A MINUTE BRIEF ON ELECTROSTATIC GENERATOR THEORY

A MINUTE BRIEF ON ELECTROSTATIC GENERATOR THEORY OF OPERATION:

Electrostatic generators come in two categories: friction (triboelectric) and influence (induction) machines. In both cases the generators convert mechanical energy into electrical energy by separating electrical charges and moving them against the electric forces to a collection point where the charges are stored. The quantity of charges separated and the amount of force it takes to keep them apart is equivalent to the stored electrical energy (plus any losses).

TRIBOELECTRIFICATION:

Sliding across a car seat with nylon pants on generates electricity by what is commonly called friction. More accurately, the seat and pants are charged by triboelectrification. When two different materials come into intimate contact, the surface molecules of the materials share electrons, that is, the motion of the electrons swarming around in the molecules cross paths. Some materials have a stronger “hold” on the electrons than others. When the two materials are pulled apart, some of the electrons are trapped on the material that has the stronger hold, giving both materials an equal charge in opposite polarities. This charge is only on the surface of the materials and usually, since the triboelectric materials are also insulators, these charges are not free to move laterally on the surface.

  

 FIGURE 1

The triboelectric series is a grouping of materials by their ability to hold excess electrons. A partial listing from negative to positive (the most negative materials have the strongest hold on electrons) is: silicone rubber, teflon, PVC, polyethylene, synthetic rubber, brass, copper, paraffin, steel, aluminum, wool, nylon, and glass. There are many circumstantial factors that affect the triboelectric charging process: surface roughness, humidity, and contamination just to name a few. Due to the influence of these external factors, there may be instances when the order listed may be violated (usually only where the two materials are close together in the series though).

CHARGE BY INDUCTION:

An electrophorus is an induction generator and will be used for illustrative purposes for induction because of its simplicity. An electrophorus is a special capacitor. What makes it special is its dielectric material, and its removable capacitor plate. The dielectric material is an “electret” or ferroelectric material with hysteresis. This material has properties similar to its magnetic counterpart; magnets and ferromagnetic materials. The electret will become “electrified” (similar to magnetized) when placed in a strong electric field. This is due to the electric dipoles in the material aligning themselves with the applied electric field. Note that in this case, the effect of the electrification is throughout the material. The dipoles are "locked" into position by subatomic forces acting within the material. The electric field will remain about the electret even after the polarizing field is removed. If an electrically neutral conducting plate is placed on either side of the electret and a wire is then connected between them, charges will flow from one plate to the other until the field created by the displaced charges reaches an equilibrium with the field across the electret. Once the charges are in balance, the wire connecting the two plates together can be removed and the electrophorus looks no different than any other capacitor charged up to a potential equal to the amount of charge displaced by the electret’s intrinsic field.

Figure 2

The amount of charge stored (or displaced) in a capacitor is measured in Coulombs and has the following mathematical relationship with the capacitance and voltage:

(equation A) Q = C V

where: Q = charge in Coulombs, C = capacitance in Farads, and V = potential in Volts

Also, the capacitor has energy stored in it when charged. The energy relationship can be expressed mathematically:

(equation B) J = (V² C)/2

where: J = energy in Joules

Now if the removable capacitor plate of the electrophorus is removed, the value of the capacitor decreases inversely with the separation of the plates (approximately). Also since the wire connecting the two plates was removed (or the charging voltage source for a capacitor was removed), the charges on the plates are trapped, and remain constant. If equation A is rearranged, it can be seen that the voltage on the capacitor goes up as the capacitance is decreased:

(equation C) V’ = Q/(C - C)

where: C = change in capacitance and V’ = new voltage across capacitor .

If the new voltage and capacitance is substituted into equation B, it can be seen that the energy stored in the resulting capacitor also went up:

(equation D) J’ = (Q/(C-C))² (C - C)/2 = Q² /{2(C - C)}

where: J’ = new energy stored in the capacitor

Of course the natural law of conservation of energy says that you can't get something for nothing (don't know why it is, it just is), so it must have taken work to reduce the capacitance. This work is done by separating the capacitor plates against the electrostatic force pulling them together. Work can be defined in terms of applying a force through a distance. This relationship can be expressed mathematically:

(equation E) J = 1.356 F d

where: J = energy in Joules, F = force in pounds, and d = distance in feet.

Essentially, induction generators go about the process of generating static electricity by repeatedly charging up a capacitor, separating the plates, and “collecting” the charge off of the plates using a method described below in the Van de Graaff generator, and discovered by Michael Faraday in the early nineteenth century. The induction generators with metallic plates have brushes that come in contact with them when they are in the “inducing” electric field. This allows the charges to flow between the capacitor plates to reach equilibrium. As the plates are moved away from each other, the brush contacts open up, trapping the charges on the plates. These plates are moved against the electrostatic force to the collectors where another brush makes contact with the charged plate and removes the charges. Once the charges are removed, the plates no longer have a force between them.

