Why is this Induction motor so small?

Discussion in 'General Electronics Chat' started by strantor, Jul 23, 2015.

  1. strantor

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    Oct 3, 2010
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    Check out this subsea inverter-duty induction motor:
    ikmmotor.png
    It's rated 42kW (56HP), 60Hz, 3,000V, 11A.
    It is 15" X 10" and weighs 191lbs.
    It is designed for 3,000V because it's at the end of a 3km cable.

    Now compare that to a similar industrial AC induction motor:
    [​IMG]
    It's rated 60HP, 60Hz, 460V, 70A.
    It is 33"" X 20" and weighs 869lbs.

    The industrial motor weighs 4.5X as much as the subsea motor, and takes up 8X the volume of the subsea motor.

    Why is the subsea motor so much smaller? I know you can decrease the size by increasing the frequency, but they're both rated 60Hz. I've never heard that you can decrease motor size by increasing voltage, and actually it seems there would be no gain from that. Sure, the winding wire woul be smaller, but it seems that would be counteracted by the need for thicker insulation (especially being inverter duty rated). And that theory seems to be supported by others:
    So what's the deal? The only other thing I can think of is the amount of iron in the frame. An industrial induction motor requires a frame with more mass, and cooling fins, to dissipate the heat. The subsea motor is submerged in cooling water so less frame mass is needed. But can that one difference account for such a huge disparity in size? I mean, even the rotor is half the length.
     
  2. MaxHeadRoom

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    What I have seen is some spindle motors that although have the regular specifications for base speed at 60hz 2/4 pole, they are actually ran at much higher frequency via VFD allowing a much smaller physical and winding size, the down side is they cannot be taken down to 60Hz because of the low inductance at that frequency and they tend to over heat or burn out.
    Not sure how that fits in with the motor in question.
    Max.
     
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  3. strantor

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    I don't think we are dealing with frequency >60Hz here.
    The subsea motor is rated 42kW, 4poles, 1744RPM, 230Nm.
    Synchronous speed for 4 poles @ 60Hz is 1800RPM, so the 1744RPM rating indicates 60Hz to me.

    Formula for rotational power:
    P = T ω (1)
    where
    P = power (W)
    T = torque or moment (Nm)
    ω = angular velocity (rad/s)
    ω = 182.63rad/s = 1744RPM
    182.63rad/s * 230Nm = 42,005W.
    So I believe the motor is rated 56HP @ 60Hz, not higher.
     
  4. GopherT

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    @strantor - I think this is the right path...
    Also, it looks like the big green motor can be face-mounted or bottom mounted and 60 HP of frame-twisting power wont do anything to it. I am guessing that the little sub-sea motor must slide into a big frame to prevent it from bending itself in half under load.
     
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  5. MikeML

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    Does "sub-sea" mean immersed in water? If so, cooling is assured...
     
  6. tcmtech

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    I also suspect that being submerged in water has a lot to do with the high power to size ratio.
     
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  7. cmartinez

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    Yup... that motor looks very similar to some submersible pumps I've encountered.
     
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  8. strantor

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    The subsea motor doesn't slide into a frame. It's face mounted and self-contained. It's also oil-filled and pressure compensated. The oil isn't circulated through the motor for "active cooling"; it's just there for pressure compensation. But I suppose even though it isn't circulated for cooling purposes, it does have the added benefit of providing a much higher thermally conductive medium than air, for which to conduct heat from the rotor to the surface of the frame. Thus allowing not only the stator/frame to have less mass, but also the rotor.
     
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  9. cmartinez

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    Yeap, that's why they're designed that way... and if you try to run them outside the submersed environment for which they were desiged... well... kablooey!
     
  10. strantor

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    I can run it just fine unloaded. Never tried with a load, and never will.
     
  11. strantor

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    So everyone agrees that the only factor lending to the size being 1/8th and the weight being 1/5th of a comparable standard in-air motor, is cooling? The disparity seems so large to me that I still wonder if there might be something else at work here.

    However I must admit, the cooling is non-negligible. The efficiency is rated at 83%. For 42kW mechanical output, that's 8.6kW worth of waste heat!
     
  12. cmartinez

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    I would say yes... all you have to do is check out other brands of submersible pumps and compare their size vs performance with standard motors. Remember also that there's a little power being drawn by the motor due to the fact that it's working immersed in oil, and it has to constantly stir that oil, overcoming its viscosity... an advantage of that is that the oil is continuously flowing, and the heat is more easily transported to the metallic enclosure, where the water surrounding it (which is also moving, due to the pumping action below it) dissipates it.
     
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  13. MaxHeadRoom

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    Also I forgot to add, the 24,000 rpm (100Hz- 400hz) 2 pole induction motor spindles are all water cooled.
    Max.
     
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  14. tcmtech

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    Not really if you compare a TEFC (Totally Enclosed Fan Cooled) motor like the green one against a equally rated OFC) Open Frame forced air Cooled (I hope I got that right o_O) motor you will see the OFC type motor will be nearly half the physical size.

    So yes how well a motor can be cooled directly relates to its physical size which is why submersible pump motors can pack some pretty serious power into a surprisingly small package.
     
  15. KJ6EAD

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    I didn't see anything in the data sheet suggesting a duty cycle or burst use but then I don't read German. If the motor was intended for a thruster or manipulator on a submarine robot, for example, it wouldn't be required to operate continuously.
     
  16. strantor

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    Good thought, but it is rated continuous duty. It's a saw motor.
    P.s. thats Norwegian
     
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