An Introduction to Sustainable Energy Systems: Motor, Generators and Transformers Last time I...

An Introduction to Sustainable Energy Systems: www.virlab.virginia.edu/Energy_class/Energy_class.htm

Motor, Generators and Transformers

Last time I claimed I'd covered ~ all the electrical science needed for this class

But I qualified that claim by ALSO saying:

"Trick is just to figure out all of the sneaky ways it can/must be applied"

One of the sneakiest / weirdest ways it can be applied, is magnetic induction

Which happens when charges meet up with moving magnetic fields

Today we'll see how magnetic induction is used at the heart of power systems

In electrical motors, electrical generators, and electrical transformers

But first we need to refresh our memories a bit:

From first lecture, on Electric Fields (E):

Electric Fields are way of mapping Electric Force

Electric force can be exerted only ON an object carrying a net charge

BY another object also carrying a net charge

Point charge (at 2) generates an electric field E (at 1) = q / |x1 – x2|2 ê12

Use 1st Maxwell Equation for more complex charge distributions

Force due to Electric Field = q E = (net charge on object) (Electric Field at object)

This is the first half of the Lorentz Force Law

To ACQUIRE a net charge (in even a microscopic region) a material MUST either:

Allow charge to move in or out of it (be an "electrical conductor") OR:

Allow charges to shift in different directions (be "polarizable" / "dielectric")

From first lecture, on Magnetic Fields (B):

Magnets (and their fields) act on other magnets, and iron, but not much else

UNLESS that something else has MOVING CHARGE

OR IF magnetic field is moving with respect to charges

Unlike Electric Fields, Magnetic field maps are NOT simple force maps

But they can be used to infer the forces between magnets

Or infer force of fixed magnetic fields upon moving charges

Or force of moving magnetic fields upon fixed charges

To figure out force on moving current, apply 1st Right Hand Rule

Reversing direction of force force for minus charges

To figure out magnetic field created BY current, apply 2nd Right Hand Rule

Reversing direction of induced magnetic fields for minus charges

Recalling those two right hand rules:

1st Right Hand Rule (part of Lorentz Force Law) => Force on moving current:

Thumb = direction of charge motion,

Fingers = Magnetic field

Positive charge: pushed out of palm

Negative charge: pushed into palm

2nd Right Hand Rule (Lenz's Law) => Magnetic field INDUCED BY current

Thumb = direction of charge motion

Which then induces a magnetic field:

Along curled fingers (positive charge)

Opposite curled fingers (negative charge)

Charge

Flow

Force on Plus Charges

Reverse B for electron flow:


Now using this information to explain electric motors & generators:

Here we must deal with all sorts of charge and magnetic field motion

Which can be represented only very poorly static figures

So I have animated these explanations on my "UVA Virtual Lab" website:

Link to AC and DC Electric Motors webpage

http://www.virlab.virginia.edu/VL/Motors.htm

Excerpts from scenes 1-6:

How one magnet can rotate another:

How a "magic" pole-switching magnet

could end up rotating continuously


Excerpts from scenes 7-12:

How an electromagnet CANswitch its poles every half turn,via paired contacts plus DC power:

Excerpts from scenes 13 – 17:

How another electromagnet CAN do the the same, via single contacts + AC power:


Subtle problem: Motor's torque is not even during rotation

Thickness of yellow PowerPoint arrow ends up representing torque vs. rotation!


Start of turn:Nearly parallel

magnetismLOTS of repulsion

LOTS of torque¼ turn:

Perpendicular magnetism

MINIMUM repulsionMINIMUM torque

Nearly ½ turn:Nearly anti-parallel

magnetismLOTS of attraction

LOTS of torque

Using all permanent magnets, let's run through a half turn of motor rotation:

A solution: Multiple rotor electromagnets

Keep turning NEW electromagnets onWHEN they are oriented to push most

strongly!


At ¼ turn:

Turn YELLOW electromagnet OFF

=> No torque

Turn ORANGE electromagnet ON

=> Torque

During first 1/4 turn:

YELLOW electromagnet ON

~ Parallel magnetismMaximum torque

ORANGE electromagnet OFF

No magnetism, NO torque

During second 1/4 turn:

YELLOW electromagnet OFF

No magnetism, NO torque

ORANGE electromagnet ON

~ Parallel magnetismMaximum torque

Taking that to an extreme:

Assembly minus the tool's outer shell (which took with it part of contact assembly):


LEFT: Single split outer "stator" electromagnet

RIGHT: Rotor with MANY electromagnets

Photographs I took of the motor from an old woodworking router tool:

Close-up of rotor:

Figure at left from: http://www.mlcswoodworking.com

Electromagnets are wrapped around green-tinted rectangles (pole pieces)

Their wires then run down to the sections of split ring near the bottom

But THIS ring has not been split in half

It has been divided in to 28 contacts!

