Yes, Rich brings up a good point. The most common winding failure is that the insulation on the windings fail (usually Field windings), causing the Field to be grounded to the motor frame. One of the most common and basic tests for suspected bad motors is to simply measure the resistance from the winding to the metal motor frame. It should be nearly infinite ohms. You could use an ohmmeter for this (or a multimeter on the Ohms scale). Professionals use a device called a "megger". It is similar to an ohmmeter, except that it operates at a much higher voltage to get a more reliable reading. A regular ohmmeter probably uses a 9-volt battery for power. But meggers often have selector switches so they can put out 100 / 250 / 500 / 1000 volts DC, which is what I have. Some special ones at work go up to 10,000 volts DC to test high voltage cable insulation. Anyway, the purpose of meggers is to test the resistance of wiring to ground. If the ohms are small, then the insulation has failed. A grounded Field coil usually needs to be rewound. As Rich said, the motor might still work with a ground at only one spot in the coil (though it would be dangerous to touch the motor frame because of the voltage on it). But two grounds cause a section of the coil to be shorted out. For a DC (or Universal motor), any motor shop's basic tests would check that the brushes are OK, then megger the motor to see if the Field needs to be rewound.
Jim Nichol On Feb 15, 2013, at 11:52 PM, Rich <[email protected]> wrote: > It does prove frustrating. > > Your best bet is someone who will show up to the next phonograph show who can > spell "electric motor". These things are not hard to fix actually. The most > likely problem is that the field is grounded and as it is only a 2 wire > device the case is also hot. It could also be the resistor has seen better > days. The field coils are wrapped with linen strips and may or may not be > then coated with tar. Either way when running on AC the coils flex at a 60 > cycle rate and over time the wrapping fails and then the enamel on the wire > wears through. This usually occurs at one or more of the 4 corners of the > pole shoe. > > You can verify a field ground by connecting the motor to power and then > checking the frame to ground for voltage. Any reading of voltage indicates a > field ground. I have attached a drawing of a universal motor, showing DC > supply but the AC is identical, and you can see that at least 2 grounds are > required to bypass any field windings. This is what you usually find when > digging into these, multiple points of failure. > > On 02/15/2013 10:07 PM, Vinyl Visions wrote: >> Jim& Rich,Thanks for the info. My motor is definitely a Universal motor >> with carbon brushes. The brushes are in excellent shape and the armature >> runs true. The main problem I'm having is shorts, as opposed to opens. The >> motor runs until a load is applied, then you can basically shut it down >> with your fingers. The "REAL" problem that I have had is due to our >> geographical location - North Carolina. There are some very good motor >> shops, but they won't mess with a small problem motor that doesn't generate >> any return business. If you find a mom& pop operation, they work whenever >> they get around to it... I left it with one place and the owner promised he >> would get to it in a week - two months later I went back to check and it was >> still sitting on his desk in the same place I left it. Another said he could >> rewind it, then quit his job. Another guy sent it to Michigan to someplace >> that was willing to work on it, but wanted $900 with no guarantees that they >> could fix it. As far a > s >> electrical engineers go, 99.9% of the population probably don't know what >> electrical engineers do for a living and they are not in abundance around >> here, except at the nuclear power plant. The guy who works on your motor may >> or may not have graduated high school and knows how to fix one or two common >> motors that are used commercially - give him a challenge and it ends right >> there. I am tired of being asked by people in motor shops - "What are you >> going to use it for?" or "What's wrong with it?" If I knew what was wrong >> and how to fix it, I wouldn't be asking them. What difference does it make >> what I'm going to use it for? I just want it to run... AND if I did explain >> that I wanted to use it in a lamp phonograph... you get the picture. Do I >> sound stressed? > -------------- next part -------------- > A non-text attachment was scrubbed... > Name: Selection_131.jpg > Type: image/jpeg > Size: 31347 bytes > Desc: not available > URL: > <http://oldcrank.org/pipermail/phono-l/attachments/20130215/09c25c36/attachment.jpg> > _______________________________________________ > Phono-L mailing list > http://phono-l.org _______________________________________________ Phono-L mailing list http://phono-l.org
