"Deane Williams" wrote in message
...

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.
These are inexpensive 20 inch box fans with induction motors. I have tried
several brands and styles of fans.
The latest was a Home Depot unit that only lasted 2 months.
Does anyone know of a reason why I can't run an induction motor from a
SS relay? What is killing the fans? I
have taken two apart and there is no smoke smell, no visible damage and if
there is a capacitor it tests OK.
Thanks for any ideas. I am going to switch over to a mechanical relay.

Dean


Some induction motors will have a thermal fuse incorporated onto the main
winding.
In the absense of a thermal fuse, this is my amateur speculation:
The switching of the SSR creates voltage spikes. Normally these are easily
absorbed by the induction motor. With enough time and heat, the lacquer
insulation on the motor windings gets brittle, thermal cycling compromises
its ability to insulate, then the voltage spikes begin jumping across high
potential areas (between winding ends or where wire leads are tied to
windings). If there is a capacitor involved, the arc is that much more
spectacular, and the motor's demise quicker.
Scott