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Real Inductors and a couple of real life stories about them.

The magentic field around a coil of wire.

The magnetic field around a coil of wire.

When I started down this path of talking about inductors in the post “Through the looking glass – Duality – The Inductor”, I chose to make a big deal about how the magnetic fields build and collapse.  That was done for several reasons.  First the magnetic fields themselves are very useful and when we talk about elector-mechanical devices such as solenoids and motors and even loud speakers these fields will be very important.  The second and equally important part is the fields are made stronger by providing a path for them to flow inside.   Air is not a good path for the fields but iron provides a good path.

As shown in the picture, the magnetic fields want to complete a circuit.  If we want to provide a good path for the magnetic field, the iron material must also make a complete loop. This is called the magnetic core of the device.  But, “Houston, we have a problem”.  Iron is also conductive, so as the fields change, a voltage would be induced into the iron and create a current.  This current is called an eddy current and this is a loss.  It is even given a name… “eddy current loss.” (That makes sense doesn’t it?)    To make a good magnetic path, but a bad electrical path in the core the core is made up of flat sheet metal pieces of steel called laminations.  These have a thin coating of varnish on them to act as an insulator to prevent electrical current from flowing from one lamination to another one.

For example, in the coil in the picture, an “E” shaped piece of sheet steel might be inserted with the open part of the E facing upward.   Then a straight piece of steel sheet, an I, would be placed on the top to complete the loop.  When the next layer is added it would have the E facing downward and the I closing the bottom.  This would continue until the whole core was built.  In this case the coil would be oval shaped instead of a circle when looking down from the top.

The lamination method works good at low frequencies.  It is almost always used on power line frequencies, 60 Hz in USA and 50 Hz in Europe.  (I am sorry, I don’t know about the rest of the world.)   Most audio frequency circuits I have seen also use laminated cores but the inductors are a lot smaller.   Remember, higher frequencies have higher inductive reactance for the same size inductor, so smaller inductors are used.

As the frequency gets higher, even laminations are too lossy, so the core is made from very small particles of iron glued together. This is usually referred to as a ferrite material and sometimes actually uses iron oxide (rust) for the material.   Because the individual particles are separated by the glue, the magnetic path is not as good or in other words it has less permeability.   An inductor of the same size of inductance would require more turns.  Often this ferrite material is shaped in the form of a donut  and is called a toroid core.

There are other problems with the iron core.  The iron has a memory once it has been exposed to a magnetic field.  It takes work to make it forget and change directions in the case of AC.  This memory is called hysteresis and creates a loss.   Again this loss is with each change of direction so the higher the frequency the bigger this is a problem.  In high frequency work no core at all is used because of this.  People that do radio work often wind their own coils on open frames or on an insulated material.  The coils do not have to be all that big.

Real wire has a resistance.  There is a measurement of inductors called the Q value.  (Q for quality)  It is the XL/R.  This value is often used in power studies, but it is very important in tuned circuits as we shall see later when we combine LR&C into one circuit.

To summarize everything talked about so far.   At low frequencies, Inductors are normally big heavy devices because of the iron cores.  As the frequency gets higher the size of the inductor gets smaller and less iron is used until eventually no iron at all.

Ok .. now a real life “war story” because it felt like war while I was working through it.  I worked on a machine.  The machine had a 250 Hp motor that controlled the unwinding of a large coil of aluminium sheet.  When the machine was stopped, especially in an emergency stop there was a large brake on the back of the motor.  This brake had a coil about the size of a person’s head.  A spring closed the brake and power (250 Vdc) was applied to this coil to overpower the spring and release the brake.  The problem was we used a remote controlled switch (something called a contactor) to open the circuit to the coil.  When the power was cut to the coil and the fields collapsed the coil wanted to continue to flow the same current and since there was an air gap in the switch it had to produce enough voltage to jump the gap.  The energy stored in the coil had to go somewhere. Needless to say the switch did not last very long with this arcing going on.   We tried several things,  all involved using a diode and the final fix was a thing called a varistor.  I will go into detail about those in a future post, but here I just wanted to show how the codgety old inductor can cause problems.  We will be dealing with that but on a much smaller scale in the future.

The second example is wires themselves and another kind of arc.   When power source wires are shorted together lots of current flows.  Protection devices open the circuit and stop the flow of current.   The problem is these device open while the current is flowing so they experience an arc.  If the arc is too strong it can overpower the circuit protector and just arc across the open contacts.  This is BAD NEWS… VERY BAD NEWS.  Things continue to arc and burn.  Luckily the wires feeding the circuit have some resistance and some inductance limiting the current available to feed the circuit with the protection device.  Power engineers do studies on circuits to make sure the Arc Interrupting Rating (AIR) of circuit breakers and fuses can handle the available fault current.   In addition to the power being provided from the power lines collapsing fields on other equipment in the circuit may be contributing to the short term fault current.  In this case the inductance of the wires is helpful to reduce the fault current.  It is way beyond anything we will be doing with our playing with electronics so we will not go into detail on that analysis.   Besides it is anything but fun.

However, the attached video of a failed High Voltage Circuit Breaker is interesting.  The story behind this picture is they were troubleshooting the breaker and had a second switch at another location eventually open to shut it off.  One very very big arc.

I hope you found this post enjoyable and educational.





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2 comments to Real Inductors and a couple of real life stories about them.

  • Brian Voss

    Hi Gary,
    Many years ago I started work developing telephone switching control circuits using wire-spring relays. Of course the relay windings are inductive and the current through them is controlled through relay contacts. Every circuit involved contact protection devices (RC networks) across the coils, to dissipate the inductive energy.

    On your video, it is interesting to watch the arc rise in the air: the arc creates plasma, the plasma is hot, convection carries the arc path upwards.


    • Gary

      Hello Brian,
      You are exactly right on both of your statements. On PLC controlled machines we usually put those RC networks on outputs controlling coils for exactly the same reason. In this case it was assumed by the equipment designer that a contactor (a very big relay) could handle the “kick back”. When I got involved it was already “an issue” so I was given the “opportunity”. (Political speak… my back was against the wall.) I had no idea what the inductance of the coil was but my first attempt had too large of a resistor and I still had a large arc. The 2nd attempt was a smaller resistor, but this time the time constant was too long and the brake did not set quickly enough and it was impossible for the operators to thread the metal in the line. Finally I was able to locate a surge suppressor that let the voltage go high enough that I absorbed the energy in the coil quickly but did not fry the contactor. The surge suppressor was large enough to handle the hit without frying. (It could dissipate the energy without overheating and destroying itself to say it a little more technically correct although the first way is how I normally say it.)

      Many circuit breakers and some contactors use the property of the arc rising to force the arc into some insulators and extinguish the arc. The old Frankenstein movies always had a device in the background also using that property.

      Thanks for the comments. Please feel free to comment any time.


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