Alternating currents ....

I set the clouds in motion
Turn up light and sound
Activate the window
And watch the world go 'round --

Anything can happen

("Prime Mover," by Rush, 1987)

 

PRIME MOVER/DRIVER DEFINED ONCE MORE

The time has come for the PTOA Focus Study on Prime Movers/Drivers.

Prime Movers/Drivers are the machines that convert some kind of energy into motive power ... power that rotates around and around or pulsates back and forth.

 

The Prime Movers/Drivers that are used in the Processing Industries fall into two historical categories, Motors and Engines.

 

The rotational power that is generated by a Prime Mover/Driver is thence transferred from the Driver's Shaft to the Shaft of the Rotating Equipment that it is coupled to.

 

The Rotating Equipment's Shaft spins the Load that performs the work that needs to be done.

Although many knowledgeable people use the terms Motors and Engines interchangeably ...

A Motor is a device that converts electric (or hydraulic) energy into motion (aka motive power).

An Engine is a device that uses combustion or heat to produce motion (aka motive power).

The definition of Motor is likely to broaden throughout the lifetimes of PTOA Readers and Students as alternative forms of energy are used to convert energy into motion.

Another general difference between Motors and Engines is the identifying hardware that will be found on each type of device:

Motors have Rotors and Stators.

Engines have Pistons and Cylinders and are connected to Crankshafts.

Now that PTOA Readers are aware of the fundamental differences between Motors and Engines ...

This PTOA Segment #187 begins the focus on Motors.

 

 

 

Both Induction Motors (aka AC Motors) and DC Motors are Prime Movers/Drivers which:

• Must have electrical utility power supplied to them and
• Create rotary motion with Rotors and Stators.

 

Understanding the difference between Induction Motors (aka AC Motors) and DC Motors begins with understanding the difference between Alternating Current (AC) and Direct Current (DC).

Yep!

All this preparation is needed before PTOA Readers and Students can be expected to understand how Induction Motors (aka AC Motors) and DC Motors can generate the Torque that turns the Shaft of a Prime Mover/Driver ...

which will be explained in the next PTOA Segment!

 

DIRECT CURRENT VS ALTERNATING CURRENT 

Direct Current (DC)

The most successful PTOA Readers and Students are reading the PTOA Segments in the intended sequential order ... ergo ...

These PTOA Readers and Students previously learned  in PTOA Segment #106  how Ohm's Law governs the electricity that flows in one direction around a basic DC circuit.

In  PTOA Segment #113 PTOA Readers and Students learned some of the common electronic hardware found in DC circuits, and probably noticed that all the circuits shown had Batteries.

For example, there is a Battery (labelled Cell in the nearby graphic) that features a DC circuit for a rheostat.

PTOA Segment #115 continued to explain to "Instrument Techie" PTOA Readers and Students that a Battery supplies the Direct Current to the Wheatstone Bridge circuitry that is incorporated into automatic instrumentation that measures both the PV Pressure and the PV Temperature.

So "Instrument Techie" PTOA Readers and Students will recognize the Battery in the lower left corner of the nearby schematic of a Wheatstone Bridge.

The Battery is drawn as 4 vertical hash marks with positive pole on the left (+) and negative pole on the right (-).

Of course all brilliant PTOA Readers and Students who read the PTOA Segments in the intended sequential order already know that the Battery is essential to the circuit because no current can flow without a Voltage Differential.

The Battery's job is to provide the highest Voltage in the circuit so that the current can keep flowing and energizing whatever needs to be energized!

In summary ...

Batteries are required in every DC circuit because they continuously provide a voltage which makes it possible for current to flow through the circuit in one direction.  The voltage provided by the Battery will decrease as the current flows through the circuit.  

 

And how does the Battery generate the DC?

A chemical reaction is ongoing within the Battery and once the reactants for the reaction are spent, the battery can no longer generate DC.

The other ways to generate DC involve using "commutators and brushes" or the more modern "rectifier"  which convert an originating AC power source to DC power.

