Get your motor runnin'
Head out on the highway
Lookin' for adventure
And whatever comes our way

("Born To Be Wild," by Steppenwolf, 1968)

 

SIMILARITIES AND DIFFERENCES BETWEEN AC AND DC MOTORS

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already learned the following in PTOA Segment #187:

• Unlike Alternating Current (AC), Direct Current (DC) "flows" in one direction.
• Unlike AC circuits, DC circuits must have a battery.
• Just like Induction Motors, DC Motors must have electrical utility supplied to them.
• Just like Induction Motors, DC Motors have Rotors and Stators.
• Unlike Induction Motors, DC Motors have additional hardware called  Brushes and Commutators (and more modern DC Motors have Rectifiers instead of Brushes).

 

QUICKIE REVIEW OF AC MOTORS

In PTOA Segment #187 PTOA Readers and Students learned how a current is created … aka "induced"... whenever a magnet is moved within a coil of copper wire … or a coil of copper wire is moved about a stationary magnet.

Otherwise stated, PTOA Readers and Students learned what the phrase "Electromagnetic Induction of a current" means.

Next ... in PTOA Segment #188 ... PTOA Readers and Students learned how the theory of Electromagnetic Induction has been commercially applied in the "real world" to create Torque, the rotating movement that turns the Shaft of a Primary Mover/Driver.

Ergo ...

All brilliant PTOA Readers and Students already know the steps that make it possible for an Induction Motor to create Torque:

A source of AC power is applied to the Stator's coils which makes it possible for the coils to generate a Rotating Magnetic Field (RMF).

The RMF induces a current within the Rotor.

The Rotor's current generates a different, temporary magnetic field around the Rotor which …

becomes interlocked with the Stator's RMF …

hence causing the Rotor and its Shaft to spin … thus creating Torque!

Now that all of the above is understood about Induction Motors …

 

WHAT MAKES THE SHAFT OF A DC MOTOR SPIN AND CREATE TORQUE?

Guess what?

That same dude named Faraday who had observed and described Electromagnetic Induction also figured out how a DC Motor would work!

Since the term "DC" is in there, PTOA Readers and Students should automatically presume:

 

• A DC Motor must have a DC circuit... a current that flows in one direction.
• There must be a Battery in the electrical circuitry of a DC Motor.

 

Correctamundo on both assumptions!

And in the case of small household DC Motors, the magnet that is used to induce current via Electromagnetic Induction is a true "permanent magnet" … just like the "Stator Magnets" labelled in the nearby graphic of a "Brushed DC Motor."

To recap...  the important parts of a DC Motor are:

• A true "permanent magnet" (like a bar magnet with a North and South Pole) … which is obviously surrounded by its own magnetic field.
• A "coil of copper wire with at least 3 windings" situated near enough to the magnet to be impacted by its magnetic field.
• A battery that has its positive and negative terminals connected to the ends of the coiled copper wire … thus making a circuit through which DC flows.

 

Time to "You Tube and Chill"
with Vithalreddy Reddy's"Simple DC Motor You Tube"

Thank you to Vithalreddy Reddy for creating the You Tube below which simply explains how to make a very Simple DC Motor.

Here's a preview of what to look for:

1. A DC current flows from a battery through the copper wire and copper wire coil and this flow of electrons  creates what Your Mentor calls a "Temporary Magnetic Field" (TMF) around the coil. The invisible TMF is "temporary" because if the DC power provided by the battery were removed, the coil's TMF would cease to exist.
2. The TMF repels the permanent magnet's magnetic field … and Voila! …
3. The invisible repelling action between the permanent magnet and the energized coil's TMF is what causes the coil of copper wire with 3 windings to spin … thus creating Torque!

 

Okay! Be sure to Thumb's Up to Vithalreddy Reddy's You Tube to show gratitude for the great visual aid!

Here's the link to Vithalreddy Reddy's Simple DC Motor.

