fluxeb

A great write-up by Tesla (2007) [1]

  In an ideal brushless drive, the strength of the magnetic field produced by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum – so that inverter and motor currents are maintained at their lowest possible values. This minimizes the I² R (current² resistance) losses and thereby optimizes efficiency. Likewise, when torque levels are low, the B field should be reduced such that eddy and hysteresis losses due to B are also reduced. Ideally, B should be adjusted such that the sum of the eddy, hysteresis, and I² losses is minimized. Unfortunately, there is no easy way of changing B with permanent magnets.

The losses in the shaft coils are low when B is low so the gains in efficiency for an "eco mode" would be excellent as you modulate B for low torque and you would still retain the ability to crank up the power in a "sport mode" at the cost of efficiency.