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Author Topic: 400 volt cap goes from 5mfd to 500mfd?  (Read 1322 times)
W7ETA
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« on: May 11, 2009, 08:29:23 AM »

According to the repair tech, a run capacitor for a fan on my heat pump, increased from 5mfd to 500mfd?  
Is that possible?
I looked at the cap.  Other than being dusty, and a few light rust spots, it looked fine.  It was around 7 years old.
I didn't pay to much attention to his Fluke meter--it looked as if it was autoranging.

It was a dual cap, 35 and 5 mfd at 400 volts.  His meter read the 35 section as 31.8.

Its a good thing my roof is flat, cause he quoted me a price of $225 for the replacement cap--I almost fainted!  I ordered one on line, as a replacement part for my heat pump for $38, and chucked the old one when it arrived.  For some reason, I just threw the old one out without measuring it.  Guess the effects from having the temps in my house at 88F lingered for a few days?

Best from Hot Tucson
Bob
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AC5UP
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« Reply #1 on: May 11, 2009, 09:07:16 AM »

According to the repair tech, a run capacitor for a fan on my heat pump, increased from 5mfd to 500mfd?
Is that possible?
_____________________________________

Yeah... If you're reading the x100 scale on the meter. Wink

AC motor run caps tend to go a long time before they fail, but they are mortal. Assuming the reading was correct the more likely scenario is that it developed a partial short and faked out the meter. I have yet to see any cap gain capacitance as it ages but that doesn't mean it can't happen. (?)

BTW: I have a beer can electrolytic that I used to trot out to ARC meetings for amusement purposes. Came out of a 70's vintage Telex power supply with a +5 VDC bus rated at 50 amps or better. Looks and feels perfectly normal in all respects until you shake it... Then it sounds like there's a golf ball inside.

Whoa!
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K5DVW
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« Reply #2 on: May 11, 2009, 09:15:05 AM »

I guess it's possible to get a reading like that if you had a short between the plates that is now barely open. That doesnt say anything about the integrity of the capacitor tho.

The meter might read higher in capacitance, but if you tried to pass current thru the capacitor or put voltage on it, you would have a problem.

Have you ever seen a car battery read 12V unloaded even if one of the cell plates has fallen apart? It's fairly common. You can't get any current out to turn the starter motor, so the battery is bad, but reads good on a volt meter. It will read very low voltage under load. I guess my point is a meter reading doesnt tell you the whole story.

Keep cool.
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WA3SKN
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« Reply #3 on: May 11, 2009, 12:11:02 PM »

Can a capacitor value change?... Yes!
In fact, capacitors are second only to electron tubes when calculating "Mean Time Before Failure".  And the tolarence values of electrolytics were -50 to +100% of value, brand new... right out of the box!  That is, before "computer grade" was invented.
Ahhh... "The Good Old Days"!
That doesn't mean he measured correctly though, too bad you threw it out... but you can't keep everything!
73s.

-Mike.
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W5DWH
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« Reply #4 on: May 11, 2009, 12:22:27 PM »

In the old days caps were rated +80%/-20%.

In over 25 years in electronics I have never seen a cap increase in value by that amount. As a cap gets older and starts to dry out it decreases in value, until it finally reaches open (infinity)
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KA5N
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« Reply #5 on: May 11, 2009, 12:29:51 PM »

Better question: "Have you ever seen a repair technician who greatly overcharged for a simple repair and made up a story to cover the lie?"

Allen
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VK1OD
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« Reply #6 on: May 11, 2009, 04:49:02 PM »


Bob, you said mfd, did you mean µF or mF, 1mF=1000µF?

A run capacitor would not usually be electrolytic, so the explanations related to electrolytic caps are unlikely to be relevant.

A simple multimeter capacitance test is not a comprehensive test of a run capacitor. Some simple capacitor testers reliably produce good answers on very low leakage capacitors, but their readings on other capacitors may be less useful which questions the worth of measurements. If you start testing a range of capacitors, you may be surprised that they are not the ideal component we usually assume.

Owen
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KC8VWM
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« Reply #7 on: May 11, 2009, 05:13:57 PM »

Neat trick...

The "repair tech" must have been a magician in his former career.

Always pay attention to the magician's hand. There are various levels of misdirection that magicians use to trick an audience using diagnostic metering equipment to create the perfect technical illusion for the unsuspecting consumer.

I usually respond to this with an illusion of my own.

First, I take my right hand placing it carefully into my back pocket. Next I pull out my billfold and exclaim "Viola.. Look!" ... all the money that was previously inside, has now somehow magically disappeared!

