Lessons In Electric Circuits -- Volume VI Chapter 4 (PART II)

Sensing AC electric fields

PARTS AND MATERIALS

• Audio detector with headphones

CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 7: "Mixed-Frequency AC Signals"


LEARNING OBJECTIVES

• Effects of electrostatic (capacitive) coupling.
• Electrostatic shielding techniques.

SCHEMATIC DIAGRAM



ILLUSTRATION



INSTRUCTIONS
"Ground" one lead of the detector to a metal object in contact with the earth (dirt). Most any water pipe or faucet in a house will suffice. Take the other lead and hold it close to an electrical appliance or lamp fixture. Do not try to make contact with the appliance or with any conductors within! Any AC electric fields produced by the appliance will be heard in the headphones as a buzzing tone.
Try holding the wire in different positions next to a good, strong source of electric fields. Try using a piece of aluminum foil clipped to the wire's end to maximize capacitance (and therefore its ability to intercept an electric field). Try using different types of material to "shield" the wire from an electric field source. What material(s) work best? How does this compare with the AC magnetic field experiment?
As with magnetic fields, there is controversy whether or not stray electric fields like these pose any health hazard to the human body.

Automotive alternator

PARTS AND MATERIALS

• Automotive alternator (one required, but two recommended)

Old alternators may be obtained for low prices at automobile wrecking yards. Many yards have alternators already removed from the automobile, for your convenience. I do not recommend paying full price for a new alternator, as used units cost far less money and function just as well for the purposes of this experiment.
I highly recommend using a Delco-Remy brand of alternator. This is the type used on General Motors (GMC, Chevrolet, Cadillac, Buick, Oldsmobile) vehicles. One particular model has been produced by Delco-Remy since the early 1960's with little design change. It is a very common unit to locate in a wrecking yard, and very easy to work with.
If you obtain two alternators, you may use one as a generator and the other as a motor. The steps needed to prepare an alternator as a three-phase generator and as a three-phase motor are the same.


CROSS-REFERENCES
Lessons In Electric Circuits, Volume 1, chapter 14: "Magnetism and Electromagnetism"
Lessons In Electric Circuits, Volume 2, chapter 10: "Polyphase AC Circuits"


LEARNING OBJECTIVES

• Effects of electromagnetism
• Effects of electromagnetic induction
• Construction of real electromagnetic machines
• Construction and application of three-phase windings

SCHEMATIC DIAGRAM

An automotive alternator is a three-phase generator with a built-in rectifier circuit consisting of six diodes. As the sheave (most people call it a "pulley") is rotated by a belt connected to the automobile engine's crankshaft, a magnet is spun past a stationary set of three-phase windings (called the stator), usually connected in a Y configuration. The spinning magnet is actually an electromagnet, not a permanent magnet. Alternators are designed this way so that the magnetic field strength can be controlled, in order that output voltage may be controlled independently of rotor speed. This rotor magnet coil (called the field coil, or simply field) is energized by battery power, so that it takes a small amount of electrical power input to the alternator to get it to generate a lot of output power.
Electrical power is conducted to the rotating field coil through a pair of copper "slip rings" mounted concentrically on the shaft, contacted by stationary carbon "brushes." The brushes are held in firm contact with the slip rings by spring pressure.
Many modern alternators are equipped with built-in "regulator" circuits that automatically switch battery power on and off to the rotor coil to regulate output voltage. This circuit, if present in the alternator you choose for the experiment, is unnecessary and will only impede your study if left in place. Feel free to "surgically remove" it, just make sure you leave access to the brush terminals so that you can power the field coil with the alternator fully assembled.


