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Induction motors are not as power efficient as permanent magnet (PM) motors. Induction motors also weigh more compared to an equivalent PM motor and have more torque at lower RPMs. The reason induction motors are used is that they are cheaper than PM motors due to only using materials like aluminum and copper. PM motors use rare earth metals which are pricey and not great for us since China currently controls most of the world's production supplies.

For more detail read this article: http://electronicdesign.com/article/analog-and-mixed-signal/...

Thank you! That link is very informative.

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