If those two speeds are really all that's needed, a true two speed motor would probably be a good choice. A VFD will give you many more speeds in between that can be used for more savings in some applications.
Also, if there's already a single speed pump installed, adding a surplus VFD is cheaper than replacing a perfectly good motor. If it's due for replacement anyways, it would probably make more sense to just replace it.
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To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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