Electric Power, Alternating Current, Transformers, Magnetic Flux, Turns Ratio, Electric Panel and Circuit Breakers

Posted by PITHOCRATES - February 6th, 2013

Technology 101

AC Power is Superior to DC Power because it can Travel Farther and it Works with Transformers

Thanks to Nikola Tesla and his alternating current electric power we live in the world we have today.  The first electric power was direct current.  The stuff that Thomas Edison gave us.  But it had some serious drawbacks.  You needed a generator for each voltage you used.  The low-voltage of telephone systems would need a generator.  The voltage we used in our homes would need another generator.  And the higher voltages we used in our factories and businesses would need another generator.  Requiring a lot of power cables to hang from power poles along our streets.  Almost enough to block out the sun.

Another drawback is that direct currents travel a long way.  And spend a lot of time moving through wires.  Generating heat.  And dropping some power along the way due to the resistance in the wires.  Greatly minimizing the area a power plant can provide power to.  Requiring many power plants in our cities and suburbs.  Just imagine having three coal-fired power plants around your neighborhood.  The logistics and costs were just prohibitive for a modern electric world.  Which is why Thomas Edison lost the War of Currents to Nikola Tesla.

So why is alternating current (AC) superior to direct current (DC) for electric power?  AC is more like a reciprocating motion in an internal combustion engine or a steam locomotive.  Where short up & down and back & forth motion is converted into rotation motion.  Alternating current travels short distances back and forth in the power cables.  Because they travel shorter distances in the wires they lose less power in power transmission.  In fact, AC power lines can travel great distances.  Allowing power plants tucked away in the middle of nowhere power large geographic areas.  But there is another thing that makes AC power superior to DC power.  Transformers.

The Voltage induced onto the Secondary Windings is the Primary Voltage multiplied by the Turns Ratio

When an alternating current flows through a coiled wire it produces an alternating magnetic flux.  Magnetic flux is a measure of the strength and concentration of the magnetic field created by that current.  When this flux passes through another coiled wire it induces a voltage on that coil.  This is a transformer.  A primary and secondary winding where an alternating current applied on the primary winding induces a voltage on the secondary winding.  Allowing you to step up or step down a voltage.  Allowing one generator to produce one voltage.  While transformers throughout the power distribution network can produce the many voltages needed for doorbells, electrical outlets in our homes and the equipment in our factories and businesses.  And any other voltage for any other need.

We accomplish this remarkable feat by varying the number of turns in the windings.  If the number of turns is equal in the primary and the secondary windings then so is the voltage.  If the number of turns in the primary windings is greater than the number of turns in the secondary windings the transformer steps down the voltage.  If the number of turns in the secondary windings is greater than the number of turns in the primary windings the transformer steps up the voltage.  To determine the voltage induced onto the secondary windings we divide the secondary turns by the primary turns.  Giving us the turns ratio.  Multiplying the turns ratio by the voltage applied to the primary windings gives us the voltage on the secondary windings.  (Approximately.  There are some losses.  But for the sake of discussion assume ideal conditions.)

If the turns ratio is 20:1 it means the number of turns on the primary windings is twenty times the turns on the secondary windings.  Which means the voltage on the primary windings will be twenty times the voltage on the secondary windings.  Making this a step-down transformer.  So if you connected 4800 volts to the primary windings the voltage across the secondary windings will be 240 volts (4800/20).  If you attached a wire to the center of the secondary coil you can get both a 20:1 turns ratio and a 40:1 turns ratio.  If you measure a voltage across the entire secondary windings you will get 240 volts.  If you measure from the center of the secondary and either end of the secondary windings you will get 120 volts.

The Power Lines running to your House are Two Insulated Phase Conductors and a Bare Neutral Conductor

This is a common transformer you’ll see atop a pole in your backyard.  Where it is common to have 4800-volt power lines running at the top of poles running between houses.  On some of these poles you will see a transformer mounted below these 4800-volt lines.  The primary windings of these transformers connect to the 4800-volt lines.  And three wires from the secondary windings connect to wires running across these poles below the transformers.  Two of these wires (phase conductors) connect to either end of the secondary windings.  Providing 240 volts.  The third wire attaches to the center of the secondary windings (the neutral conductor).  We get 120 volts between a phase conductor and the neutral conductor.

The power lines running to your house are three conductors twisted together in a triplex cable.  Two insulated phase conductors.  And a bare neutral conductor.  These enter your house and terminate in an electric panel.  The two phase conductors connect to two bus bars inside the panel.  The neutral conductor connects to a neutral bus inside the panel.  Each bus feeds circuit breaker positions on both sides of the panel.  The circuit breaker positions going down the left side of the panel alternate between the two buss bars.  Ditto for the circuit breaker positions on the right side.

A single-pole circuit breaker attaches to one of the bus bars.  Then a wire from the circuit breaker and a wire from the neutral bus leave the panel and terminate at an electrical load.  Providing 120 volts to things like wall receptacles where you plug things into.  And your lighting.  A 2-pole circuit breaker attaches to both bus bars.  Then two wires from the circuit breaker leave the panel and attach to an electrical load.  Providing 240 volts to things like an electric stove or an air conditioner.  Then a reciprocating (push-pull) alternating current runs through these electric loads.  Driven by the push-pull between the two bus bars.  And between a bus bar and the neutral bus.  Which is driven by the push-pull between the conductors of the triplex cable.  Driven by the push pull of secondary windings in the transformer.  Driven by the push-pull of the primary windings.  Driven by the push-pull in the primary cables connected to the primary windings.  And all the way back to the push-pull of the electric generator.  All made possible thanks to Nikola Tesla.  And his alternating current electric power.

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