[GreenKeys] Measuring Electromagnet Saturation?

[Bob Camp]

Hi

Well assuming we are still trying to measure, rather than compute the  
value:

Drive the coil with a high impedance constant current source. Then  
either measure the inductance or impedance. A fancy way would be to  
apply fixed AC + variable DC current. Then you could monitor the  
voltage on the coil with your 'scope.

Bob


On Jul 30, 2007, at 9:14 PM, Brooke Clarke wrote:

> Hi Robert:
>
> The core is made up of a stack of laminations each about 0.030"  
> thick.  The central core is 0.533" x 0.420" for an area of 0.224 sq  
> in. (144 sq mm).
>
> I don't know specs or who made the Electrical Silicon Steel  
> laminations.
> The coil is 6 Ohms DC and I've been told was driven from 120 Volts  
> at 1/4 amp.
> No luck so far measuring the inductance.
>
> The wire is enameled about 0.029" dia, or maybe AWG 22.  6 Ohms  
> would take (Cooner Wire) 6/16.2 = 370 feet.
>
> The coil form might have an ID of 0.68" and OD of 1.6" with a  
> length of 2.3" this gives a rectangle for the turns that's 0.46"  
> high x 2.3" long or 1.058 sq inches.  If each wire was a square  
> 0.03" on a side then 1,175 turns would fit.  The mean radius is  
> 0.522, mean circumference is 3.27" which times the number of turns  
> is 3,850" or 320 feet, not too far from the computed 370 feet.
>
> Any WAG on what's needed to saturate it?
>
> Hi Bob:
>
> I've tried to use the turn off voltage kick waveform into a known  
> value shunt resistor to back into the inductance, but the numbers  
> are not consistent as I change the resistor value.  But this may be  
> because I'm not saturating the coil, hence the effort to learn  
> about saturation.
>
> Can't use your two coil method, since there's only a single coil.
>
> Hi Randy:
>
> The next setup will be one where I can monitor the current on a  
> scope.  I expect that if a big cap charges the coil through a  
> resistor to set the peak current to 1/4 amp the current will be  
> flat during charge and then drop off when the coil is full.
>
> Have Fun,
>
> Brooke Clarke
> http://www.PRC68.com
> http://www.precisionclock.com
>
>
>
> WA5CAB at cs.com wrote:
>> That will work.  And is probably what an undergraduate would be  
>> made to do in a class on the subject.  But in practice (back when  
>> I was in practice), I would have either used a Gaussmeter and  
>> adjustable current supply to get an empirical answer or if I was  
>> sure that I knew what the core material and the number of turns in  
>> the coil was, measured its cross section, and used a B-H curve for  
>> the material to calculate the ampere-turns (NI) required to  
>> saturate it.
>> In a message dated 7/30/2007 6:06:00 PM Central Daylight Time,  
>> ham at cq.nu writes:
>>> The setup is pretty simple.
>>>
>>> Take two coils in series and run current through them. Ground one  
>>> end  and bypass the end the power supply is connected to.
>>>
>>> Then you measure the inductance and the "center point" with a  
>>> fairly  normal inductance bridge.
>>>
>>> The net result is that you can run the current up and watch the   
>>> inductance drop.
>>>
>>> There also are ways to saturate the coil and then watch the spike  
>>> as  you break the circuit. The pulse contains the energy stored  
>>> in the  coil ...
>>>
>>> Bob
>>>
>>>
>>> On Jul 30, 2007, at 12:02 PM, Brooke Clarke wrote:
>>>
>>>
>>>> Hi Bob:
>>>>
>>>> I've been doing my homework on electromagnet saturation and  
>>>> found  Electrical Silicon Steel as used in transformers, motors  
>>>> and newer  electromagnets.  Not only higher saturation but also  
>>>> less remnant  magnetization.  That's what's used as the core of  
>>>> the  Synchronization coil in the WU 37SS clocks with the Sweep  
>>>> Second  hand.  They were synchronized once a day using what  
>>>> looks like a  Teletype loop running at 120 Volts and adjusted to  
>>>> 1/4 amp.  A one  second pulse that ends at the top of the hour  
>>>> was used.
>>>>
>>>> In trying to come up with a circuit to drive the sync coil I'd  
>>>> like  to have a way to know that the electromagnet is in  
>>>> saturation.
>>>>
>>>> Any thoughts on a test that would demonstrate saturation of the   
>>>> electromagnet?
>>>
>> Robert & Susan Downs - Houston
>> <http://www.wa5cab.com> (Web Store)
>> MVPA 9480
>> <wa5cab at cs.com> (Primary email)
>> <wa5cab at comcast.net> (Backup email)
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