Grinder that makes the hummmmmmmmm sound

In other words, replace everything and hope for the best. Gotcha. Take the lazy way out and never learn anything.

How many companies do you think make motor start and motor run capacitors? Especially the ones in the molded plastic cases? How much variation in ESR do you expect between those brands? If any, then you would have to use the exact same capacitor as the OEM for every repair. A house number is meaningless, if you know the capacitance and operating voltage. All the house number does is make it easier in house for the assembly drones to put in the right parts. also, the capacitance can be dead nuts on spec, yet a high ESR will cause the capacitor to heat. Even new capacitors can be defective.

Really? How much variation in ESR do you calculate for a 30% change in capacitance?

Neither grandmother said any such thing. One said to work hard and do your best at whatever you do. The other grandmother rarely said anything, but then she was an old grouch, like you.

I would expect that huge bug up your ass would cause you enough discomfort that you would want to have it removed, but I guess you've grown fond of it instead.

You seem to know it all, have done it all, from building space craft to transmitters.. OH, and a dd form 214. But, you're clearly misinformed about capacitor ESR.

Heat isn't the major circuit problem with borderline ESR readings in caps. High ESR caps essentially appear as open-circuit/reduced capacitance, failing to filter the ripple. In SMPS electrolytic caps, internally generated heat is a result of high frequency, high ripple currents by design.

The ripple currents create internal heat in the caps, accelerating drying out of the electrolyte, and then the cap's ESR begins to increase. There is going to be internal heating before high ESR even contributes to circuit problems, not the other way around.

The cap being discussed is operating at 60Hz, not tens of kHz.

The primary B+ filter caps in SMPSs, operating at 120Hz don't commonly have high ESR readings, even after many of the secondary-side caps have become ineffective as filters, and the SMPS shuts down (or melts down).

Replacing the start cap in the grinder motor that's been showing signs of difficult starting, is being thorough. To not replace it would be ineffective, lazy, and hoping for the best outcome.

The OP should halt the simple, inexpensive repair and buy an ESR meter that he might find a use for about 10 time per year, because you insist that the repair isn't done properly because he doesn't know the ESR of the already stressed cap, that costs $4 (he stated).

What he won't know is the leakage current, value change or dielectric absorption characteristics.

So genius.. what ESR do you have in your lookup data for a motor start cap marked 435-580uF 125VAC, or a 125-270uF 377VAC cap, or one that's just marked 136uF -10/+20% 120VAC? You're not going to find any ESR numbers that are published by the manufacturer, such as 3 ohms max, or any other specific ESR reading. What you may find is a vague statement such as less than 20 ohms, even if 2 ohms would be ideal for any particular value and series of their caps. The value and voltage markings don't offer a clue as to what materials the motor start cap is made of.

Caps operating at 60Hz aren't especially susceptible to high ESR.

Maybe next time you can insist that the ESL be checked.

My thinking exactly. The bearings are 6204, as I found out, they cost $7 a piece from McMaster-Carr, and the cap costs $3.64. It would be a much easier sell if I said that I replaced both bearings as well as a cap, and I would feel good about what I have done, so it is a no brainer. Hopefully tomorrow I will have both.

i

On Tue, 13 Oct 2009 07:53:26 -0500, the infamous Ignoramus14507 scrawled the following:

Both bearings + new cap allow you to put "Fully Rebuilt Motor" in your sales text, too.

-- The best and safest thing is to keep a balance in your life, acknowledge the great powers around us and in us. If you can do that, and live that way, you are really a wise man. -- Euripides

Awesome. I am a sucker for fixing machinery that needs light fixing. It is not the most profitable activity, compared to alternatives, but very satisfying.

i

Wild Bill, replacing the starting cap took care of the starting problem quite nicely. Now the grinder starts strong every time.

Now all I have left is changing the bearings, which I can hopefully do tomorrow when they arrive. Then this grinder will be as good as new.

:wq

Yawn... Nice little rant, but boring.

Most important of all, I am willing to learn new things, unlike a lot of people. I have talked with engineers about the quality problems and was told companies like Sprague (Before they were swallowed by Visahy) that they could make better capacitors, but no one wanted to pay the price as long as something would last through the warranty period.

No. I have studied the problem quite thoroughly.

Bullshit. When the ESR first starts to rise, the higher resistance causes the capacitor to run hotter. That causes the electrolytic to dry out faster, which raises the ESR. Over time the rate increases, till the capacitor no longer allows the circuit to function properly. Of course by the time you notice it, it will be high enough to be an effective open circuit.