Or directly access the You Tube below:

Second verse … same as the first!

PTOA Readers and Students should take a second look at Vithalreddy Reddy's "Simple DC Motor" You Tube and this time be alert to the following facts:

1. The ends of the coiled copper wire with 3 windings must be directly opposite from each other … aka 180 degrees apart.
2. One side of the ends of the copper wire is shaved.  If that step had been missed, the coiled copper wire with 3 windings would not keep rotating but rather spin half a rotation and then rotate back the other way because of the invisible attraction/repulsion between the North and South poles of the permanent magnetic field and the energized coil's TMF.
3. Notice that starting the Simple DC Motor takes an initial "push." You know what that means? ...DC Motors are not self-starting! Yep, that's right!

 

DC Motors ARE NOT self-starting!

 

Once again … access Vithalreddy Reddy's "Simple DC Motor" You Tube HERE.

 

 

INDUSTRIAL-SIZED DC MOTORS

Industrial-Sized DC Motors Have Electromagnets 

Big, industrial-sized DC Motors do not have permanent magnets.

Industrial-sized DC Motors substitute coils of copper wire in the Stator …

which every PTOA Reader and Student already knows will become an Electromagnet that generates a magnetic field once the coil is energized.

 

 

The Rotor of the Industrial-Sized DC Motor

In an industrial-sized DC Motor, the components of the Rotor are

• The Armature.
• Commutator Rings and Brushes.
• A Shaft onto which to transfer the created Torque.

 

So what exactly is an "Armature?"

The coils of copper wire in the Rotor are the Armature.

For example, the single coil of copper wire that appears as the Armature in the below Simple DC Motor graphic is gold.

 

PTOA Readers and Students will soon understand why more than one coil is needed in the Armature to make sure that it rotates smoothly and continuously through 360 degrees.

A see-through schematic of a DC Motor is shown in the nearby graphic. The Armature shown has 16 coils.

The Armature is the part of a DC Motor which changes the electrical power supplied to the Motor into the mechanical power.

This mechanical power generated by the Armature creates the spinning motion known as Torque … which is subsequently transferred to and spins the Shaft of the DC Motor.

Got it, Fred?

The Armature of a DC Motor creates the Torque that spins the DC Motor's Shaft!

Furthermore …

Just exactly like what would happen in a "Simple DC Motor" …

The positional changes of the North and South poles of the Stator's Electromagnetic Field and the North and South Poles of the Rotor's TMF would cause the Armature to rotate one way and then back the other way!

Heck, that action would not be very helpful with respect to spinning a Shaft!

 

Hardware components called Commutator Rings and Brushes keep the Armature rotating smoothly and continuously in the same direction.

The coil ends of the Armature are connected to their separate Commutator Rings.

Brushes provide the interface between the Commutator Rings and the wire leads that connect to the Battery terminals.

Ergo …

Commutator Ring(s) and Brushes are part of the DC circuit that energizes the coils in the Armature.

 

 

The interaction between the Brushes and Commutator Rings successively aligns … and then intentionally misaligns … the Temporary Magnetic Field (TMF) that is created in the Rotor's Armature with the Stator's Electromagnetic Field.

The continuously controlled invisible action between the two magnetic fields keeps the Rotor spinning smoothly in one direction. 

 

Okay! Now that PTOA Readers and Students have learned all about the form and important function of Commutator  Rings and Brushes ...

be aware that the modern DC Motors are "brushless" meaning they use modern technology to make certain that the Armature spins continuously and smoothly in one direction.

Hey! There have just been too many words describing how Torque is created between the Rotor's Armature and the Stator of a DC Motor!

Time to "You Tube and Chill" again!

 

LEARN ENGINEERING YOU TUBE EXPLAINS  "HOW A DC MOTOR WORKS" 

Thanks again to LearnEngineering for explaining what the industrial-sized  "real world" DC Motor would look like!

