RSS
Home
Improved AC induction motors power EVs and grounded airplanes
Print
Email
RSS
Subscribe
Hybrid and electric vehicles are in a state of technology flux and will be for a long time. That helps explain why DC motors with precious, rare-earth materials inside their permanent magnets (PM) have been eclipsed by improved AC induction technology for full EV and PHEVs. That may include hybrids before too long; and with good reason. PMs cost a lot of money and are subject to rare-earth supply shortages (read my Neodymium article).
As it stands today, hybrids are powered mostly by DC brushless drives, while most PHEVs and full electrics like the Chevy Volt and the Tesla are driven by AC induction motors. The fact that major automotive manufacturers like GM, Toyota and Tesla have chosen AC rather than DC drives for full EVs and PHEVs says volumes about the technology. Word on the street says the feel of the AC drives resembles the ICE cars we have in our garages now, but with greater off-the-line torque.
According to engineers, though, DC brushless motors produce great torque but have no starting torque when directly connected to what they call “fixed frequency, utility power.” To achieve high torque at 0 rpm, DC brushless drives require the aid of an inverter, whereby the “phase” is maintained in step with the angular position of the rotor to produce that instant torque feel upon rotor rotation.
One problem, according to engineering sources, is that PM motors degrade in performance at higher temperatures and, in some instances, may require a cooling system in the design. And, as noted in my previous article, permanent magnets with rare earth materials like neodymium are expensive; and, many other articles report that supplies are being dominated by China, much like Cobalt was years ago in the Congo.
AC vs DC
It's time for all of us to get acquainted with and understand the EV lingo and the technologies, especially the AC/DC relationship. According to gmvolt.com three main options for series hybrid electric motors are availed to the auto industry. They include:
1) DC Permanent Magnet (PM)
2) 3-Phase AC Induction
3) Multi-Phase AC Induction
For reference sake, permanent magnet motors were initially found in the Prius, while GM‘s EV-1 (now canceled) was powered by AC induction motors. As you may surmise, before too long the technologies behind automotive electrification may get almost as numerous as with internal combustion engines. After all, we've had wedge heads, hemi-heads, fuel injection, carburetion, direct supercharging, turbocharging, diesels, nat-gas, etc. Point: Don't be shocked (pun intended).
AC Induction Motors, the EV OEM's Choice
Not surprising, technology changes may also make this article obsolete very soon; but, the latest available, AC induction motor information tells me the industry is developing AC induction motors with more and more utility. I can almost hear the wheels humming at GM’s Research Laboratory and other places. Even the aircraft industry is involved (more on that at article's end).
Surprisingly, the stators (the stationary part of any motor system) for the 3-phase induction motor and the DC brushless motor are virtually identical, except the DC has permanent magnets. Both have three sets of “distributed windings” that are inserted within the stator core. The essential differences between the two machines are with the rotor.
According to my engineering research, DC brushless motors are especially subject to diminishing returns as motor size grows. That means magnetic losses increase proportionately while part load efficiency drops. With induction motors, when machine size grows, losses do not necessarily grow in concert.
An inverter is still key, though, to powering an AC induction motor from a battery or some other DC source. The variability of the speed rotation becomes possible simply by adjusting the inverter's frequency. In simple words, it’s a variable electric field creation that the rotor literally chases about its axis.
Examiner Final Comments
The idea behind an AC induction motor is to produce the variable-speed performance of a PM motor without the high cost of the rare earth materials. Furthermore, with all the geo-political games being played by China and other nation suppliers of rare earth materials, America and the world would be better served.
Some propose that an AC Multi-Phase motor over a 3-phase unit would be better for EVs, and would emancipate the auto industry from rare-earth jeopardy. Others think the torque gain at the low end (5 mph) wouldn’t justify the cost or the complexity. Their point is duly noted: Electric vehicles already have superior, off-the-line torque than cars powered by combustion engines.
Nonetheless, engineering technology is meeting the challenge to free us from future blackmail. Witness an AC Multi-Phase motor-drive like the new, Chorus(R) Meshcon(TM) motor drive system, with its "mesh-connected" winding. It is certainly unique in the industry.
Instead of a fixed three-phase design, the Chorus' winding configuration enables the inverter to rewire the configuration on the fly, so to speak. That means the software can change the number of magnetic poles, thus simulating as many phases as needed to get the maximum startup torque while maintaining high-speed performance capabaility. Imagine programability up to 12 or perhaps18 phases.
According to the company's website, rather than fighting a counter-acting brake torque, the drag effect from the fifth harmonic, the company's system reprogams the fields to make the fifth harmonic become a boosting torque.
Advantage: At low speed it provides big DC-type torque, as much as ten times more; but, at high speeds, it delivers stellar AC-induction performance. Also think virtual transmission here, one that requires no mechanical transmission parts, no cooling circuit, and no expensive, rare-earth materials.
In fact, that’s similar to the system that will be used on the new 737s to allow aircraft to taxi themselves without external hookups to ground vehicles, and without burning/wasting jet fuel while approaching the terminal area. It's called WheelTug (R) and is supposed to be available the end of 2009. Their site is http://chorusmotors.com/ Cool stuff.
Siemens AC Electric Motor Picture Source: electric-cars-are-for-girls.com (a great resource for getting up to speed with EV knowledge)
Related Articles:
Add a Comment
.jpg)
