Van keeps blowing blower motors

The front blower motor went out on my van. I replaced it and it worked for about 6 months then went bad again. I bought a replacement that was rebuilt and cheap. I thought maybe it was a case of you get what you pay for. I spent more money and got a quality blower fan. Worked great until 6 months later. It has stopped working. What is causing me to burn through blower motors?