Re: Slow projector fans - Lubrication?

[P3D John W Roberts <roberts@xxxxxxxxxxxxxxxxx>]

>Date: Mon, 16 Jun 1997 21:21:09 -0500
>From: "P3D  Dr. George A. Themelis"  <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
>Subject: Slow projector fans - Lubrication?

>I have a question regarding motor fans in projectors.
>I run into slow motors quite often.  Slow means that the 
>motor turns slowly for a number of seconds and then it takes 
>off.  Now I have a TDC projector motor that takes a full 10
>seconds to start.  I tried lubrication and did not work.
>Then I sprayed the motor with the cleaner used in shutters
>to dissolve oils.  At first things looked OK.  However,
>after the motor sits for a day or two, it goes back to its
>slow start.

>Questions:  Should the motor be lubricated and how? (some
>motors don't have any holes for oil)  What causes this
>slow start?  Is the projector in any risk when the fan
>does not kick it for 10 seconds?

You didn't mention what kind of motor. Low voltage DC? 110VAC?

Low power AC motors typically are induction-type (no carbon brushes).
When driven by normal single-phase AC, there's a problem in getting
enough torque at startup - sort of like getting a foot-pedal powered
sewing machine going. Larger inductive motors use a second set of coils
with a capacitor in the circuit to get the current ~90 degrees out of
phase, to create a rotating magnetic field to get the motor started -
a centrifugal switch cuts off the second circuit above a certain speed.
The small induction motors I've encountered usually have some asymmetry
built into the structure to put a sort of "twist" in the electromagnetic
field, to help get the motor started. This also reduces efficiency and
causes the motor to run hotter, but it's more compact and cheaper than
the capacitive start. If there's a lot of friction, the limited torque
at startup could take a while to get the motor going. Also the heating
could warm up the lubricant or change the dimensions to reduce friction.

You didn't mention some other information that would be useful for
diagnosis:
 - Does the motor buzz or vibrate during the process of starting up?
 - After it's been running for a while, if you turn it off for a few
     seconds (long enough to stop completely) and then reapply the power,
     does it start right up, or is there another delay?
 - Can it be persuaded to start running sooner if you give it a little
     nudge (being very careful)?
 - With no power applied, does the shaft seem to rotate more freely after
     the motor's been running a while?

I've had some luck taking small induction motors apart and lubricating them 
from inside. Some are held together with long machine screws parallel to the
motor shaft, others have metal tabs that have to be bent. There may be metal
cups around the shaft containing felt rings for oil. I would ordinarily use
sewing machine oil. Given your extensive experience with Realist equipment,
I assume you know to watch for small parts that fall out and wires that
come loose, and you are aware that despite all precautions a repair attempt
will occasionally end in failure.

A few times I've encountered motors where the *windings* failed - sometimes
they short out in such a way that the motor turns very slowly and has very
low power. If that happens, I don't know of any repair short of replacing the
windings (which I don't know how to do).

Note that some higher powered motors are packed with grease instead of light
oil. Some extremely small motors appear to use teflon (or other plastic?)
bearings, and never need to be lubricated. Putting oil remover in the motor
sounds very risky (in terms of reducing the lifetime of the motor).

John R


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