Model Forum / General / Railroads / April 2009

That is the typical symptom of a motor that has lost its magnetism, it
can most likely be cured by remagnetising it. There was an article on
how to do that in MR in 1972. I could provide a scan if you contact me
off list. (Reply won't work get my address from my website).

Could be, they take a lot more current when the magnet is weak,
usually they also go faster but with less torque as you observed with
the first one. This second one is probably not so far gone.

Keith

Make friends in the hobby.
Visit <http://www.grovenor.dsl.pipex.com/>
Garratt photos for the big steam lovers.

Check the armature windings.  It could be you have one open winding, the
remaining windings are enough to make it run under light load, but you
need all the windings intact to have real torque.  The fact that you
have to give it a spin by hand to start it is a hint that one of
windings is open.  With luck you can spot a broken wire, they often
break right near where they solder onto the commutator. Put your ohmeter
across the brushes, it should read a ohm or so.  Rotate the armature by
hand slowly.  If you see the resistance go "open" you are missing one
winding.
It is possible to rewind an armature, assuming some soldering skills on
your part. You have to find replacement wire of the same gage.  Then
it's just a matter of carefully winding the wire neatly onto the
armature and getting the same number of turns into each little slot.  If
the replacement wire is bigger, or your winding technique is less than
neat, it may not fit.

Yes, if the permanent magnet looses its strength, the motor looses
torque.  On the other hand  I have never had this particular problem
happen to me.  You can make a rough check by feeling how hard the magnet
tugs at an iron tool.  Compare the pull of the suspect motor with the
pull on a good motor.  If the suspect motor's magnets are weakened
enough that you can  feel a difference, then you might be onto
something.
It is possible to "recharge" or remagnetize the older Alnico magnets.
In principle this can be done by putting the magnet into a very strong
magnetic field, and perhaps tapping the magnet lightly.  I've never
tried to do this in the basement and I have a feeling that creating the
necessary strong magnetic field could be tricky with just homebrew
equipment.
 What's the current draw like?  A good open frame motor ought to run
unloaded pulling something like .25 amps.  Motors try and run at the
speed where the back emf of the motor matches the driving source
voltage.  They pull current as required to get up to that speed.  When
you load them down, they pull more current to keep their speed up.  If
the field is weak, it takes more current to create the same torque.  You
mentioned that this one gets hot, which is a sign of pulling too much
current.

 
David J. Starr

Sounds to me like both motors have VERY weak magnets.

It is possible to remagnetize them, find a hobby shop that specializes in
slot cars.  I recall from years ago remagnmetizing motors was "sop" after
rewinding them.

Don

--
don.dellmann@prodigy.net
http://www.geocities.com/don_dellmann
moderator: WisModelersAid@yahoogroups.com
moderator: MRPics@yahoogroups.com
moderator: vintageHO@yahoogroups.com
co-moderator: SCENERY@Yahoogroups.com
http://groups.yahoo.com/group/MRPics
http://groups.yahoo.com/group/vintageHO

Hi:

Before testing old motors they should be cleaned an lubricated.
The commutator plates can be cleaned with a soft eraser, lightly
applied.  I generally use Labelle 101 on bearings and spacer
washers, if no plastic parts are close.  A small drop on plates
improves brush contact.  

Question 1:  In many older motors poor solder joints were not
uncommon and centrifugal force at high RPM threw wire ends
outward, breaking connection.

To determine if there is a open winding on an open
frame motor, start motor, then reduce throttle until it stops.
Mark the armature with a soft pencil.  Repeat several times.  If
it stops at the same point constantly, a connection is broken.
Usually this is at the commutator plate and may show up as arcing
in a darkened view.  Examination under magnification might reveal
the break.  Plates are skewed with respect to poles, so the break
will not be aligned with mark.  The wire end will probably be
close to original solder connection on plate.

In most cases, the connection can be resoldered.  Since the wire
is insulation coated, the end should be carefully scraped to
avoid breakage.  If too short, a fine wire jumper can be added.
I have repaired many this way, especially Revell.

If the stopping point is not consistent, the problem is usually a
weakened magnet caused by application of AC or rapid reversal
while running.   This is indicated by excessive RPM, running with
no load and drastic drop in speed under even light loading,
accompanied by excessive current.  Speed can be detected by ear.
Higher pitch equals higher speed. Remagnetizing is possible but
dangerous.

The Alnico magnets used do not deteriorate with age unless
subjected to high temperatures, strong opposing magnetic fields,
interuption of loop path or hard jolts.  More recent experiments
have revealed methods to replace these magnets with Neodymium
types yielding lower RPM and current.  These can be removed from
pole pieces without deterioration.

Question 2:  The AHM FM C-Liners were originally produced by
Rivarossi with ballbearing motors, then farmed out to Tempo of
Jugoslavia with poorer motors.  Some had one carbon brush and one
rolled-up copper mesh.  These were prone to wear.  Lubrication may
solve problem.  If not, the best solution is to find a
replacement, since recharging would be very difficult.  Since these
locos were very inexpensive and common, they can be easily found.

For details and extensive discussion of problems and
solutions, see first site below.

Hope this helps.

Thank you,

Budb

Author of:

MODELRAILROAD TECHNICAL INFORMATION

http://www.geocities.com/budb3/

PROTOTYPE TECHNICAL INFO FOR MODELRAILROADERS

http://www.freeyellow.com/members4/budb/

Moderator of:

MR TECHNICAL HELP GROUP

http://groups.yahoo.com/group/mrtechhelp

COUPLER HELP GROUP

http://groups.yahoo.com/group/mrcouplers