Electromagnetic force design

[forteh]

I know there are some clever people on here (in one form or another) but is anyone familiar with designing electromagnetic coils?

Basically I want to know if it's feasible to produce an electromagnet that can apply a constant force of around 3.5-4kN to a steel rod (think of it as a linear accelerator in a stalled condition); power available is 415v 3ph although that could be passed through a transformer if lower voltages are needed.

Who is really, really clever?

[ben_travis]

[dann2707]

Yes

[Dave Anscombe]

Tesla coil ?

[RobinJI]

I can't help with formulas or anything, but I've spent a while at work messing around with magnetic locks recently. I tried to do a little research into whether the design of the winding had an effect on the magnetic fields size/strength ratio (we didn't need much strength but needed it to work over a slight distance). I drew a bit of a blank but from what I saw while looking, for the sort of thing you're talking about the calculations didn't look too tricky (depending on the orientation of the rod to the coil.) Sadly I didn't go into it enough to be much help to you though . We were restricted to budget off the shelf part's, so beyond seeing if I could find any trends I didn't need to worry about numbers too much as the products spec sheets told me what I needed to know. 

[David]

This has some formulas that may be useful:

https://www.google.co.uk/url?sa=t&source=web&rct=j&url=https://www.stcharlesprep.org/01_parents/vandermeer_s/Useful%20Links/Honors%20Physics/pdf%20lectures/Torque%20in%20Magnetic%20Field.pdf&ved=2ahUKEwiKzpbXxe_dAhVCXxoKHah2DykQFjAOegQIBBAB&usg=AOvVaw29lgmNWSg74mBoa_-0-yqL

 

Are you trying to build a solenoid or just a normal electromagnet? (Albeit essentially the same).

[forteh]

A normal electromagnet in effect, we have tested with solenoids in the past but they only produce sufficient force at 100% stroke.

Because this is actuating a pair of wearing pinch wheels a solenoids would need to be constantly adjusted which isn't going to happen.

We need a constant applied force across a range of positions not just at one point.

[monkeyseemonkeydo]

2 hours ago, forteh said:

Because this is actuating a pair of wearing pinch wheels a solenoids would need to be constantly adjusted which isn't going to happen.

We need a constant applied force across a range of positions not just at one point.

You should give a guy called Clive Evans from Scale Engineering in Swanage a ring. It's not specifically something he's done before but he's had a lot of experience working on some serious bits of kit for big steel rolling mills where everything has to be incredibly accurately positioned and the forces involved are considerable... Can't guarantee he'll have the answer but I bet he'd have some useful thoughts.

[RobinJI]

I think you'll really struggle to get a constant force over a varying distance. I'm fairly sure that the power of attraction varies as an inverse square function of distance, and as such it'll be linear-ish at larger distances, (although relatively weak), but never quite a constant force. I don't believe there's much you could do about this other than using control systems doing something clever with relating the voltage to the distance. 

[GavLawson]

Do the pinch wheels have to be actuated regularly or is it just for wear compensation? 

The force increases in a solenoid to 100% at full stroke because as the plunger is drawn into the coil the number of turns increases. To get the force to be constant(ish) you need the plunger to have the same number of turns for 100% of the stroke. So the plunger needs to stay the same diameter and extend through the coil at both ends of the stroke.  Providing the coil is sized correctly and the current in the coil and opposing force remains the same, it will stay in one position. 

you will need to be able to vary the current in the coil using a controller to move the plunger for adjustment. 

I would suggest you take the problem to a decent electrical engineering firm with a winders workshop, you will probably want them to wind the coil anyway. It’s a pig of a job!

 

 

[forteh]

Thanks all for the inputs, I think in reality the practicality of the design given the application and environment is preventing it from being a realistic proposition.

The adjustment is only to allow for wear in the polyurethane wheels, if the coil could be such designed so as to be able to provide the force across a range of positions then that could work.

Even if a suitable coil could be wound to supply the force required I suspect the power requirements would be far higher than our current electromechanical solution (a small stallable electric geared motor that drives a cam and linkage; it works very well but we're having some issues with supplier capacity and investigating possible alternatives). Given that the design life of this unit is 20+ years, the less power used the better.

I do actually have access to a coil winding machine, my dad was a mechatronic engineer/inventor/instrument maker and built himself one but unfortunately he's deep in the grip of alzheimers so couldn't help with the design.