On Wed, 10 Jul 2013 14:09:24 -0500, Karl Townsend wrote:

I have pretty much the same thing in my house. i think the motors are
20 years ould now.

Very unlikely to have that many defective motors. I'd look to another
problem in mounting or gearing. My first guess would be the motor is
too small for the pulley gearing you have, check a new one with an amp
meter and see if its pulling more than name plate. If so, change
pulleys or go to larger motor.

Also possilbe that mounting is poor taking the bearings out.

I suppose you'de know it you have low voltage problems at your site.
Even occasionally kills motors.

Karl

House voltage is running a little above 120V; low voltage does not
seem to be the issue.

A meter suggests that the fans are drawing 5.5A to 7A; the latter
number is right up toward the nameplate rating but not way over it.

Some more background, in case it suggests anything:

The salesman who sold us the greenhouse ordered fans with 220V motors
for it, even though he had ordered a 120V controller system. He then
tried to correct it by ordering new motors only, not complete new fan
setups.

The motors were changed on-site by the local electrician who was
putting the system together.

So it is certainly conceivable that the belts may be too loose or too
tight. I have no sure-fire way to know what the right tension is; they
are tight enough that the belt doesn't flap; the adjustment method is
so awkward that having the belts too loose seems to be more likely
than too tight.

It is also possible that the pulleys are not right in some way. My
understanding is that they are the original pulleys from the original
motors that were nominally the same HP rating...

Some other sort of problem that I haven't thought of is also possible.

Bob