On Monday, July 27, 2015 at 7:06:47 PM UTC-7, Deane Williams wrote:
I set up a thermostat in my garage that comes on at 80 degrees F. and sends a 6 VDC battery voltage to control a 120 VAC solid state relay which powers up the vent fan. The fans always quit within 2 months to a year with no obvious defects.,,, there is no smoke smell, no visible damage and if there is a capacitor it tests OK.

Three possibilities: your thermostat could be chattering (sending a bouncy-switch signal
to the SSR) and that is causing rectification. Your SSR could be a zero-crossing type, which
is exactly the most stressful motor-start situation (and an inexpensive motor might
take a magnetization at turn-off, then saturate at turn-on, and melt its fusible protector).
Third, the SSR might be susceptible to some other signal than your thermostat (RF
or maybe even input/output feedback oscillation).

If you use an AC relay, it cannot operate fast enough to rectify; if you feed that
AC relay with a triac-output optoisolator, with a smallish capacitor shunting
the input terminals (1 uF?), there ought to be no drive-side problems.