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AC Synchronous motor is actually quite a wide statement, as it can be a PM, reluctance, self-excited, seperately excited motor and still be synchronous. Actually, their is about 3 reluctance type motors. Just branding it as synchronous doesn't really tell you much. It most probably is a self-excited synchronous motor (the type used in most generators), but that still leaves a couple of choices on how the excitation works. These motors generally have a field winding on the rotor, which would be excited by a small generator on the shaft (can be 3phase, in which case the shaft would also carry the rectifiers). This is the setup used for most gensets (which is basically a synchronous motor driven by a machine or engine).

In my opinion, if it is a self-excited synchronous motor, it loses out against both ACIM and PMSM, as it is a very good generator, but it has lower pull-out torque than either of the other types. I prefer more torque rather than better regen.

Dawid
 

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I thought the Leaf was using a BLDC motor but I've seen references that it's a DC excited synchronous motor that does not use rare earth magnets, I'm not familiar with those. Anyone know more info?
What you're talking about is also known as a wound rotor synchronous motor. They're similar to sepex motors in the sense that the field strength is directly controlled.

Typically they have high efficiency across a wider RPM range than PM motors but slightly lower peak efficiency.
 

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This is probable. Renault guys officially confirmed they will use wound rotor synchronous motors in their cars. So Nissan would.

Is the control side any easier than an ACIM?
Generally - a bit. With separately controlled field, you can operate this motor at PF 1.0 all the time and avoid some calculation steps in control algorithm. But hardware is more complicated (field controller and its power stage) and costs more. This is offset to some extent by greater power density of a synchronous motor when compared to an IM.

Just thinking of reasons Nissan might have chosen it over ACIM.
I don't think that control difficulties have played any role. AC IM control is well established and widespread.
 
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