Absolutely amazing.. isn't it?...

...I usually ask these magicians if they wanna see my trick again just in case they missed it the first time.
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N6AJR
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« Reply #8 on: May 11, 2009, 11:13:56 PM »

I had a  once trusted tech over to do some minor repair on my AC/heater.  then he said he needed to replace capacitor in my AC starting motor because it was loosing farads. OH, said I, how many has it lost,  he says several hundred, a lot, capacitors  that loose farads  are a real fire hazzard too.

Now understand he had been in the house for about a half hour, and had seen the many antennas on the roof and the shack inside with 7 hf rigs and many mobiles and ht's, several computers and so on.  

So you think he would have figured I might have known something about electricity.  So he flips the heater on and points to the piezo igniter which is now sparking and says look,  that capacitor is really bad.


AT this point I finally told him to pack his lying A** up and leave.  I then called his boss on the phone as he was packing and told him what happened.  The boss  said, wow good thing he caght that bad capacitor. I told him he just lost a customer for lying to me.

Ya think with the radios and antennas  he would have figured I might have known what a capacitor was.   and look, baby farads running around,  all over my carpet....

I should have reported them to the BBB .....
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NE5C
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« Reply #9 on: May 12, 2009, 04:00:14 AM »

These days, "A truly qualified" AC Technician, who has gone to College and earns a Degree in Air Conditioning, spends quite an amount of hours and invested money... to even get started in the business.

Many of "The Best" earn anywhere from a beginning labor rate of $17.00/Hr on upwards with experience to $28.00-$30.00 per hour. That does not account for their Provided Company vehicle and Gas Cards, Health Insurance Package, provided and cleaned weekly uniforms, Paid Vacation time and/or Sick leave days, Tool replacement programs, 401K retirement packages, Government EPA Certification license to handle Refrigerants, and/or possibly even a State License which alone can represent around a $1200.00 per year investment. The digital meter to check capacitance (for example made by Supco)can cost around $200.00!

Honestly, when you then add a reliable Fully Insured, Service Van or Truck with GPS Tracking equipment, with Ladder Racks, all the needed various sized step or Extension Ladders, Torch kits and Welding Brazing supplies with Vacuum Pumps, EPA Certified Reclaim machine with EPA Certified reclaim Drums... and anywhere from $5000.00-$8,000.00 on-Board Stock, on this Vehicle and last but not least the Furnished Company Cell phone, and/or Commercial Radios for Dispatching???

Sure...I imagine you can call a few "PO-DUNK SHADE TREE" greenhorns, and YOU CAN get ripped off.
But if you honestly do the Math then you will begin to see what has to be charged, for a TRUE COST of DOING BUSINESS to begin with! That is when you realize that you could have called the BBB to start with and selected a True Professional to support your needs.

It's sort of like that conversation about the cost of a simple step ladder. It really shouldn't cost but about $30-$50 yet...because of the Lawsuits by a few who didnt use it properly to begin with, good ones sell easily for $100 plus. Of course if you didn't provide your workman with the Quality product and it malfunctions and he falls...then He owns your company anyway for providing whats called SUB-STANDARD equipment, which must have caused his or Her, injury!

Got a Gripe? Oh yes I can agree! How about having to go sit in a Dr.'s Office for an hour, before being seen, just because you have to refill a common everyday perscription, trapped in there with 20 other FLU patients, wheezing and gagging with Fever! That is one of my, pet Peeves!  

Yes, Capacitors break down, and in regards to Fan motors, they can cause the motor not to start properly and Over-heat and Bake, and yes, in some cases have caused some fire hazards. You know...one mark of a "PROFESSIONAL" is someone who has done their Job so EXTREMELY WELL, in such a manner that they somehow please many people in the performance of their service who gladly call them again, if the need arises.
Why? because THEY ARE WORTH the service that they provide.

There's a Funny story about the Man who said you charge TOO MUCH - when he called to ask about Service Costs. The Manager then said "OK I'll send you a Man that will cost you $5.00 per hour - How is that? The man asked wow that is great - does he know anything about my Problems? The manager replied "OH NO, but at $5.00 per hour we will send him and let him stand around there all day - GUESSING - about what your problem might be!" But...you'll get, what you paid for!

The Man, quietly, hung up the phone.
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W7ETA
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« Reply #10 on: May 12, 2009, 08:30:42 AM »

Hi Owen
Labeling was MFD, 35 + 5
http://www.cozyparts.com/Lennox-Parts/ProductInfo.aspx?productid=53H27

When I saw the cap, I asked if it was oil filled?  The tech replied it was acid filled.

