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Old 01-24-2009, 04:23 PM   #2 (permalink)
Ryland
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120 volt DC motors are not very common and changing an AC motor to DC is not an easy, practical, or efficient way to go.
Using only part of your battery pack for low speeds is a bad idea unless you really like buying new batteries as some of your batteries will always be used and some will only be used at high speeds and putting partially charged batteries of different states of charge in the same string will damage your batteries, it might even reverse cell polarity on the first few batteries in the set.
Before spending any money on this project I highly recommend that you made a trip to your local used book store and look for a book with a title like Understanding Electricity and Electronics (I own two copies of this book, one is to loan to people with questions like this).
Last of all, at low speeds you want peek amp output of the battery pack, not the amp output of a single battery in the pack, this is where solid state speed controllers have made electric vehicles a practical thing to build, switches have arcing ever time they open and close, they also become very complex when trying to vary voltage like you want to do.
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