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As a background to this project, I have taken an off the shelf bldc stator and incorporated it into a small pump, and then built my own rotor using discrete neodymium magnets. At first, everything went fine, and experimentation showed that higher magnet levels increased efficiency proportionately. I then purchased custom n52 magnets that maximized space. However, I noted that the motor then ran funny. Throwing the setup on the scope like thistest attachments

and spinning the rotor with a drill showed this odd waveform. No steel core After adding a small steel core, note how this changed.enter image description here

femm plots:

Showing current with steel coreCurrent with steel core

Showing current No steel coreCurrent no steel core

No current with steel coreNo Current

No current, no steelNo current no steel

Addition: I am comparing bemf voltages produced by original rotor and replacement rotor. OE is around 5.2V@1250rpm, the handbuilt replacement rotor is around 4.05V@1250rpm

Femm plot if you want to try it yourself

Question #1. Am I dealing with eddy currents, or how could these odd scope waveforms be explained?

Question #2. I suspect that efficiency and field strength go up with steel placement, but why don't the plot field strengths seem to reflect this?

Question #3. Whats the best way to understand these plots in a way that translates to real world physical force on the shaft?

Edit: I am leaving the links to the referenced items in the answer about the coupler, but these have caused misunderstanding about the issue, so I am removing the pictures from the direct view.

https://i.sstatic.net/1Fr6v.jpg

https://i.sstatic.net/P8Ken.jpg

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  • \$\begingroup\$ Can you explain your pictures a little bit? Are the 8 circles the neo magnets? And the 10 rectangles with rounded edges are your windings? Can you point to what is air and what is steel (the reason I ask is that I can't tell where the air gap is)? \$\endgroup\$
    – Eric
    Commented Oct 25, 2013 at 16:22
  • \$\begingroup\$ Also a comment about your general approach. I wouldn't expect efficiency to go up by merely changing the magnets. You said you had some success at first ... how did you measure efficiency? Assuming the motor was originally designed well, then adding stronger magnets is just going to saturate the steel and not do much for you. And if you've changed your air gap, then all bets are off on what will happen The way to make a more efficient motor is to reduce your losses. I^2R losses, core losses, and friction/windage losses are the main losses in a motor. Stronger magnets won't help those. \$\endgroup\$
    – Eric
    Commented Oct 25, 2013 at 16:27
  • \$\begingroup\$ The 8 round circles are the neo magnets. The ring inside the 8 magnets is the steel ring in test. \$\endgroup\$
    – Erik Friesen
    Commented Oct 25, 2013 at 22:01
  • \$\begingroup\$ I expect to lose some efficiency to the air gap. The stock gap is around .025", I am around .075". I went down the road of trying to have a custom winding made, it was too pricey for the quantity we want. This is working, I am in the process of fine tuning it. \$\endgroup\$
    – Erik Friesen
    Commented Oct 25, 2013 at 22:04
  • \$\begingroup\$ To measure efficiency, I am comparing amp draw at the controller necessary to drive the pump at a given rpm. I am within 10% or less of the previous design that used a hobby motor and magnetic drive. \$\endgroup\$
    – Erik Friesen
    Commented Oct 26, 2013 at 2:20

1 Answer 1

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I'm hardly a motor expert, but since no one else has jumped in here, I'm going to offer a few thoughts as more of an "extended comment" than an answer.

Question #1. Am I dealing with eddy currents, or how could these odd scope waveforms be explained?

It's impossible to say, since you didn't say where these waveforms come from. How is the stator wired, and how are you driving the motor?

Question #2. I suspect that efficiency and field strength go up with steel placement, but why don't the plot field strengths seem to reflect this?

It did — inside the steel. Were you expecting the fields in the gap between the rotor and stator to change substantially?

Question #3. Whats the best way to understand these plots in a way that translates to real world physical force on the shaft?

Motor design is a complex subject, and as I said, I'm hardly an expert. The torque exerted on the rotor is a function of how much the energy of the magnetic field changes as the rotor turns. A static analysis of a single position isn't really going to tell you much.

The biggest problem I see with your design is the huge air gap between the rotor and the stator. You can see that lots of field lines are running directly between the poles of each, rather than coupling across the gap. This means that the total field energy is only mildly affected by rotor position, which makes the motor extremely inefficient.

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  • \$\begingroup\$ I was afraid of adding those last two pictures, they muddy the waters a little bit. Those are magnetic couplers. The gap of the bldc system is right at .075" I will add more information on the test method. \$\endgroup\$
    – Erik Friesen
    Commented Oct 24, 2013 at 1:23
  • \$\begingroup\$ Re - question 2, I do expect so, why does every outrunner on the market have a nice steel jacket around the outside? \$\endgroup\$
    – Erik Friesen
    Commented Oct 24, 2013 at 1:45
  • \$\begingroup\$ It's the flux ring, it keeps completes the flux "circuit" and keeps it from exiting the motor (hand waving explanation). \$\endgroup\$
    – Matt
    Commented Oct 25, 2013 at 1:47
  • \$\begingroup\$ Ok, so what is the difference, in an inrunner, does it need a steel core for the same reason? \$\endgroup\$
    – Erik Friesen
    Commented Oct 25, 2013 at 19:18

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