"daestrom" wrote in message
...
Anon wrote:
"Deodiaus" wrote in message
...
I found one for $65 shipped at:
http://boatandrvaccessories.com/43587.htm

Does anyone know where I can buy one for $30?

ps1. I am interested in the start capacitance, (the motor makes the
humming sound trying to start up).
ps2. my friend the electrician is not the same as the AC repairman
and not the same as the earlier guy who put in the 7.5 MFD cap (which
seems to be the one that the sites sell with it [A.O. Smith recommends
10 MFD]).
ps3. The reason I was looking for the calculations is that I can see
how sensitive they are to a change in capacitors.

The motor you referred to is not a cheap motor, it's just not an
exessively
inflated priced motor.
New surplus blower motors can be found at numerous surplus dealers for
about
$30 or less, plus shipping.
snip

Induction motors are greatly affected by the load upon them. If the load
is less than the maximum, a smaller capacitor value is used to keep the
rotation speed within reasonable parameters. An induction motor run
without any load will run too fast and the windings will overheat and
break down the lacquer insulation causing an internal short.


Not even close. Induction motor speeds vary little from no-load to
full-load (slip from no-load to full-load of typical induction motor [not
some ceiling fan junk] is 1 to 3 % of synchronous speed). Running
completely unloaded is perfectly fine and the windings of an induction
motor will *not* overheat in that situation. (hint, the motor draws less
current when it's unloaded)

Ah, I should have qualified that with "low-torque/low speed" induction
motors (less than 300 RPM) often found in smaller squirrel cage blower fans
and in some ceiling fans. I have not worked on many high torque, high speed
induction motors to consider them typical of induction motors. Low torque
motors are designed to carry a specified load and can run up to 25% faster
without that load. But I concede, a blower motor requiring a 10µF running
capacitor probably is a high torque induction motor that will not suffer
from heat-rise unless it is from the bearings.


If the motor just hums and does not rotate - there is something else
wrong. A failed capacitor results in this same symptom and is the easiest
solution to try, hence the reason it was replaced. Even a 4µF or 5µF
capacitor should turn the motor. Check the bearings by rotating the
unloaded motor shaft - it should rotate fairly easily. Usually sealed
ball bearing sets are used in these motors, pressed onto the shaft -
replacement will require a bearing puller.

If the blower is directly attached to the shaft, the only bearings to
check are in the motor. If it is a belt driven set-up (older), the
bearings on the squirrel cage will be suspect.

If the motor spins freely, but still only hums on start up, either a bad
connection to the capacitor (or other component) or a short or break in
the motor windings, not to mention another capacitor failure. These
motors tend to have very fine and compact winding wraps and then take a
lacquer bath, making service difficult if not impossible. If there is a
short in the motor, the first place to check is the bundle where the
insulated wires are soldered to the winding wires, if that is the
connection method.


These are good troubleshooting points. But your statement earlier about
motor speed/load is off.

daestrom