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by fluxeb
1019 days ago
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It's more than that. 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.
Essentially, this new tech combines the best of Permanent Magnet Brushless DC and Induction motors. It allows you to change the magnetic field B without contact.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. [1]https://www.tesla.com/blog/induction-versus-dc-brushless-mot... |
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There’s the old semi-permanent magnet trick, although this only allows a couple of discrete field strengths, not continuous tuning. Basically, a neodymium (or other hard) magnet and alnico (or other soft) magnet are put in parallel, surrounded by a pulse winding. A pulse can flip the magnetization of the alnico, but not neodymium, so you get field strengths of either neo + alnico or neo - alnico. (This is most often used sized for similar strengths to turn a “permanent” magnet on or off, but can be used in non-equal strengths as desired.)