I'm an AC/DC man
You can read my circuit diagram
I feed on electric jolts
I need fifty-thousand volts

Make a circuit with me

("Make a Circuit with Me," by The Polecats, 1983)

 

FOCUS ON INDUCTION MOTORS

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

• Motors are Prime Movers/Drivers which must have electrical power supplied to them.
• Induction Motors convert that electrical utility power into rotary motion using Rotors and Stators.
• An Alternating Current can be electromagnetically induced when a magnet is moved toward and away from a good conductor (like a copper coil) and vice versa ...
• An Alternating Current can be electromagnetically induced when a good conductor (like a copper coil) is moved toward and away from a magnet.

Heck! All those bullet points sound very important!

Since this PTOA Segment #188 explores the commercial application of Electromagnetic Induction to generate Torque in the Shaft of the Primary Mover/Driver known as an Induction Motor ...

PTOA Readers and Students who are bewildered by the above bullet points and emboldened words have the opportunity to return to PTOA Segment #187 HERE .

Your Mentor will wait.

Okay , now that every PTOA Reader and Student is "refreshed" regarding the differences between DC Motors and AC Motors ... let's get one thing straight:

 

Nobody uses the term "AC Motor" ... including PTOA Readers and Students from this point onward.

The phrase "Induction Motor" spoken by a human being is short hand for a specific model of Motor ...

the Squirrel Cage Induction Motor design ....

even thought the Squirrel Cage Induction Motor design is not the only type of Motor that converts the supplied electrical utility into rotational power via Electromagnetic Induction.

The Squirrel Cage Induction Motor is just so dominantly popular that it has adopted the name of the classification as its own.

Which is kind of like how the descriptor "Kleenex" is commonly used to mean every kind of facial tissue sold within the USA market.

 

TWO INDUCTION MOTOR DESIGNS

The two designs of Induction Motors are:

• The Squirrel Cage Induction Motor
• The Wound Rotor Induction Motor

 

The Wound Rotor Induction Motor is explored briefly at the end of this PTOA Segment.

The majority of this PTOA Segment is dedicated to the much more simple, rugged and thus most popular Squirrel Cage Induction Motor.

The vast majority of Prime Movers/Drivers shown in PTOA visual aids have been Squirrel Cage Induction Motors.

 

TWO TYPES OF SQUIRREL CAGE INDUCTION MOTORS

The two types of Squirrel Cage Induction Motors are:

• The Synchronous Motor.
• The Asynchronous Motor  ... this would be the model that everyone is referring to when they say "Induction Motor."

 

So remember!

The descriptors "Synchronous Induction Motor" and "Asynchronous Induction Motor" only apply to the Squirrel Cage Induction Motor design!

And ...

The phrase "Induction Motor" refers to the Asynchronous Squirrel Cage Induction Motor ...

because that's what most everybody means even though there is a Synchronous Induction Motor design and a Wound Rotor Induction Motor design.

Clear as mud?

Do not stress!

All of the information that has been introduced in the last few paragraphs will be understood "more better" by the time PTOA Readers and Students finish reading this PTOA Segment and watching the upcoming You Tube.

Until then ...

The main difference between Synchronous Motors and Induction Motors is bulleted on the nearby graphic.

 

YOU TUBE TO THE RESCUE AGAIN!

Brilliant PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order remember that in the recent PTOA Segment #186 Fred the Stickman was left wondering how the Torque of the Driving Shaft is initially created so that the rotational movement can be transferred to a Driven Shaft and the Load attached to it.

Kudos to the Learn Engineering "How an Induction Motor Works"You Tube!

The link to this magnificent You Tube can be accessed below after reading several more paragraphs.

The You Tube will enlighten Fred and all PTOA Readers and Students how the Electromagnetic Induction observed by Faraday was commercially applied by Nikola Tesla to create the spinning motion which ultimately creates Torque in a (Squirrel Cage) Induction Motor.

This Learn Engineering "How an Induction Motor Works" You Tube is worth a million written words that PTOA Readers and Students would otherwise have had to muddle through!

PTOA Readers and Students should give a "Thumbs Up" to Learn Engineering to show their appreciation to the creators of "How an Induction Motor Works."

 

For sure, the Learn Engineering "How an Induction Motor Works" You Tube uses some very fancy, confusing language!

For the first viewing of the You Tube, just tune out the fancy verbiage and focus on how the Squirrel Cage Induction Motor produces Torque.