At two contacts per electromagnet

28 contacts => 14 electromagnets

(confirmed by count of green rectangles)

Manufacturer did this because wood routers

drive HUGE cutting bits into wood

and they thus require LOTS of torque!

So my router motor was actually more like this:

OUTER ("stator") AC electromagnet, switching polarity 120 times a second

INNER ("rotor") AC electromagnets (14 rather than the 4 that I fit into drawing)

Only ONE of which receives AC power at any time

That ONE "selected" by its contacts rotating under the fixed "brushes"

AC

BIG ENERGY SYSTEM TAKEAWAY MESSAGE #1:

In ALL of the above: Cause and effect are effectively interchangeable

Moving magnetic fields induce currents (via Lorentz Force Law)

OR:

Moving currents induce magnetic fields (via Lenz's Law)

CONSEQUENCE: Motors and Generators have SAME structure

One is just the other "being run backwards"

With motors using permanent magnets all you need to do is:

Turn DC motor => Get DC POWER OUT = DC generator*

Turn AC motor => Get AC POWER OUT = AC generator

*(for really steady DC power may need to add a capacitor for smoothing)

Although it's a bit more complicated with electromagnets

To produce power from a generator, you must START with a magnetic field

If doesn't come from permanent magnet (impractical in large systems) then need:

a) External power applied to exterior "stator" electromagnets of generator

OR:

b) External power applied to interior "rotor" electromagnets of generator

Hydroelectric dam's huge turbine generators have powered rotor electromagnets

And YES that implies that if a hydroelectric plant lost ALL power

Including its locally generated auxiliary power

Then water turning the generator's rotors would NOT produce power out!

Rotation + a little power in => Lots of power out

BIG ENERGY SYSTEM TAKEAWAY MESSAGE #2:

For motors AC power is much more versatile / much more controlled

DC motors change speed anytime the voltage wanders or load changes

AC motor speed is pretty much locked into their design

With little shift caused by changes in voltage or load

You will ALSO soon learn how AC is also more versatile in power transmission

1st Electrical Power Systems => HUGE BATTLE between DC vs. AC

Thomas Edison (ruthlessly) pushed DC

Nicholas Tesla & George Westinghouse pushed AC

Because of motors, generators, transmission, transformers . . .

AC won out . . . EVERYWHERE!

I mentioned that BIG permanent magnets are not practical:

For BIG magnetic fields they are just too damned BIG/HEAVY

Especially if try to spin them in rotors at tens of thousands of RPM!

Thus in big motors/generators almost All magnets = electromagnets

But that creates another problem in large and/or long lifetime motor/generators:

Those rubbing contacts ("brushes"), either DC split ring or dual AC ring

They wear away!

Especially with split rings (which can also spark when slide over gaps)!

So we'd often like to get rid of the rotor magnets completely

But what would outer magnets then push AGAINST?


Nicholas Tesla figured out that they can push against plain metal

If you think about it (the way Tesla did), you have already seen the proof

When we dropped magnet down (non-magnetic) copper pipe its decent was slowed

We explained this (in detail) as downward moving

magnetic field inducing loop of current in pipe

With current loop then producing counter magnetic field

that pushed up on falling magnets

That explanation was correct, but we still have Newton's: "action = reaction"

So as magnet is being pushed upward, pipe must have be pushed downward

From scientific suppliers, I got two other rigs that show this clearly:


First is an enhanced version of the earlier apparatus:

TWO pipes, one copper one clear plastic

Both hanging together from a scale

Drop metal slug down either pipe: No deflection on scale

Drop magnet down plastic pipe: No deflection on scale

Has no free electrons to be induced into flowing!

