency, or inverter, drives, Edelson said.

Who Will Stop the Reign?

The lowly ac induction motor, long the prime mover of choice for most applications because of its rugged, simple design and an innate dislike for coddling, can vary its speed according to the frequency of the ac electricity that feeds it. An induction motor rotor, Edelson said, "is simply short circuited loops of wire with no electrical connection to the outside. It doesn't need permanent magnets. It doesn't need electromagnets. It doesn't need slip rings."

Because the ac induction motor embodies such elegant design, it has become the workhorse of industry. Technology has followed by devising better controls for it. Hence, the invention of the inverter drive decades back, Edelson explained.

The Chorus demonstrator motor, shown on the dynamometer, was wound in high-phase order configuration from a standard three-phase motor. Motor leads were left accessible for experimentation. On the right, a cradle-mounted three-phase induction motor with regenerative drive serves as an absorber.

An inverter controls the speed of a motor by converting ac power to a series of dc voltages, then varying the frequency of the dc pulses to approximate a sinusoidal ac waveform. Inverter designers have worked so hard at making faster switching that today the current waveform going from inverter to motor is indistinguishable from a true ac sinusoid, he said.

An inverter produces harmonic-rich power because of fast switching that creates voltage spikes, Edelson explained. Ordinary line-frequency harmonics, while capable of causing havoc with sensitive electronics, don't bother induction motors much. But harmonics generated by an inverter drive certainly do, by lowering a motor's operating efficiency. Or if a drive is designed to reduce the formation of harmonics, the cost of its electronics shoots up.

"The technology to date has been to use the same old motor and improve the fidelity of the inverter output," Edelson said. Variable-speed drives are optimized for three-phase motors. Chorus has made a fundamental change to the old motor. "We're looking to break the three-phase paradigm," Cox said.

The Chorus motor actually benefits from a harmonic-laden waveform, Edelson said. Since all the harmonics up to the phase count contribute to rotor rotation, a motor can be made more efficient by as much as 3 percent. To lay ears, that might not sound like much. But the typical 10-hp three-phase motor is already 90 percent efficient, he said. A 3 percent efficiency gain equates to a 30 percent reduction in the losses. From the start, users of high-phase order machines are using less electricity.

"Those are real dollars saved," Cox added.

What's more, a low-cost inverter, whose output can be rich in harmonic content, is actually the preferred drive because it will get more from a given motor. For most continuous applications, the cost of the motor and the drive are about the same, Cox explained. If you could reduce the cost of the drive by worrying less about the fidelity of the output, more applications could open up to inverter controlled motors.

"We are not looking at fixed-speed applications," Edelson said. Instead, the company is focusing on uses that could benefit by going to lower-cost drives and variable-speed ac motors. Traction motors are a big target. Diesel-electric and all-electric locomotives for freight or commuter rail use traction motors, as do electric vehicles, conveyors, and assembly lines.

The hybrid Honda Insight, for instance, uses a costly dc brushless motor, Cox said. An ac induction motor controlled through a low-cost drive could offer a cheaper traction source, he said.

Traction motors are not sized by their continuous torque, Cox said, but by their peak torque. When it comes to using that peak power, the more closely the inverter waveform approaches a sine wave, the less time the electronics are operating at peak levels. A square wave—cheaper to produce but loaded with harmonics—remains at peak amplitudes far longer than a sine wave does, Edelson explained. In the case of the Chorus motor, the more harmonics the better, because, up to the phase count, they are adding to rotation rather than taking away from it.

During the demonstration, Edelson and Cox ran a locked rotor test on the Chorus motor. "Watch the load cell carefully," Edelson warned, saying it would display a maximum torque value, then drop quickly as the motor tripped off line. The meter peaked at around 259 ft.-lbs. before the breakers kicked out. Compare that, Edelson said, with about 90 ft.-lbs. locked rotor torque for a conventional 10-hp three-phase ac induction motor, using a class B (80°C) temperature rise. For traction motors, it's locked rotor torque that casts the deciding vote, he added.

Heat is the important consideration, Bourne explained. For the short demonstration test, the temperature rise was only 20 to 30°, he said. "Anybody can get massive torque out of a motor just by running it very hot, but this will quickly destroy it," he said. "Inefficiency displays itself in overheating. The unused energy has to go somewhere. If your energy use is more efficient, you don't have as much heating. That's why the 30 percent fewer energy losses give us such a big margin when it comes to pushing the motor hard at startup," he said.

A Long Time Coming

Chorus Motors is a subsidiary of Borealis Exploration Ltd., originally a Canadian mining company now based in Gibraltar. The company began exploring high-phase motors as a way to increase efficiency in mining operations, Edelson said. It soon realized, however, that the Chorus motor had applications well beyond the mines.

The company doesn't manufacture motors, however. For that reason, it is talking with various motor makers and industries about technology licenses.

From a manufacturing perspective, the new motor is not far from a conventionally wound induction motor, Edelson said. Any motor maker could adapt its manufacturing expertise to the new design and come up with a product that stands out from the commodity parade of three-phase machines.

Still, if the Chorus motor is truly going to change the game as it's been played for the last hundred years, what's taken it so long? The first shift came quickly, Edelson said. Early induction motors were two-phase machines. Then, engineers realized that both two- and three-phase current needed three wires anyway to flow. From there on, three-phase machines dominated. Three-phase induction machines were, and continue to be, wildly successful, Edelson said.

To be sure, drive technology itself has had something to do with keeping the nearly virgin terrain of high polyphase motors unexplored, Cox said, at least for the last couple of decades.

Chorus uses field programmable gate array, or FPGA, control chips in its inverters. They offer the capacity to control many more channels than digital signal processors, or DSPs, which have been the standard in the motor control industry, Cox said. Because of their limited number of channels, DSPs made it extraordinarily inconvenient for motor developers to get off the three-phase habit.


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