Ode to an induction motor

There are three basic concepts of nature that you need to know.
If you want to understand what makes an induction motor go.

First, whenever current through a conductor does flow,
Lines of flux around the conductor will definitely go.

This creates a magnetic field these lines of flux;
A mystery of the cosmos, same as why gravity sucks.

Why mass creates gravity even Einstein did not know.
The same is true of why flux is created by current flow.

By accepting this, a magnetic field we can create.
And control its strength with the current’s rate.

For every turn we add to a current carrying coil of wire.
The strength of the magnetic field will surely go higher.

Thank you very much Michael Faraday;
For figuring this one out along the way.

At each end of the coil like an open mouth.
Will exist a pole, one north and one south.

If we alternate the currents direction to go back and forth;
The magnetic poles will flip north to south, and south to north.

Lines of flux through the center of the coil go;
Then out both ends, before going to and fro.

Around the outside of the coil nice and neat;
A toroid-shaped circuit they will complete.

Second: Place a conductor in the presence of a magnetic field.
Push a current through that conductor and on itself a force it will yield.

The larger the current or stronger the field;
The bigger the force the conductor does yield.

With a large enough force, it is simple to prove,
That most certainly the wire will start to move.

Change the direction through the wire of the current flow.
It will result in changing the direction that the wire will go.

Third: Push a conductor through lines of flux at any rate.
Across the conductor a voltage it will certainly generate.

The relative direction between conductor and flux that does persist;
Determines the voltage polarity across the conductor that will exist.

As more lines of flux cut through the wire.
The voltage induced will for sure go higher.

Now if you would please be so kind,
As to keep these 3 concepts in your mind.

And I will do my best,
To explain the rest.

A rotating field and a sturdy base is the purpose of the stator.
Don’t worry about the rotor just yet; we’ll talk about it later.

A thick-walled cylinder hollow in the middle like a topless and bottomless can.
Parallel slots on the inside running end to end to hold our coils, that’s the plan.

Three-wire coils in the slots around the stator are placed.
Making sure that from each other they are equally spaced.

If an ac sine wave phase to each coil we connect.
A rotating magnetic field the coils will then project.

For every complete cycle the three-phase sine wave does make.
A lap around the stator the rotating magnetic field will take.

Change the number of coils per phase the stator does carry.
And the speed of the rotating magnetic field will also vary.

For each coil per phase there will be 2 magnetic poles.
1 coil creates 2 poles; 2 coils 4; That’s how it goes.

Using 60 Hz 3-phase with a stator that is 2-pole, 4-pole, 6-pole or 8.
3600, 1800, 1200, or 900 rpm is the magnetic fields rotational rate.

No matter the number of coils the speed of the rotating magnetic field does rely.
Upon the frequency of the three-phase power which to the stator we do supply.

Take two round copper discs that are parallel and from each other spaced.
Between them near the edges and separated, copper bars evenly placed.

You have just constructed what for the time being looks like a hamster wheel.
Now fill in the gaps between the rotor bars and in the center with insulated steel.

A protruding shaft through the center to connect a load;
This will also provide a good place for the bearings to hold.

Now that you have built yourself a fine-looking rotor;
Place it inside the stator to complete your motor.

Apply a 3-phase ac sine wave power supply to the stator and it will energize.
There is now a rotating magnetic field, though you can’t see it with your eyes.

Through the rotor bars the rotating magnetic field will glide.
Cutting down to up in front, and up to down on the other side.

With rotating lines of flux, concept three will now come into play.
Across the rotor bars there will be a voltage generated right away.

The polarity of the voltage generated on opposite sides of the rotor will not be the same.
Since the direction of flux through bars is different on each side; concept three is to blame.

The way the voltage polarity on opposite sides of the rotor goes.
Copper rotor end discs complete the circuit, thus current flows.

So through the bars and end plates around the rotor current will flow.
Left to right on one side and right to left on the opposite side it will go.

Current through a conductor in the presence of flux produces force.
This is the second concept we talked of; you remember, of course.

Related to current through a conductor Concept 2 also does say.
Change the direction, and the force will go in the opposite way.

Current in opposite directions through conductors on opposite sides of a rotor;
The resulting opposing forces will cause it to spin, and “Ta da,” you have a motor.

If the rotor were ever to spin as fast as the rotating magnetic field;
No flux would cut through the rotor bars so no torque would it yield.

This difference in speed between the field and the rotor;
We refer to as slip, which is unique to an induction motor.

And if ever the shaft of the rotor is pushed enough by an outside force.
The rotor could move faster than the rotating magnetic field, of course.

If this occurs instead of consuming energy, your motor will generate.
Pushing energy back to the power supply; reducing your energy rate.

To Nicola Tesla we credit the induction motor, an invention so great.
It has survived for well over a century, being awarded a patent in 1888.

Greg Davies is training engineer and poet, Yaskawa America Inc. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media and Technology, mhoske@cfemedia.com.

KEYWORDS: Induction motor tutorial, induction motor design

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