Alternator belt keeps breaking

[L33h]

Right I recently changed my alternator/fan belt due to it squealing. Fitted a new one and realised a slight nock. Sprayed it up with wd40 as i thought the belt was try or whatever. About 50 miles down the road the outside of the belt ripped. I thought it was down to me not getting the belt centralised on one of the pullys but I've changed it and the same thing has happened.

What do you think it could be? The tensioner?

 

[lol4fun]

With the belt off spin the tensioner, idler and air on pulleys and report back. Also the tensioner should move, I think a 17mm open ended spanner on the square bit ontop of the tensioner, could be thinking of something else though lol

 

[@udi A3]

Alternators have clutches on them, is that ok? mines just been replaced, was seized and juddering like a goodun.

 

[Alais DJA]

tensioner not set properly

 

[s3dave]

Got Aircon? could be a seized compressor