What advantage do permanent magnets provide that it isn't the case that all motors are made without them?

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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snovv_crash an hour ago | parent | next [-]

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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Rapzid 5 hours ago | parent | prev [-]

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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_kulang 4 hours ago | parent | next [-]

It’s a near-doubling of energy loss - probably a healthier way to understand it when the efficiencies are all 90%+

	▲	Maxion 2 hours ago \| parent [-]
		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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tbrownaw 3 hours ago | parent | prev | next [-]

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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handstitched 4 hours ago | parent | prev [-]

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.

	▲	namibj 4 hours ago \| parent [-]
		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.