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I have several small DC motors where if I place an extra magnet to back-up the included magnet, the performance seems to improve significantly.

These motors are 30-40 years old & their performance is degraded. I've been told that it's possible to restore their performance by re-magnetizing the magnets.

How can this be done?

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I am not an expert here, only having done this once or twice a long time ago, but the "usual" method is to send a very short pulse of high current (e.g. >10A, <1us) through the winding, in order to saturate the magnet completely.
All you need is a large capacitor bank and a switch (plus a frame/shield to hold in in place in case anything decides it wishes to depart in a rapid fashion from the motor) You can work out the voltage needed to charge to by measuring the resistance of the windings (roughly, as it will be a fast rising pulse as opposed to DC you are sending through, but it will give you some idea)

I had a quick Google and found this page which discusses the method. I suggest you try it out on and old motor or magnet first, as it is likely to be a rather empirically judged process. This document looks interesting and useful also.

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    \$\begingroup\$ I'm amusing myself picturing an "instant steam engine" on the model railroad. \$\endgroup\$
    – Anindo Ghosh
    Commented Jan 19, 2013 at 18:13
  • \$\begingroup\$ @Anindo - yes, hopefully the engine will remain an electrically operated one :-) On the topic of steam engines, I actually have a little toy steam engine here (a steam roller type, around 30cm long - you use a firelighter as fuel), I recall having much fun with it as a child. I wonder if they ever use (or used) real steam engines for model railroads? \$\endgroup\$
    – Oli Glaser
    Commented Jan 20, 2013 at 10:49
  • \$\begingroup\$ yes, real steam model train engines are a rather expensive but popular segment in g and s scales. \$\endgroup\$
    – Anindo Ghosh
    Commented Jan 20, 2013 at 21:31
  • \$\begingroup\$ Interesting - I just took a look and noticed that Hornby and others make them, according to Wikipedia there are even some diesel/gasoline powered models (interesting thread with pics here). \$\endgroup\$
    – Oli Glaser
    Commented Jan 20, 2013 at 22:16

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