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Question about power capacity of electrical motors

  1. Jul 10, 2017 #1
    I read that the Tesla electric motor is able to achieve its remarkable power to weight ratio of 8.5 kW/kg, about 10 times better than a typical gasoline engine, because it tightly winds its copper wires in its coils therefore being able to carry more current in the same space. If so, then we should be able to produce more power if we have a material that can carry more current.

    Carbon nanotubes are such a material. In tests they can carry orders of magnitude more current than ordinary conductors such as copper, aluminum, or gold. The problem is they've only be able to be produced for short lengths, a few centimeters at most.

    But an interesting research on nanotube-copper composites showed they can carry a 100 times more current than standard copper. So for the same size electric motor we could produce 100 times more power than currently:

    Posted: Aug 06, 2013
    Novel CNT-copper nanocomposite delivers a 100-fold increase in current density
    http://www.nanowerk.com/spotlight/spotid=31710.php

    For the Tesla that would not be as major a deal because of the small size of the motor at only about 32 kilos for the Tesla S, a vehicle that weighs thousands of kilos.

    But that would be major for large scale motors used in other scenarios. These can weigh thousands of kilos, and could then be reduced to only tens of kilos. It would also be especially important for proposed electric aircraft that many groups are working towards.

    So is it really true we can increase electric motor power levels simply by using wire of higher current capacity?


    Bob Clark
     
    Last edited: Jul 10, 2017
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  3. Jul 10, 2017 #2

    f95toli

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    Yes, sort of.
    This is one of the reasons why electric motors made using high-Tc superconducting wire just might be commercially viable for ship propulsion; you need some extra equipment for the cooling but you save a LOT of weight since the motor itself is much smaller than a conventional motor made using copper.
    Note that there are already a few working prototypes and this is one area where they seem to be making some progress.
     
  4. Jul 10, 2017 #3

    Baluncore

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    Copper or gold wires with internal carbon nanotubes may well have less migration of metal atoms and so be more stable over time. The application mentioned in the referenced article seems to be for the very small wires that are used to connect a semiconductor die to a package connector frame.

    Metal atom migration is more important for very small conductors than it is for the much thicker conductors used in powerful motors. A new composite material may survive short high current pulses, but electric motors are in continuous operation so the limitation on motor construction is rate of heat removal rather than the wires “wearing out” like old fuses that eventually fail at their rated current.

    Superconduction is more likely to yield improved performance in large electric motors.
     
  5. Jul 10, 2017 #4
    95%+ ( a guess) of all electric motors, cost is the #1 decision factor. There are applications where size, weight, efficiency or other factors have precedence, but of the 20 or so motors in your home, how many would benefit from this? So there will be little market unless the cost is comparable to copper, not that it will not be developed, but I do not see it as a market wide game changer.
     
  6. Jul 10, 2017 #5
    bold mine -
    I'd like to first mention, that your comparison is rather odd. If I were looking to understand the benefits of their copper coil winding, I'd compare it to other electric motors. Why compare winding methods of electric motors to an ICE? It's a bit like saying, "I gave this race horse a shot of Vitamin-B, and it was 1000 x faster than a turtle". Tells me nothing about the effect of the Vitamin-B.

    If you are interested in power-to-weight ratios, electric motors stink. Listen to Elon:

    https://chargedevs.com/newswire/elo...io-is-the-challenge-with-ac-induction-motors/

    If I converted that right, that's about 197 kW/kg, so an electric motor's power to weight ratio is not remarkable all in comparison. Or ~ 5,000 HP from the 70 # Tesla motor!


    Yes, diminishing returns for a 70 # motor in a car.

    The 100x number you cite appears to be for the conductor capacity, but that is not the only weight in an electric motor is it? There must be a frame to hold all that, and an axle, and bearings and cooling, etc. - especially in a very powerful motor that must withstand such large forces. What % of an electric motor's size/weight are the copper conductors? It might be significant, but it isn't 100%.

    Still very interesting, but seems a long way off. But maybe a reality someday.
     