Wow!

That must mean that when there is an industrial need for continuous DC to power Rotating Equipment there must first be a dependable source of AC.

Righteeo!

So how is Alternating Current created?

 

 

Alternating Current (AC)

Unlike DC, Alternating Current is not created by chemical reaction but rather by the interaction of fluctuating magnetic fields near metal conductors.

Unlike DC that flows in one direction, Alternating Current moves back and forth with rapid frequency ... hence the name ...

Alternating Current!

The human who gained everlasting fame for being the first to document his observations that a moving magnetic field can create a voltage that thence creates a current in a closed circuit ...

even though the circuit did not have a battery ...

was named Faraday.

And Faraday decided to label his observation of a Human-Made current as an "induced current."

That phrase has stuck around ... and admittedly flows off the tongue easier than "human created current."

Then Faraday decided to label the process of creating a current with a moving magnetic field with a real high fallutin' name...

Electromagnetic Induction ... which is explained "more better" below.

By the way, the human who figured out the commercial value of using Faraday's findings to induce currents in an Induction Motor  was a brilliant madman named Tesla ...

And the human who is currently famous for using Nikola Tesla's Induction Motor design in electric cars and giving away the patent for free is a dude named Elon Musk of Tesla Motors!

 

UNDERSTANDING  ELECTROMAGNETIC INDUCTION AND AC

Electromagnetic Induction creates Alternating Current

So .. the very first step toward understanding how rotational movement is created in an Induction Motor begins with understanding this phenomenon Faraday called "Electromagnetic Induction."

Many Thanks to eLearnIn's You Tube For Explaining  Electromagnetic Induction!

The You Tube produced by eLearnIn that simply explains Electromagnetic Induction can be accessed in a link that is provided after reading a few more preparatory paragraphs.

Please be certain to  "Thumbs Up" the video while you are there to show appreciation for the creator's work.

Here's a preview of what to look for in the eLearnin You Tube:

The eLearnIn You Tube illustrates Electromagnetic Induction with a coil of copper wire which has its ends attached to a galvanometer.

Instrument Techie PTOA Readers and Students learned way back in PTOA Segment #115 that a galvanometer measures very small currents.

Note that there is not a battery or anything else to make current flow ... the circuit is just made of a coil of copper wire and that galvanometer.

A simple bar magnet is moved toward and then away from the coil of copper wire, first with its north pole leading and next with the south pole leading.

While watching the eLearnIn You Tube be certain to observe:

• A needle on the galvanometer indicates that a current is created ... or "induced" by the movement of the magnet toward and away from the coil.
• The direction of the current changes depending upon if the magnet is moved toward or away from the coil.
• The direction of the current changes when the north or the south pole of the magnet approaches and then is pulled away from the coil.

 

The movement of the galvanometer explains why Faraday described his observation of an "induced Alternating Current."

Next, the bar magnet is held in place while a hand appears and pushes the copper coil toward and then away from the magnet.

Shazam!

Moving the coil toward the stationary magnet also creates ... aka "induces" ... an Alternating Current that can be detected by the galvanometer!

Okay! It's time to "You Tube and Chill!"

Access the eLearnIn You Tube about Electromagnetic Inductions using the below link or direct access to the You Tube:

Electromagnetic Induction

https://www.youtube.com/watch?v=tC6E9J925pY

The "take-home message" from watching the eLearnIn You Tube is ...

"The relative movement of a magnet and a coil of copper wire creates a current."

Which means the wire can be moved toward and then away from the magnet ...

Or the magnet can be moved toward and then away from the wire ...

Either way an Alternating Current can be created ... aka "induced."

 

AC Frequency

So it is the fluctuating magnetic field that creates Alternating Currents (AC) that change direction in regular, periodic intervals.

The Frequency of the changing current direction is measured in Hertz (Hz). The nearby graphic shows the AC Frequency in a USA wall socket is 60 Hz. The AC Frequency in Europe is 50 Hz.

