Electromagnet Question

[gyulasun]

...
When I move the magnet in and out of the charged solenoid the voltage reading is steady at 9 volts.  If the battery is not connected I can induce voltage spikes of ~4 Volts. 

Hi Mondrasek,

The 9V battery also has a very low inner resistance like a good power supply so any voltage drop caused by changing a current in the circuit it feeds is also very low, hence you cannot see it with a 3 and a half (or even a 4 and a half) digit resolution your multimeter has.

...   I also see no change in current with any of the experiments, but here is the weakest part of my test set up.  I have no current probe to use with the o-scope.  So I have a Fluke multimeter reading current and that may miss any transients.  But I cannot get it to read anything but the expected 120 mAmps even while pushing the magnet through the solenoid field (which should be generating +- 4 volts).

Well if you have the true RMS type Fluke multimeter like Fluke 87, then you may try it reading AC current in its most sensitive (ACmA) range. In this AC current range the 120mA DC current will not show up of course but you may be able to cause some small mA change when moving quickly the magnet away or towards the solenoid.  You can test this without the battery first, (especially if you do not have the RMS measuring type Fluke) by setting the most sensitive  ACmA range and connecting parallel the tips with the solenoid, then quickly moving the magnet in and out. You will hopefully see some peak mA change...  EDIT: If you see about 4V induced peaks across the unloaded solenoid, then you close the solenoid with the ACmA meter, the AC peak current will approximately be 4Vp / 75 Ohm=53mAp in the solenoid.

Here is some hint on making an AC current probe if you happen to have a higher permeability toroidal ferrit core... :
http://cappels.org/dproj/aciprobe/ACCurrentProbe.html
No need for calibration and the low value resistor termination, only would be useful for indication with your scope...

rgds,  Gyula

[mondrasek]

Gyula,

I had a bit more time to play before leaving work.  The Fluke MM I am using says it is an RMS type so I tried to set up for ACmA as you said in parallel with the solenoid. But it would not read anything when I introduced the permanet magnet to the solenoid.  It also would not energize the solenoid.  I placed it in series and I was able to see very large readings while moving the permanent magnet in and out of the solenoid, both with and without power to the solenoid applied.  I then switched back to DC and was able to witness small fluctuations to the current readings while moving the magnet in and out.  I realized that I had added some more magnets to my original permanent magnet in order to increase the mass and keep it from firing so high since I had lost it in my work area several times after failing to catch it.  So the increased length of the magnet, and possibly the strength (I added a much larger diameter magnet, one that could not enter the solenoid, to the top) are allowing me to finally get readings.

I will work on making a toroidal current probe.  I assume I can use a core from an old circuit board.  We have a junk bin of various boards in our repair department that might have one.

Any reason this type of probe will not work with DC current?

I am still thinking the sample rate on my available o-scope is not high enough to show the diference in the current traces when charging the solenoid with and without the permanent magnet present, but it is all I have now.  My father teaches at a local University and he agreed to check with their Engineering Department to see if I can use some better equipment if I need to go further.

M.

[mscoffman]

That lead me to what I believe is the problem with my test set up and measurements.  I am using an industrial 24 V power supply.  This supply is likely adjusting current/voltage for the changing load and not allowing me to see the real differences between charging the solenoid with and without the permanent magnet in place.

Actually that is the job power supply to keep it's voltage as constant as possible even as it's input current changes.
And yes, current probes will help. Unless you have a "crappy" scope it should be fast enough for these inductor speeds.
Also if you has any choice it would be better to use FET field effect transistors like IRF511 as they have very high off
resistance and can have very low on resistance like .01 Ohms on for specials, much closer to relay contacts. FET are
voltage mode triggered and that makes them easier to understand bipolars. Bipolars (2nxxxx) are current mode controlled
and harder to design with and that control current has a cost - they are often used less efficiently.

If you want to do power measurements you need to put a resistor across the coil when you disconnect all else - make
it identical to the DC resistance of inductor.  That way you transfer maximum power, when source and output resistance
(impedance) are equal. <= read this; it answers why you seem to see no effect when you have power connected vs nothing.
A battery's job is to have very low internal impedance.  As Guyla said. Good Luck!

:S:MarkCoffman

[gyulasun]

Gyula,

I had a bit more time to play before leaving work.  The Fluke MM I am using says it is an RMS type so I tried to set up for ACmA as you said in parallel with the solenoid. But it would not read anything when I introduced the permanet magnet to the solenoid.  It also would not energize the solenoid.  I placed it in series and I was able to see very large readings while moving the permanent magnet in and out of the solenoid, both with and without power to the solenoid applied. 

@Mondrasek,

Sorry if I was not a 100% clear with my suggestion but I meant first do a induction test without any power supply or 9V battery  and then with the battery (or the power supply), ok?
And in the without battery test, I meant you connect the ACmA meter's tips directly to the coil's wire endings (to me this means a parallel connection, there is nothing in between the meter and the coil endings, ok?) and move the magnet. This way you test the induced current because the meter as a load simply places a short circuit onto the coil and the induced current is limited only by the coil own copper resistance + some inductive reactance.
You wrote you did not see anything in this parallel connection and placed the meter in series with the coil?  You lost me here, sorry.

If you happen to have access to better instrumentation by your farther's help,  it would save you a lot effort and time on not 'tinkering' with AC current probe building, I think.

Such type of current probes do not work on DC because their principle is based on normal transformer operation so a DC current can only bias them in one or other direction on their cores' B-H curve, that is all.  It takes a current change like an AC does to get transformation on the secondary coil.

rgds,  Gyula

[mondrasek]

Gyula,

I see that I misunderstood your suggestion.  I look forward to trying it correctly.  Unfotunately I must visit a customer tomorrow and will not be able to play with my simple test set up.

When I modified your suggestion to "in series" it was only a guess that you had accidentaly said parallel and truely meant in series. 

I have access to quality ferrous toroids and built a simple AC current probe this morning but of course it did not work on this DC circuit.  I suspected as much.

At least with every failure I learn something new!  Maybe that is what I enjoy the most?

Thanks for all your support.

Any other way to see the current trace of a DC electromagnet when it is initially energized?

M.