Here's a preview of what to look out for in the Learn Engineering "How an Induction Motor Works" You Tube:

Once 3 Phase AC Power is supplied to the 3 separate copper windings of the Motor's stationary (non-moving) Stator ...

A uniform "rotating magnetic field (RMF)" is created and pulses toward the air gap between the stationary Stator and the able-to-rotate Rotor.

This RMF plays the role of the bar magnet being moved about in the  eLearnIn Electromagnetic Induction You Tube which was featured in PTOA Segment #187.

Don't spend a moment stressing over the part in the LearnEngineering You Tube that shows how the three colorful Stator windings generate a uniform rotating magnetic field (RMF); to grossly simplify the situation the 3 Phase AC Power energizes pairs of magnetic poles and since they are evenly spaced ... the RMF is uniform. So now you know!

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order will remember that the eLearnIn Electromagnetic Induction You Tube that was featured in PTOA Segment #187 had a coil of copper wire in which a current was induced by a moving magnet.

In the Learn Engineering "How A Induction Motor Works" You Tube the role of the copper wire is played by the Rotor Bars with their End Rings.

The RMF that is created by the energized windings of the Stator cuts across the air space between the Stator and Rotor Bars ...

And this RMF induces a voltage between the Rotor Bars, which all PTOA Readers and Students know will ... TADAH!... "induce a current"!

 

 

Yep! Just exactly what Faraday observed with his bar magnet and coil of copper wire!

So now there is a current flowing through a closed circuit that has developed in the Rotor Bars ...

And since this "induced current" is surrounded by and thus being impacted by the rotating magnetic field (RMF) generated by the Stator ...

This "induced current" in the Rotor Bar cannot help producing its own magnetic field!

So who would be surprised to learn that ...

The magnetic field created by the circuit in the Rotor Bar impacts the rotating magnetic field (RMF) created by the stationary Stator! How and why the two magnetic fields impact each other was mathematically explained by a dude named Lenz ... but who cares?

Just accept that ... upon being energized... the stationary Stator creates a rotating magnetic field (RMF) which is magnetically interlocked with the spinning Rotor's magnetic field and thus ...

The Stator's RMF drags the Rotor around with it ... creating rotational movement and thus Torque!

Okay, Fred!

Know what to look out for in the You Tube?

 

Okay, PTOA Readers and Students it is time to "You Tube and Chill"!

Access the Learn Engineering "How Induction Motors Work" You Tube HERE or directly below:

 

SQUIRREL CAGE INDUCTION MOTOR "SLIP"

Once is not enough.

Second verse ... same as the first!

Deja Vu all over again!

PTOA Readers and Students must  access the Learn Engineering "How Induction Motors Work" You Tube yet again after first being alerted to observe the following:

 

The rotation rate of the Rotor cannot keep up with the rotating magnetic field (RMF) created by the energized Stator windings.

The difference between the rate of rotation of the RMF and the Rotor is called "Slip."

"Slip" varies from a low of 2% to a high of 6%.

"Low-Slip" Squirrel Cage Induction Motors are efficient when the Load that must be spun about is steady ... which describes the Load on most of the Rotating Equipment in a well designed Processing Plant.

The Synchronous (Rotational) Speed of a Squirrel Cage Induction Motor is measured at the air gap between the Stator and the Rotor when there is no load on the Motor's Shaft.

 

Here's an example of how the jargon "Slip" applies to Squirrel Cage Induction Motors:

A NEMA compliant three-phase Squirrel Cage Induction Motor that is rated to have a Synchronous Speed of 1800 rpm with No Load will be rated to rotate at 1750 rpm at Full Load." The Slip for this motor can be calculated to be 2.8% ...

Which means the Rotor lags spinning with the RMF by 2.8%.

The nearby chart compares the Synchronous Speed of the Rotor with No Load to whirl around to the rated Squirrel Cage Induction Motor Speed with a Maximum Load that must be whirled around.

Fred is in the back of the room wondering what the term "poles" in the nearby graphic means.

The number of poles refers to the number of "north and "south" poles in the Stator's windings that are energized with AC and thus able to create a RMF.

There will always be an even number of poles in an Induction Motor. The more poles, the more Torque generated (and less rpm).

One last point:

The voice in the Learn Engineering "How Induction Motors Works" You Tube keeps muttering that Induction Motors are "Self-Starting."

"Self-Starting" means that the Process Operator just presses the "Start" button that will connect the AC Power to the Stator's Windings.

Almost immediately Torque is generated and the Pump or Compressor or whatever the Load is that is attached to the Driven Shaft will begin doing work!