VAN DE GRAAFF GENERATORS:

The Van de Graaff generator was invented by Robert J. Van de Graaff in the 1930s (US patent # 1,991,236). The VG1 is a Van de Graaff generator that separates the charges by triboelectrification and then uses the charged drive pulley to induce charges on the transport belt. The drive pulley on the motor is made of teflon. It drives the rubber transport belt which is in intimate contact with the pulley. According to the triboelectric series, teflon is more negative than rubber. This means that when the up going side of the rubber belt separates from the teflon drive pulley, a net negative charge exists on the teflon pulley and a positive one on the rubber belt in the localized area of separation. Note that since both materials are insulators, the charges are not free to move on the surface of them. After the drive pulley has turned a few revolutions, it has a uniform negative charge all around its circumference. The rubber belt traveling up to the collector sphere has a positive charge on the inside of surface of it (side contacting the drive pulley). It travels up to the collector where the charges are removed as explained later. When the belt comes down it is neutral (has the charges on the inside removed).

A conductive brush with many sharp points is placed on the opposite side of the rubber belt where the negatively charged pulley and belt are in contact. The sharp points allow the surrounding air to ionize easily in the presence of an electric field. The brush has a wire connected to it which connects to the inside of a metal hemispherical base cover. Both this cover and the hollow collector sphere at the top of the Van de Graaff generator behave in such a way that any charge that is placed in it or taken away from it from the inside, will have an immediate affect on the apparent total outside charge. That is, any charge that is brought inside the sphere (or hemisphere) will immediately be seen as adding to or subtracting from the total charge on the outside of the sphere even though it still resides inside. Another thing, if the charge that was brought inside is allowed to come into contact with the inside of the conductive hollow sphere, the charge will immediately be transferred to the outside, and no net charge will then reside inside the sphere. This will be the case no matter how great the charge outside the sphere or how small the brought in charge was.

Now at the brush points where the rubber belt and teflon are in contact, the teflon pulley has a negative charge and the rubber belt is neutral (the inside of the belt doesn't obtain the positive charge until the belt and pulley separate further around the pulley, and the down going side is neutralized prior to leaving the collector globe). The negative charge on the pulley will cause electrons to be pushed off of the brush points on the opposite side of the belt . When the field is strong enough, electrons from the air molecules in the vicinity of the brush jump off to the brush points, creating positively ionized molecules. These molecules are free to move and are attracted to the negatively charged pulley on the other side of the belt. They move until they come into intimate contact with the top side of the belt (since they cannot travel through it). They become attached to the belt in a way similar to the attachment of the triboelectric materials to each other, although these attachment forces are not fully understood. Once attached, it requires a force to separate the ions from the belt even after the field that caused them to become attached is removed. It is this force that allows the ions to remain attached to the belt while it is being moved up to the collector, and it is this force that when exceeded will cause the ions to detach and limit the amount of charge the generator can output. The action is such that a net positive charge per unit area will reside on the top side of the up going belt. The charges are then moved up toward the collector sphere against the electrostatic forces. They are brought inside the collector sphere, and then are allowed to come in contact with the inside of the sphere with the aid of a collector brush, at which time they are transferred to the outside of the collector. The rubber belt is neutralized at this point and then continues back out of the sphere carrying no charge out with it. This process continues with the voltage building up on the sphere by the addition of charge until breakdown occurs. In the case of the VG1, that is around 70,000 volts. 

Figure 3

DISK INDUCTION MACHINES:

The Wimshurst machine, named after the British inventor James Wimshurst, is an induction generator. It has two collectors, one positive and one negative, and two counter rotating disks with metal sectors attached to them. The sectors act as movable plates of a capacitor. In operation, an electric field exists between the two collectors. Two of the sectors which are displaced off the collector center line by some angular distance, come into contact with a diagonal shorting bar (also commonly called neutralizing bars). This bar connects the two plates together electrically and causes charges to move from one plate to the other while charging up the capacitance between each plate and its respective collector. The same thing is happening to the sectors on the counter rotating disk on the opposite side. As the disks continue to rotate, the shorting bar brushes break contact and trap the displaced charges on the sectors. These sectors are then rotated away from the charged collector that induced the charge on them and are forced to move (mechanical energy is converted to electrical energy) toward the other collector which is charged in the same polarity as the moving sector. When the charged sector reaches the collector, a collector brush contacts it and transfers the charge in a similar manner to the description given in the Van de Graaff generator.

In a “plateless” generator like the WM1, ions take the place of metallic plates. Another difference in the WM1 is that it only has one rotating disk, and it has separate inductor pole pieces instead of using the collectors as the inductors. Other than this, the principal of operation is the same as the Wimshurst machine. Ions from air molecules (oxygen and nitrogen) are formed at the neutralizing combs due to the high field strength induced by the field pole pieces on the sharp edges of the combs. These ions are attracted toward the disk by the pole pieces and get “stuck” on the disk similar to the ions attaching to the Van de Graaff rubber belt. The disk rotates around to the collector combs where the charged ions are stripped off in a manner similar to the metal plate and Van de Graaff generators, adding the charge to the collectors. As the charge builds up on the collectors to a level higher than that on the field pole pieces, the electric field between the probe rods and the collector pole pieces causes ions to be formed at the probe rod tips. The ions are mobile and can transfer more charge to the pole pieces, increasing the field strength. This process continues until an equilibrium is reached from leakage, corona, or breakdown.

Typically higher voltages can be generated with a “plateless” generator than one with plates. This is because the plates have edges and will cause the field on the plate to be uneven. As the voltage on the plate increases the higher field strength on the edges will ionize the air around it and will carry off some of the charge on the plate. This energy is then lost and winds up as chemical energy (ozone, etc.) or is lost to heat.    
   Small Van de Graaff Generator  in operation (video) 
Induction Machine Operation (Video of Induction Machine)