Hi guys,

I'm new here. It seems to have lots of resourcefull people here and that's what I need.

I designed and fabricated a small assembly. It's basically an electrical linear actuator. I have an electro magnet that when powered moves a small iron core that pushes an alumnium plunger... When I remove the power, I have a spring recoil that bring the plunger and iron core to home.

I bought a magnet wire by the foot in some local electro store that is I guess 36 gauge as I measured it to be 0.008" diameter.

So here is the deal, my coil wound on small brass cylinder about 0.200" OD. Inside that brass cylinder is the iron core that moves when activating the power.

My power source is 14V DC (fixed cannot be changed) and I have as much amp needed (basically working on a car battery with alternator system), the resistance of the coil I've been able to do is 7.2 OHMS. I'm limited by the dimention the coil can takes as I'm limited in space. Therefore, I should be able to modify my assembly a little bit to gain space for coiling if required. Maybe double it to go up to 14 OHMS. (more wire more resistance right?)

My thing works and do what it have to do but I'm on the low side with the magnet force. To make something real good, I would need more power. I have to use my lowest force recoil spring that is not what is the best.

Therefore the main problem of my set-up is the huge amount of heat created by the coil. I have no choice to make it able to handle a 100% duty. It becomes hot afer only few seconds. I extend the time with the power to about one minute to see if it would have stabilised to somehow tolerable heat. But after one minute with the power on everything goes smoking out and I'm not able to hold it in my hand... If fact touching it will burn skin instantly.

Now I need help because I make my thing machined and they cost me quite a lot and cannot afford to go trough many batches of prototypes. I would like to try the next batch to be the final one so I need you guys.

Here are my questions:

1- Do the design with a coil winded over the non ferrous brass with small iron moving core will produce heat more than the same coil winded on iron with moving iron core.

2- If I want to reduce dramatically the heat it produces AND increse the force of the magnet by the same manoeuvre, on what should I work if my design (question 1) is OK. My voltage is FIXED at 14V. Should I change wire diameter? But I'm also limited in space. Increase or decrease the number of loops?


Thanks a lot for you help!

 

Magnetism is proportional to amp-turns. More turns, more magnetism. More turns, less current. Less current, less heat.

and get the metal out of the gap between the coil and the plunger. The smaller the gap, the more effective the coupling. If a .005 inch thick plastic tube was strong enough, that would be excellent.

 

Magnetism is proportional to amp-turns. More turns, more magnetism. More turns, less current. Less current, less heat.

and get the metal out of the gap between the coil and the plunger. The smaller the gap, the more effective the coupling. If a .005 inch thick plastic tube was strong enough, that would be excellent.

Thank you for the reply.

I've tried to keep the wall between the coil and the plunger as thin as possible but my brass cylinder is capped one side and it acts like a plug. There is oil into that assembly. I think my walls are like 0.030".

Base on what you said, I should't change the gauge on my wire? If I get more turns it will do fine? It should even be even better because the wire will heat even less.

Just a question also, you said more turns, less current... does it really a questions of turns or of lenght or wires that I wind and eventually of increasing the resistance of the coil. Because if I change my design maybe I'll increase my brass OD and ID a little bit to increase also the iron core diameter to make it stronger into the magnetic field.

Coiling on a bigger OD will mean more lenght of wire but also less turns for the same lenght of wires. Overall sure it ill have more turn but I will loose some of the turns that was tight on a smaller OD.

Thanks

 

Yes, the less current is because of the longer wire and, hence, higher resistance.

Keep in mind that 14V*2A=28W, which is a LOT of power! Look at the size of a 10W resistor and you will get some idea.

You want to put just as many turns on that coil as you can (assuming that speed of operation isn't a huge concern since you haven't said anything).

But, given a constant diameter coil and the same wire size, you won't see a signficant difference in magnetic force because as you double the turns you will double the resistance and, hence, halve the current giving you the same ampere turns. Going to a bigger diameter coil/plunger will increase the force. The brass sleeve is acting as a shield, so try to replace it with something non-metalic. Going to a larger wire diameter will increase the current for a given number of turns or, conversely, let you use more turns for the same current. Either will increase your force (and make your coil bigger, of course).

 

Most solenoids are not designed to be "on" for long periods. Look at the guts of a relay to see how to make a coil that can run continuously.

 

That's actually something I've wondered about from time to time. I've never looked into it. I didn't think that the difference between having a movable core and a fixed core would account for it.

 

The logic in my head says: a solenoid, when active, pulls the core into the coil. Therefore, it would resemble a fixed core when activated.

 

That's largely my reasoning. But another part argued that it had to create more force in order to move the core compared to a relay to pull in a contact. But there are some pretty heavy duty relays out there.

My first exposure to the difference was when I was in high school and put a dual electrical system and dual batteries in my truck. I used solenoids to switch the batteries between the systems and they burned out very quickly. I then discovered RV battery isolators that looked just like my solenoids yet worked like a champ for years. I have no idea what the difference between them was.

 

If you put a soft iron tube over the coil with a magnetic end caps, you will provide a low impedance return path for the solenoid flux coming from the center of the tube. That should significantly increase the flux density in the center of the coil and thus also increase the solenoid force. Look at how a commercial solenoid is designed for ideas on how best to do this.