  7. Jul 11, 2017 #6
    I don't think the higher power would be particularly useful for the Tesla since the motor is already so lightweight compared to the weight of the vehicle. But looking at some high power electric motors used in industry I was surprised to see that to get in the range of 400 hp like the Tesla motor, they commonly weigh thousands of kilos. So such weight saving would be important then.
    Also making a comparison to the gasoline engine is relevant because if the motor was of comparable mass to an equivalent power gas engine it would then weigh hundreds of kilos and would then be a significant mass of the total mass of the car. Again, typical electric motors of such high power for fixed industrial uses, commonly are of such heavy weight as the gas engines.
    Do a web search on AC motors at high horsepower as the Tesla. You'll find they commonly weigh thousands of kilos. But it is true that the Tesla motor is not the only electric motor offering such high power at lightweight, but it does rank among the highest. See the list here:

    https://en.wikipedia.org/wiki/Power-to-weight_ratio#Electric_motors.2FElectromotive_generators

    Another important application is for electric powered aircraft, where achieving a lightweight powerplant would be paramount.

    Bob Clark
     
    Last edited: Jul 11, 2017
  8. Jul 11, 2017 #7
    Again, I think you are "falling in love with a technology" rather than analyzing the cost/reward/benefits in an application.

    No, I don't think weight saving is very important in a 400 HP electric motor. They are used in stationary applications, mostly. Maybe big ships, but those motors are a tiny fraction of the weight of the ship.

    For most aircraft, there's not much sense in electric power due to the low kWh/kg of batteries. It isn't even close to hydrocarbon fuel. Niche application, drones, etc.
     
  9. Jul 11, 2017 #8
    Also - a car only needs 400HP ( you don't really NEED 400HP) for a short period unless in a race car maintaining very high speed. You can drive a smaller electric motor VERY hard for a few seconds, to minutes, well over it's continuous duty rating. The Mission Profile for an EV is quite different then most electric motor applications - driving down the highway only needs 30 HP or so.

    Electric motors - from a performance and efficiency standpoint are already a good solution for a vehicle, since you can generate torque VERY quickly and at essentially any speed and they have a very wise speed range, this torque can be applied for both acceleration AND deceleration. The energy conversion ( efficiency ) is pretty high.

    The challenge is all about the batteries and has been for 120 + years.
     
  10. Jul 11, 2017 #9

    russ_watters

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    Good points, but I want to reiterate this because I'm not sure it is well known:
    Right:

    Gas engines actually start losing both power and torque above a certain RPM, so their performance is pretty much just what the torque and power curves dictate.

    Electric motors aren't so limited and can give you more and more performance until they burn-up if you ask for it. At start, they can, if you want, pull a huge current and generate a huge excess torque (which is why your lights dim in your house when your air conditioner turns on) or run them at higher speed a nameplate torque. If you want to pretend a 5 HP motor is a 10 HP motor you can -- it will perform like one....at least until it dies prematurely.

    This dovetails very will with how electric cars use motors, allowing you to do essentially that: you put in a smaller motor to handle the continuous duty and it performs like a bigger motor for acceleration. That's part of the reason why electric cars outperform what their horsepower ratings imply when compared to gas cars.
     
  11. Jul 11, 2017 #10

    Baluncore

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    Arguing over the power to weight ratio of IC and electric motors is pointless; hydraulic motors will beat both every time.

    The total weight of motor and fuel must be considered for vehicles that carry their own fuel supply.
    An IC motor with half a tank of fuel needs to be compared with an electric motor plus the weight of the battery.

    The range of the vehicle and the economy of transport are more important and meaningful than maximum possible power.
     
  12. Jul 11, 2017 #11
    You are correct that for an electric vehicle to make a fair comparison you have to include the weight of the battery. This can be quite heavy such as for the Tesla, where it makes up a significant portion of the total weight.

    But what do you mean by a hydraulic motor?

    Bob Clark
     
  13. Jul 11, 2017 #12

    Baluncore

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  14. Jul 12, 2017 #13
    Do you have references for that?
    I'd like to see what the power to weight ratio is for that case.

    Bob Clark
     
  15. Jul 15, 2017 #14
    I've been informed also that an additional problem is that even with high current you run into the problem of magnetic saturation, where the magnetic field produced levels off:

    418px-Magnetization_curves.svg.png
    https://en.wikipedia.org/wiki/Saturation_(magnetic)

    But according to this wiki article the best you can get is with iron alloys at 2.2 T (Tesla). However, I've heard of cases of high intensity magnetic fields where you can get continuous, sustained fields significantly higher than this:

    Orders of magnitude (magnetic field).
    https://en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field)

    So how do they produce these high magnetic fields?

    Bob Clark
     
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