Aha! PTOA Readers and Students now fundamentally understand:

• How Alternating Current is induced.
• How AC differs from DC.

 

 

DC MOTORS AND INDUCTION MOTORS (AKA AC MOTORS)

Now that all PTOA Readers and Students understand the difference between Direct Current (DC) and Alternating Current (AC), who is surprised to learn that there are two types of electrically powered Motors ...

• Direct Current Motor (DC Motor).
• Induction Motors (aka AC Motor ... but no one calls them that).

 

Induction Motors (aka AC Motors) and DC Motors will be separately featured in the next few PTOA Segments.

As was stated above and thus is already known ...

Both DC Motors and Induction Motors (aka AC Motors) have hardware called Stators and Rotors.

However, DC Motors need the additional hardware that was also mentioned above ...

 

Brushes and Commutators (or the more modern Rectifier) to convert AC to DC.

These hardware components require maintenance, reduce the speed of rotation achievable, and generally reduce the life expectancy of the Motor.

 

Since Induction Motors (aka AC Motors) require less maintenance and last a long time ...

Which PTOA Reader or Student would be surprised to learn that Induction Motors are most popular in the Process Industries?

 

One other big difference between AC Motors and DC Motors is how they can be modified into Variable Speed Drivers.

Don't stress over it ... because on a daily basis no Process Operator needs to be expertly aware of how controlling the rotational speed of a DC Motor differs from controlling the speed of an Induction Motor (aka AC Motor).

But if anyone ever asks about it, you heard it here first:

The speed of a DC Motor can be controlled by varying what is called the armature winding's current.

However, the speed of an Induction Motor (aka AC Motor) is controlled by varying the frequency of the current with an adjustable frequency drive control.

Either adaption converts a Prime Mover/Driver into a Variable Speed Driver.

Brilliant PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already predicted the impacts of Centrifugal Pump Performance as the spinning of an Impeller was increased and decreased.

Predicting changes in Centrifugal Pump Performance was featured back in  PTOA Segment #170.

Time to connect some dots!

The changes in the TDH, BHP, and Efficiency of a Centrifugal Pump as the Impeller spins faster or slower are all made possible by the varying the  Frequency of the current supplied to an Induction Motor (aka AC Motor).

Wow! PTOA Readers and Students are much smarter than the average Process Operator!

 

TAKE HOME MESSAGES: The two broad categories of Prime Movers/Drivers are Motors and Engines.

Engines convert the combustion of fuel into motive power and have Pistons and Cylinders and are connected to Crankshafts.

Motors convert electrical energy in the form of DC or AC into motive power. So Motors must have electrical utility supplied to them. Motors also have Stators and Rotors.

DC moves in one direction only. DC can only be created by batteries or by converting AC to DC with Brushes and Commutators or with a Rectifier.

AC alternates direction ... meaning the current moves back and forth at a rate that is described as Frequency in Hertz (Hz).  The Frequency of AC generated power is not uniform throughout the world but may vary from country to country.

AC is an "induced" current that is created by the relative movement of magnets and metals ... like copper coils ... that are good electricity conductors.

A dude named Faraday was the first to write his observations that the either the magnet can be moved toward the copper coil or the copper coil can be moved toward the magnet to create ... or "induce" ... a current that can change direction. He's the one who started calling the process "Electromagnetic Induction."

Thanks to Elearnin for the You Tube that simply explains Electromagnetic Induction.

The two main categories of Motors are DC Motors and Induction Motors (aka AC Motors ... but no one calls them that).

AC Motors require less maintenance because they do not need Commutators or Brushes to convert AC to DC as the DC Motor does.

AC and DC Motors also vary in how they can be modified into Variable Speed Drivers.

©2018 PTOA Segment 0187

PTOA Process Variable Pressure Focus Study Area
PTOA PV Pressure Rotating Equipment Focus Study
PTOA PV Pressure Prime Mover/Driver of Rotating Equipment Extension Study

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