# Electromagnet coil

Discussion in 'General Electronics Chat' started by cmartinez, Dec 4, 2014.

1. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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I have this electromagnet with an iron core completely embedded in resin that I'm trying to reverse engineer. I had it tested for inductance with a specialized company that owes me a few favors, and they told me that when its coil is exited with a 1Khz@1V sinusoidal signal, their instruments would read a value of 1.0964 mH . And that when they measured its internal resistance, they'd read a value of about 0.60Ω ... I'm saying about this value, because it actually oscillated between 0.65Ω and 0.55Ω when measured with a high-accuracy multimeter... I'm thinking that the fluctuations have something to do with the reactance in the coil due to the small voltage/current that the multimeter needs to apply when making a measurement.
Is there a way to calculate the wire gauge and/or number of turns with this information? Or will I need to take it appart and find out?

2. ### MaxHeadRoom Expert

Jul 18, 2013
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If it is intended for electromagnet, it most definitely is intended for DC, you could apply an increasing DC via a Variac and a bridge if you wanted to experiment with a use you had in mind for it.
Taking apart is most likely going to destroy it.
Max.

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3. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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You mean assemble a wheatstone bridge with the coil being the unknown resistor?

4. ### GopherT AAC Fanatic!

Nov 23, 2012
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No, a rectifier bridge.

5. ### RichardO Well-Known Member

May 4, 2013
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Are you using a 4-wire ohmmeter? At these low resistances it is more likely that the fluctuations are due to changing resistance from the meter probes to the coil connections.

If you don't have a 4-wire meter there are a couple of ways to do the resistance measurement. The easiest is to power the coil at it rated voltage and measure the current through the coil. Ohm's law gets you the resistance. Keep in mind that as the coil heats up, the resistance will go down.

To prevent the heating problem you can use a power supply and a low value power resistor. Choose a resistor that will put as much current through the coil as you can without it heating up. Now measure the current through the coil and the voltage across it -- again followed by using Ohm's law.

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6. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Oh! I see what you mean... use a rectifier bridge after a Variac to make sure that the coil's getting only DC current ... I have all I need with me to do just that...
Thanks Richard, for the suggestion; it sticks quite well to the KISS principle...
The downside about the variac-rectifier is that it will deliver a series of halfwave pulses to the coil, instead of a stable voltage... it's probably gonna take some serious capacitors to stabilize the output ... maybe I'd be better off using one of these?

7. ### Brownout Well-Known Member

Jan 10, 2012
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You can easily destroy the coil with DC. I'd use a resistor to limit current.

ie 5V/.6= 8.3A = "poof" BTW, you can just use a battery to get DC.

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8. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Not without knowing the permeability of the electromagnet core.

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9. ### MaxHeadRoom Expert

Jul 18, 2013
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Did you take a resistance measurement yourself?
Have you any idea as to its original purpose?
Material lift magnets and brake/clutch type are usually higher resistance than you show.
Max.

10. ### wayneh Expert

Sep 9, 2010
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What function are you trying to reverse engineer? It might be easier to reproduce the field strength with a given power supply than to determine the coil properties.

That said, you might look at the Wheeler formula for a solenoid inductance. There are comparable formulas for other shapes. They can give some insight into the relationship between winding details and inductance.

11. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Thanks Max and Wayneh for your replies. The electromagnet is part of a valve used in a high pressure pump (about 20 Kpsi) for oil in an industrial application. This pump has a controller that governs the valve, whose behavior I'm trying to tweak a little. What I'm trying to do is bypass the controller 's signal to the valve and introduce my own, while at the same time mimicking the electromagnet's response so as to make the controller believe that there's nothing wrong with the valve... I'm trying to avoid as much guesswork as possible, but I also know that an electromagnet's coil changes its resistance and inductance value (don't know by how much) with frequency and temperature. I'm learning as I go...
Here's an image of the waveform going into the coil.

12. ### MaxHeadRoom Expert

Jul 18, 2013
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Would this be a coil of a proportional valve by any chance?
Max.

13. ### cmartinez Thread Starter AAC Fanatic!

Jan 17, 2007
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Not really... the pump works in pulses and not in a continuous mode. The input pressure at the valve remains constant, so the amount of oil being delivered depends on the amount of time that the valve remains open.
From what I gather, the valve receives a series of 120V pulses lasting around 3ms, and then remains open for a lapse that can last between 5 and 14 ms, and at the end of the cycle a negative 75V pulse is sent by the controller. I'm guessing it does that so as to close the valve more rapidly and to avoid magnetizing the puppet assembly in the long run.