Tuesday, September 18, 2012

Can a Pulse Transformer Solve World Hunger?

Small electric pulses, processed by a
pulse transformer, stimulate growth

One of our customers sells a system to stimulate plant growth.  The stimulation is accomplished by passing small electric pulses through the ground.  These pulses stimulate the plant roots to grow faster and a larger.  This more robust root network allows the plant to grow faster.  It is more complicated than just electrically stimulating the roots; maximum performance is when the pulses of electricity vary over a wide range of frequencies – processed by a pulse transformer.

The problem our customer brought to us was that the waveform was not the correct shape, and the electrical efficiency of the system was poor.  Since this is a product that is in the market, the customer’s requirement included a limitation on the physical size.  It could not increase because the customer committed to keeping the same outer dimensions.

The restrictions on physical size, tight waveform criteria, and the wide range of operating frequencies made us rethink the entire design.  The first step was to change to a high efficiency ferrite core.  This did two things; it allowed the system to operate at a high level of electrical efficiency and the physical size was actually reduced.  Yes, sometimes the best things come in small packages.  The windings were carefully calculated to ensure that the core did not saturate.  Core saturation would cause poor efficiencies and also variable and unpredictable performance.

The end result was a pulse transformer with a ferrite core that does a superb job in a smaller package.  This made the customer, and lots of plants, very happy.

Written by
Denny Wist
President of ButlerWinding

Friday, July 20, 2012

Ideal Transformer – Fact or Dream?

We all have dreams.  You may dream of the perfect vacation, winning the lottery or just having a wonderful day in the sunshine.  My dream is a lot more farfetched.  I dream of the ideal power transformer.

Like all dreams, my dream is about as likely to occur as finding the “ideal” man or woman.  A lovely concept but never actually experienced by anyone.

The ideal transformer is a physics concept that refers to perfect magnetic coupling where there is 100% efficiency in the transfer of power between the primary and the secondary windings.   The physics equation for an ideal transformer is;

The ratio voltage of the primary (Vp) to the voltage of the secondary (Vs) is the same ratio as the number of turns on the primary (Np) to the number of turns on the secondary (Ns).

A transformer that takes the primary voltage and increases it is called a “step up” transformer because the secondary voltage is higher than the primary because it has more turns on the secondary.  Likewise, a transformer that has fewer turns on the secondary (compared to the primary) is called a “step down” transformer because the voltage is reduced.

So if you want the voltage to step down by exactly 50% then you need 50% of the windings on the secondary compared to that of the primary.  If only it were this simple.  Apparently God has a keen sense of humor and decided that if things were this easy then all of us would pass physics and that would mean that there would be rampant unemployment in the ranks of teachers and teaching assistants!

There are two factors that interrupt the perfection of an ideal transformer; imperfect coupling and core or power losses.  In order to achieve perfect coupling, all of the magnetic flux produced by the primary must be transferred and captured by the transformer core.  One problem that quickly arises is simply a matter of geography.  The shape of the windings and the core would have to be infinitely close in order to have all of the lines of flux completely cut through the core.  While the transformer and core manufacturers struggle mightily to come up with shapes that maximize the coupling, it is impossible to get all of the flux transferred into the core.  Stray flux lines radiate in all directions thus reducing the efficiency of energy transfer from the primary to the core.

Even if we could get perfect flux transfer from the primary to the core, the core has issues itself.  The purpose of the core is for it to be a conduit of flux to the secondary.  The core absorbs flux form the primary and then by producing a magnetic field for the secondary, creates a voltage in the secondary.  Over the years better and better core materials have been developed to reduce power losses but we still are far from perfection.  Even tiny imperfections know as grain boundaries in the crystals that make up the core slow down the flux and resist its movement.  This resistance becomes heat.  If you want a real life experience of this, Just touch any transformer under power and you will see that they range from warm to very hot (depending on the efficiencies).

I will continue to dream of the ideal transformer while the rest of the world dreams of more achievable goals like world peace or the perfect enchilada.