According to the tech, the caps should measure within 6% of spec, 10% out of spec and they want to replace the cap.  The claim is that more than 10% out of spec causes the motor to generate more heat.  

I've been wondering how a 10% out of spec cap can cause more heat in the AC motor?

Best from Hot Tucson
Bob
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VK1OD
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« Reply #11 on: May 12, 2009, 01:37:09 PM »


Bob,

In this part of the world, M is a prefix for x 10^6 and m is a prefix for x 10^-3. Yes, older practice is to use MFD for MicroFarad, but most of us write µF or uF. (You can buy mF capacitors today.)

I am not saying that the Fluke doesn't read correctly on a low loss capacitor, and that if you get a new, low loss capacitor, that it won't correctly show the correct capacitance give or take a little.

What I am saying is that if it reads some other value, you need to know the loss of the capacitor to know if you should trust the reading.

The problem is that most multimeter capacitance measurements assume the capacitor is lossless and read the capacitance, but if the capacitor is not lossless, the reading is meaningless.

If the capacitor is of a different capacitance, the current in its winding will be almost certainly different in magnitude, and at least I^2R loss in that winding will be different. More often than not, the capacitor has become leaky, and so it will generator heat itself. If the departure is sufficient, the motor might not even start, and its locked rotor current might overheat the motor (which has lost its cooling). As to whether 10% is sufficient, I doubt it in most cases... it would be a quite marginal design that was adversely affected by just 10% change in capacitance alone.

Again, the issue is often one of increasing capacitor loss... and the 10% tolerance dimension doesn't capture that.

If they replace a cap that indicates over 10% error on the Fluke, they are probably playing pretty safe, selling capacitors when in doubt... and they want to fix the device and not get a return call under warranty, so they are probably not too concerned about replacing caps prematurely.

Owen
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AC5UP
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« Reply #12 on: May 12, 2009, 09:49:03 PM »

On a related note, for the past few days I've been tearing down Boat Ankers that have been ratholed in the garage for 20+ years. I'm talking about tubular test equipment with enough mass to have their own gravity field.

The current piece on the bench is an ME-62 audio phase monitor. Excellent build quality and loaded with precision components. Pulled about two dozen Sprague Vitamin-Q foil caps (.22 uf @ 400 wvdc) of the mil type that bolt to the chassis and feature epoxy end caps. Also pulled eight 35 uf 400 wvdc electrolytics plus another two dual section 35 + 35 uf 400 wvdc jobbies, all made by Sprague with a 10-57 date code. Like the smaller caps they bolt to the chassis and are not the axial or twist-lok consumer type.

Before I tossed them I ran them through the HP LCR meter.

Every one of the electrolytics checked good and the Vitamin-Q's were almost spot-on.

We're talking 50+ year old parts and for all I know they'd crap out muy pronto if I took them up to 350 volts, but considering the less than optimum storage condx I am impressed. Worst electrolytic in the bunch came in at 38 uf, best was at 49 uf. Average for the lot was a shade over 40 uf and none show any sign of leakage.

Someday I plan to restore some old console radios and that's why I'm stashing vintage parts. Could end up being a waste of time, but... Ya' never know...

BTW: Tore down a late 50's vintage universal counter / timer over the weekend that used a buttload of 5963 tubes in the decade counters. Interesting parts included a power transformer with an HV winding rated 620 VAC CT @ 210 mils and a pair of 6.3 VAC filament windings @ 10 amps plus the usual 5 volt winding for the 5U4.

The clock oscillator used a genuine Bliley "Ovenaire" crystal with an octal socket and a smokin' frequency of 100 kc. Can you imagine... Best guess is the counter topped out around 500 kc's. Wink
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W7ETA
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« Reply #13 on: May 12, 2009, 09:58:27 PM »

Jeeeze Owen!

You drive on the wrong side of the road, AND you use the wrong term for mfd!

The one that got me was uuf.

I made a list of parts to a two tube xmitter from a 1948 ARRL HandBook, went to a local parts supplier, bought all the parts, but they choked at uuf.  An enterprising person there gave me a 1980s ARRL HandBook so that I could look up what value uuf is.  Nope.  No mention of uuf.

Thanks for all the info.

Best from Hot Tucson
Bob
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AC5UP
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« Reply #14 on: May 12, 2009, 10:03:14 PM »

Correction: The Bliley crystal is a TCO-2L "Temperature Stabilized" unit. Ovenaire is a whole 'nother outfit.

BTW: The socket has 1-57 marked on the bottom in orange ink.

Whoa.
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