Hi Im trying to add 4 equal 1" long electromagnets to an Adafruit motor shield 2.3(only 3 running at any given time, it switches). Im wondering what the length of the coil I should use on each should be. It says it can run up to 1.2A for each motor. I have a 3s 2100mah 11.1V battery. The wire I have is 30AWG coated copper. And a .2" soft iron core
Now I read you want High turns, High resistance, less amps to make it not get as hot, maintain battery life, and have strong magnetic push/pull. Correct me if Im wrong or you suggest differently.
Now this part is where Im starting to not be sure whats right
So for the coils using ohms law V=IR should it be 11.1V=(<1.2A)(R)
Online I read Resistance per 1000 ft (ohms) @ 20 C for 30AWG Cu is 103.2 so .1032Ohms/ft
So 11.1/1.2=9.25 so >9.25=R=L.1032 so 9.25/.1032=89.6ft
so I need at least 90 ft for each electromagnet? That seams absurd. Do I need to split the voltage into 3 because there will be three on at any given time? They are each in a separate motor driver so I can handle each of them separately. Does the motor shield do any of this managing current or am I doing this right where my length should depend on this stuff.
Idk please help! Much appreciated!
so I need at least 90 ft for each electromagnet?
Yep, if you use that fine gauge wire. Much more for heavier gauge wire.
However, the resulting 30AWG wire coil will be giving off (1.2A)^2*9.2 Ohms=13.2 Watts of heat, and will smoke rather handsomely before (perhaps) burning up completely.
The maximum current for point to point wiring or power transmission is discussed in this wire table. It is not trivial to design and operate an electromagnet.
Edit: it does not appear that the Adafruit motor driver can do automatic current limiting, which can be used to control power consumption and heat generation. The Pololu stepper drivers can.
So each "motor" in this case electromagnet will get 11.1V or will it divide because I have 3 running at any given time? Should I possibly get a 7.4V battery instead?
or maybe I put a resistor in series with the electromagnet? I only have room for a 1in long electromagnet with radius of a 1/2". Im not sure how many turns or wire length that could fit. maybe 500 turns like 700ft max maybe idk. But also id rather keep the radius smaller that is just max but can do it
So each "motor" in this case electromagnet will get 11.1V or will it divide because I have 3 running at any given time?
Each "motor" gets slightly less than the voltage that you are supplying to the motor driver.
Should I possibly get a 7.4V battery instead?
You can use any voltage you want, within the restrictions for the motor driver.
Im not sure how many turns or wire length that could fit.
Wind the coil and see.
Wait how much less voltage does each motor get? so if I have a 7.4V or a 11.1V battery how do I know how much voltage goes into each motor? because isn't that needed for the math to work?
So I can use any voltage I want for this and it will work the same.
Or I can use either voltage but should probably use one more specific to this?
Sorry for all the questions I really appreciate your help!
Wait how much less voltage does each motor get?
Carefully study the motor driver documentation, or ask Adafruit.
However, it would probably be safe to assume a voltage drop within the motor driver of less than 0.5 V.
ok great thank you so much!
However, it would probably be safe to assume a voltage drop within the motor driver of less than 0.5 V.
Some of the transistor H-Bridges can loose 2V per transistor, so with one at the top and one at the bottom your voltage loss is likely to be in the order of 4V.
ohhh wow I appreciate it! Also would it be wise to use the same battery to run the Arduino. I read on Adafruit that this can cause problems but it just seams simpler than using two sets of batteries. In which case how many volts would that take. The only other thing ill be using is 1 Hall effect sensor
In which case how many volts would that take.
You seem to be obsessed with volts where as it is current you need to worry about.
Can you post a link to this motor driver you have?
Also would it be wise to use the same battery to run the Arduino.
Yes.
But do you have any decoupling in the wiring?
See http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
Electromagnets do not generate nearly as much interference as motors do, but you get some spikes on switch on and off that might cause you trouble.
Exactly how much magnetic field do you need from your electro magnets?
Some of the transistor H-Bridges can loose 2V per transistor, so with one at the top and one at the bottom your voltage loss is likely to be in the order of 4V.