Drop magnet down copper pipe:

Scale deflects while magnet falling through pipe

Rising back when magnet falls out bottom

CLEAR evidence of magnetic field pushing (non-magnetic) metal

"Lenz's Law Apparatus" (#32520) from The Science Source (www.thesciencesource.com)

Or a bit more subtle:

Strong permanent magnet, with pole pieces

Three flat (non-magnetic) aluminum shapes hung from trapeze like bar

Positioned they can hang in gap between magnet's pole pieces

I have enhanced this apparatus by mounting the magnet on a sliding stage

Magnet can then be driven INTO position of hanging aluminum shapes

With result that: When magnet drives in comb shape ~ nothing happens

But square shape is dragged along with magnet

Slotted shape is also dragged along with magnet

WHAT THE HECK IS GOING ON!Pasco Scientific: Magnetic Force Accessory (EM-8642A) / Variable Gap Magnet (EM-8618)

To answer, figure out why square paddle is dragged along

Magnet's direction of movement (right)

Magnet's field

Magnet's field points into page

When magnet moves right:

Magnet "sees" paddle's electrons moving left

1st Right hand rule: thumb left, fingers into page

Electrons pushed into palm = upward

But as field passes, electrons will scurry back downward

Charge Flow


So passing magnet "induces" loops of electron flow:

Via 2nd Right Hand Rule:

Green loop of electrons induces opposing magnetic field INTO the page

Induced field is parallel to magnet's field

So magnet & induced fields repel one another

Magnetic field

movement

Can still get induced green loops in the slotted aluminum shape

So it ALSO fights passing of magnet

Versus other shapes:

But cannot induce current loops in comb shape:

Magnetic field

movement

By blocking the induced electron current loops in the comb shape (only!)

A counter magnetic field is not set up in the COMB

Hence there is NO resistance to passing magnet field through it!

In our recycling plants, we capture magnetic metals with magnets

But in last lecture I pointed out that NOT ALL METALS ARE MAGNETIC

A magnet will not grab copper or brass (which is a copper / zinc alloy)

A magnet will not grab aluminum – which is a big deal because:

Aluminum refining requires HUGE amounts of electrical power !

A (possibly) interesting digression: Metal recycling

http://www.syracuse.com/news/index.ssf/2014/02/

report_roth_steel_in_syracuse_closing_after_century_in_business.html


Answer: Via something very much like my rolling magnet!

Which worked via magnetic induction:

Magnet tried to push its magnetic field into the conductive metal

Magnetic force pushed electrons => loop of electrical current

Current loop => Opposing magnetic field

Which pushed metal away from magnet

So embed magnets in the roller of a trash conveyer belt

To flick the metal pieces UP off of the belt!

(And then catch them before they can fall back)

So how DO we capture non-magnetic metals?


Magnets are embedded in the final roller of the trash conveyer belt

Those magnets flick all metals up off the belt

Where they are caught and diverted by a barrier:

Schematic of non-magnetic metal picker:


From the "American Recycling Center:"

Copy of video cached on this lecture's "Supporting Materials" Webpage:

Movie of actual metal picking conveyer:

https://www.youtube.com/channel/UCurq7PjCvPBx4zZ6v0i0wGw

Supporting Materials: Metal Sorter

http://www.virlab.virginia.edu/Energy_class/lecture_notes/Magnetic_Induction_Supporting_materials.htm%23Sorter



To Nikola Tesla magnetic induction instead suggested new motor:

Which would involve (only!):

A simple, non-magnet, non-magnetic, rotor

Immersed in an imposed rotating magnetic field

Via 1st Right Hand Rule:

Rotating magnetic field should induce loops of electron current in rotor

Via 2nd Right Hand Rule:

Those induced loops of current should induce a new magnetic field

Parallel to and repelled by the imposed rotating magnetic field

Causing the rotor to turn

But requiring no permanent magnets or electrical connections to that rotor!


How to generate a rotating magnetic field:

Use "three phase" AC current = Three AC signals, offset 120° from each other

Which you generate by putting three sets of coils around spinning (electro) magnet:

AC2

AC1

AC3

This is actually the standard way of generating AC power

It makes the best (most continuous) use of the magnet's rotating field


Not sure it's that easy? Take it in smaller steps:

Draw out each of the voltage/current for each of the three phases

Record when each phase reaches a positive (negative) peak

90 (270)

(30) 210

(150) 330Peak positionsin degrees:

Or numbering them: (1) 2 (3) 4 (5) 6

Now APPLY those phases to 3 coils rotated 1/3 circle apart:

Timing of (+) peaks: Timing of (–) peaks

Cumulative magnetic fields peaking (at 1/6 cycle = 1/360 second intervals):

Magnetic field rotating in circles (essentially recreating what happened in generator!)