The link to the LearnEngineering You Tube is right here:

DC Motor You Tube.

 

Otherwise …

PTOA Readers and Students can directly access the You Tube below.

As usual, be sure to Thumbs Up the LearnEngineering "How a DC Motor Works" You Tube to show appreciation and gratitude for the creator's work!

 

PTOA Readers and Students should make certain they understood these finer points which were featured in the LearnEngineering  "How a DC Motor Works" You Tube:

• The loops of copper wire in the stationary Stator are called "Field Windings." Upon being energized, the Field Windings become an Electromagnet.
• The coils in the Rotor's Armature are called   "Rotor Windings." When energized and rotating, the Rotor Windings create the TMF.

 

The voice in the LearnEngineering You Tube  also explained that:

• The Rotor Windings are connected to the Commutator Rings.
• Both the Stator's Field Windings and the Rotor's Rotor Windings are energized by the same DC circuit that connects to a Battery.

 

The design of the DC electrical circuit between the Stator's Field Windings and the Rotor's  Rotor Windings makes all the difference in the world!

 

The design of the electrical circuit between the Battery, Field Windings, and Rotor Windings determines the Torque and rotational speed characteristics of the DC Motor.

 

SHUNT DC MOTORS AND SERIES DC MOTORS 

The Learn Engineering "How a DC Motor Works" You Tube describes how two different types of industrial-sized DC Motors are defined by the structure of the electrical circuit that energizes the Rotor's Rotor Windings and the Stator's Field Windings.

In a Shunt DC Motor …

The Rotor's Rotor Windings are in a "parallel electrical circuit" with the Stator's Field Windings.

Fred! Don't stress!

Process Operators are not responsible for electrical circuit engineering!

Knowledge is a different kind of power, though … so read on!

The middle circuit in the nearby electrical circuit graphic illustrates a Shunt Circuit which would be used for a Shunt DC Motor. This electrical one-line graphic illustrates how the copper wires in the DC Motor would be wound with respect to the DC current flowing between the Rotor Windings and the Field Windings.

The letter "A" is labelling the Armature. The letter "F" is labelling for the Shunt Circuit Field Windings.

The positive and negative leads to and from the Battery are at the bottom of the graphic.

 

The Shunt Circuit design results in a low DC Motor starting Torque but relatively consistent rotating speed while transitioning from No Load status to Full Load status.

The above statement is illustrated in the nearby graphs which show the "Torque versus Current" and "Speed versus Torque" characteristics of a Shunt Motor.

 

In a Series DC Motor …

The Rotor's Rotor Windings are in a "series electrical circuit" with the Stator's "Field Windings."

The left hand side circuit shown in the nearby graphic represents the circuitry for a Series DC Motor. Once again the "A" represents the Armature. And in this graphic the "S" represents Series Circuit Field Windings.

 

The Series DC Motor has high starting Torque but a speed of rotation that rapidly decreases as the Load is increased.

The below graph illustrates the "Torque versus Speed" relationship of a Series Wound DC Motor with constant voltage.

PTOA Readers and Students probably stressed over the detail description of the "Back EMF" in the Learn Engineering You Tube. Albeit a unique and worrisome phenomenon of DC Motors, just file the following "Take Home Warning" about "Back EMF:"

There must be some means to regulate voltage while an industrial DC Motor is started up.

Otherwise the Rotor Windings can be burned up with too high of a current flowing through them.

Wow! This has been too many words!

Just access the LearnEngineering DC Motor You Tube again!

 

 

DC MOTOR SERVICE  IN PROCESS INDUSTRY

The Power Plant at any industrial facility is going to be generating AC power and/or the purchased electrical power from the local utility will also be AC.

So why would a DC Motor ever be used instead of an Induction Motor?

Most definitely when a Prime Mover/Driver is needed for constantly spinning its Load, the go-to Motor will be the Induction Motor.