ILLUSTRATION



INSTRUCTIONS
First, consult an automotive repair manual on the specific details of your alternator. The documentation provided in the book you're reading now is as general as possible to accommodate different brands of alternators. You may need more specific information, and a service manual is the best place to obtain it.
For this experiment, you'll be connecting wires to the coils inside the alternator and extending them outside the alternator case, for easy connection to test equipment and circuits. Unfortunately, the connection terminals provided by the manufacturer won't suit our needs here, so you need to make your own connections.
Disassemble the unit and locate terminals for connecting to the two carbon brushes. Solder a pair of wires to these terminals (at least 20 gauge in size) and extend these wires through vent holes in the alternator case, making sure they won't get snagged on the spinning rotor when the alternator is re-assembled and used.
Locate the three-phase line connections coming from the stator windings and connect wires to them as well, extending these wires outside the alternator case through some vent holes. Use the largest gauge wire that is convenient to work with for these wires, as they may be carrying substantial current. As with the field wires, route them in such a way that the rotor will turn freely with the alternator reassembled. The stator winding line terminals are easy to locate: the three of them connect to three terminals on the diode assembly, usually with "ring-lug" terminals soldered to the ends of the wires.

I recommend that you solder ring-lug terminals to your wires, and attach them underneath the terminal nuts along with the stator wire ends, so that each diode block terminal is securing two ring lugs.
Re-assemble the alternator, taking care to secure the carbon brushes in a retracted position so that the rotor doesn't damage them upon re-insertion. On Delco-Remy alternators, a small hole is provided on the back case half, and also at the front of the brush holder assembly, through which a paper clip or thin-gauge wire may be inserted to hold the brushes back against their spring pressure. Consult the service manual for more details on alternator assembly.
When the alternator has been assembled, try spinning the shaft and listen for any sounds indicative of colliding parts or snagged wires. If there is any such trouble, take it apart again and correct whatever is wrong.
If and when it spins freely as it should, connect the two "field" wires to a 6-volt battery. Connect an voltmeter to any two of the three-phase line connections:

With the multimeter set to the "DC volts" function, slowly rotate the alternator shaft. The voltmeter reading should alternate between positive and negative as the shaft it turned: a demonstration of very slow alternating voltage (AC voltage) being generated. If this test is successful, switch the multimeter to the "AC volts" setting and try again. Try spinning the shaft slow and fast, comparing voltmeter readings between the two conditions.
Short-circuit any two of the three-phase line wires and try spinning the alternator. What you should notice is that the alternator shaft becomes more difficult to spin. The heavy electrical load you've created via the short circuit causes a heavy mechanical load on the alternator, as mechanical energy is converted into electrical energy.
Now, try connecting 12 volts DC to the field wires. Repeat the DC voltmeter, AC voltmeter, and short-circuit tests described above. What difference(s) do you notice?
Find some sort of polarity-insensitive 6 or 12 volts loads, such as small incandescent lamps, and connect them to the three-phase line wires. Wrap a thin rope or heavy string around the groove of the sheave ("pulley") and spin the alternator rapidly, and the loads should function.
If you have a second alternator, modify it as you modified the first one, connecting five of your own wires to the field brushes and stator line terminals, respectively. You can then use it as a three-phase motor, powered by the first alternator.
Connect each of the three-phase line wires of the first alternator to the respective wires of the second alternator. Connect the field wires of one alternator to a 6 volt battery. This alternator will be the generator. Wrap rope around the sheave in preparation to spin it. Take the two field wires of the second alternator and short them together. This alternator will be the motor:

Spin the generator shaft while watching the motor shaft's rotation. Try reversing any two of the three-phase line connections between the two units and spin the generator again. What is different this time?
Connect the field wires of the motor unit to the a 6 volt battery (you may parallel-connect this field with the field of the generator unit, across the same battery terminals, if the battery is strong enough to deliver the several amps of current both coils will draw together). This will magnetize the rotor of the motor. Try spinning the generator again and note any differences in operation.
In the first motor setup, where the field wires were simple shorted together, the motor was functioning as an induction motor. In the second setup, where the motor's rotor was magnetized, it functioned as a synchronous motor.
If you are feeling particularly ambitious and are skilled in metal fabrication techniques, you may make your own high-power generator platform by connecting the modified alternator to a bicycle. I've built an arrangement that looks like this:

The rear wheel drives the generator sheave with a long v-belt. This belt also supports the rear of the bicycle, maintaining a constant tension when a rider is pedaling the bicycle. The generator hangs from a steel support structure (I used welded 2-inch square tubing, but a frame could be made out of lumber). Not only is this machine practical, but it is reliable enough to be used as an exercise machine, and it is inexpensive to make:

You can see a bank of three 12-volt "RV" light bulbs behind the bicycle unit (in the lower-left corner of the photograph), which I use for a load when riding the bicycle as an exercise machine. A set of three switches is mounted at the front of the bicycle, where I can turn loads on and off while riding.
By rectifying the three-phase AC power produced, it is possible to have the alternator power its own field coil with DC voltage, eliminating the need for a battery. However, some independent source of DC voltage will still be necessary for start-up, as the field coil must be energized before any AC power can be produced.