Written by Denny Wist
President of Butler Winding

Tuesday, May 15, 2012

The Power Transformer’s Ugly Stepsister - the Pulse Transformer

By definition, transformers convert AC power from one voltage to another.  For example, a typical step down transformer will take 480 volts to 220 or 110 volts for home use.  The laws of physics dictate that a transformer will not work with DC power.  If you ask a physicist why DC power will not work in a transformer, they will mumble something about quantum mechanics, electron spins and electron coupling and then quickly look down at their shoes until you walk away.

So if you accept that a transformer converts AC power then the wave form is a typical sinusoidal alternating current shape as shown below.   The frequency determines the number of peaks per second, and typical US AC power is 60 cycles per second.

Sometimes this smooth oscillating wave will not do what you want therefore you may need a square wave form.

Why would anyone prefer the odd and ugly boxy square wave form over the smooth eye-appealing sinusoidal shape?  The answer is usually that a nice square wave, that is a pulse, can be used in different ways than the “s” shaped curve.  A pulse is of great use in applications such as data communications or signaling a power semiconductor to turn on or off.    One of the practical applications of a pulse transformer is to produce high power pulses that feed into radar to produce the sound waves that prove I don’t understand that a 55 mile per hour speed limit does not mean 55 miles per hour give or take 20.

A pulse transformer thus has a different design than the typical power transformer.  (See pulse transformer operating principles)  The pulse transformer designer needs to be concerned about such issues as low coupling capacitance and excellent insulation properties to prevent high breakdown voltages.  Pulse transformer design can give a range of “square” shapes that can be actually rectangular with varying degrees of length and amplitude.   These variations allow for giving stronger or weaker pulses and vary the length of time of the pulse.

If you need a circuit to produce a signal that is not the typical analog signal then a pulse transformer may fit your needs quite nicely.  At Butler Winding, we manufacture custom pulse transformers just like the one mentioned above.  View additional Butler Winding pulse transformer projects.

Written by Denny Wist
President of Butler Winding

Tuesday, April 17, 2012

When Exciting Current Gets Exciting – Redesign the Transformer

A transformer is basically a voltage changing device that takes an incoming voltage (primary) and raises or lowers it (secondary).   The process used is taking advantage of the unique electromagnetic property of electric current creating magnetic fields and magnetic fields creating electric currents. 

The process of raising or lowering incoming voltage is very efficient, that is, the power that’s fed into the primary side is nearly equal to the power on the secondary side.  But there are power losses, which cause small inefficiencies.  One of those power losses comes from what is called “exciting current”.  Exciting current is the amount of current required on the primary side to create and hold the magnetic field even though there is no secondary current flowing.  That may sound confusing, but think of it as the amount of gasoline your car uses to idle when not moving.

One of our customers supplies railroad equipment.  The railroad routinely monitors the railroad traffic signal lights, which tell the engineer on the train which track(s) is available and if there is oncoming traffic.  The railroad traffic management system monitors the electric current consumed on the primary side to determine if their signaling lights are operable or burned out.  If a current is flowing then obviously the light is working.  No current would indicate that the light was burnt out, and the management system would send out the repair crew to replace the bulbs in the signal light.

Our customer discovered that the transformer used in the signal light had such a high exciting current that the primary side was showing it was consuming power all of the time.  This prevented them from being able to tell if the signaling light was working or not.  As you can imagine, signaling lights malfunctioning can be devastating by causing trains to take the wrong tack and lead to disasters such as derailing or collisions.

At Butler Winding, we design and manufacture transformers.  In this case, the solution was a conventional E I laminated power transformer redesign that focused on lowering the exciting current to a minimum.  This was done by making the core larger by about 30%.  The larger core meant that the magnetic field could energize the core more easily, thus reducing the exciting currents by over 50%.  The exciting currents were then small enough that they were easy to distinguish and the railroad could be absolutely sure if the signal light was on or off.

A special thanks to Tom Clapper at Butler Winding who put in extra effort to complete this project, and thank you to everyone for solving this problem.  View examples of typical power transformer designs Butler Winding has completed.

Written by Denny Wist
President of Butler Winding