On Fri, 19 Jul 2013 15:59:29 -0700, "Bruce L. Bergman (munged human
readable)" wrote:

On Thu, 11 Jul 2013 22:39:38 -0400, wrote:

On Thu, 11 Jul 2013 15:13:27 -0500, dpb wrote:

On 7/11/2013 12:22 PM, Jim Wilkins wrote:
...

Just checked with a Kill-A-Watt. A muffin-sized centrifugal blower
draws 24W open, 18W with either the intake or exhaust blocked.

OK, let's recast what I was driving at...you're saying there's no fan
curve/as-shipped design point for which it's possible the OP has done
something to his ducting or louvers or the like that can result in
raising motor load above design so he shouldn't/needn't consider that as
a possible root cause/factor?
I'd say that sounds right. Short of physical restriction (drag) it
is difficult to overload a fan. Restricted air flow COULD possibly
cause a fan motor to everheat due to reduced cooling, but not due to
increased load.

Muffin fan motors are simple shaded-pole driving a small blade, not an
induction driving a large blade.

It's easy to rig a belt-drive so the motor is overloaded - You have
to look at the Full Load Amps and the Service Factor, and make sure
the actual draw is under the nameplate ratings. If it's not, you have
to make the motor pulley smaller to drop the fan speed and the load.

If it doesn't have the variable-diameter pullies, you need to add
them. Or you have them and they've rusted solid at the old setting,
which means dismantle and derust.

And check the line voltage available at the fan motors - If they are
120V motors, anything below 110V is trouble. You could have a bad
wirenut that has rusted out halfway to the greenhouse too. This is
why they use the 240V motors - half the current means half the voltage
drop. You might have 120V at the house, but the Greenhouse is 250'
from the house which means 500' of wire...

If the pipe is big enough you can run #6 Copper or #4 Aluminum wire
and solve the voltage drop issues - if it's 1/2" conduit and already
at fill, you'll need to bump the voltage up instead.
Are these 1725 (1800) rpm motors when 1125 (1200) should be used?