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by xeonmc17 hours ago |
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upvoteby sitharus17 hours ago|
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A lack of wear components.

A permanent magnet motor uses permanent magnets on the rotor, but an electrically excited synchronous motor has an electromagnet on the rotor. This requires a rotating electrical contact which has normally been made with slip rings and carbon brushes. These wear over time and need replacement.

Most large electric generators are externally excited synchronous generators using carbon slip rings, so it's a well understood field.

This can be made contactless using inductive coupling and a rectifier - since inductive coupling needs AC but the excitation coil needs DC - at the expense of some efficiency.

You can see the efficiency difference - Renault claim 92% efficiency but permanent magnet motor EVs have touted efficiency over 95% in the motor.

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upvoteby ipbrown4 hours ago|
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I am a little surprised that Renault is only claiming a drive cycle efficiency of 92% (unclear for which drive cycle). It is possible to design EESM with brushless high frequency rotating transformers and rectifiers for WLPT drive cycles with greater than 94% almost 95% efficiency.
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upvoteby Rapzid17 hours ago|
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To a layman that seems like a really small efficiency tax if you can't get your hands on the magnets for some reason.
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upvoteby _kulang16 hours ago|
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It’s a near-doubling of energy loss - probably a healthier way to understand it when the efficiencies are all 90%+
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upvoteby Maxion14 hours ago|
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Funnily enough if enough of that energy loss (heat) can be scavange, this wouldn't be nearly that bad for us living up here in the cold.
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upvoteby antonkochubey10 hours ago|
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In most EVs motors are watercooled, so that energy can indeed be scavenged – problem is, during low-speed driving, the heat output is not high enough to get noticeably above ambient temperature.
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upvoteby tbrownaw15 hours ago|
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You can get about 2/3 as much output power for a given amount of waste heat and cooling capacity.

It's like how laptop power bricks used to be big and get hot, and now they aren't and don't.

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upvoteby handstitched16 hours ago|
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It's a small difference, but if you had a choice between "more efficient AND less maintenance" and "less efficient and more maintenance" then it's easy to see why the permanent-magnet solution is preferred.
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upvoteby namibj16 hours ago|
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The actual alternative is induction motors, which are just a bit less efficient than PMSM and otherwise basically the same. Except that the frequency fed to them isn't exactly proportional to speed.

They've been used to great success since we had the needed power electronics to drive the electric trains of Europe.

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upvoteby mcdonje7 hours ago|
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Another comment said they're not using brushes, so they shouldn't need more maintenance.
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upvoteby snovv_crash13 hours ago|
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You can also make squirrel-cage rotors that are auto-inductive in the sense that they resist slip from the rotating field of the stator. This is also extremely simple to manufacture and doesn't require driving separate fields or anything similar.
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upvoteby adrian_b10 hours ago|
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This is mentioned in the parent page, where it is also mentioned that their disadvantage is a lower energy efficiency than either electrically-excited synchronous motors or permanent-magnet motors.

The lower efficiency means a lower range for the same battery, which is why the companies that have used them in the past, like Tesla, have abandoned them.

Permanent-magnet motors have the highest possible energy efficiency, followed by electrically-excited synchronous motors, than by the induction motors mentioned by you.

Both permanent-magnet motors and induction motors do not contain parts that need frequent maintenance, while this property is more difficult to achieve for electrically-excited synchronous motors.

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upvoteby hulitu9 hours ago|
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> The lower efficiency means a lower range for the same battery,

And some heat which must be dissipated or else they will dethrone the BMW as the leading burning car. /s

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