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Discussion Starter · #1 ·
I like to think I know my electrical theory really well but this one has me stumped and it bugs me that I don't have a answer for my tech that worked on it.

motor is in a furnace its single phase 120v 60hz motor 3/4hp. the furnace is twinned with a identical unit in the same room. (same control, shared ducting)

The capacitor went bad and was preventing the motor from starting, pretty common issue. the capacitor that was bad was a 7.5microfarad capacitor 400v achttp://www.tenta.com.tw/prodp019.html. tech replaced it with a 7.5 370v (the standard electorlytic type that comes standard with almost all motors)

although now with the new capacitor when the motor is running it makes a odd humming noise(i have not heard it myself just going by the techs description) that stays as long as the motor is running.

Now of course i said it was probably the bearings were on there way out but he said he swapped the capacitors between furnaces and the other one started making the same noise while the orignal furnace became normal sounding...

All capacitors were metered out on several flukes to make sure they were 7.5 microfarads and both are AC rated.

Any ideas?
 

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THE "BIG RED MACHINE"
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maybe centrifical switch is bad not letting motor come out of start
maybe might have to change the internal switch not not necessarily the centrifugal component.
 

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Yes...you need to get the proper capacitor.

Even though the 7.5mfd is the same, the construction is not. The original was a
metallized polypropylene film and the replacement is standard electrolytic.

That is critical, they are totally different in electrical properties, that is why the electrolytic one creates a hum.

The electrolytic one will eventually burn up the motor, so change it sooner than later.

 

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It shouldn't be an electrolytic if it's a motor cap. Pick up another cap or maybe a different brand and give it a try.
 

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Discussion Starter · #6 ·
Guess calling it a electrolytic capacitor is wrong.(gonna blame my useless instructor in school for this as this is what he showed us as an example for electorylic type) its just your standard silver round or oval capacitor that comes with almost every furnace motor you order.



Now obvously the correct answer is to get the correct part but I would like to understand the theory behind it. Why is it causing the motor to make this sound? What other electrical properties are there to a capacitor? I understand there are many different ways of constructing a capacitor but you see the silver can style on probably 90% of motors. I'm sure weve replaced the film type with these before with no issues.
 

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Probably similar to transformers with alternating magnetic fields causing it to change shape each half cycle resulting in a humming noise
 

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Put a volt meter across the terminals and measure the peak voltage that the capacitor is exposed to. The voltage rating is just the quality of insulation inside .Try a different cap. And please let us know how it turns out.
 

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Crazyjoker77 said:
Guess calling it a electrolytic capacitor is wrong.(gonna blame my useless instructor in school for this as this is what he showed us as an example for electorylic type) its just your standard silver round or oval capacitor that comes with almost every furnace motor you order. Now obvously the correct answer is to get the correct part but I would like to understand the theory behind it. Why is it causing the motor to make this sound? What other electrical properties are there to a capacitor? I understand there are many different ways of constructing a capacitor but you see the silver can style on probably 90% of motors. I'm sure weve replaced the film type with these before with no issues.
Did he do a load test to see if that changes as well?
I don't why it would, as they said VAC shouldn't matter that much
 

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that's kinda weird. i have a friend that has been in the electric motor repair for over 40 yrs, he told me the uf's have be close but the voltage had to be the same or hi'er . lower would result in something not pretty!:whistling2:
 

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Discussion Starter · #12 ·
So you're saying he replaced one film capacitor with another film capacitor, both of the same capacitance, but the one with a slightly different voltage rating causes a hum?
this is exactly the case and not only that but he can repeat it on the other identical furnace hooked up in the same room. That tenta capacitor I linked is the exact one he removed.

He did say he took current readings and both furnaces were inline with each other and just under nameplate draw. He also took supply voltage readings and said they were fine and solid at 122v(which would be 172v peak well under the 370v that the cap is rated to)

I don't know if he tried another cap but I can't see him running into this issue and not trying another cap especially since we usually have 10+ stocked in each truck. He did however tell me he measured the actual capacitance with 2 different meters to make sure it was within tolerance.

I may take a run out there myself and hook my scopemeter up(fluke123). one probe across the motor the other across the cap and do both furnaces and see if I can see anything different in the waveforms and just to hear this noise myself.

Theres not much that makes me scratch my head but I cant even come up with a wild theory to explain why. I was hoping someone ran into this before.
 

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I'm with lep on this one. Most likely the problem was a bad centrifugal switch to start with that left the cap in the circuit until it fried. That caused the first cap to die, now you have replaced the cap, but the bad switch is still there. The old one probably made a noise until it died, it's just that nobody was listening.
 