Frequency has nothing to do with it, but it is more common in SMPS output circuits because of the tiny packages with poor seals and not enough surface area to radiate the waste heat. That causes the higher failure rate.

Back in the tube days a lot of electrolytics used in voltage doublers in transformerless TVs died the same way. The only difference was the ESR was measured at 60 hz, and was called 'Power Factor' on the most common capacitor analyzers. If you looked at some of the old datasheets, you would see some electrolytics that listed the rated ripple current. The most common value was 160 uF @ 250 VDC. Most were easy to spot. They vented, exploded, or caused hum bars on the screen. They also had a paper insulating sleeve over the ungrounded aluminum can. We used to buy them by the dozen, because we saw so many failures.

It doesn't matter. Ripple current is ripple current. The difference is that motor capacitors are designed for a longer service life.

They do fail with high ESR in some designs. I have a pile of dead ATX power supplies with bulging electrolytics on both sides of the transformer. Think about it. The input current is a couple amps with a couple large capacitors, and the output can be 35 amps or more, with smaller physical capacitors that can't dissipate the heat. then they are grouped together and usually covered with the bundle of wires that run to the motherboard.

Where did I say you shouldn't replace it? I said to test it to see if it was failing.

BUZZZZZZ!!!! A regular ESR meter won't work at 60 Hz. All you need is a scope or AC voltmeter, a low voltage 60 Hz source and a few parts. Here is an example:

that costs less than a dollar to build. The same circuit can be used with a 1 KHz source to measure speaker impedance.

Sigh. There is nothing vague about the published specs. You are NEVER going to achieve the 'ideal' ESR with an electrolytic. Its the nature of the beast. If you want to approach ideal ESR you need to use a good grade of film capacitor, and make sure it can handle the ripple current involved. As far as dielectric adsorption and leakage, how are you going to measure it? The reverse biased capacitor is a rectifier, with some forward voltage drop. Any dielectric adsorption will disappear into the run winding when the power is removed.

Bull. If this were true, you would never have a bad motor run capacitor.

If you want to test it, go ahead. It isn't much of a problem at low frequencies, and doesn't change as an electrolytic ages. Some low ESR electrolytics are built from four parallel capacitors to reduce the ESL. Show me a motor run capacitor built that way.

Repairs can be very gratifying, and better yet when they're easy and inexpensive.

As others commented, the capacitor wasn't the only possibility for the starting problem, but the new cap is a good value for yourself and the next owner.

I have a hard time passing up a deal on a used split-phase motor. They generally don't need much in the way of parts or effort to return them to a condition of completely reliable operation.

For split-phase motors, examination and cleaning of the centrifugal switching components, and particularly the condition of the switch contacts, can often prevent future problems. If the contacts are in good physical condition, just cleaning them to remove oxidation may be all that's required. I avoid using files or abrasives whenever possible, as the contact surfaces last longer when smooth and slightly crowned.

The least damaging method I've found is to polish the surfaces with cardstock (file folder cut into strips) saturated with DeoxIT. I place the strip between the contacts, apply light finger pressure to close the contacts, and pull the strip thru numerous times, then use a dry strip.

My thoughts exactly.

Another fortunate thing is that the new cap is exactly the same form factor as the old one, which means that it will nicely fit into the bracket.

I have hard times passing up ANY motor. I am a sucker for used motors. I almost always make money from them one way or another, or just have a good use for them, so being a sucker does not hurt as much, but it may seem strange to some people.

i

Motor caps are AC caps, not DC. DC you can charge up and play with blowing off the tip of your tongue. More than likely it is open or shorted, but leaky can cause more issues. DC testing would have to be at the rated voltage. Chemicals within are not set up for DC.

They sometime have internal resistors (ballasts).

Not hard to change.

Mart> Larry Jaques wrote:

They are a pair of DC caps connected in series. The reverse polarized capacitor acts as an elecrolyitc recifier. The two capacitors exchange their function with each half cycle.

If you have one on hand. Using the proper test will tell you if its bad before spending time & money to locate one. You can jury rig a 60 Hz ESR meter for under a 'pre Obama dollar'.

Maybe it just doesn't know the words.

Let's see, $600 for a new ESR Meter, or $4 for a new capacitor that you substitute in and see if it fixes it - or just change it out on general principles because the original is 20 - 40 years old...

Unless you are doing this all day, every day, 300 or so days a year, that seems like a true No-Brainer to me. You end up stocking a few spare $4 capacitors, and swap it out to test.

And you wait to find a used ESR Meter at an estate sale for $10.