4 2

6

3

5 1


In center of those coils, now put a plain metal ring

And, once again, haul out the 1st Right Hand Rule

Using this to figure out currents induced in metal ring:

Red: Rotating magnetic field

Blue: Motion of charge, relative to field

Green: Force on electrons => movement

Charge Flow



Then use 2nd Right Hand Rule to figure out induced magnetic field:

Induced (blue) magnetic fields, inside the ring,

Are PARALLELING the rotating field

Meaning that they will be REPELLED

So rotating fields will drive ring to rotate

Reverse B for electron flow:

As depicted in "www.LearnEngineering.org" animation

Screenshots from the almost five minute long animated tutorial:

(One of MANY excellent teaching animations on this website)

Link to original: www.learnengineering.org/2013/08/three-phase-induction-motor-working-squirrel-cage.html

YouTube posting: www.youtube.com/watch?v=LtJoJBUSe28&feature=kp

http://www.learnengineering.org/2013/08/three-phase-induction-motor-working-squirrel-cage.html

http://www.youtube.com/watch?v=LtJoJBUSe28&feature=kp

"Slip?"

Did you catch that animation's reference to slip?

The idea that rotor had to rotate more slowly than rotating applied field?

This did NOT come up in our earlier all magnet or electromagnet motors

But it has to occur in induction motors because to have an effect on charges (rather than on other magnets)

applied magnetic field must be MOVING relative to that charge

If rotor kept pace with rotating field, there would be no relative motion!

So rotor HAS to rotate more slowly

So that rotating magnetic field is always moving towards it

Which THEN induces current in rings => Induced magnetic field

Which pushes back against rotating applied field


Magnetic induction also allows us to TRANSFORM AC power

What exactly IS electrical power?

Water flow analogy is useful here: How do you get power from water?

Imagine alternative pipes supplying water:

Pipe #1 supplying lots of water (flow) but at miniscule pressure OR

Pipe #2 supplying miniscule flow, but at at high pressure

Limitations of both mean NEITHER would be capable of doing much work

For work, you need BOTH flow AND pressure. In fact:

Work = Flow x Pressure

Electrical equivalent = Current x Voltage

OK, but what current and what voltage?

Shortcoming of LARGE CURRENT:

Current = electron flow through a material

But flowing electrons bump into atoms, loosing kinetic energy

(=> vibration of those atoms = "heat")

So current flow => LOSS of electrical energy / power

Shortcoming of LARGE VOLTAGE:

Voltage is analog to water pressure:

High pressure water requires thicker pipes

High electrical voltage requires thicker/better insulation

Insulation adds cost / takes up more volume

Too little insulation => chance of failure => electrocution

So optimum choice depends on situation:

When TRANSMITTING electrical power, want to minimize losses

So there is a strong incentive to REDUCE CURRENT

Holding power constant by using INCREASED VOLTAGE

When USING electrical power (especially in homes and/or appliances):

Often want to minimize bulk by minimizing thickness of insulation

But to keep chances of insulation failure to minimum

Need to DECREASE VOLTAGE

Holding power constant by using INCREASED CURRENT

THESE CONCLUSIONS APPLY TO BOTH AC (alternating) AND DC POWER

But conclusions are contradictory UNLESS you can easily TRANSFORM between

How to develop such a voltage/current "transformer?"

We've seen (from 2nd Right Hand Rule) how loops of current => magnetic fields

Earlier, those loops of current were induced by approaching magnetic fields

But can also DRIVE current through loops by pushing current with a voltage

Can also increase magnetic field by increasing number of loops:

One full circle: Versus four full circles:

But more loops => More resistance to current flow ("longer pipe")

So will also need higher applied voltage ("pressure") to get same flow

Right, multi-turn, magnetic field building & storing structure has MANY applications

And is given a a variety of special names (depending on application):

"Inductor" / "Solenoid" / "Electromagnet" / "Coil"

Place two such coils side by side

Close enough that magnetic field from first can impinge upon the second

What will happen in second, lower, coil?

If current is constant (due to DC voltage), NOTHING will happen!

There IS charge in lower loop

But no relative movement of the top (constant) magnetic field


But if voltage/current are CHANGING in top coil:

Time 1: Time 2 (increased voltage/current):

Side view: Note how growing field from top coil sweeps across lower coil!


Field sweeping across lower coil => Current induced in that coil

Current/Voltage applied to TOP coil Current/Voltage induced in BOTTOM coil

So electrical power is actually being transmitted across gap between coils!

Is this the current/voltage transformer we were looking for?

Not yet, at least not with these ~ identical coils

Need mismatched coils:


Coils with different number of turns

Again apply AC (changing) voltage/current to top coil:

Time 1: Time 2 (greater voltage/current):

Side views:

More coils affected at bottom, expect MORE (something) to be coming out

But MORE what coming out?

