There is not belt tensioner, per se. The motor is on a sliding mount and as it builds speed will slide back, creating more tension on the and increasing the spin speed. If the tub is sipinning fast, I suspect you may have an issue with the drain pump, perhaps a faulty pump or a partial obstruction. The drain pump is accessed from the front of the machine for inspection. There are a couple tension clips between the front and top of the machine, released by sliding a putty knife in between the panels. Once released, the top will hinge up, allowing access to hidden screws at the top of the front panel, inserted from the back. Once those are out, the front panel will come off, allowing full access. Let me know what you find with the pump once you get a look at it. If you do find it to be the issue, the correct one for your model is available at the link below.

Whirlpool 35-6465 Drain Pump Assembly - AppliancePartsPros.com