# Electromagnet: Current or Voltage??

Discussion in 'Electronic Basics' started by Randy Gross, Sep 5, 2005.

1. ### Randy GrossGuest

I'm trying to determine what gives an electromagnet its strongest
power: high current or high voltage.

I ask this question because of L-reactance in the coil when it is
energized which limits the current through the coil.

Should the initial power be more current with low voltage or, higher
voltage with low current?

rg

2. ### BobGGuest

Its a resistor. A coil of wire. It has x ohms. Put V volts across it,
it draws V/x amps.

3. ### Bob EldredGuest

Current creates force in an electro-magnet. Voltage develops current through
resistance. You cannot have one without the other. So your question makes no
sense. Reactance is not an issue unless in an AC circuit where the reactance
does limit the current. But, that's not the usual case. It's the DC
resistance that establishes the current by ohms law and the current
establishes the force. The resistance of the wire will dissipate heat and
that limits the amount of power that can be placed on a given magnet.
Bob

4. ### John PopelishGuest

By power, I assume you mean flux. Magnetic flux, for a given core
permeability, is always proportional to ampere turns (total amperes
circulating around the core).
Inductance limits how long it takes for the field to reach full
strength but resistance limits the ultimate current. Are you in a hurry?
Voltage gets current going, in spite of inductance, and keeps it going
in spite of resistance. For a given coil volume and wire resistivity
there is an optimum wire size that gets the most ampere turns in that
volume for a given amount of total power (volts times amperes). The
optimum combination will come out to about the same power, regardless
of the voltage (neglecting the space taken up by wire varnish).

If you are in a hurry (if inductance is still controlling the rise of
current when time is up), then the optimum balance will be different.

5. ### Ralph MoweryGuest

The strength is determined by the amp-turns. That is how many amps and how
many turns. The voltage will be determined by how many amps you want. That
is you can use the same number of turns and large wire that will draw more
amps for the same voltage or you can use smaller wire and draw less amps and
it will be weaker.

There is a short period of time that it takes for the current to reach its
maximum. If you put a resistor in series with the coil and make the voltage
larger so the final current is the same , it will reach the maximum strength
sooner. Not much like a couple of miliseconds or so for a small coil.

6. ### Randy GrossGuest

Yes. I'm looking to get maximum flux density. The electromagnet will
only be used in a 12 vdc momentary burst so I need maximum capability
in an instant for an instant.

The width of the coil, including Iron core, is one inch. The length can
be up to one inch. I have 18 awg. magnet wire to wind the coil.

rg

7. ### vicGuest

Current and voltage are linked by this equation :

U = L di/dt + R i

You can't have one without the other.

8. ### Randy GrossGuest

[email protected]

I think you misunderstood. I know that for a given volt/amp, lets say
12VA for demo., I have a range of choice. I can use 1 amp @ 12 volts up
to 12 amps @ 1 volt. I was just trying to determine where along this
and turns appear to be the answer.

rg

9. ### John PopelishGuest

I'm not familiar with that measure of time, ;-)
Can you put any number on how long an instant is? I don't understand
if we are talking about a second or a nanosecond.
If you can explain more about what you need this flux for, I could
make better suggestions. After that it is a matter of writing and
solving an equation.

10. ### Randy GrossGuest

This is an experiment in Torque.

I'm building a motor from a different point of view. Instead of
rotating the coil in a magnetic field, I'm mounting the magnets
(1"x1/8" NEO disks) on the rotor and spinning the magnetic field.

I want to use the coils, mounted in planetary fashion, like spark plugs
in an engine with an adjustable "distributor" (armature) for energizing
the coils mounted on the rotor. When the rotation of the magnets reach
approximately 75 degrees to the coils, The coils (electromagnets) will
energize wrenching the magnets toward 90 degrees but will de-energize
before the poles try to cog on each other.

At this time, I have no Idea what the RPMs will be so time for me is a
best guess. I'm thinking low milli for about 13 degrees of rotation.

The adj. feed is to time for maximum torque and to compensate for my
shortcommings;-)

rg

11. ### John PopelishGuest

I think the ideal place the magnets should be with respect to the
coils is that the coil should be exactly half way between two opposite
magnet poles when the coil current peaks. That way, one magnet pole
is pulling toward the coil and one is pushing off it. The coil
current is ideally zero as the magnet pole passes directly beneath it.

