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revelations22.11

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Discussion starter · #1 ·
I recently went to a job where the exhaust fans was not working. If you spin the fan, the motor continues to work. The motor would not turn the fan when you turned the switch to on. I naturally thought it to be the capacitor. I took the MFD reading and read 5 MFD, so it seemed to be in good condition. I still decided to goto the supply house and get another one and if that was not the problem save it for another time. I switched the capacitors and it started working. When I got home I switched my meter to read ohms and it still seems to give out readings as if its working properly. I didnt check my amp draw, though when taking the fan out, I had to disconnect wires from a brah adapter piece. The only other thingI could think it may have been, was a bad connection on the wires and when I reconnected it, it had a better connection. Open to suggestions, but this does boggle my mind.
 
#3 ·
Sometimes a capacitor appears functional when tested with a capacitor checker and fails when placed under full load. It is possible to have an intermittent short in a wire, a faulty capacitor is more likely.
 
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#4 ·
I've had this problem before as well. In my case, the capacitor terminals were corroded enough to prevent proper contact to the connectors. Cap tested good but fan wouldn't work. After some filing and brushing, I reconnected and it worked fine. Were the terminals on your old cap corroded?
 
#5 ·
Is it possible the capacitor has high ESR? I've seen this a LOT with electronics, not so much testing done with motor caps. High ESR caps will check good with many cap checkers but have a high impedance when an AC load is applied.
 
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#7 ·
revelations22.11 said:
When I got home I switched my meter to read ohms and it still seems to give out readings as if its working properly.
Click to expand...
What readings from the "ohm" setting tells you it's working properly ???

What does ohming the cap tell you??

Curious.
 
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#8 ·
In my opinion, the new cap being final resolution or conclusion on replacing a motor soon is the flip of a coin.

I advise my techs to test a cap in this procedure.

Amp draw first, prior to even wiggling a wire off the cap, if no amps, most likely open cap, verify next with cap tester. If open, def replace, if upon removing wires, they are rusty, our techs can clean terminals with small file and test again. Even at this point, we recommend new cap and terminals for longevity as once you clean them, they will rust even quicker as now the protective coating, on the terminals, in the production of the cap is removed by cleaning.

If amp draw present, disable power, verify residual power remaining in cap is discharged, remove one (common if possible) to test with MFD tester. Removing only one terminal eliminates the possibility of damaging cap terminals by 50 percent on a cap or more on a dual cap, when removing the wires.

We prefer to use cap tester vs multimeter, for this reason only. When we remove the cap and if it is taken it to the consumer, they seem to have more trust in a device connected to the cap with MFD tester on it, such as Supco MFD tester, than they do a multimeter which looks very complicated to them. Clean and cut, if the readout they see is other than such the rating on the cap, replace. Always advise the motor may still have winding damage to be tested after a new cap is installed. Some consumers enjoy being involved in the visual verification of a bad component, and further builds trust with them.

After new cap, another amp test to verify current to winding, if current is not right, and voltage is correct, replace motor, which would have required a new cap anyway, so no real waste of time.

This method of testing, was endorsed and published in the Amana servicers handbook July of 2007 as their preferred method of verification of PSC motor/cap testing.

We all have different ways of procedure, I push our techs to be consistent as to help remove doubt in their decision after a repair.

"Procedure vs OCD"
 
#9 ·
revelations22.11 said:
When I got home I switched my meter to read ohms and it still seems to give out readings as if its working properly.
Click to expand...
This test will not give you the current capacity vs rating of the cap, only tell you if it's cycling properly, i.e. Open, shorted, Charging. A multi-meter or MFD tester can tell you the MicroFared's currently being detected.

"Procedure vs OCD"
 
#10 ·
View attachment 259251

Start winding or on cond fan and indoor blowers with cap use neutral side cap line for amps X 2650 divided by active volts under load of cap. This is the real deal of is the cap shunting over to the dual side and a drop of volts is happening etc. Using this method may be inherently dangerous to measure but is spot on for "no callbacks". Motor shell temp after 10 min's run time is also a must to leave the motor and walk away!
 
#11 ·
CaptHook said:
After new cap, another amp test to verify current to winding, if current is not right, and voltage is correct, replace motor, which would have required a new cap anyway, so no real waste of time.
Click to expand...
I'm assuming you are speaking of current of the start winding and voltage across the capacitor?
What should the voltage and current be? If the cap is good, aside from an improper frequency, how would one be correct and not the other?
 
