alternator

have a 1990 3.0 v6 toyota and the alternator is getting extremely hot. when the truck is running you can hear the alternator working harder than it should. when the truck is running and you disconnect the positive batter cable the truck will die but it has a brand new alternator and new battery. when checking the voltage, everything is at working and normal volts. when you unplug the plug on the back of the alternator you can hear the truck run better and not bogging down. the last alternator failed and basically melted into itself. wondering if it is a short or going to ground? is there a wire not completing a circuit? any help would be great.