Induction motor

PARTS AND MATERIALS

• AC power source: 120VAC
• Capacitor, 3.3 µF (or 2.2 µF) 120VAC or 350VDC, non-polarized
• 15 to 25 watt incandescent lamp or 820Ω 25 watt resistors
• #32 AWG magnet wire
• wooden board approx. 5 in. square.
• AC line cord with plug
• 1.75 inch dia. cardboard tubing (toilet paper roll)
• lamp socket

• AC power source: 220VAC
• Capacitor, 1.5 µF 240VAC or 680VDC, non-polarized
• 25 to 40 watt incandescent lamp or 820Ω 25 watt resistors
• #32 AWG magnet wire
• wooden board approx. 15 cm. square.
• AC line cord with plug
• 4.5 to 5 cm. dia. cardboard tubing.
• lamp socket

CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 13: "AC motors", "Single Phase induction motors","Permanent split-capacitor motor".


LEARNING OBJECTIVES

• To build an AC permanent capacitor split-phase induction motor.
• To illustrate the simplicity of the AC induction motor.

SCHEMATIC DIAGRAM


ILLUSTRATION



INSTRUCTIONS
There are two parts lists to choose from depending upon the availability of 120VAC or 220VAC. Choose the one for your location. This set of instructions is for the 120VAC version.
This is a simplified version of a "permanent capacitor split-phase induction motor". By simplified, we mean the coils only requires a few hundred turns of wire instead of a few thousand. This is easier to wind. Though, the larger few thousand turns model is impressive. There are two stator coils as shown in the illustration above. Approximately 440 turns of #32 AWG (American wire gauge) enameled magnet wire are wound over a one inch length of a slightly longer section of 1.75 inch diameter toilet paper tube. To avoid counting the turns, close-wind four layers of magnet wire over a one inch width of the tube. See (b) above. Leave a few inches of magnet wire for the leads. Tape the beginning lead near the end of the tube so that the windings will cover and anchor the tape. Do not cut the final width of the cardboard tube until the winding is finished. Close wind a single layer. Tape or cement the first layer to prevent unwinding before proceeding to the second layer. Though it is possible to wind additional layers directly over existing layers, consider applying tape or paper between the layers as shown in schematic (b). After four layers are wound, glue the windings in place.
If close winding four layers of magnet wire it too difficult, scramble wind 440 turns of the magnet wire over the end of the cardboard tube. However, the close-wound style coil mounts more easily to the baseboard. Keep the windings within a one inch length.
Cut the finished winding from the end of the cardboard tube with a razor knife allowing the form to extend a little beyond the winding. Strip the enamel from an inch off the ends of the pair of lead wires with sandpaper. Splice the bare ends to heavier gauge insulated hook-up wire. Solder the splice. Insulate with tape or heat-shrink tubing. Secure the splice to the coil body. Then proceed with a second identical coil.
Refer to both the schematic diagram and the illustration for assembly. Note that the coils are mounted at right angles. They may be cemented to an insulating baseboard like wood. The 25 watt lamp is wired in series with one coil. This limits the current flowing through the coil. The lamp is a substitute for an 820 Ω power resistor. The capacitor is wired in series with the other coil. It also limits the current through the coil. In addition, it provides a leading phase shift of the current with respect to voltage. The schematic and illustration show no power switch or fuse. Add these if desired.
The rotor must be made of a ferromagnetic material like a steel can lid or bottle cap. The illustration below shows how to make the rotor. Select a circular rotor either smaller than the coil forms or a little larger. Use geometry to locate and mark the center. The center needs to be dimpled. Select an eighth inch diameter (a few mm) nail (a) and file or grind the point round as shown at (b). Place the rotor atop a piece of soft wood (c) and hammer the rounded point into the center (d). Practice on a piece of similar scrap metal. Take care not to pierce the rotor. A dished rotor (f) or a lid (g) balance better than the flat rotor (e). The pivot point (e) may be a straight pin driven through a movable wooden pedestal, or through the main board. The tip of a ball-point pen also works. If the rotor does not balance atop the pivot, remove metal from the heavy side.

