Brushless motor design with additional separation layer at the air gap

Question

As a thought experiment, I was thinking about how to actuate an passive impeller inside a tube from outside, contactless. Imagine a tube equipped with an impeller inside it. The impeller is mounted in a passive way such that it can freeley rotate around the tube's axis. That setup can be used to generate air or water flow for example but the impeller has to be actuated in order to do so.

What I was thinking about is to mount permanent magnets at the impeller's tips and surround the tube outside with a stator containing many electromagnets.

The interesting question here is how does such a setup behave knowing that additionally to an air gap (in the case we want air flow generation), we have now an additional layer of material separating the rotor and the stator (the tube's material).

Does the tube's material play a role? If yes, what are the properties of that material that have an effect (magnetic permeability? magnetic capacitance?, thickness?), I can imagine that it would behave differently when using PVC, Stainless Steel, Iron, Copper.

How does the performance compare to a normal brushless motor? Which affects the system's performance more: the increased distance between stator and rotor, or the additional material layer between them?

Do such actuation systems already exist? If yes how are they called?

Answer 1

Yes, these drive systems exist.

If the separation material is non-conductive, then the effect is no more than an additional airgap.

The effect of an increased airgap is to require more H field (longer or stronger magnets) to get the same B field in the gap, or conversely you get a lower B field for the same expenditure on magnets. This results in a different Kv, and a small reduction in efficiency.

If the separation material is conductive, then it will slow the rate of change of magnetic field due to circulating currents within it, limiting the maximum speed of the motor, and it will get hot, reducing efficiency. Stainless steel is the metal of choice, as it has a much higher resistivity than copper, and high alloy grades are essentially non-magnetic.

Answer 2

In the event that the sleeve is stationary and attached to the stator, PVC, ceramic, or other nonconductor will have the same effect as an increased air gap. A magnetically permeable material will act as a flux path for the stator teeth "shorting out" the field and significantly degrading motor performance. A conductor like non-mag stainless steel or copper will work, but eddy currents from the rotating magnetic field will lead to losses in the tube and some heating of the tube, so if you use this approach non-magnetic stainless is preferable to copper because of its higher resistivity.

Answer 3

There are drive systems that have a rotating magnet that couples with the driven magnet on the other side of a stationary casing - Here's an example of one from Iwaki

Alternatively the rotating field can be generated by a stator assembly, exactly like any regular motor, with either a non-conducting sleeve, or a low-conductivity metallic sleeve. The advantage of the metallic sleeve is that they can be far thinner than most non-conducting sleeves, so the additional airgap penalty is small, but is balanced by eddy current losses. On small water pumps for circulating water heating systems, the sleeve is often brass, and the rotor is a typical induction machine rotor. For more exotic (higher speed or high power density) applications, such as in aerospace, steel or nickel alloys are used.

Here's one used in an aerospace application. The sleeve (34) between the stator (28) and fluid-filled bore in that application is a PEEK material (if I remember correctly). This one is a BLDC motor with a pump attached that circulates glycol coolant.

Answer 4

The pictures below show a pump similar to the design described in the question. The pump impeller is connecter to the rotor. The rotor is a permanent magnet. The nails held by the rotor demonstrate the location of the magnet poles. The impeller and rotor turn on the shaft which is supported by a structure in the center of the plastic ring that directs the inlet water. The opposite end of the shaft fits loosely in the motor housing. Water is discharged radially through slots in the pump housing.

Motor Rotor and Pump Impeller

The motor stator is completely encapsulated inside a plastic housing. A tube molded into the housing extends through the stator. It presumably fits tightly through a hole in the stator iron. The "air gap" consists of the thickness of the plastic tube and the water between the tube and rotor. The shape of the stator housing suggests that the coil is wound around the stator laminations off to the side and the laminations extend to surround the rotor to form a "C-frame" stator.

Motor Stator and Pump Housing

This pump is used for an aquarium filter. Pumps like this are probably sold in a lot of retail stores that sell pet supplies.