window a/c intermittant fan problem


  #1  
Old 10-04-02, 11:00 PM
mthornblad
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window a/c intermittant fan problem

I have an old Kenmore 28,000 BTU window a/c.
The fan motor runs for about 5 minutes and goes
off. The compressor continues to run. In approx.
5 minutes the fan motor comes back on.

Please help..

Thanks

Mark
 
  #2  
Old 10-05-02, 06:22 AM
bigjohn
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Hi Mark:

Inside the motor is a device called the thermal protector. If the motor overheats or draws excessive current, the TP opens and shuts off the electricity to the motor to keep it from burning up. When the TP cools off, it resets itself and turns the juice back on. It sounds like your motor is cycling on and off on the TP. Most likely scenarios are the fan motor capacitor is bad, the motor bearings are dry, or the motor is bad. Pull the unit out, [get help it will be heavy] and look for some oil ports on the top of the motor at the end bells. Use some 3 in 1 or similar oil [not automotive oil/the appliance parts places sell a small plastic bottle of oil with a telescoping spout. they're handy for hard to reach places] and try oiling the motor. Don't drown it in oil. After the first shot of oil, spin the blades around and give it another squirt. Put the unit back in and try it out. If it's still acting up, follow the motor leads back to the capacitor. Try replacing the capacitor. [about $10] If it's still acting up, then you'll have to decide about replacing the motor. The motor won't be cheap. It's dealers choice as to how much money to put in an older unit. Let us know how you make out.
 
  #3  
Old 10-06-02, 01:51 AM
mthornblad
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bigjohn,

Thank you so much for your help. You are the
man!!

I followed your advice and determined that the
fan motor capacitor is bad. I tested it with
an ohm meter and the needle didn't budge. To
make sure, I tested a good capacitor and the
needle rose and then dropped.

I looked up the model number of my 1971 a/c
unit and was able to come up with a part number
for the capacitor. The only problem is, Sears
wants $22.50 + tax + shipping + handling.

If I knew the rating for the capacitor, I could
buy it for 5 dollars. I can't read the numbers
on the defective one. All I can read is
??/370 vac. I can't make out the numbers before
the 370 vac.

Is there any way to determine the ?? in the above
??/370 vac based on the BTU of the unit or the
fan motor specs?

One last question. What is the basic function
of a fan motor capacitor and what is a micro-
farrad? (in layman's terms).

I guess that was 2 questions.

I greatly appreciate you sharing your expertise
and knowledge with me.

Thanks

Mark
 
  #4  
Old 10-06-02, 12:32 PM
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Question Fan motor capacitor ??

The fact that it will run for 5 minutes suggests a motor bearing is getting hot and seizing up.
if the cap was bad the motor wouldn't start at all without a "push" to get it going.

If it was me I would replace the whole unit, a 1971 28,000 BTU unit uses a LOT of juice.
Unless you are trying to cool a warehouse or your house is completely uninsulated 28,000 is overkill, you would be much better off with a smaller unit that ran longer cycles, your electric bill wouldn't be $200+ either ...
 
  #5  
Old 10-06-02, 01:49 PM
bigjohn
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Hi Mark:

First thing to do is look on the label on the motor and see if they indicate what size it takes. If you know the motor horsepower, you can sometimes guess, but it's not a good idea. At 28,000 btu, your unit probably has a 1/2 or 3/4 horsepower fan motor. A 1/2 hp motor usually uses a 7.5 or 10 mfd capacitor while a 3/4 hp motor could be 10, 12.5 or 15 mfd. It all depends on the design of the motor. When replacing capacitors, there is a + or - 10% tolerance allowance of the mfd rating. So you really need to use the exact one for your motor. Even with the shipping/handling your cost shouldn't be much over $30 which is still a lot cheaper than a service call. The capacitors job is to regulate the current flow thru the start winding. Farads are units of measurement of capacitance and one farad is a large amount. One microfarad is .000001 fardad. I think the farad is named after Micheal Faraday. Since you have the unit out anyway, now would be a good time to oil the bearings and wash out the coils. I don't want to argue, but my experience has been that a motor with a bad run cap will start on it's own- maybe not the motors produced today, but a motor made in 1971 will have a lot more copper in it than todays motors. Let us know how you make out.
 
