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Why do Electric Motors have High Torque?

  1. Mar 22, 2010 #1
    Simple question. I've been reading qualitatively about the torque and power curve of electric engines versus internal combustion engines. I'm led to understand that electric engines get much higher torques even at lower speed. One argument for why electric cars are inherently more efficient than IC cars, in particular for urban travel.

    I'm wondering why, is it due to the fact that the high energy content of fossil fuels can be surpassed by that of x amount of current and voltage? Thanks in advance.
     
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  3. Mar 22, 2010 #2

    mgb_phys

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    Electric motors generate maximum torque at zero speed, that's perfect for cars where you need the most torque to pull away from a stop.

    IC engines generate peak torque about half-way to maximum revs (depends on the design) so you need a gearbox to allow an engine to run at 3000rpm with the wheels turning at 1rpm

    For a given power/size/cost the IC engine will probably generate more peak torque - which is why you don't see many hybrid bulldozers
     
  4. Mar 22, 2010 #3
    Why do electric motors have high torque? Is kind of the wrong question.

    Some electric motors don't have high torque. The one spinning a hand fan certainly doesn't. It also has nothing to do with the energy content of a fuel, as thats to do with power. To get more power, you use more fuel. Just like to get more power from an electric motor you up the voltage or amps.

    What you mean is why do they have a flat(ish) torque curve?

    Electric motors generate rotational motion by design. Internal combustion engines generally dont, they transfer a linear motion to a rotation via the cranksahft.

    The reason why electric motors have a constant torque, is becuase it used a magnetic field to rotate it. If you apply a set current, you generate a set torque.

    EDIT: I think it's current that determines the field, but i'm not 100% on that.
     
  5. Mar 22, 2010 #4
    Right I'm aware that electric gets more torque. I'm curious about why.

    I have this link comparing the torque from a Tesla EV, which is indeed about flat up to 6000RPM but does steadily reduce after that http://www.velvetron.com/blog/wp-content/uploads/2008/05/tesla_torquegraph.jpg [Broken] , to an IC engine. I'm aware that the torque is higher, I'm just unsure why. Chrisx indicates that it's a matter of efficiency?

    Comparing torque curves from a similarly efficient high power engine (i.e. high fuel consumption) to the torque curve of a low power engine, the high power clearly gets more torque. Is it then not logical to assume that it's a matter of useful energy content? Torque is a moment dependent on force, and force is dependent on energy. For a rated power, electric motors have higher efficiency and so better utilization of the input energy and therefore higher torque. As speeds increase, the magnetic fields between rotor and stator (or slip) begin to misalign, and so the efficiency and torque drop (as in that graph in the above link). Right?
     
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  6. Mar 22, 2010 #5

    mgb_phys

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    Not sure quite what you are asking.
    Both engines generate torque, without torque you don't get any motion.
    You can't say one technology generates more torque - it depends on the size, there are very big ICE and very small electric motors for instance.

    Comparing which is best depends on the application. There is a difference in the relationship between torque, power and speed which is useful depending on the thing you are trying to build.
    There are different ratios of torque/power even in ICE, a 300hp 4cylinder turbo sportscar engine will generate much less torque than a 12cylinder pickup truck engine of the same power.
     
  7. Mar 22, 2010 #6
    I've been led to understand that for the same power, you'd get more torque in an electric engine assuming they both had the same rated power (e.g. 100bhp). So to get the same power from an ICE and EE, the EE could run at lower revs because it has higher torque. I was asking why, wondering if it's just a matter of EE's having significantly higher efficiency. Is it an illogical question or am I fundamentally misunderstanding something.
     
  8. Mar 22, 2010 #7

    mgb_phys

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    Not sure that's true for maximum torque, as I said the huge advantage for most moving vehicles is that electric motors have peak torque at zero revs.

    But it's just a factor of the engine design not a fundemantal law of efficency.
    Diesel engines have more torque than gasoline which have more torque than a gas turbine - you pick the one you want.
     
  9. Mar 22, 2010 #8
    An automobile ICE engine, by design, has to be spinning to make torque. It has to suck in air, compress it, and expel it, and it's not going to do that at 0 RPM. It's not going to do it at 10 RPM either, or even 100 unless it's some huge marine engine. Remember, a piston only makes power 25% of the time, and this power has to pay for the remaining strokes, including compression, which slows it WAY down. So in general, it has to spin at a decent speed to make enough power to be able to even pay for itself. Once it gets there, it has to spin faster to make extra power, and even faster to make the kind of power you want. Depending on design, you'll usually end up being in the thousands of RPM before you get there....

    Electric motors, on the other hand...they're just some wires and magnets. You run a current through a wire and it becomes a magnet, which pushes against another magnet and vuoila! you've got force.

    The more current you supply, the more force. I'm guessing the greatest current usually runs at 0rpm, since there's practically no resistance in the wires. Once the motor starts spinning, the moving magnetic field slows the current down.

    That's my layman's explanation of it, it's extremely simple and obviously will differ as there are many ways to make electric motors, and it may not even be 100% correct...in which case I invite someone to correct me.
     
    Last edited: Mar 22, 2010
  10. May 16, 2010 #9

    You did pretty good at explaining why electric motors have greater torque than ICE's for the majority to understand, but I would add that AC induction motors do not make max torque at zero RPM (except for the NEMA design D and C AC induction motors) even though they pull in the most current at stall, but have what is called breakdown torque which occurs at about 95-97% of the full load speed (the "knee" in the curve) when the motor is loaded down to this point (not by varying the speed to this point). If the load torque exceeds the breakdown torque point, the motor torque will start to diminish to a lower value resulting in quick deceleration to a stall. The breakdown torque though is usually higher than ICE's peak torque for the same full load HP at RPM as well though. So the electric still wins.
     
  11. May 16, 2010 #10
    can you compare , as how the fuels energy and electric current is converted to useful torque in IC engine and electric motor respectively. In high rpm there is more magnetic repulsion cycle in electric motor same as IC engine has more combustion per cycle. I think the mechanism of IC engine has some flaws these don't convert the peak pressure into peak torque due to inefficiency of crank to do so. For eg: when you are cycling at a high speed pedalling slowly does not affect the speed of cycle, it doesn't even slow the cycle, I mean lower torque than the threshold to do work does not affect the speed however negative torque does. So peak torque even for a fraction of time can be useful.I think in electric motor the magnetic force pulse is converted to torque as it is, but in piston engine pressure pulse is not converted so. if you consider a single power stroke of IC engine the pressure has peak at TDC but turning moment of piston crank system will be peaking at 90 degree after TDC. OR at TDC any amount of pressure can not generate any torque but at 90 degree after TDC all pressure at that time will be converted to torque.

    Note: IF I am wrong correct me
     
  12. May 16, 2010 #11
    Seems pretty much spot on to me.
     
  13. May 17, 2010 #12
    sr241, since you have a interest in electric motors I recommend that you start with this book:

    Electric Motors and Drives: Fundamentals, Types and Applications (ISBN: 978-0750647182 ).

    Then if you want a more advance, deeper understanding, then you will need to purchase these two:

    Electric Machinery (ISBN: 978-0073660097 )
    Electric Machinery Fundamentals (ISBN: 978-0072465235 )
     
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