My acer aspire turns itself of after about 30min of using

Usually,, something like this is caused by a heat problem.
Things that usually heat up in the laptop are:
- the CPU (check the fan, may have to take apart to really clean)
- the Video Card (this may have a fan but usually a heat-sink, also may be dirty and full of dust)
- the RAM (usually nothing much to do here)
- the Hard Drive (this may be the problem but check other things first)

I would check the following forum as they describe how to take it apart for cleaning and also refer to a "build up" of dust in this laptop model.
http://www.avforums.com/forums/laptops-netbooks-tablets/1057228-acer-aspire-7720-turns-off.html

Another thing would be to check the BIOS version and also check at what levels the alerts and actions are set.

Also, when using a laptop - do not cover the fan vents which may be underneath.

Good luck