If the coils have iron cores that extend toward the magnets, there
will be cogging, just because iron is attracted to a magnet. This can
be reduced if there is a different number of magnet poles and coil
poles. For instance, you might drive a pair of magnet poles with 3
coils. That way, all 3 magnets do not cog at the same time. The
current switching arrangement gets more complicated, though. You
might look into hall effect switches as a means of detecting rotor
position to switch the coil currents. That is the way brushless
For first effort, since you don't have a speed goal, I think you
should forget speed and try to maximize stall torque per watt of DC power.
It is commonly used to advance the coil voltage timing to get the
current peaks where they are needed in spite of the inductive delay,
at rated speed. This is like adjusting the timing on a gasoline
engine to produce maximum power. There is a best setting for each speed.

You should be able to come up with a mathematical expression for
winding resistance versus turns, based on the dimensions of the
winding volume and your wire gauge. Ohm's law converts that to
current. Pick a DC power and you have the current allowed for a given
DC voltage.

Current times turns gives you flux which is proportional to torque.

Combining the ampere turns with the power formula gives you torque per
DC watt.

It should come out that above some number of turns, the coil diameter
is so big that the resistance is going up faster than the flux, so it
may not be best to fill all available space with coil.

12. ### Bob EldredGuest

So, you are reinventing the permanent magnet DC motor??? In a traditional DC
motor, the communtator does the switching to direct the current to the
appropriate winding as it passed the magnets. What you have described sounds
similar to this. In a brushless DC motor, the switching is accomplished by
transistors or FETs turned on and off by hall effect or optical means as the
magnets or coils pass each other. It doesn't matter which moves, the coils
or the magnets. Motors have been built both ways, rotating coils and
rotating fields. The important thing is the magnetic path, it's reluctance
and the air gap. Get a book on motor design and study how they work, you'll
save your self a lot of frustration and avoid reinventing what was done 100
years ago.
Bob

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13. ### Randy GrossGuest

I agree!
I've debated the inclusion of Iron cores for this unit because of the
attempts at cogging and reduced potential power. I may settle for a
synthetic (resin/Iron powder) to reduce attraction an retain some
benefit from the core.
On the back burner for now. I'll center on torque since load support is
expected.
Understood!

I think I have enough now to set me on the right path. I'm more than
sure that before I finish this build, I'll have a question or two.
Thanks for the help!

rg

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14. ### Randy GrossGuest

No Mr. Eldred. What I am doing is akin to building a replica of the
KittyHawk or a 65 Mustang only with variations. It's the same soup,
just a different bowl.

I guess you could say that It is built in honor of those that gave us
the technology. I for one am fascinated by what they accomplished and,
the completed units, if not done hastily, are works of art.

rg

15. ### Bob EldredGuest

I give up...If you argue with idiots you begin to sound like one...no
thanks!

16. ### Jasen BettsGuest

both...

the more current you can get circulating close to the centre of the winding
the stronger the electromagnet.

if you use a high current you need thicker wire, if you use a low current
you can use thinner wire but will need more voltage to push the current
through the wire.

Bye.
Jasen

17. ### Jasen BettsGuest

How long is an "instant", you could use 8awg wire (and a supply capable of a
few hundered amps - like a car battery) if you only want to run the
electromanet for a small fraction of a second

hmm you'd get somewhere around 500" of 18awg in your coil
so resistance 0.3 ohms. from 12V peak curent would be about 40
amps so about 500 watts disipation... (and this is best case)

a coil about 1" in size can't stay cool enough to survive and disipate more
than about 5 watts (unless you have heastsinks and cooling fans etc)

so I'd reccomend a duty cycle of 1% (1:100) so if you run it for 1 second
rest it for 100 etc...

as for how strong it'll be I have no idea, pretty strong I expect.

I'm no engineer so wear safety glasses and have a fire extinguisher on hand
etc...

Bye.
Jasen

18. ### Randy GrossGuest

This is always good advice, thanks Jasen ;-)

rg

19. ### Randy GrossGuest

Pull your horns in Bob, you were heard. Think of this as building a
scale model only, you can't buy a kit.

rg

20. ### Jasen BettsGuest

all else being equal thin wire 1200 turns and 12V x 1A will get you the same
efficiency as thick wire, 100 turns and 1V x 12A
turns take length of wire that means resistance and resistance reduces the
amps. you can't have both.

Bye.
Jasen