Double check the wiring. Check that any bare wire has been insulated. The circuit may be powered-up without the rotor. The lamp should light. Both coils will warm within a few minutes. Excessive heating means that a lower wattage (higher resistance) lamp and a lower value capacitor should be substituted in series with the respective coils.
Place the rotor atop the pivot and move it between both coils. It should spin. The closer it is, the faster it should spin. Both coils should be warm, indicating power. Try different size and style rotors. Try a small rotor on the opposite side of the coils compared to the illustration.
For lack of #32 AWG magnet wire try 440 turns of slightly a larger diameter (lesser AWG number) wire. This will require more than 4 layers for the required turns. A night-light fixture might be less expensive than the full-size lamp socket illustrated. Though night-light bulbs are too low a wattage at 3 or 7 watts, 15 watt bulbs fit the socket.

Induction motor, large

PARTS AND MATERIALS

• AC power source: 120VAC
• Capacitor, 3.3 µF 120VAC or 350VDC, non-polarized
• #33 AWG magnet wire, 2 pounds
• wooden board approx. 6 to 12 in. square.
• AC line cord with plug
• 5.1 inch dia. plastic 3 liter soda bottle
• discarded ballpoint pen
• misc. small wood blocks

CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 13: "AC motors", "Single Phase induction motors","Permanent split-capacitor motor".


LEARNING OBJECTIVES

• To build a large exhibit size AC permanent split-capacitor induction motor.
• To illustrate the simplicity of the AC induction motor.

