Re: Why do electric powered automobiles need to be so complicated?

Jim Thompson wrote:
On Mon, 05 Jun 2006 19:08:46 -0400, John Perry <jp@xxxxxxx> wrote:

...
As I pointed out above (and in my previous post), the power is not
constant with rpm. Only the torque is. This is a serious difference.

John Perry

The torque is constant ??

In a DC motor the torque is maximum at zero RPM and zero at maximum
RPM's, due to back EMF.


Yes, a series motor in particular does this at a given voltage. For a
parallel motor it is less true. But that's a practical matter of how
commutation is done. In both cases you can get closer to the maximum
torque over a wide range of speeds by increasing the voltage as the
speed increases (exactly as you can in an AC motor by modulating the
drive voltage as speed increases).

Think about the fields between the rotor and stator of any motor. The
maximum torque occurs when the ferromagnetic material of the cores
starts to saturate. If we can choose the type of motor, we certainly
won't choose for a vehicle a mechanically commutated motor that has a
voltage/current limit determined by considerations other than maximum
torque and efficiency over a wide range of speeds.

There are good reasons why no one considers building a vehicle with
anything other than AC induction or synchronous PM motors.

jp
.

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