  #6  
Old 10-06-02, 04:07 PM
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Question Startup

All of the motors I have seen with a bad cap will continue running once they are started by hand.
If you have a bad cap (hence AUX/Start winding gets no power) how does the motor know which way to start turning ??
Isn't magnetic field rotation determined by phase of start/AUX winding ??
I guess eh cap could be SHORTED causing excessive current to go through the start/AUX windings and cutting off the thermal protector ??
Check and see if the motor spins freely after the fan "shuts off"
Maybe the old motors are different ....
 
  #7  
Old 10-07-02, 01:07 AM
mthornblad
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54regcab,

I appreciate your attempt to help, but you are
mostly incorrect.

1. The fan motor starts even when the capacitor
was removed.

2. The motor is over-heating but is not seizing.
When the thermal protector stops the fan
motor, the shaft still spins freely.

3. I am cooling a 24 x 50 foot (1200 square ft.)
storage/work building. Believe me, 28,000
BTU is not over-kill.

You are 100% correct that the unit is totally
ineffecient. 7.2 EER to be exact. However, I
only need to use it for a couple of hours on
weekends for a total of 12 hours max a month.
At 7.5 cents per KWH, I figure it will cost me
less than 5 bucks a month to run the "dinosaur".

I am convinced that replacing the fan motor
capacitor will solve the over-heating problem.


Mark
 
  #8  
Old 10-09-02, 01:50 AM
mthornblad
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bigjohn,

I replaced the capacitor. I bought a 7.5/370
for 8 dollars. You were correct. The plate
on the motor, when I finally was able to read it
said to use the 7.5/370.

With the new cap, I noticed that the motor got
up to full RPM almost immediately. However, the
thermal protector is still shutting it off after
a couple of minutes. The motor is still getting
hot. Starting from cold, I put my hand on the
motor and felt it getting hotter and hotter. It
gets to the point that it is too hot to touch and
then it kicks off. When it does stop running,
I spin the blade by hand and it spins about 10
times. It seems to spin freely even when it is
hot. It doesn't make any unusual noise when it
is running.

I find it hard to believe that this much heat
is being generated so quickly by friction. If
the shaft was binding, then maybe. But it seems
to me (a total electrical moron) that the heat
has to be caused by an electrical problem. I
don't know, but it can't be friction.

At this point, I don't care about the cost to
fix it. It is now a matter of solving the problem. If a new motor would solve the problem,
it wouldn't matter. I want to know why this motor
is over-heating !!

Please let me know. I would appreciate it.

Mark
 
  #9  
Old 10-09-02, 06:10 AM
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Unhappy Shorted winding

Sounds like you have a shorted winding in the motor, this would also explain why it would start running without a capacitor

Open the motor up and you most likely will see the burned area where it has shorted, if not an ohmmeter check will reveal which winding is shorted.

In a way it makes me think the cap going bad somehow overheated the motor and shorted the winding ... ?

Sometimes if the shorted winding is one of the low speed windings you can cut it loose and the motor will work (just only on high)

I did this to a 1965 Delco A/C and it was still running when I sold the house 2 years later

Since the motor is trash anyways it's worth a try
 
  #10  
Old 10-11-02, 01:51 AM
mthornblad
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54regcab,

You made me think. Oh no!

When I was moving the a/c, while trying to pick
it up, it slipped and it dropped about 2 inches
right onto the selector switch. The switch that
controls low, medium, or high fan speed.

Needless to say, this broke the switch. Maybe
by crushing the switch, this caused 2 or more
speeds to be selected at once.

I believe the switch has 5 contacts total. 1 for
the power, 1 for the compressor, and 1 each for
each of the fan speeds. A properly functioning
switch allows only 1 speed to be selected at a
time.

The question is... What happens if you apply
power to 2 or 3 speeds at one time? I think that
is what may have happened.

In your previous post, you told me to take the
motor apart and look for a bad winding and cut
it away.

Now that you know what happened, please advise
on the best way to solve the problem. I will
be happy if I can use only 1 speed. The motor
still runs and if I can eliminate the short so it
won't overheat, I will be forever grateful...

Humble but determined...

Mark
 
  #11  
Old 10-11-02, 06:47 AM
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Thumbs up Two speeds at once

Will cook the motor every time !!
You will need to take some ohm readings on the windings and "dig through" the motor.
Finding the "low" windings can be difficult, sometime you will get lucky and they will be wound with a lighter gage wire.
I was lucky in the fact the motor i had was only a 2 speed with one set of windings using a lighter gage wire.
I just clipped the lighter gage wire on all 6 poles and it worked.
I couldn't belive it actually ran but who was I to question it
 
 

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