 

Think about forced cooling with a small fan.
Maybe even a suitable heatsink assembly.

 

It looks to me that the maximum force you can get out of a 0.2" diameter core when you will use the maximum induction ( getting the maximum magnetization in the iron core) of 1.5T (or around) is about 18N( 2kgf).Even if you increase the current in the solenoid, you will not get more "striking" force from the plunger.
If you need more than 2kgf on it you will have to increase the plunger diameter.

I did not calculate the solenation for your small device, but it looks like you will be saturating the core around few hundred mA, the rest of the current is just going to generate heat and no work.

If you can find a way to put more turns on your coil it will help by increasing the dc resistance therefore reducing the current.The magnetic field will be huge, but above a certain value it does not increase the plunger attraction force.Maybe a variable resistor in series with the solenoid would help to determin how much current is actually needed to achieve saturation.

Also, make sure the iron core does not totally come out of the solenoid and during rest there is at least some 10-20 % of it inside the solenoid core.

 

It sounds like the AC ripple on your power supply is heating your brass sleeve to excess; (eddy currents). You are getting some iron core heating due to the copper coil, but the brass sleeve is the big heater. Try something non-metal for a sleeve or filter your power supply AC ripple with a capacitor.

Note that your power of 28 watts is rather high; 14 VDC @ 2A. for a small solenoid. Many commercial solenoids (solenoid valves) have power ratings of only 10-12 watts. Use this as a design guideline.

Cheers, DPW [Spent years turning op-amps into heaters.]

 

It sounds like the AC ripple on your power supply is heating your brass sleeve to excess;

Uh... no... it really doesn't sound like that is what happening. I base this on the fact that he is using a 14V DC supply. Joule heating from dumping 28W of power into the coil sounds like the more likely culprit.

 

Must you really make your own solenoid? You can buy them cheaply and the materials they use will give you a device that's far more efficient than anything you can make yourself. And if you're thinking of production rather than prototypes, you'll pay far more to have a craftsman make solenoids than you would if you bought something mass-produced in a factory.

 

Most solenoids are not designed to be "on" for long periods. Look at the guts of a relay to see how to make a coil that can run continuously.

A relay doesn't use a 'solenoid', or not a true solenoid. The use an "E" core and a coil, and pull the moveable pole in to make a complete magnetic circuit. Its when there is no return path for the magnetic lines of force that you get a lot of heat.

 

I guess I should have looked at the guts of a relay first!

 

I guess I should have looked at the guts of a relay first!

Ever apply power to a induction motor with the rotor out of it? The coils get hot real fast.

 

A relay doesn't use a 'solenoid', or not a true solenoid. The use an "E" core and a coil, and pull the moveable pole in to make a complete magnetic circuit. Its when there is no return path for the magnetic lines of force that you get a lot of heat.

But answer this (and it's a legit question -- I don't know much about magnetic circuits).

I make a solenoid using a coil of wire whose DC resistance is 7Ω. I then power that coil by a 14V DC supply and leave it energized.

I then make a relay using that same length of wire for it's coil and leave it energized from the same supply.

How will any details of the construction of the solenoid or relay affect the amount of heat that is produced? It seems like the current in both cases will quickly settle in to 2A DC and that the thing will be dissipating 28W of power from the resistance of the coil alone. How can any heat be coming from the core of the magnet? Where is that power coming from?

Now, notice that I'm not saying anything about whether you need to use the same length of wire to get the relay to work versus the solenoid to work. I just don't understand all of the statements about the heat coming from the magnetic part of the circuit when we are talking about a DC system.

 

But answer this (and it's a legit question -- I don't know much about magnetic circuits).

I make a solenoid using a coil of wire whose DC resistance is 7Ω. I then power that coil by a 14V DC supply and leave it energized.

I then make a relay using that same length of wire for it's coil and leave it energized from the same supply.

How will any details of the construction of the solenoid or relay affect the amount of heat that is produced? It seems like the current in both cases will quickly settle in to 2A DC and that the thing will be dissipating 28W of power from the resistance of the coil alone. How can any heat be coming from the core of the magnet? Where is that power coming from?

Now, notice that I'm not saying anything about whether you need to use the same length of wire to get the relay to work versus the solenoid to work. I just don't understand all of the statements about the heat coming from the magnetic part of the circuit when we are talking about a DC system.

I'm wondering the same thing. Therefore, in my current assembly the core is not totally out but so small, I guess it can be just teasing the magnetic field and maybe can cause some higher heat.

To answer one question, I have to coil it myself because it's inside the part I'm machining, so cannot use it as an add-on.

I've tried different spring recoil and I barely compress a spring that is 4#/po. The spring I use actually is about 1#/po that works quite well but as you can imagine, I'd rather to have more power in my device.

The speed reaction of my projet has not impact. Under one second or about is all good for me.

I'm gonna post you a small sheme of the thing.

Thanks all again.

 

I don't know the answer either to that. It's just how it was explained in a book on solenoids and electromagnets. And contactors and relays that are on for long periods use the E core movable pole type of construction instead of a solenoid type.