Comment by userbinator
2 days ago
A historical pioneer in the complex technology of electric motors without magnets
Those who know the history of electric machines will find the title and verbiage very amusing. Motors with no permanent magnets were the first practical ones, and at this point wound-rotor motors are over a century old.
It's worth noting that some of the biggest motors have always been designed this way, because the size of magnets required would make them both too expensive and dangerous, and still not powerful enough for their size; a field coil can generate a field that's only limited by the current and resistive heating of the winding, but rare earth magnets have fixed limits on field strength.
Long ago, when I was in Cub Scouts, one of the projects was to build an electric motor. The parts list was:
1. a plank to form the base
2. several 6 inch nails
3. wire
4. a tin can (as a source of sheet metal)
5. tape
No magnets. But it worked perfectly fine when connected to a dry cell. Adventurous science lad that I was, I decided it would work better when connected to AC. So I attached a power cord and plugged it in.
A loud vibration ensued, and then it burst into flames. My mom wasn't happy.
P.S. I still use tin cans as a source of sheet metal. There was a big storm a while ago, with tree branches whistling by at high speed. (Not a good time to be outside.)
Three holes were punched in the house by the branches, 1-2 inches in diameter. What to do, what to do. I took a coke can, slit it and unrolled it into sheet metal. Then cut a disk bigger than the hole, and epoxied it into place. Worked like a charm, and cost nothing.
I've used coke can metal for shingles and flashing, too. They don't rust.
I like that story. I fixed a microwave door latch with a beer-can shim and some decorative ribbon; we used it another 11 years.
there's also a plastic liner on them that I'm sure helps.
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That 60Hz sound is a sure sign we did something very wrong. By the time you hear it it’s usually too late to say “Uh oh”
One of my favorite sayings:
"Good judgement comes from experience; experience comes from bad judgement."
I commend your excellent use of bad judgement there, WalterBright (despite your mom's lack of enthusiasm)!
Username checks out.
Been there. Im gonna guess that 90% of HN folk have similar stories to tell.
The Cub Scouts in the 1960s were a lot of fun. Each den meeting involved a project. The other one I remember was we each built a kite from scratch.
Mine was a bit fragile, and the first gust of wind shredded the sticks and plastic film.
But it was still fun!
As a teen I built a flame thrower. No, I'm not going to explain how to build one. My dad told me that God looks out for little boys, because otherwise they'd never survive to adulthood.
When I was 9, I found a book of his "Rocket Manual for Amateurs". The opening sentence was something like "if you're fascinated by things that burn and explode, this book is not for you." Who could resist a teaser like that? I promptly read it cover to cover. He wouldn't let me buy any of the necessary materials.
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You're right about the verbiage being amusing.
All big generators have an exciter coil that is used to generate the magnetic field. It has the advantage of allowing voltage regulation through adjustment of the field, rather than after the fact, which would be far less efficient.
In both motors and generators, there is an efficiency hit related to the need to supply power in order to generate the field, but when you scale up the system, it actually becomes more efficient to use the electromagnet. With the rare-earth mineral shortage, it makes even more sense.
> field. It has the advantage of allowing voltage regulation through adjustment of the field, rather than after the fact, which would be far less efficient
That and not having huge strong magnets is nice when doing maintenance.
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.
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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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.
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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Not quite true: you're also limited by the mechanical strength of your windings and core (this is the upper limit on superconducting magnets like at CERN and in fusion plants).
And if you also ignore iron saturation.
Verbiage? What about the _nounage_?
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