That is not true of the Adafruit V2.3 motor shield, mentioned in the OP. That shield has TB6612 MOSFET drivers.
From the TB6612 data sheet, the "output saturating voltage" drop is typically 0.5V at 1 A current draw.
Your right I had volts all in my head from the previous issue and volts is directly related to current. I am worried about the current thats why Im lowering my voltge because I dont have room to make the resistance big enough in the coil. So it seams like the. board can handle 7.4V in that ill be using it will just take more current from the battery but will not effect the voltage going in to the motor shield.
So I think I need to turn the one red and one black battery wire into two red and two black to put two in the motor shield and two in the Arduino vin/gnd unless there's an easier way possible going from the motor shield 5V to the Arduino. This may be an absurd thing to say and totally wrong forgive me.
I don't have any decoupling I don't think not entirely sure what that means I know you posted the link on it and Ill look more into but for spikes I may need to address that as well. I've heard the words relays and transistors capacitors I sound like an idiot saying that haha but maybe one of those things are what I would need to incorporate
the motor shield Im using is that one just posted. This is more in depth information on the 2.0 version I have the 2.3 but I think its pretty similar
And I'm not sure exactly how strong I need it to be but I think this should be enough and if not ill make adjustments
And I'm not sure exactly how strong I need it to be
What do you want it to do?
From affecting a compass needle to lifting several kilos?
Normally anything involving lifting takes a very lot of current.
oh more than move a compass needle. Hopefully be able to lift a small n52 magnet from an inch away.
Also how does the motor controller adjust speed? Does it limit the current or something? By adding resistance or something?
I actually just wound it up and managed to get 2000 turns haha way more than I expected. I don't have a multimeter with me to test its resistance but tomorrow I will. Its about diameter of iron core .2" diameter of electromagnet about 3/4" length 1"
Idk the length of the wire because idk how to measure that but I made a basic servo with uh the light reader whatever its called and counted once every turn and did 2000. I have 3 more to do but am going to wait until tomorrow when I measure the resistance. hopefully get around 13ohms and I can use a 11.1V battery with that if my maths correct. Else around 8.5ohms will work with the 7.4V battery.
Also how does the motor controller adjust speed? Does it limit the current or something? By adding resistance or something?
It normally works by turning the motor on and off rapidly using PWM. The ratio of the on to off time gives you the speed control.
I ended up doing around 2000 turns on each which measured between 25.2 ohms for the lowest and 26.8 ohms for the highest. then I took off from the high ones so they were all 25.2-25.5ohms.
With an 11.1V battery I should get about 10.5V actually being used on the electromagnets(after the motor shield voltage drop) so I will have V=IR 10.5 volts = (.42 amps)(25 ohms) so .42 amps drawn for each
I think everything looks great here. If for some reason its too strong I can use a 7.4V battery and get 6.9 volts=(.276 Amps)(25 ohms)
Oh that makes sense on speed. Thanks!
If for some reason its too strong
My guess is it won’t be strong enough. Let us know.
I hope not haha. Well I have to wait until my lipo batteries come in to test. Hopefully this week. In the meantime I'm going to get a feel for the motor shield and get it working
I've gotten the batteries and guess what. you're right too weak.
Remember they are 26ish ohm electromagnets running around .4 amps with 11.1v battery through Arduino motor shield. 30awg magnet wire. I need more current I believe. so I ordered 22, 26, 28 awg magnet wire.
The 22 I already got and it was only like 2 ohms in the same space that the 30awg was 26ohms so that would be way too much current to run through the motor shield but I put the wires right into a 7.4v battery(idk if thats recommended) but it was pretty strong(so like 3.7amp going though).
I think the 26 or 28 awg may create an optimal balance where I can get more current running through hopefully around the 1.2A level(what the motor shield is rated for) while not getting too hot but keeping them small enough to fit in the space I have
Otherwise maybe I'll look into a motor shield that can handle more current and do 4 dc motors like the Adafruit one. hopefully I don't need to though because I like the simplicity of this shield