Can't get MORE power out of bottom coil then put into top coil

That would be creating power (and thus not conserving energy)!

But CAN change what made up that power = current x voltage

Which goes up, which goes down?

Hint: Longer length of wire at bottom resists flow of current

So here get higher voltage but lower current flow out of bottom coil


More generally:

For an AC transformer (remembering that this only works for changing AC) with:

N in = number of input turns N out = number of output turns

Voltage in vs. Voltage out (for an "ideal" transformer):

Voltage in x N out = Voltage out x N in OR:

Voltage out / Voltage in = N out / N in

Which, in order to have Power in = Power out requires that:

Current out / Current in = N in / N out

OK, but what then does "ideal" operation imply or require?

To be "ideal" must be very effective coupling of magnetic fields:

It's the part of the magnetic field sweeping through second coil that does the work

And in our "transformers" above, most of magnetic field is way out in space

Doing very little at all to couple the input and output coils together!

Which brings us to the topic of "pole pieces"

Remember how in last lecture we covered "ferromagnetic materials?"

Materials, like iron, that tended to "suck in magnetic fields"

Because of the counter-oriented "spins" on unpaired electrons

We can use configurations of such materials to redirect magnetic fields

Thereby increasing coupling between transformer coils


Using "pole pieces" to redirect magnetic fields:

Permanent magnet without pole pieces =>

Permanent magnet with pole pieces =>

AC powered electromagnet with pole pieces:

Mangetic Induction - Supporting Materials - Pole pieces animation 1

Note how the pole pieces STRETCH OUT the magnetic field

http://www.virlab.virginia.edu/Energy_class/lecture_notes/Magnetic_Induction_Supporting_materials.htm%23Pole_pieces_1

Using more effective designs to totally trap magnetic fields:

Two coils side by side on pole piece (with B field still spilling out ends) =>

Coils on opposite side of ring (B field almost entirely IN ring)

Coils wrapped OVER one another (B field IN double ring)

Mangetic Induction - Supporting Materials - Transformers animation 1

Final structure: REALLY effective coupling between coils, very compact, B field has more room to spread out => Standard design

http://www.virlab.virginia.edu/Energy_class/lecture_notes/Magnetic_Induction_Supporting_materials.htm%23Transformers

But this is still far from an "ideal" AC transformer

Because, unless we make further changes, we'll waste power heating pole pieces:


Growing/ Shrinking magnetic field MOVES through metal pole pieces

INDUCING loops of current (here called "eddy currents" – in blue)

Current, colliding w/ atoms, looses energy (to heat). Potentially a LOT of power!


To reduce eddy currents (and power lost to heating):

Make pole pieces out of stacks of metal plates (insulated from one another)

Breaking up paths over which large eddy current loops want to flow


(It's easy to insulate plates: Just paint both sides with insulating lacquer!)



Result: An almost ideal AC transformer

In senses that:

Voltage out / Voltage in = N out / N in And that:

Power out ~ 98 to 99% Power in (for large, well designed transformers)

But a few percent loss still means that AC transformers get warm

Leading people to label chargers (using AC transformers)

"Power Vampires," and to insist they be unplugged when not in use!

Also there is a funny consequence of using laminated pole pieces (or "cores"):

Transformers hum (sometimes rather loudly, as in utility rooms/yards)!

Small eddy currents remaining => Small induced magnetic fields

That drive plates together / apart as magnetic fields grow / shrink!

So from the phenomenon of magnetic induction:

We get:

- DC electric motors

Which, if we drive their rotation, become DC generators

- AC electric motors

Which, if we drive their rotation, become AC generators

- Nikola Tesla's clever AC induction motor

Which requires nothing other than a piece of metal for the rotor

(And for which, yes, there is an analogous generator design)

- AC transformers

Which allow us to change AC voltages and currents

And to do this both easily and hyper efficiently

Credits / Acknowledgements

Some materials used in this class were developed under a National Science Foundation "Research Initiation Grant in Engineering Education" (RIGEE).

Other materials, including the "UVA Virtual Lab" science education website, were developed under even earlier NSF "Course, Curriculum and Laboratory Improvement" (CCLI) and "Nanoscience Undergraduate Education" (NUE) awards.

This set of notes was authored by John C. Bean who also created all figures not explicitly credited above.

Copyright John C. Bean (2015)

(However, permission is granted for use by individual instructors in non-profit academic institutions)


An Introduction to Sustainable Energy Systems: Motor, Generators and Transformers Last time I...

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