Some of these motors have round brushes constructed from tightly rolled
up brass screen. If this motor has that type of brush they are most
likely dirty and oxidized. They are the first thing to check and they
are cleaned by removing and soaking in carburetor cleaner. Do not sand
them or the commutator surface.
As to how to get them fixed you have to find some old retired person who
has the time to fiddle with it and possibly make new parts. This will be
someone who collects early electrical equipment and keeps it running.
On 02/15/2013 03:15 PM, Jim Nichol wrote:
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 mechan
ic
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
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