blower motor surging?

[lakerd]

couldn't find anything about this on site or google so throwing it out there to see if it happened to anyone else. i have a dayton shaded pole motor on my blower and speed is controlled by a plug in rheostat. if i set the speed at about 3/4 it runs at constant and expected speed for a minute or two then unexpectedly speeds up to max all on its own. it works fine at lower speeds but i avoid setting it anywhere near the 3/4 mark before i leave the house because i worry about this runaway speed control. i tried replacing the rheostat with one from the neighbor and the same thing happens so apparently it is something with the motor. anyone else experience this phenomenon?

 

[Sodbuster]

Have you checked voltage when this is occurring?

 

[lakerd]

i have a multi tester but i don't know how i would check the voltage. the blower plugs into the rheostat which plugs into the wall. there are no wires exposed. ready to learn something new if you want to teach.

 

[Sodbuster]

Check the voltage after the rheostat, to see if it varies. Also check the voltage directly from the wall plug. I've had surging fans before, and the whole house voltage was running between 103v-110v, very low.

 

[lakerd]

line voltage is 120. unloaded rheostat output varies continuously from 76 to 90 volts.