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#12 ·
mgenius33 said:
I'm assuming you are speaking of current of the start winding and voltage across the capacitor?
What should the voltage and current be? If the cap is good, aside from an improper frequency, how would one be correct and not the other?
Click to expand...
Exactly, good point, one can only verify at the moment results of active volt & amps to gain under load uf. If is rated at 10uf/440v is it at 122^ ? It happens to be 75^ so what now? Plus most tech forget a cap is really just a incredibly fast on/off switch of spiking release voltage to jump ahead of the same bus line constant voltage, this may or not be at 60 cycles hertz. Most think why bother dude, replace the cap and motor and move on!
 
#13 ·
Ohmmeter readings

superd77 said:
What readings from the "ohm" setting tells you it's working properly ???

What does ohming the cap tell you??

Curious.
Click to expand...
If the needle of the meter shows a deflection (analog) to the right end of the scale and back to infinity, the capacitor is probably good. If the needle comes to rest on 0 ohms, the capacitor has a short. If the needle doesn't move, it's open. This is pretty much right out of the book...
 
#14 ·
superd77 said:
What readings from the "ohm" setting tells you it's working properly ???

What does ohming the cap tell you??

Curious.
Click to expand...
he didn't ohm the cap, he microfarad tested the cap, but he ohm tested his leads on the meter...

at least that's how I read his post.:cheers:
 
#16 ·
yes, Sorry for being unclear,, speaking of the voltage line to the motor being correct, and the load on the motor correct, then if the amps to the winding are off, there is probable internal damage to the motor. (bows)

Nice Catch,, (bows)
 
#17 ·
CaptHook said:
We prefer to use cap tester vs multimeter, for this reason only. When we remove the cap and if it is taken it to the consumer, they seem to have more trust in a device connected to the cap with MFD tester on it, such as Supco MFD tester, than they do a multimeter which looks very complicated to them. Clean and cut, if the readout they see is other than such the rating on the cap, replace. Always advise the motor may still have winding damage to be tested after a new cap is installed. Some consumers enjoy being involved in the visual verification of a bad component, and further builds trust with them.
Click to expand...
Bingo. Anytime I have the opportunity to show the customers the problem whether it's the heat exchanger, fan motor, circuit board I do it. Seeing is believing. Especially in this business. The trust it builds is so rewarding.
 
#18 ·
JBM1000 said:
Exactly, good point, one can only verify at the moment results of active volt & amps to gain under load uf. If is rated at 10uf/440v is it at 122^ ? It happens to be 75^ so what now? Plus most tech forget a cap is really just a incredibly fast on/off switch of spiking release voltage to jump ahead of the same bus line constant voltage, this may or not be at 60 cycles hertz. Most think why bother dude, replace the cap and motor and move on!
Click to expand...
Love the chart you linked to in your last post. Caps should not significantly change in value based on temperature (if they do they are bad). Frequency is determined my the mains power, not capacitor or motor, will always be 60hz in the USA. The amp/volt test is very accurate, and even accounts for ESR in the cap. A capacitor is NOT switch, it simply changes the PHASE of the power going to the start winding. The difference in phase is what creates the rotating magnetic field required to run the motor.

http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/circuitsphase.htm

http://www.allaboutcircuits.com/vol_2/chpt_13/9.html

All that being said when it's 100 degrees outside it's easier just to replace the cap and test. If the new cap doesn't fix, install the motor.
 
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#19 ·
[QUOTE The difference in phase is what creates the rotating magnetic field required to run the motor.

You are correct it is only a phase shift for a capacitor, however getting a Tech to not be a deer in headlights I've used the "incredibly fast on/off switch" to at least try to "fix stupid"! One has to find methods of madness that may work to get a Tech to understand. Mention Wet Bulb, Sub Cooling, Saturation Point or anything other than "beer can cold" and your back to a deer in headlights all over again.
 
#20 ·
Yes, I believe it. Unfortunately this is what causes many of the installation/design mishaps that we go around fixing. Supply and return air are something EVERY tech should understand, if we get that done peoples A/C units would deliver a lot more comfort. When I see an install I can immediately tell who did the work. Sometimes it's the top techs, sometimes it's the guys just starting out. On builder installs few companies can afford to pay their top techs to do the install and still make a profit on the job. The best companies in our area don't even do new construction work because they have plenty of more profitable work.
 
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