The niche of the DC Motor is a process service that requires a wide range of speed control and starting Torque … in that situation the DC Motor will be more economical than an Induction Motor. 

 

And by far the most popular use of a DC Motor is the DC Generator.

 

 

DC GENERATORS (aka Compound Motors)

With just some tweaking of the circuit design between the Stator's Field Winding and the Rotor's Rotor Winding, a hybrid known as a Compound Motor  (aka DC Generator) is created.

A Compound Motor-DC Generator Circuit combines the architecture of the Shunt parallel Circuit and the Series Circuit design with respect to connecting the Stator's Field Windings and the Rotor's Rotor Windings.

The resulting Compound Circuit  includes both:

• a Field Winding in series with the Armature.
• a Shunt Field Winding that is in parallel with the Armature.

 

An electrical schematic of the Compound Circuit that is incorporated into a DC Generator-Compound Motor is shown on the right hand side of the nearby graphic.

The benefits of the DC Generator-DC Compound Motor Circuit are:

• The Torque characteristics of the Series DC Motor with …
• The ability to regulate the speed characteristic of a Shunt DC Motor.

 

Otherwise stated,

DC Compound Motors have high starting Torque which can be maintained over a variation of rotating speeds.

In summary …

The DC circuit design through the Field Windings and Rotor Windings is what determines the service of a DC Motor and what the DC Motor will be called.

As the below graphic shows, a Shunt Motor and Shunt Generator can appear to be physically identical yet they have totally different purposes!

 

Instead of converting electrical energy into mechanical energy as the Shunt and Series DC Motors do ...

The DC Generator converts mechanical energy into electrical energy!

 

TAKE HOME MESSAGES:  Both Induction Motors and DC Motors require electrical utility  to be energized and have Rotors and Stators.

DC Motors are energized by a DC circuit in which electrons flow in one direction from a Battery → through a Brush & Commutator Ring to the Rotor Windings in the Armature and also the Field Windings in the Stator → through an exit Commutator Ring and Brush → back to the Battery.

The Torque of a DC Motor is created when an energized coil of copper wire … sometimes called an "Armature" and sometimes called "Field Windings"... generates a Temporary Magnetic Field (TMF) which is then repelled by the magnetic field of a permanent magnet.

Small household DC Motors have a permanent magnet but large industrial-sized DC Motors have an Electromagnet.

The coils of copper wire that make up the Field Windings are collectively called the Armature of the Rotor. The Armature is THE part of the DC Motor which converts the electrical DC energy into mechanical energy which causes rotating motion … and Torque.

Unlike Induction Motors, DC Motors:

• Are not self-starting.
• Require Commutators and Brushes or some other type of technology to keep continuously and smoothly rotating 360 degrees in the same direction.
• Require some means to regulate voltage during startup otherwise the Rotor Windings can be burned up.

 

The circuit arrangement between the Field Windings and Rotor Windings of a DC Motor determine what type of DC Motor it is and what its service will be.

Shunt DC Motors have a parallel circuit between the Field Windings and Rotor Windings. Shunt DC Motors have a low starting Torque but relatively consistent rotating speed while transitioning from No Load status to Full Load status.

Series DC Motors have a series circuit between the Field Windings and Rotor Windings. The Series DC Motor has high starting Torque but a speed of rotation that rapidly decreases as the Load is increased.

Compound DC Motors are called DC Generators. DC Generators convert mechanical energy into electrical energy.

The Compound Circuit which connects the Field Windings and Rotor Windings of a DC Generator/Compound Motor incorporates both a parallel Shunt Circuit and a Series Circuit. The Compound Circuit results in a high starting Torque which can be maintained over a variation of rotating speeds.

Vithalreddy Reddy rocks and helped PTOA Readers and Students understand the components of a Simple DC Motor.

Learn Engineering rocks and helped PTOA Readers and Students understand how an industrial-sized DC Motor works.

 

©2018 PTOA Segment 0189

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