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I'm with lep on this one. Most likely the problem was a bad centrifugal switch to start with that left the cap in the circuit until it fried. That caused the first cap to die, now you have replaced the cap, but the bad switch is still there. The old one probably made a noise until it died, it's just that nobody was listening.
He put the new cap on another identical running OK motor and it made that one sound funny also. I'm guessing it's a run cap?
 

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Nice translation from the description in your link. :laughing:

Capacitors are made of metallized polypropylene film and packed by difficult burned plastic box and filled with epoxy
 

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Now obvously the correct answer is to get the correct part but I would like to understand the theory behind it. Why is it causing the motor to make this sound? What other electrical properties are there to a capacitor? I understand there are many different ways of constructing a capacitor but you see the silver can style on probably 90% of motors. I'm sure weve replaced the film type with these before with no issues.
I just caught that you changed the cap with a different style, but still a correct cap. I think you are going to find that the different style of cap is just changing the audio characteristics (resonance) of the motor.
 

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I'm with lep on this one. Most likely the problem was a bad centrifugal switch to start with that left the cap in the circuit until it fried. That caused the first cap to die, now you have replaced the cap, but the bad switch is still there. The old one probably made a noise until it died, it's just that nobody was listening.
Do we know that it has a centrifugal switch?
 

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Now obvously the correct answer is to get the correct part but I would like to understand the theory behind it. Why is it causing the motor to make this sound? What other electrical properties are there to a capacitor? I understand there are many different ways of constructing a capacitor but you see the silver can style on probably 90% of motors. I'm sure weve replaced the film type with these before with no issues.
Among other things, the different capacitor types have differing "ESR" (Effective Series Resistance) which WILL affect their performance in a given application. (Remember a capacitor will pass AC current through it and effectively block DC current through it. So that ESR will change how much current can flow through the capacitor.)

Also, IIRC different caps change how they behave in concert with a reactive load (in this case your motor's windings) so that will change how the motor acts.

So you're saying he replaced one film capacitor with another film capacitor, both of the same capacitance, but the one with a slightly different voltage rating causes a hum?
'Tis not the voltage rating but the dielectric material that is the issue here.

this is exactly the case and not only that but he can repeat it on the other identical furnace hooked up in the same room. That tenta capacitor I linked is the exact one he removed.

He did say he took current readings and both furnaces were inline with each other and just under nameplate draw. He also took supply voltage readings and said they were fine and solid at 122v(which would be 172v peak well under the 370v that the cap is rated to)

I don't know if he tried another cap but I can't see him running into this issue and not trying another cap especially since we usually have 10+ stocked in each truck. He did however tell me he measured the actual capacitance with 2 different meters to make sure it was within tolerance.

I may take a run out there myself and hook my scopemeter up(fluke123). one probe across the motor the other across the cap and do both furnaces and see if I can see anything different in the waveforms and just to hear this noise myself.

Theres not much that makes me scratch my head but I cant even come up with a wild theory to explain why. I was hoping someone ran into this before.
So he needs to get that Tenta cap and I suggest you stock up on them. :)

The scopemeter should show some differences between the two motors..try to post screenshots of what you find on here if you do the test.

I'm with lep on this one. Most likely the problem was a bad centrifugal switch to start with that left the cap in the circuit until it fried. That caused the first cap to die, now you have replaced the cap, but the bad switch is still there. The old one probably made a noise until it died, it's just that nobody was listening.
True, the bad switch will fry the cap..BUT OP says BOTH motors make the noise with the new cap in place (second motor had the proper cap evidently) when they were swapped to test so I don't buy that the switch is causing the noise..unless BOTH motors have bad switches.

Still worth taking a look at that c-switch on the first motor for sure. :thumbup:
 

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He put the new cap on another identical running OK motor and it made that one sound funny also. I'm guessing it's a run cap?
I missed that he had swapped it and the noise followed the cap.

As Rosanne Rosannadanna used to say;

"Never mind"
 

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The noise following the cap makes no sense provided that the replacement cap is the right type (non polarized/AC cap) and value. The slight decrease in voltage rating should not affect performance.

It sounds like he's taking someone else's word that the noise followed the cap. I'd need to see for myself. If it is in fact true, then it sounds like the new cap is defective in some indeterminate way and needs to be replaced.

I am assuming that putting the old cap from the good motor/cap on the "bad" motor causes it to run silently?
 
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