The "Self-Starting" feature is a big deal!

The "Self-Starting" feature makes it a lot easier to get Rotating Equipment back on line after an emergency power outage.

Neither the Synchronous Induction Motor nor the Wound Rotor Induction Motor featured in a few more paragraphs have this Self-Starting feature.

Okay, "You Tube and Chill" with Learn Engineering one last time!

Access the Learn Engineering "How Induction Motors Work" You Tube HERE or directly below:

 

"High Slip" versus "Low Slip" Induction Motors

The lower the Slip, the more efficient an (Asynchronous Squirrel Cage) Induction Motor uses the raw material of AC power charged to it.

Yet how much Slip to design into an Induction Motor depends upon the Torque-Load requirements of the Rotating Equipment package.

If the Full Load is generally steady, then the more efficient Low-Slip Induction Motor is desirable.

However, small industrial Air Compressors are an example of Rotating Equipment that has frequent Torque-Load impulses.

So an older model Air Compressor driven by an Squirrel Cage Induction Motor would be designed for High Slip  ... and with flywheels.

 

SYNCHRONOUS INDUCTION MOTORS

Understanding the definition of "Slip" makes it automatically possible to understand the definition of a Synchronous Motor because ...

Synchronous Induction Motors are designed to have no Slip.

Synchronous Induction Motors have very nearly the same rate of rotation between the RMF and the Rotor.

So ...

Synchronous Motors run at a fixed rotational speed determined by the AC power frequency and the number of poles in the machine.

But what happens to the interfering magnetic field generated by the Rotor that causes Slip in an Asynchronous Induction Motor?

The answer is really boring ... but here goes:

The Rotor of a Synchronous Motor substitutes what are called Damper Windings that do the job of the Squirrel Cage Induction Motor Rotor with respect to bringing the Motor from startup to nearly Synchronous Speed.

And that means the RMF generated by the Synchronous Motor's Stator is locked onto an externally excited DC-created field.

How can that be so?

A  small source DC power ... so small it is called "an exciter" ... is provided by:

• 

  The Synchronous Motor has a DC exciter current and does not generate a combating magnetic field.

  A small DC generator in the Synchronous Induction Motor design that includes Brushes.

• Or nowadays by solid-state and silicone controlled Rectifiers (SSR and SCR) which would be mounted on the Shaft of a Synchronous Induction Motor.

 

Wake Up, Fred! Don't  you want to know that ...

Once the Stator's RMF is in sync with the Rotor ...

The Stator will maintain a constant rotational speed as long as the Torque required by the Load does not exceed the maximum Torque generated by the Motor.

So now you know!

 

Pros and Cons of Synchronous and (Asynchronous) Induction Motors

Synchronous Motors require an external DC excitation and are thus not Self Starting.

Synchronous Motors also have Rectifiers that will no doubt require more maintenance.

 

 

So why not just put up with a little Slip from an (Asynchronous) Induction Motor instead of purchasing a Synchronous Induction Motor that requires more maintenance and cannot Self-Start?

The answer is always about not wasting money!

Synchronous Motors are 2-3% more efficient than Asynchronous Induction Motors or DC Motors of the same size and speed.

Over the operating life of the Motor, the loss of efficiency translates into wasting a lot of money on:

• The electrical utility supplied to the Squirrel Cage Induction Motor.
• Degradation of the Power Factor of whatever type of AC power generator is providing the AC power to the entire Processing Plant

Degrading the Power Factor of the originating source of electrical AC power generation is a big deal!

The AC power generator that supplies AC to all of the electricity users can generate Useful Power and Useless Power.

Degrading the Power Factor of the AC generator means the AC power generator will generate less Useful Power and generate more Useless Power ... and that will impact the power for all of the electrical utility users in the Plant.

The above statement explains why, generally speaking, a Synchronous Induction Motor will more than likely be selected instead of an (Asynchronous) Induction Motor when:

• The Induction Motor is designed for 6000 HP and above.
• The Induction Motor is designed for 2000 HP and above with a rotational speed of 3600 rpm.
• The Induction Motor is designed for 700 HP and above with a rotational speed between 500-1200 rpm.

 

In the Induction Motor design range of  200-700 HP, the selection of an (Asynchronous) Induction Motor over an Synchronous Induction Motor would depend upon:

• The cost of generated AC power and/or purchasing AC power from the local utility.
• The impact on the Power Factor on the generated source of AC power.
• The expected hours of continuous Induction Motor operation.
• The type of enclosure needed.

