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Tesla Motors of course looked at both and went with AC induction, citing better average efficiency of induction over BLDC as one of the reasons, cost being one of the others.
Likewise, when torque levels are low, the B field should be reduced such that eddy and hysteresis losses due to B are also reduced. Ideally, B should be adjusted such that the sum of the eddy, hysteresis, and I² losses is minimized. Unfortunately, there is no easy way of changing B with permanent magnets. In contrast, induction machines have no magnets and B fields are “adjustable,” since B is proportionate to V/f (voltage to frequency). This means that at light loads the inverter can reduce voltage such that magnetic losses are reduced and efficiency is maximized. Thus, the induction machine when operated with a smart inverter has an advantage over a DC brushless machine – magnetic and conduction losses can be traded such that efficiency is optimized. This advantage becomes increasingly important as performance is increased. With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. With induction, as machine size grows, losses do not necessarily grow. Thus, induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.
http://www.teslamotors.com/blog4/?p=45
 

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I guess that's the problem with comparing differences of a few percentage points under varying conditions. You'd have to pick your application, pick your power requirements, pick your driving conditions, and then compare different systems within those parameters, which is what Tesla did. You can't just say "X system is the most efficient" if it won't actually work in your specific situation.
 
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