Can I use an old vacuum cleaner electric motor for a homemade windmill generator?

A vacuum cleaner motor will not produce electricity like a generator unless you were to do some modifications so you could excite it. It would be better to use something that will directly work as a generator. A PMDC motor is a good bet (Permanent Magnet Direct Current motor) but most are pretty small. A car alternator would be a simple solution if you are looking to charge a battery with it, the Delco alternators made in the 1970s-80s have a built in regulator, easy to connect. Not as efficient as it could be but quick, easy and relatively inexpensive.  more