3
\$\begingroup\$

I am attempting to build a relay from constituent parts of Meccano Elektrikit. There are instructions for such a device in the set's manual, but the finished item (and all modifications I have tried) has a problem. If I arrange the parts so that the coils have enough force to pull in the armature, then the armature is not always released when the current is switched off. It sticks. If I try to manually release it at this stage, I can feel the 'stickiness'. If I then immediately manually move the armature back to its closed position, the stickiness has disappeared so there doesn't appear to be any residual magnetism in the core at that stage. If I don't manually release the armature, it will stay stuck for several minutes.

The relay assembly has two coils connected in series (I'm wanting the relay to switch over two separate circuits) and the fixed end of the cores are bolted to a steel base plate.

I have tried arranging the circuit with the coils' fields in opposite directions and in the same direction but have observed the problem in both cases. I have also tried many variations on gap spacing, the mass of the armature and the springiness of its mounting.

\$\endgroup\$
1
  • 2
    \$\begingroup\$ Hysteresis, try using a different core material. \$\endgroup\$
    – Nedd
    Nov 13, 2022 at 20:57

2 Answers 2

Reset to default
4
\$\begingroup\$

You are stuck with your materials in the magnetic circuit. Remanence is the problem.

You could place a small nonmagnetic spacer like a bit of paper on the relay pole which means that the gap when pulled in never goes to zero. You can try more layers of paper which should stop the sticking.

Also you can increase spring tension and increase coil current to ensure pull in. I have done this on other stuff but not your relay.

Another possibility is to demag the coil at switch off. I have not had to do this but others have. This can involve a resonant capacitor across the coil that gives a decaying AC wave that assists coil demag.

\$\endgroup\$
1
  • \$\begingroup\$ A small bit of paper does the trick, thanks. Demagnetization may have also worked but dismantling everything and then finding a safe low-voltage AC supply would have been a bit of a pain. \$\endgroup\$
    – quilkin
    Nov 14, 2022 at 9:33
3
\$\begingroup\$

Remanence is a material property. Soft iron is better than steel. As is Silicon-steel (eg. transformer laminations).

\$\endgroup\$
3
  • \$\begingroup\$ The cores are soft iron but are >50 years old. Would remanence change with age? If so, is there a way of rejuvenating them? \$\endgroup\$
    – quilkin
    Nov 13, 2022 at 21:29
  • \$\begingroup\$ Shouldn't. Heating will demagnetize but the Curie temperature is probably pretty high for iron. \$\endgroup\$
    – Spehro Pefhany
    Nov 13, 2022 at 21:35
  • \$\begingroup\$ You can demagnetize with a slowly reduced AC magnetic field that is strong enough. Generally you put it in a strong mains-frequency field and slowly remove it from the field by physically withdrawing it. I've seen home-brew ones made from 1/4-HP motor stators. with series strip heaters to limit the current. \$\endgroup\$
    – Spehro Pefhany
    Nov 13, 2022 at 21:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.