SCHEMATIC DIAGRAM


ILLUSTRATION



INSTRUCTIONS
This is a larger version of a "permanent capacitor split-phase induction motor". There are two different stator coils. The 1.0 inch wide 3200 turn L2 winding is shown in the illustration above (b), wound over a section of 5.1 inch diameter plastic 3-liter soda bottle. L1 is approximately 3800 turns of #33 AWG (American wire gauge) enameled magnet wire wound over a 1.25 width of a section of soda bottle, wider than shown at (b). Mark a 1.25 inch wide cylinder with 0.25 inch margins on each end. The wire will be wound on the 1.25 inch zone. The form is cut from the bottle on the outside edges of the margin. Cuts of 0.25 inch from the margin to winding zone are spaced at 1 inch intervals around the circumference of both ends so that the margin may be bent up at 90^o to hold the wire on the form. To avoid counting the 3800 turns, scramble wind a 1/8 inch thickness of magnet wire over the one inch width of the form. Else, count the turns. Scrape the enamel from 1-inch on the free end, and scrape only a small section from the lead to the spool. Do NOT cut the lead to the spool. Measure the resistance, and estimate how much more wire to wind to achieve 894 Ω. Apply enamel, nail polish, tape, or other insulation to the bare spot on the spool lead. Continue winding, and recheck the resistance. Once the approximate 894 Ω is achieved, leave a few inches of magnet wire for the lead. Cut the lead from the spool. Secure the windings to the form with lacing twine or other means.
The L1 winding of 3200 turns is approximately 744 Ω and is wound on a 1.0 inch wide form as shown at (b) in a manner similar to the previous L2 winding.
Strip the enamel off 1-inch of the ends of magnet wire leads if not already done. Splice the bare ends to heavier gauge insulated hook-up wire. Solder the splice. Insulate with tape or heat-shrink tubing. Secure the splice to the coil body. Then proceed with the second coil. The coils may be mounted in one corner of the wooden base. Alternatively, for more flexability in use, they may be mounted to movable pallets.
Refer to both the schematic diagram and the illustration for assembly. Note that the coils are mounted at right angles. L2, the smaller coil is wired to both sides of the 120 Vac line. The capacitor is wired in series with the wider coil L1. The capacitor provides a leading phase shift of the current with respect to voltage. The schematic and illustration show no power switch or fuse. Add these additions are recommended.
If this device is intended for use by non-technicians as an unsupervised exhibit, all exposed bare terminations like the capacitor must be made finger safe by covering with shields. The switch and fuse mentioned above are necessary. Finally, the enamel on the coils only provides a single layer of insulation. For safety, a second layer such as an insulating wrapping, Plexiglas box, or other means is called for. Replace all wooden components with Plexiglas for superior fire safety in an unsupervised exhibit.
The rotor must be made of a ferromagnetic material like a steel vegetable can, fruitcake can, etc. A too long vegetable can may be cut in half. The illustration for the previous small induction motor shows rotor dimpled bearing and pivot details. The rotor may be smaller than the coil forms as in the case of a cut down vegetable can. It can even be as small as the can lid rotor used with the previous small motor. It is also possible to drive a rotor larger than the coils, which is the case with the fruitcake can. Locate and mark the center of the rotor. The center needs to be dimpled. Select an eighth inch diameter (a few mm) nail (a) and file or grind the point round. Use this and a block of wood to dimple the rotor as shown in the previous small motor A fairly long can balances better than a flat rotor due to the lower center of gravity. The tip of a ball point pen works well as a pivot for larger rotors. Mount the pivot to a movable wooden pedestal.
Double check the wiring. Check that any bare wire has been insulated. The circuit may be powered-up without the rotor. Excessive heating in L2 indicates that more turns are required. Excessive heat in L1 calls for a reduction in the capacitance of C1. No heat at all indicates indicates an open circuit to the affected coil.
Place the rotor atop the pivot and move it between both energized coils. It should spin. The closer it is, the faster it should spin. Both coils should be warm, indicating power. Try different size and style rotors. Try a small rotor on the opposite side of the coils compared to the illustration.
Three models of this motor have been built using #33 AWG magnet wire because a large spool was on hand. AWG #32 magnet wire is probably easier to get. It should work. Although the current will be higher due to the lower resistance of the larger diameter #32 wire. If a 3.3µF capacitor is not available, use somenting close as long as it has an adequate voltage rating. A discarded AC motor run capacitor (bath tub shaped) was used by the author. Do no use a motor start capacitor (black cylinder). These are only usable for a few seconds of motor starting, and may explode if used longer than that.
Try this: It is possible to simultaneously spin more than one rotor. For example, in addition to the main rotor inside the right angle formed by the coils, place a second smaller rotor (can or bottle lid) near the pair of coils outside the right angle at the vertex.
It is possible to reverse the direction of rotation by reversing one of the coils. If the coils are mounted to movable pallets, rotate one coil 180^o. Another method, especially usefull with fixed coils, is to wire one of the coils to a DPDT polarity reversing switch. For example, disconnect L2 and wire it to the wipers (center contacts) of the DPDT switch. The top contacts go to the 120 Vac. The top contacts also go to the the bottom contacts in an X-crossover pattern.

Phase shift

PARTS AND MATERIALS

• Low-voltage AC power supply
• Two capacitors, 0.1 µF each, non-polarized (Radio Shack catalog # 272-135)
• Two 27 kΩ resistors

I recommend ceramic disk capacitors, because they are insensitive to polarity (non-polarized), inexpensive, and durable. Avoid capacitors with any kind of polarity marking, as these will be destroyed when powered by AC!


CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 1: "Basic AC Theory"
Lessons In Electric Circuits, Volume 2, chapter 4: "Reactance and Impedance -- Capacitive"


LEARNING OBJECTIVES

• How out-of-phase AC voltages do not add algebraically, but according to vector (phasor) arithmetic

