Why are Halbach Arrays useful in PM motors?

Question

I understand how Halbach arrays strengthen the magnetic field on one side of the array. I also know that, in a permanent magnet (PM) motor, there is already a limit on the air gap flux you can design for, as beyond a point, there is magnetic saturation in various parts of the stator steel. If you can achieve this flux using a regular array of strong magnets with high remanence, what is the point of the Halbach array? Are they only used for cheaper magnets to get to that near-saturation level of flux? Or is there something I am missing?

Answer 1

If you can achieve this flux using a regular array of strong magnets with high remanence, what is the point of the Halbach array?

None, which is why you don't often see Halbach arrays used in motors.

Actually there is a case for Halbach arrays - when you don't have Back iron. In most motors the magnets are mounted on an iron or steel ring which performs the task of coupling flux between magnets. If for some reason you need to use a non-magnetic rotor material then Halbach arrangement may be necessary to get the desired flux.

In the graph below we see the effect on flux of having Back iron, Halbach, both or none. In this example the Halbach array is better than a conventional magnet arrangement with no Back iron, but worse than a conventional arrangement with Back iron. If Back iron is present then Halbach provides no improvement.

Advantages and disadvantages of Halbach array in BLDC motor

The site above also mentions two other possible reasons to use Halbach arrays - to reduce eddy currents and cogging torque. But this can also be achieved with a conventional design. So why are Halbach arrays so often used in DIY designs? Probably because (as the article suggests) it avoids having to machine steel parts.

Answer 2

A Halbach array motor, no steel:

The shape of the magnetic field in the Halbach array is such that it can close its path if two rotors are used. In a classic motor, you have to use a ferromagnetic material both for rotor and stator to be able to close the magnetic path.

With only one Halbach rotor, you would still need a ferromagnetic stator, with two such rotors, even stator can be made coreless.

As you can see, the flux can have its complete path. In the pictrure there are two Halbach array rotors, in the mid part the stator is inserted.

At first look, it is a miracle of science, since it is lightweight compared to steel rotor and stator. The drawback is that magnets still need a support, could be a aluminium plate, carbon,...the stator also need a support, like epoxy resin. Without the metal parts also the heat dissipation is low, so it tends to overheat.

Yet another coreless motor, full building details

Answer 3

I've actually been analyzing the benefits of using a halbach array with a FEMM modelling tool recently, so you might be interested in my findings.

I've been using FEMM(Finite Element Magnetic Modeller) to analyse Halbach arrays because I want the best bang for my buck in my hobby motors, and halbach arrays are great, especially when you're building lightweight, can't machine your own iron cores, or where field strength per magnet cost is an issue for you. Air is harder to compress field into, but it doesn't saturate the way iron does.

Note that all the magnets in a halbach array try to push apart, so assembly is a big challenge, especially for larger magnets, and if you "Push" the fields of the magnets against eachother and move them too much during assembly(Goal is to smoothly put the magnet in place and never have it move again relative to neighbouring magnets) they can be partially demagnetized.

My experiments have been to establish the strongest possible magnetic field at the center of the pole face(Prime area because flux lines are perpendicular here, so I'll present some of my findings.

First, at least one complete magnetic circuit must be present for maximum field projection from the magnet, so I analysed the center two poles of a set of four poles modelled. All magnets are N40.

First a conventional pair of poles with no iron back.

Next a conventional pair of poles with iron backing note the backing is as thick as the magnets and still almost saturated. Note that the strongest parts of the field are concentrated center between the poles where they do little good.

Now a pair of poles in a halbach arrangement using the same mass and strength of magnet: Note that in the halbach arrangement, not only is the overall field stronger, but the stronger parts of the field are moved closer to center pole face where they are more useful to a rotating machine.

Finally a pair of halbach poles with iron backing. Note how little of the iron is used and therefore how little would be required to contain the magnetic field in situations where that is an advantage.

Finally because the most relevant for my project is field strength at a given distance, and center pole field strength matters most, I prepared a side by side comparison in the same order and re-colored several bands for illustration purposes. Horizontal lines are 1mm each from the face of the magnet. You can see that the normal magnets with no iron backing performed the worst. Adding an iron backing pushed the same field strength out twice as far, from 2 mm to 4mm by completing the magnetic circuit where possible. The halbach array, even with no iron back, because of the cancellation and short circuiting of the field on the bottom side and reinforcement of field on the top side, outperforms the iron backed array by 1.5mm and predictably sees only a tiny improvement when iron is added.

Note that thus far I've only evaluated back side iron and single sided arrays, but you can see from the simulation that the halbach array gives not only improved overall field strength and range, but has its field concentrated more usefully distributed compared to an equal mass of conventionally arranged magnets, with or without iron backing.

I've also simulated sandwich type arrangements where the air gap to magnet ratio is improved further, but I see that was covered in another answer so I won't add it here.

I'm also working on simulating an 8 pole radial flux halbach array based on 3/8" by 3/8" by 3/4" N40 magnets to see if the little triangles of space between the square magnets become problematic without iron.