Author Topic: what is the best way to run a powerful electromagnet - test your knowledge!  (Read 3500 times)

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Offline user1235423643

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what is the best way to run a powerful electromagnet ?

most magnetic power vs power used. (efficiency)

ac? dc? (mostly asking about dc)

linear dc or a pwm dc.. etc


--- test your knowledge ---

>lets say the setup is x2 18650 cells. (how ever you like series or parallel)
>how would you make and config your magnet. winds? thickness, core?s

how will it lift???

keep it simple !

i may have a thank you price for the best idea \ setup

 




 

Offline jitter

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Well, it's current that develops the Lorentz force, so I might run the two cells in parallel at the highest current they can safely deliver.
Magnet: copper wound around a weak iron core with a length and diameter combination that would lead to the right current from the batteries. Then you'd only need a switch to switch the electromagnet on or off.
« Last Edit: May 11, 2016, 05:32:00 pm by jitter »
 

Offline danadak

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Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 

Offline VK5RC

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If you have a superconducting magnet,  a 'little bit'  of DC eg 500-1000A (at bugger all voltage drop)   for a MRI magnet (c 1-3Tesla)   eg http://mriquestions.com/how-to-ramp.htm   HiHi
Whoah! Watch where that landed we might need it later.
 

Offline user1235423643

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Well, it's current that develops the Lorentz force, so I might run the two cells in parallel at the highest current they can safely deliver.
Magnet: copper wound around a weak iron core with a length and diameter combination that would lead to the right current from the batteries. Then you'd only need a switch to switch the electromagnet on or off.

what method would you use to control the current. what is your theory on coil \ wind \ size
 

Offline user1235423643

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Magnetic Fields Inside Solenoids
 

Offline EPTech

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Hi there,

Interesting thread.

I guess one would achieve maximum efficiency by using a lamilated core and ecually important, placing the coil as perpendicular and as close as possible to that core.

I would only use a single layer or two layers of windings at the most. I would also make a water tight enclosure and use a glycol or liquid nitrogen cooling system, depending on the budget and explosion safety requirements.  ;)

If it has to be just an ON/OFF magnet, I would use a transformer with a fat diode rectifier, like the old welding transformers.

If you want maximum control, I would supply it with a fat switch mode supply. The feed-back would not be the current in the secondary but the magnetic flux, measured with a sensor around the core, close to the apperture of the magnet. Of course there has to be an overcurrent and an over temperature safety. The magnetic flux itself being the reference for the supply would not be affected by the construction of the magnet or the temperature (resistance) of the coil.




Kind greetings,

Pascal.
 

Offline Ian.M

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Its all down to Ampere turns (n*I).   For a particular winding cross sectional area, ignoring core saturation, there's a fairly firm limit on the Ampere turns you can use. I2R losses heat the coil, and the limit is set by the insulation temperature rating and the thermal resistance to ambient, which is to a great extent set by the surface area, bobbin construction and thermal conduction to the core.  However R is proportional to n and inversly proportional to the wire CSA, so the losses in the coil itself are to a first approximation independent of n while n*I is held constant.  This breaks down at the extremes - on the small n end, large round wire doesn't pack well at the bottom and sides of the former, reducing the effective winding area (though a copper foil or sheet winding can fully fill the former) and at the large n, thin wire end, the enamel insulation on the wire becomes a significant proportion of its CSA again wasting winding area.

Therefore, given a free choice of wire gauge for the winding, losses in the control circuit become the deciding factor.  Resistive losses go with I2R, and semiconductor losses are I*V where V is Vf for diodes or Vce_sat for transistors, so there is a clear advantage to make n large and I small, within the limits of the available supply voltage and the voltage ratings of the semiconductors in the control circuit.

You also have to consider turn-on and turn-off time - the closer the supply voltage is to coil I*R, the slower it will be to ramp up to the desired operating current, and when you shut it off the stored energy has to go somewhere, and unless you drive it from an H-bridge that can be controlled to dump the energy back into the supply, it will all have to be lost in the anti-parallel back EMF protection diode and the coil resistance.

Assuming ultra-fast turnon/off wasn't required, I'd put the 18650 cells in series and design the coil(s) to run at rated current at 6V for two cells, then PWM it with current feedback to maintain the same current from fully charged to your typically 3V/cell cutoff point.  6V minimum isn't too bad for MOSFET gate drive, though for lower RDS_on and lower I2R losses, three or even four series cells for a cutoff point of 9V or 12V and a coil designed to match would be preferable.
 

Online T3sl4co1l

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Voltage and current don't matter.

Turns and wire size are open variables. You can only put on as much copper as the assembly will hold, period.  And you can only dissipate so much heat as the assembly will allow (given that you can fill the winding with conductive potting, and heatsink it, or add cooling tubes and water flow if you're desperate).

Thus, the amount of magnetic field intensity you can obtain, from a given quantity of copper, requires a fixed power input.  The copper's resistance is the conversion factor between power (volts * amps, heat dissipated) and magnetic field intensity (amps * turns / magnetic path length), which is also an energy figure (energy density is B^2 / (2*mu)).