 

WOUND ROTOR INDUCTION MOTORS

Industrial Use of Wound Rotor Induction Motors

Here's a scenario:

A crane or hoist is needed to do industrial work.

Both the crane Motor and the hoist Motor will be rated for intermittent duty cycles of 30 or 60 minutes.

So use of the crane or hoist is going to intermittently put a heavy Load on the AC power generator that provides power to the crane or hoist Motor.  Wherever the originating AC power comes from ... the generator typically has limited capacity.

In summary, here are the criteria that the crane or hoist Motor must achieve:

• Needs high starting Torque ... but must do so with a low starting current.
• Needs speed control.
• Needs Slip under peak Loads

 

The above criteria describe the industrial domain of the Wound Rotor Induction Motor.

Because of the ability to control Slip, Wound Rotor Induction Motors are also used for

• Ball Mills and Hammer Mills.
• Chippers and Crushers.

 

The Wound Rotor Induction Motor Design

The Wound Rotor Induction Motor is accurately named because ...

The Rotor has a 3 phase Winding that is connected in the same way as the Stator Windings are.

However, this Rotor Winding has a connection to external power through what are called Slip Rings.

This connection to the Rotor Winding through Slip Rings makes it possible to adjust an external source of electrical resistance for the purpose of controlling the Speed-to-Torque output. Hey, that's a kind of Variable Speed Driver!

The nearby graphic concisely describes how the Brushes and Slip Rings of the Wound Rotor Induction Motor work together to control rotational speed, and thus Torque.

Although the Wound Rotor Induction Motor has a niche service application, the (Asynchronous Squirrel Cage) Induction Motor with Variable Speed Drive has very nearly replaced it in industry.

Hey!  High Five with Your Mentor!

PTOA Readers and Students are done learning about Induction Motors ... the kind of Motors that use Faraday's observations to create Torque!

 

TAKE HOME MESSAGES: The three types of Induction Motors are:

• The Asynchronous Squirrel Cage Induction Motor
• The Synchronous Squirrel Cage Induction Motor.
• The Wound Rotor Induction Motor.

 

The (Asynchronous Squirrel Cage) Induction Motor is so dominant that it is what human beings are referring to when they say "Induction Motor."

(Asynchronous Squirrel Cage ) Induction Motors are popular because they are very low maintenance, are Self-Starting, and their speeds can be controlled.

Many thanks to the Learn Engineering "How Induction Motors Work" You Tube for explaining how Faraday's observed Electromagnetic Induction creates the Torque of an Induction Motor. Always be amazed that Nikola Tesla saw all of this in his head.

The Synchronous Speed of Squirrel Cage Induction Motors is measured at the air gap between the Rotor and the Stator when there is no Load on the Motor.

The (Asynchronous Squirrel Cage) Induction Motor's speed of rotation with Load will always be rated less than Synchronous Speed with no Load .. and the difference is called "Slip."

"Slip" is a characteristic of (Asynchronous Squirrel Cage) Induction Motors because the Rotor cannot turn at the same rate as the RMF created by the Stator Windings. Slip varies from 2-6% in NEMA rated Induction Motors. The lower the Slip, the more efficient the Induction Motor.

Synchronous (Squirrel Cage) Motors have no Slip, their fixed rotational speed is determined by the AC power frequency and the number of poles in the machine.

By eliminating Slip, Synchronous (Squirrel Cage) Induction Motors are 2-3% more efficient than (Asynchronous Squirrel Cage) Induction Motors or Wound Rotor Induction Motors.

Even though they have no Slip, Synchronous Induction Motors have higher maintenance costs due to needing to be externally excited with a DC current which involves Damper Windings, Brushes, and/or Rectifiers.

Synchronous (Squirrel Cage) Induction Motors also do not downgrade the Power Factor of the supplying AC generator. Downgrading the Power Factor of the AC Generator means the generator "will generate more Useless Power and less Useful Power."

The list of criteria to consider for choosing a Synchronous (Squirrel Cage)Induction Motor over the (Asynchronous Squirrel Cage) Induction Motor appears in this PTOA Segment.

The Wound Rotor Induction Motor must also be externally excited by a DC current so they are also not Self Starting. They also have Brushes and Slip Rings that require maintenance. The Wound Rotor Induction Motor is the best choice when the Motor service operation is as follows:   

• Needs high starting Torque ... but must do so with a low starting current.
• Needs speed control.
• Needs Slip under peak Loads

 

©2018 PTOA Segment 0188

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