Getting voltage by manually turning the dc motor.?

I notice that on some simple dc motors used for dvd players, rc cars, audio cassette players, turning the rotating thing manually produces enough voltage (depending on how fast I turn it) on the battery input wires to light up a led (3 to 5v)while some of them don't even produce .10 v. Why the difference? Does... show more I notice that on some simple dc motors used for dvd players, rc cars, audio cassette players, turning the rotating thing manually produces enough voltage (depending on how fast I turn it) on the battery input wires to light up a led (3 to 5v)while some of them don't even produce .10 v. Why the difference? Does it have to do with how thick the magnets are or the coil winding? Is there a universal way to distinguish between the two?
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