It's not truly constant because it depends on available power, dissipation capacity, geometry and pole pieces, but those are only ratios that tweak the total.  But the copper, you can't do any better with anything else.

If battery operation is required, I would seriously consider using as many efficiency enhancements as possible:
1. PWM DC, obviously.  Preferably filtered, to reduce core heating.
2. Relatively low current density, to reduce power dissipation.  Requires a physically larger winding (more copper).
3. Pole pieces.  An iron core goes through the winding and shorts out much of the magnetic path length, increasing the field intensity (in the remaining gap) proportionally.
4. Permanent bias.  Magnets are added to the core to provide a background field which is the average level required by the application.

If you need an always-on field, then you can use all permanent magnets, with only very little electrical input required for trimming it around the desired level.

If you need to go from zero to (peak) approximately equal amounts of the time, then use magnets for (peak)/2 bias, and electrical drive to +/-(peak)/2, thus achieving the 0 to (peak) range with half the electrical requirement.

If the duty cycle is uneven, bias accordingly.  If the magnet is off most of the time, use less bias (or none, since it will be inconvenient to use very tiny magnets at some point), and pulse it on to enable.  If it's on most of the time, use more bias, and pulse it to disable (opposing the permanent field with the electromagnet).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline user1235423643

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thanks for reply, how about your answer to the question

how would you setup simply and realistic


Voltage and current don't matter.

Turns and wire size are open variables. You can only put on as much copper as the assembly will hold, period.  And you can only dissipate so much heat as the assembly will allow (given that you can fill the winding with conductive potting, and heatsink it, or add cooling tubes and water flow if you're desperate).

Thus, the amount of magnetic field intensity you can obtain, from a given quantity of copper, requires a fixed power input.  The copper's resistance is the conversion factor between power (volts * amps, heat dissipated) and magnetic field intensity (amps * turns / magnetic path length), which is also an energy figure (energy density is B^2 / (2*mu)).

It's not truly constant because it depends on available power, dissipation capacity, geometry and pole pieces, but those are only ratios that tweak the total.  But the copper, you can't do any better with anything else.

If battery operation is required, I would seriously consider using as many efficiency enhancements as possible:
1. PWM DC, obviously.  Preferably filtered, to reduce core heating.
2. Relatively low current density, to reduce power dissipation.  Requires a physically larger winding (more copper).
3. Pole pieces.  An iron core goes through the winding and shorts out much of the magnetic path length, increasing the field intensity (in the remaining gap) proportionally.
4. Permanent bias.  Magnets are added to the core to provide a background field which is the average level required by the application.

If you need an always-on field, then you can use all permanent magnets, with only very little electrical input required for trimming it around the desired level.

If you need to go from zero to (peak) approximately equal amounts of the time, then use magnets for (peak)/2 bias, and electrical drive to +/-(peak)/2, thus achieving the 0 to (peak) range with half the electrical requirement.

If the duty cycle is uneven, bias accordingly.  If the magnet is off most of the time, use less bias (or none, since it will be inconvenient to use very tiny magnets at some point), and pulse it on to enable.  If it's on most of the time, use more bias, and pulse it to disable (opposing the permanent field with the electromagnet).

Tim
 

Offline user1235423643

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so this is the setup in more detail for this challenge

x2 18650

overall size 5x5 inch max

temporary switched. when needed. push button.

use: pickup all of a cup of screws spread over 30cm

peak power is the key.



 

Offline Ian.M

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18650 cells in series,  electromagnet wound with two coils such that the max power dissipation with the battery fully charged and the coils in series doesn't burn them out,  Relay + timer circuit to start with the coils in parallel for maximum 'grab' then switch over to series a second or two later before they get too hot, which will still  be plenty of flux to hold parts that have already been picked up.

Alternatively one can do it by designing the coil to run too hot if the full battery voltage is applied continuously, and PWMing it - 100% initially cutting back to whatever duty cycle provides satisfactory holding so it doesn't overheat.  Due to the losses in the back-EMF protection diode that it will need across the coil to let the current recirculate, it should actually be a MOSFET ideal diode driven by the PWM controller antiphase to the PWM (less some deadtime) if you want the circuit to run cool and acceptable battery life.

Alternatively do what the sane people do - put a big permanent magnet in an inside out tough plastic bag, pick up the screws with it then turn the bag back the right way in and pull it off the magnet with the screws inside it.
 

Online T3sl4co1l

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A turn-off coil might still be preferable, as you might like to hold it magnetized for quite a long time, but only need to drop the contents in one place.

Pole pieces, spaced at distances suitable for picking up whatever size screws you're after, will do well.  Consider the design of a "magnetic chuck" used in machining.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline user1235423643

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need more info on core\coil winding.

how many turns, how think, how many layers. what config. etc

its seem to all come down to this.

thannks
 

Offline user1235423643

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any combos that will work? tested
 


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