Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What's about no air-gap in motors?

  1. Sep 5, 2016 #1
    I saw few threads here about air-gap in motors, and I wonder what if we ignore mechanical issues and make an "iron-gap" in the motor.
    for example: in linear motor(e.g. this cute one: https://www.physicsforums.com/threads/linear-motor-equations.794499/) if we make the tube between the stator and the rotor from iron, it would be more efficient, due to reduce in the magnetic reluctance.

    but, it will produce undesirable effects like eddy currents, maybe it will be a Faraday cage.
    what do you think? there is a way to overcome those problems?

    e.g. cutting a length-wise slot down the tube can mitigate eddy currents
  2. jcsd
  3. Sep 5, 2016 #2
    but how would the rotor turn if there was no airgap? the rotor and the stator have to be mechanically disconnected , also have you ever seen a rather common problem in asynchronous ac induction motors when the bearing wears out and the rotor starts to touch the stator laminations ? even if the rotor still moves with hand the motor probably won't work because the magnetic fields interact now in a way they are not meant to.

    Maybe I misunderstood what you are trying to build.Go ahead explain more , surely folks here will be able to help you
  4. Sep 5, 2016 #3
    theoretical. I said that I ignore mechanical issues(I'll explain in a moment)
    It's interesting, why they are not meant to interact this way? they air gap is like a huge resistor in an electric circuit, isn't he? so if I want to gain more power I want to minimize the air-gap.

    I know there is mechanical limitation, and I must have air-gap. understood.
    my question is if I replace part of the air-gap by iron-gap(and that why I asked about linear motor), it's should be better since the "resistor"(magnetic reluctance) now is smaller. how it's affect the effecient of the motor? it probably will create eddy currents, and maybe more iron-losses in the core of the motor, etc.

    again, I want to ignore any mechanical lmitation(like friction, heat, etc.) to understand correctly the electro-magnetic rational
  5. Sep 5, 2016 #4
    Heat from friction is the whole problem and it can't really be ignored. Let's say thought that you could have some friction-less magnetic material and eliminated the air gap all together. You could simply remove the reluctance of air gaps from your magnetic circuit model of the motor and you would end up with a more powerful and efficient motor.

    For the record, electric motors are already pretty efficient. You can get them with efficiency in the 90's. They scale well, meaning that the bigger the motor is the more efficient it is.

    I have a side question for anyone else. Has anyone tried making a ferro-fluid lubricant for motors? That could reduce the effects of the air-gap too.
    Last edited by a moderator: Sep 5, 2016
  6. Sep 5, 2016 #5
    Hi, Thanks for your reply.

    Your side question is my question. the ferro-fluid will remove the reluctance, but what about the side-effects? such as eddy current, iron-losses and faraday cage?
    the efficiency of common motors is 85-90% due to small air-gap(among other things...), but for some applications you can't allow small air gap so you need to find another solution to improve efficiency.
  7. Sep 5, 2016 #6
    Also, here's a video which proves that friction really matters. Friction welding!
  8. Sep 5, 2016 #7
    Iron loss is an unavoidable material problem. You can reduce it by possibly using a better material or by minimizing the field strength in the material. I'm not a material scientist so I don't know if there are newer better core materials out there. However, you can diffuse the magnetic field by designing the core to have a bigger cross sectional area perpendicular to the magnetic field lines. So, there's going to be a trade off between making the core bigger/heavier/expensive and reducing losses.

    Eddy currents can be reduced by using a laminated core so that currents cannot circulate perpendicular to the field lines.

    As for how that applies when you eliminate air-gaps, I'm not sure.
  9. Sep 6, 2016 #8


    User Avatar
    Gold Member

    Needle bearings in proper spacing or if timing dictates certain spacing, then group sections of needles. Same principal as regular needle bearings.
    But I suspect something like this was tried, back around 1900 o_O
  10. Sep 7, 2016 #9
    so, if I wraps the stator with this:
    (there is no contact between the two half-pipes)
    I can say that Eddy currents have no impact on the efficient of the motor?
  11. Sep 7, 2016 #10


    User Avatar
    Science Advisor

    I think RonL has the right idea. Supporting the armature with spaced needle bearings would be one solution. The field would be short circuited between adjacent poles if adjacent needles contacted magnetically.

    The problem with bearings is high surface velocities. The smaller the shaft diameter the slower and so the more durable is the bearing. Needle rollers running on the OD of an armature would be high velocity and so have a short life. Magnetic contaminant particles would stand up on the rollers and rapidly bring about erosion failure. Any lubricant would be thrown out into the field windings so the motor would need to run flooded with a dielectric oil.
  12. Sep 7, 2016 #11
    I was thinking that you could have a sealed void between the rotor and stator where the field shoes rotate by each other. Then you just need something like an oiler system to keep the void filled with ferrofluid lubricant. Surely someone has tried this already or at least thought of it.


    What do you mean by "wrap the stator"? Are you trying to shield the stator from eddy currents? That's not how it works. Eddy currents happen inside the material of the magnetic core. You might want to read up on this some. Wiki does a good job of explaining the basics. Pay attention to how the cores are laminated in their illustrations.
  13. Sep 8, 2016 #12
    also ferrofluid or other fluids inside the motors will impose a drag on the rotor as it rotates , so one has to calculate exactly the amount of force taken away by drag but this could very well prove useless as the gain is taken away by the very thing that caused the gain in the first place.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted