Small Electric Motors - Any Hope?

Measuring from any conductor to the frame: The lower the resistance, the worse the leakage. Properly done with a "megger", any resistance reading below 500K is bad. But then a megger puts out much higher voltage than an ohmmeter, usually 500 volts. Such leakage cannot usually be repaired, it must be rewound. "Dipping & Baking" such motors is a valid repair technique, but usually is not worth the cost for smaller (<25 HP) motors.

It has been my experience that an "analog" ohmmeter gives better results than does a digital. There are circumstances where a digital meter will not show voltage when it is there. A good way to commit suicide. But rare in a "home shop" environment. But then, I have always used "high end" meters, a Fluke digital and a Simpson 260 analog. Your results may vary. . .

There is a class of motors that uses different "impedance" windings to get the phase differential without using a capacitor. Usually relagated to fans and other "light" loads, larger versions can be used for machine tools. These motors will still have a centrifugal switch. There is another class of motors that I haven't seen in a long time, ca. 1970ish. It has a commutator and brushes but there is a "shorting" ring that operates at speed and the brushes are "shorted" to each other. I bring it up because your project involves old motors.

There are many "jack leg" repairs made by "less than competant" repairmen where cord insulation has been damaged. Even at low (120V) voltage, if the insulation is damaged, replace it. There are some versions of "duct tape" that contain metallic fibers. Cheaper "duck tape" usually doesn't. In any case, neither is electrical insulation.

Using compressed air to remove sawdust and other contaminants from motors does a fair job. However, use the lowest pressure possible. My preference is ~15 pounds max. Lower is better. Washing with electrical solvent once the loose trash is blown out is an option. In any case, do not use line (~100 PSI) pressure.

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Thanks for the info on insulation testing.

I really can't justify an $500 multi-tester, but I do need to get something a lot better than what I have.

I wasn't familiar with the Simpson brand. Didn't realize they were still in production. The last time I used one of those was in physics lab - a long, long time ago. Maybe a lower cost Fluke in my future, but have to shop around.

TomKro
 
A megger is one of those instruments that if you really have regular use for, they can be found. For the average "repairman" it would be a waste of money. I got lucky, my brother found one in a "yard sale" and knew I knew what it was. Otherwise, I would still be "meggerless".

A megger can be "sort of" simulated with a microwave transformer. It is fairly easy to "hi pot" with AC. The problem is dealing with the high voltage. You need a good meter with a high voltage range to start. And dealing with "one hand in a pocket" testing. I recommend against it unless the tester is well qualified. It is dangerous. Not just hazardous, serious danger. Like smoking while reloading shot shells dangerous. My suggestion is to find someone (motor shop?) with a megger and have them test it.

Now for Simpsons, I have a couple. But had them for years, one a Series 4, the other a Series 6. Most of the time I use the Series 4, it reads the higher voltage. The Series 6 is OSHA compliant. Backward plugs, flared probes, lower voltages, the whole shooting match. The Series 4 will measure 2300 volts with ease. I do advise "hot sticks" though.

It's been two or three years since I looked, but last time I went prowling, the Simpson was still out there. But as a Series 6, OSHA compliant. Or maybe Series 7 by now. For an earlier version, you would have to prowl used sites like eBay. The big deal about the Simpson is that they will withstand being dropped from preposterous heights. Like from the bridge crane in a mill. There are many meters that are the same sensitivity, just not as rugged. 20K ohms/volt is not that sensitive. I have two or three other meters of similar sensitivity. I reach for the Simpson as much through force of habit as a need for "accuracy".

My digital is a Fluke 76. A 4 digit resolution for reading 4-20mA circuits. It also serves as a calibration meter (+1 order of magnatude) for the analog meters. I have a 4-1/2 digit (AmRel) but don't trust it. Last I looked, the Fluke was under a $100. I don't have need for a "cal lab" these days, but do like to stay reasonably accurate. FWIW, I use a digital to measure resistance. That way, I never have batteries in the analog meters.

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Bi11Hudson said:
The big deal about the Simpson is that they will withstand being dropped from preposterous heights. Like from the bridge crane in a mill. There are many meters that are the same sensitivity, just not as rugged. 20K ohms/volt is not that sensitive.

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Click to expand...
My series 6prt (roll top) did not fare well after falling out of my trucks tool compartment, about 4 feet.The guts were still good, but he case did not survive. No I keep an old military TS-352 in the truck, the simpsons stay home.


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Lots of older motors residing down here in Fallston Tom if you run out up there ! :big grin: I haven't been down my late uncle's house in years , but from what I remember , he has half of the old Crown Cork and Seal plant out in his shed .
 
I suppose the truck was moving? I have dropped a Simpson 4 feet and there wasn't enough left to repair. And have dropped one from a crane that wasn't scratched. Maintenance men lead a hard life, things get broken. Then there's "Splinter", came rushing into my (instrument) shop and grabbed my Simpson. As I attempted to tell him it was a new meter, he gave me the "Boss-Flunkie" routine. And shortly used the Series 6 to (attempt to) measure 2300 volts on a synchronous motor. The high voltage(2500 volts) was taken off when they became OSHA compliant. When he finally got back to work after 2-3 weeks, some of his face was still bandaged. Seems the glass shattered too.
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Still no decision as to what multi-meter to pick up, but now considering a more general purpose unit with a hoop ammeter. The Depot has a few models from Klein tool. It looks like some of these meters are marketed for HVAC work and can also check capacitance and temperature. Might be handy for general purpose basement tinkering. Any feedback on the Klein tool brand?

As to use of "meggers", it looks like most of these motors are headed for salvage. I opened up a nicely built 1/4 HP split phase Westinghouse motor. This one also performed double duty as a dust collector.

SmallWestinghouse_resized_cropped.jpg

The contacts for the starting windings were really burned up. The sheet metal broke apart while trying to scrape away the fried sawdust. Hard to believe it started without smoking.

Westinghouse_CookedSwitch_resized.jpg

TomKro
 
I have recovered motors in worse shape, but they were do or do without. Not something I would do as a pass time.

Re. Klein Tools: Klein has been a point of reference as long as I have been an electrician.(1965) I'm sure they have been around longer but I don't have direct knowledge from childhood. An important point is that there are (at least) two versions. Those sold by electrical supply houses and those found at Home Depot or Lowes. The "low end" tools found at HD are still far better than off brand tools. The "high end" versions sold at supply houses are without compare. I had a pair of linesman pliers that after 17 years as a working tool would still cut a hair. They were burned when a noob told me the wire was dead. . . and it wasn't. The handles that come on new pliers are worthless. Good, thick, properly insulated covers are extra. A pain in the wazoo to install but well worth the effort. They are exceptionally good tools, but must be taken good care of. Throw them in a tool box and they rust. I have used mine every day from a pouch, never had a problem.

Re. Amprobes: There are Amprobes and then there are "clamp on ammeters". I place the Amprobe in the same class as Simpson and Klein. The rest are just "the rest". An Amprobe is a magnetic device, not electronic. The newer meters all claim a digital display, with batteries that can go dead when most needed. They depend on a "Hall Effect" device to operate. As with any meter, if it reads zero, check on a known good circuit to verify. I prefer an Amprobe mechanical meter to all the bells and whistles. There are some Amprobes that have a volt meter scale, I have one. But I've lost the leads somewhere along the line. Doesn't matter, I never used that function. That's what a Simpson is for.

Amprobes are AC devices. They will not work on DC. There are DC versions but are well above the cost even a professional would pay. Think "corporate tool room" costly. An Amprobe is calibrated to 60 cycles (hertz). Reading above or below that, there will be an error. There are probably 50 cy versions but I've never dealt with one. I have used the 60 cy version on 50 cy, it worked well enough to hook up the ship. The lowest scale is 5 amps (I think). If accuracy is needed below that, the conductor can be "looped", passing through the clamp two times, to double the current.

DC current is better measured with a shunt. A Simpson can handle 200mA. Anything higher should use a resistor, measuring the voltage drop. A Simpson also is calibrated to read 60 cy in AC voltage. True RMS measurement for AC current or voltage is usually relagated to the instrument shop.

Shopping for a "multi-meter" for home use is a fuzzy area. Especially as they are used "on again-off again". The only "advice" I will offer is to have a separate VOM and Ammeter. An Amprobe is higher end but. . . For casual use, most any brand will do. Same as with a VOM. I don't like digital displays, but that's your call. For longevity, go with Amprobe analog.

M Klein is deceased, the sons are probably long retired. Who knows who owns the name now. But be advised, they never made test instruments. That's a whole nuther ball of wax. Meaningless to meters.

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Bi11Hudson - Thanks for all the advice. You certainly have me reading the specs more carefully.

Aside from the specs, I try to look carefully through user reviews. Never sure how much to trust the reviews, but when I see a distribution of ratings skewed a bit at the low end, I tend to get more cautious.

I'm definitely in the home use "fuzzy" area. I don't expect to run into anything above 240V AC, and likely limited DC, but it's important to get a good understanding of the limitations of each style of device.

Thanks again,
TomKro
 
Honestly, having -any- multimeter is better than none... I mean, it's a tool, right? ;-)

That said, cheap multimeters are only suitable for small currents. Its VERY easy to blow the fuse for the current and not know it. Diagnosing.said fuse requires some knowledge. And, it's always a pain to get to.

Clamp on ammeters are fabulous! Analog ones are nice because of the complete lack of batteries! Digital will read a wider range.

DC and ac/dc reading clamp ons are not so expensive these days. There are two methods to make these work: hall effect sensing of the magnetic field value (low accuracy), and zero sensing. In the latter, the hall sensor is only used to find zero. Current is run through a winding on the clamp core and this is used to zero out the field. I actually built one many years ago :)

Stay away from high voltage unless you know what you are doing. In this case, high should be interpreted as above "safe extra low voltage or 30ish Volts" for a total noob, and 120 volts for a small level of skill. For 120 you need to develop your "one hand measurement" technique. Clip leads are your friend here! It's best to install the meter and then power up. That way, you are well away if something goes bad.

On 120vac and up you really need to avoid having one lead in each hand. You, my friend, are a conductor! Your heart does not run right after passing current :-(

Ok, I'll stop now...

Sent from my SM-G892A using Tapatalk
 
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