Vatiable field magnet motor

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Gow864
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Vatiable field magnet motor

Post by Gow864 »

Not sure if this has already been seen, but i thought I'd post it anyway

Variable Field Magnet Motor

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Post by Electrocycle »

it's field weakening for permanent magnet motors :)

A nice way to get a wider usable rpm range out of a PMAC motor for a given voltage.
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Post by Gow864 »

Torque and high speed from the one motor? A bit of engineering in it though.
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Post by Electrocycle »

yeah it certainly adds complexity!
It's usually easier just to have a higher voltage battery pack :)
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Post by Tritium_James »

Electrocycle wrote: yeah it certainly adds complexity!


Yes, and in the wheel too! It's unsprung weight, which is bad.

It also plays havoc with any sensorless control algorithms, as the motor inductance changes with the airgap setting.

The benefit, especially for solarcars, is that you can trade off copper losses vs iron losses, and get the motor at the optimal efficiency for any operating point.

On the other hand, if they had an even more advanced motor (ironless) there's no iron losses in the first place, so no need for the adjustments...

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Post by Electrocycle »

hmm yeah that's true!
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Post by coulomb »

Electrocycle wrote: yeah it certainly adds complexity!

Surely it would be vastly easier to make the rotor an electromagnet, powered by slip ring brushes, so the field could be controlled like a separately excited DC motor.
It's usually easier just to have a higher voltage battery pack :)
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Post by Electrocycle »

when you're going for maximum efficiency though permanent magnet is the way to go...
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Post by BG »

Very cool actuation..
In that example, chances are it would work nearly as well with a higher voltage pack.. Assuming its actually radial flux, the b-field isn't reduced in the remaining (unchanged) airgap, sliding the stator out is just reducing the flux linkage to the stator (coils) which give a lower back EMF...
So the laminations (and in particular, tooth tips) that remain beneath the magnets will see exactly the same flux densities, so wouldn't necessarily increase efficiency. Though it might allow for a design that is slightly less compromised during normal operation by the higher speed operation. ah need to think more.. but it wouldn't be my first choice. neat, though.

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Post by bga »

It's a good example of a problem with PM motors.

My favorite bit had to be the "overunity" motors following it. There's obviously something magical about all those magnets spinning around.
Mostly they seem to need a crank to start them! Image

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Post by Lowly »

James,

Just curious about the sensorless comment.

Are you guys, or anyone else you know, using sensorless control for traction applications?

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Post by Tritium_James »

Yes, our solarcar controller runs sensorless, but only once the vehicle is moving faster than about 1.5km/h. You can't use sensorless from a standstill in vehicle applications unfortunately, you need some back-emf to work out what's happening with the motor. For a fan or a pump it's OK, you just start and see what happens. The problem is the first few phase steps might move the motor backwards a bit before it goes forward, this obviously isn't OK in a vehicle! If you can start with really low torque it could also work, but this can't be guaranteed in a car - you might be doing a hill start. So we start in 6-step mode using the hall effect position sensors in the motor, and once moving at a few Hz, change the modulation to sinewave and go sensorless.

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Post by a4x4kiwi »

ACmotor and myself are using sensorless control on our vehicles. Feedback would be nice but it is very expensive and not really necessary. Mine is road legal and now with about 300km since rego running fine. My blog in my sig has details.
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Post by Johny »

I am using sensorless and while it is not in the vehicle yet I have done primitive torque tests at stall with no strange results so far.
a4x4kiwi, acmotor and I are all using industrial VFDs.

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Post by Electrocycle »

sensorless is a different ball game when it's a synchronous motor rather than an induction motor.
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Post by Tritium_James »

Yeah, sorry, I should have made that more clear - I was talking about synchronous motors (BLDC) with permanent magnets, that's all anyone uses in solarcars.

Sensorless with induction motors is definitely possible.

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Post by Richo »

Tritium_James wrote: The problem is the first few phase steps might move the motor backwards a bit before it goes forward, this obviously isn't OK in a vehicle!


Actually I see ICE cars roll back quite regularly before going forward.
So this effect might feel more natural for them when driving an eV Image

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Post by Tritium_James »

The problem is that it isn't a 'roll' backwards, it's a 'drive' backwards - at whatever torque you've got your accelerator pedal pressed to!

Now, one or two poles (worst case) backwards on a high pole count BLDC motor, then through a reduction gearbox, may actually not be significant, it may be too small a rotation at the wheels to actually feel it. This would be nice, the hall sensors (in a similar way to an encoder on an induction motor) are by far the most delicate part of the motor. Going completely sensorless would be fantastic.

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