A by-pass motor is merely a hybrid of the two styles. In its simplest form one could say it is a high wattage dirty fan motor, onto which one or two sets of clean fans are fitted, and the set up with the dust bag and filters is identical to that of a clean fan vacuum. These are used mostly on wet & dry cleaners, so that if water is picked up by the motor, it is ejected out of the side of the fan unit and not blown over the motor. The suction fans are totally sealed from the motor and a separate cooling fan is used to cool the motor, all of which is identical in principal to your ordinary low wattage dirty fan upright motor.
When I said mostly used on wet & dry, there was a Goblin cylinder, the 702 I think, which used a bypass motor, but that was dry only cleaner, so I don't know what the thinking behind that one was.
