[GreenKeys] Measuring Electromagnet Saturation?

[Brooke Clarke]

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)
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