SCHEMATIC DIAGRAM



ILLUSTRATION



INSTRUCTIONS
Build the circuit and measure voltage drops across each component with an AC voltmeter. Measure total (supply) voltage with the same voltmeter. You will discover that the voltage drops do not add up to equal the total voltage. This is due to phase shifts in the circuit: voltage dropped across the capacitors is out-of-phase with voltage dropped across the resistors, and thus the voltage drop figures do not add up as one might expect. Taking phase angle into consideration, they do add up to equal the total, but a voltmeter doesn't provide phase angle measurements, only amplitude.
Try measuring voltage dropped across both resistors at once. This voltage drop will equal the sum of the voltage drops measured across each resistor separately. This tells you that both the resistors' voltage drop waveforms are in-phase with each other, since they add simply and directly.
Measure voltage dropped across both capacitors at once. This voltage drop, like the drop measured across the two resistors, will equal the sum of the voltage drops measured across each capacitor separately. Likewise, this tells you that both the capacitors' voltage drop waveforms are in-phase with each other.
Given that the power supply frequency is 60 Hz (household power frequency in the United States), calculate impedances for all components and determine all voltage drops using Ohm's Law (E=IZ ; I=E/Z ; Z=E/I). The polar magnitudes of the results should closely agree with your voltmeter readings.


COMPUTER SIMULATION
Schematic with SPICE node numbers:

The two large-value resistors R_bogus1 and R_bogus1 are connected across the capacitors to provide a DC path to ground in order that SPICE will work. This is a "fix" for one of SPICE's quirks, to avoid it from seeing the capacitors as open circuits in its analysis. These two resistors are entirely unnecessary in the real circuit.


Netlist (make a text file containing the following text, verbatim):

phase shift
v1 1 0 ac 12 sin
r1 1 2 27k
r2 2 3 27k
c1 3 4 0.1u
c2 4 0 0.1u
rbogus1 3 4 1e9
rbogus2 4 0 1e9
.ac lin 1 60 60
* Voltage across each component:
.print ac v(1,2) v(2,3) v(3,4) v(4,0) 
* Voltage across pairs of similar components
.print ac v(1,3) v(3,0)  
.end

Sound cancellation

PARTS AND MATERIALS

• Low-voltage AC power supply
• Two audio speakers
• Two 220 Ω resistors

Large, low-frequency ("woofer") speakers are most appropriate for this experiment. For optimum results, the speakers should be identical and mounted in enclosures.


CROSS-REFERENCES
Lessons In Electric Circuits, Volume 2, chapter 1: "Basic AC Theory"


LEARNING OBJECTIVES

• How phase shift can cause waves to either reinforce or interfere with each other
• The importance of speaker "phasing" in stereo systems

SCHEMATIC DIAGRAM



ILLUSTRATION



INSTRUCTIONS
Connect each speaker to the low-voltage AC power supply through a 220 Ω resistor. The resistor limits the amount of power delivered to each speaker by the power supply. A low-pitched, 60-Hertz tone should be heard from the speakers. If the tone sounds too loud, use higher-value resistors.
With both speakers connected and producing sound, position them so that they are only a foot or two away, facing toward each other. Listen to the volume of the 60-Hertz tone. Now, reverse the connections (the "polarity") of just one of the speakers and note the volume again. Try switching the polarity of one speaker back and forth from original to reversed, comparing volume levels each way. What do you notice?
By reversing wire connections to one speaker, you are reversing the phase of that speaker's sound wave in reference to the other speaker. In one mode, the sound waves will reinforce one another for a strong volume. In the other mode, the sound waves will destructively interfere, resulting in diminished volume. This phenomenon is common to all wave events: sound waves, electrical signals (voltage "waves"), waves in water, and even light waves!
Multiple speakers in a stereo sound system must be properly "phased" so that their respective sound waves don't cancel each other, leaving less total sound level for the listener(s) to hear. So, even in an AC system where there really is no such thing as constant "polarity," the sequence of wire connections may make a significant difference in system performance.
This principle of volume reduction by destructive interference may be exploited for noise cancellation. Such systems sample the waveform of the ambient noise, then produce an identical sound signal 180^o out of phase with the noise. When the two sound signals meet, they cancel each other out, ideally eliminating all the noise. As one might guess, this is much easier accomplished with noise sources of steady frequency and amplitude. Cancellation of random, broad-spectrum noise is very difficult, as some sort of signal-processing circuit must sample the noise and generate precisely the right amount of cancellation sound at just the right time in order to be effective. CONTINUE....