If this is a "Universal" motor, it can be easily identified by having carbon brushes riding on a commutator. A Universal motor is really a DC motor that has been specially designed to also work on AC. Whereas an Induction motor is AC-only, and is an entirely different thing (no brushes). Any motor repair shop that knows anything about DC motors should be able to analyze a Universal motor. As a matter of fact, the first thing to check before getting it repaired is to see if the carbon brushes are worn down, and the springs that push them against the commutator are working. Often the brushes get stuck in their brush holder slots from carbon dust and grime, and are no longer making contact with the commutator. Freeing up the brushes so they slide easily will fix such a motor.
A DC motor (and a Universal motor) have two sets of windings called FIELD windings and ARMATURE windings. The Field coils are stationary, and are often bolted to the inside of the motor frame. The Armature coils are wound onto the rotor. The brushes and commutator bars are used to connect the power source to the Armature (rotor) windings. It is possible for DC motors to be made in two configurations. The Field and Armature windings can be wired in parallel with each other, or they can be wired in series. Such DC motors are called either "parallel" or "series" wound. Universal motors are usually series wound. Series motors (unlike parallel) have huge torque at slow or stalled speeds, and very low torque at high speeds. This is because if the mechanical load stalls the motor, it slows down, which increases the current in the Armature. Because the Armature and Field are in series, they then BOTH get more current, more magnetism in BOTH Armature and Field, and thus more torque squared. This is great for home use, like drill motors, vacuum cleaners, and for phonographs with heavy tonearms. The slower it goes, the more torque is created to compensate. There are two drawbacks for series motors. At high speeds they have very low torque, and secondly, they can theoretically reach infinite speed if there is no mechanic al load and fly apart. Some even have fans to provide a little load at high speeds. Jim Nichol On Feb 15, 2013, at 3:04 PM, Rich <[email protected]> wrote: > He was advised that it was a short. You are correct that the correct > condition condition description is OPEN Circuit. If the fields happen to be > in parallel then that would be where I would start looking. > > On 02/15/2013 01:04 PM, Jim Nichol wrote: >> Al didn't say you had a "short". He said you had an "open". They are >> opposites of each other. He only mentioned "shorted turns" to imply that >> they are NOT likely to be the problem. >> >> A broken wire is an "open", resulting in zero current. A "short" is two >> wires touching each other, causing the current to take a shorter path than >> intended. In a lamp cord a short would blow a fuse, whereas an "open" would >> prevent current from flowing. I recommend not saying "short" when you mean >> "open" (though many electrical novices do). If you said you thought "the >> motor has a short" at a repair shop, it would mark you as unknowledgeable, >> maybe setting yourself up to be ripped off. >> >> The symptom of an "open" in your motor would be that nothing happens at all. >> On the other hand, saying the motor has a short (or more specifically >> "shorted turns") means that a few of the loops of wire in the coils are >> shorted together, causing the current to bypass them. This would lower the >> overall resistance of the coils, thus increasing the current. The motor >> would still run, but would have less magnetic torque (because some of the >> turns of wire are not carrying current), and the remaining turns would get >> hot. >> >> Sorry for the lecture, but as an electrical engineer, this is one of my pet >> peeves. >> >> Jim >> >> On Feb 15, 2013, at 8:53 AM, Vinyl Visions<[email protected]> wrote: >> >>> Al,Thanks for the reply. As you noted, I have been advised that it was a >>> short in the motor somewhere. The problem seems to be that this motor is >>> wound for three different types of power - AC/DC and maybe 220. So, either >>> the shops don't know what the problem is or they just don't want to mess >>> with it... I'm tending toward the latter, since the guys I sent it to are >>> very knowledgeable. There is a picture of the motor on our website: >>> www.carolinaphonosociety.com >>> A shortcut to the pic is: >>> http://open1234.wix.com/camps-site/twilight-zone-2#!__fairy-phono-lampCurt >>> >>>> To: [email protected] >>>> From: [email protected] >>>> Date: Fri, 15 Feb 2013 04:01:17 -0500 >>>> Subject: Re: [Phono-L] Turntable Motor Question >>>> >>>> >>>> Induction motors that lack torque can usually be traced to an open field >>>> coil or an open armature loop. A shorted turn will eat torque but the >>>> motor will let you know by getting hot. How about a photo of the motor? >>>> Most good motor shops can fix anything from fractional horsepower to 100 >>>> HP. Do you know of a fan collector in your area? I have repaired fan >>>> motors that lost power and have the same symptoms of your phonograph. >>>> These things are not rocket science. >>>> >>>> Best wishes, >>>> Al >> >> _______________________________________________ >> Phono-L mailing list >> http://phono-l.org >> >> > _______________________________________________ > Phono-L mailing list > http://phono-l.org _______________________________________________ Phono-L mailing list http://phono-l.org
