Can you determine how many leads are used for the motor, excluding an earth
ground lead?

Is there a low value capacitor assocated with the motor wiring? The
capacitor may possibly be more than one value, such as 2uF and 4uF.

A grinding noise may be unintended contact of moving parts. I would inspect
the motor's interior, and the blower section for signs of contact of moving
parts.

A low frequency growling noise may actually be a result of mildly worn
bearings.. a sort-of chatter that takes place as the shaft rattles in the
bearing I.D.. there is a specific term for this type of chatter, but I can't
remember it.
For a Hunter air cleaner that I use daily, standing the unit on it's side
eliminates the noise, which I'm certain is due to slightly worn bearings.
I've been using it for about 5 years, and it was neglectfully used when I
bought it.

With the motor shaft in a horizontal position, notice that the motor shaft
has a certain amount of axial movement (end play). At a point near the
center of this axial movement, apply radial force finger pressure to the
shaft or blower wheel to see if there is any significant amount of movement.
Any movement geater than barely noticeable, is bearing wear.

Small motor temperatures often seem to be exessive when checked by hand
contact. A thermocouple or other type of contact measuring device would
indicate if the motor is actually overheated.

In many types of air moving appliances, the air flow path passing over/thru
the motor provides adequate cooling if the motor vents and passages are
clear.
Additionally, most consumer appliances use motor designs with internal
temperature protection devices.. a self-resetting, temperature sensitive,
bi-metal type switch that will open at a temperature level below a dangerous
temperature.
A label on the motor generally states that the motor has temperature
protection.

Because the pass-thru air flow cools the motor, this also means that the
motor bearings are exposed to fine dirt that gets past the filter elements
in normal operation.
Periodic internal cleaning of the appliance and a drop of very light machine
oil to the bearings will go a long way to extend the effiiency and useful
life of the appliance.

Fine dust around bearings, even ball bearings, tends to wick the lubrication
out of the bearings.

I'm not saying that your motor doesn't have a thermally induced and/or
shorted turn fault, but conclusively finding out if it does, isn't a simple
matter.
If a capacitor is associated with the motor, that could be a more likely
source of the problem.

--
Cheers,
WB
..............


"Robert Macy" wrote in message
...
Originally I had asked about the single speed of the Oreck
Professional Air cleaner: Apparently NOT the bearings in the fan
motor, but the motor itself.

Turn on cold and the squirrel cage fan motor has three speeds. ANd,
runs quitely and well in the slowest speed. After running a while, the
motor starts generating a light grinding noise, and only has one
speed. Plus, and this is pure subjective, the motor metal body seems
hot.

If the fan can run well for a while, it seems it may be possible to
repair this motor.

It seems two possibilities, getting hot opens something up, or getting
hot shorts something out. Since the construction of the motor is a
transformer-like winding, it seems likely that after heating up, the
coil has a shorted turn.

Does anybody have experience with the death throes of such a motor?

Robert