I have designed and built a small solenoid and… it doesn't exert any force on the steel bolt I am using as a plunger. I could freely move the plunger back and forth without any resistance.
- I am using 0.2mm copper wire with a plastic coating.
- The coil cross-section is 15mm x 3mm (inside radius = 2.5mm, ouside radius = 5.5mm)
- I calculated that I could get about 1125 turns on the solenoid with a resistance of 14.7 Ohms. For the actual solenoid I measured 11.7 Ohms.
- The solenoid frame is made of plastic.
- The plunger has a 3mm diameter and the gap between the coil and the plunger is 1mm, of which 90% should be plastic and 10% air.
- The overall radius of the solenoid is 5.5mm.
- I applied 12V and the total current draw was 1.02A.
- The solenoid became hot after 20 or 30 seconds, as expected.
I put the values into this calculator: http://www.daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml
- I = 1.02
- N = 1125
- A = 95 mm2 (PI x 5.5mm x 5.5mm)
- g = 1mm
and it give a force of 78.598 N.
Clearly I have done something fundamentally wrong, but what?
Could it be the steel bolt?
I've looked for an iron substitute but couldn't find anything except old nails with a large taper.
What would happen if I tried a 3mm diameter neodymium magnet?
Thanks, Andy
Best Answer
No force at all sounds like the bolt might be non-ferromagnetic. Simply test it with a magnet and make sure it is strongly attracted. 400-series stainless is only slightly ferromagnetic and 300-series stainless steel is almost totally non-magnetic under normal conditions.
If you sense the force is there (Luke?) but it is very weak, then keep in mind that the calculator you used assumes the total gap around the magnetic path is 1mm. If that sounds impossible to you- consider the following images from here:
(g = 1mm)
Or this one from the same site:
As you can see, the magnetic material of the core and armature forms a magnetic path that is only interrupted by a single small air gap (not counting the minimal clearance gaps required for the armature to pivot in the first image or the plunger to slide in the second image). The total reluctance around the path adds like electrical resistance, but the reluctance of the ferromagnetic materials is much lower than air, so the air gaps dominate.