That's all well and good /if/ you have the meter.

-->--

I would think that kludging a ESR meter for motor start caps would be very simple. Since motor start caps can easily be removed and tested by themselves, I would think they could be checked at 60 hz. No real need to use high frequencies so you can test without removing the cap from the circuit. Besides the cap is going to be used at 60 hz so why not test it at 60 hz. .So I am thinking a low voltage transformer with a resistor in series with the secondary. Attach the cap to the secondary and measure the voltage across the resistor. You can calculate the impedance due to the capacitance. So you know the max voltage that you might read with the cap is the circuit. And you can short the leads without the cap to see the max voltage with a perfect cap of infinite size.

But I would probably be buying a new cap anyway just to verify that my kludge works.

Dan

I posted a link for a 99 cent ESR meter. It will tell you if the capacitor is bad before buying a new one. I have no problems with replacing a capacitor and the bearings in a complete overhaul, but why buy it if the motor may be bad? A 20 - 40 year old motor can have a bad winding too. Are you going to rewind it, just in case? How about a new start switch? Don't forget to replace those pesky terminal boards, and to repaint it, too. Here is the link, so you can ignore it, again:

Sigh. a closed mind is a no brainer. if you can't assemble and use the 99 cent ESR meter, you shouldn't own any tools.

I bought the original Bob Parker ESR meter kit about eight years ago for a little over $50, but it isn't intended for 50 or 60 Hz use.

I do. I

[ ... ]

You *are* assuming that he has an oscilloscope. If he doesn't, the cost will be well in excess of the 99 cents suggested.

I have a 'scope. (Actually, several) You (presumably) have a 'scope. But does he? You did not ask that before pushing the "99 cent ESR meter".

Enjoy, DoN.

I posted about the 99 cent ESR meter several times, and included the comment that a AC voltmeter could be used in place of the scope. I also stated that a small transformer could be used to get a low voltage 60 Hz source, in place of the function generator. Most people on the group could scrounge a usable transformer from an old wall wart, find a couple suitable resistors and a box to mount it in, for nothing.

Too many to put on a single workbench. HP, Tektronix, Heathkit, Magnavox military scope and some junkers.

Whatever you say, Don. A scope is handy, but not required as I pointed out several days ago.

You da man! You inspire us all.

You're likely aware of capacitor faults, DoN, but I've included some comments to clear up some of the misrepresentations made by another poster in this thread.

FWIW, inexpensive, improvised ESR indicating circuits have been online for about 10 years. They aren't ESR meters, and don't give a direct ohms reading. The improvised circuits are essentially for comparison to known good capacitors. Another improvised circuit:

The improvised ESR circuits require more guess-work if the user isn't experienced with the circuits' indications, also oscilloscope and signal generator experience/familiarity.

The authors of the circuits don't state any use of their circuits, or anticipated results for testing AC motor capacitors, which are manufactured differently than DC electrolytics. Also, AC motor caps have different recommended allowable ESR specs. Additionally, AC capacitors' markings and value tolerances are different than DC electrolytics. The authors do state several limitations of the improvised circuits, such as shorted or excessive internal current leakage in electrolytics.

If testing ESR at 60Hz was practical, the engineers and designers such as Bob Parker, Sencore or others wouldn't have needed to develop more complex test ciruits. ESR testing at 60Hz won't result in any worthwhile indication of in-circuit capacitor ESR, and only widely varied, inconclusive results for out-of-circuit testing.

All capacitors have a certain amount of ESR, which normally doesn't affect circuit operation (assuming that the proper capacitor is used in the circuit). The function of an ESR meter is to ignore the capacitance and indicate the Effective Series Resistance of a capacitor being tested.

My position (and lengthy experience) on testing for ESR (only) doesn't show the condition of the capacitor, and the person doing a ESR test won't know if the test results are worthwhile if they didn't have past experience in testing AC motor capacitors. Referencing available data for acceptable ESR in DC electrolytics won't be applicable to AC motor caps.

I rely as much on Leakage and Value tests, as I rely on ESR results. Testing multiple charcteristics results in a much more thorough evaluation of suspected capacitors and various other components. I have up to 1kV capability for Leakage tests, so relying upon DMMs and improvised circuits is of the question for reliable evaluations (no guessing). With the above testing capabilities, plus Dielectric Absorption testing, it's not likely that weak/borderline faulty capacitors won't be detected. With a dedicated analyzer, it's very easy to complete all the tests in less than a minute, with the exception of Leakage tests on very high Value/capacitance electrolytic capacitors which require more time to charge.