1992 Dodge Colt Alternator Issue

I had a bad alternator and replaced it with a re-built unit. 10 days later the re-built unit went bad and was emitting no output voltage , the unit was warrantied so I received an exchange. I have checked the voltage regularly and was gettin 13.5 + volts while the engine was running, now 7 days later I am getting no output voltage from the alternator. What would cause 3 alternators to go bad in less than 30 days? ( the battery is brand new and the wiring is tight and clean).

Thanks

Roye