Electrostatic motors are now common in MEMS applications, but researchers at the University of Wisconsin and spinoff C-Motive Technologies have brought macroscale electrostatic motors back. [via MSN/WSJ]

While the first real application of an electric motor was Ben Franklin’s electrostatically-driven turkey rotisserie, electromagnetic type motors largely supplanted the technology due to the types of materials available to engineers of the time. Newer dielectric fluids and power electronics now allow electrostatic motors to be better at some applications than their electromagnetic peers.

The main advantage of electrostatic motors is their reduced critical materials use. In particular, electrostatic motors don’t require copper windings or any rare earth magnets which are getting more expensive as demand grows for electrically-powered machines. C-Motive is initially targeting direct drive industrial applications, and the “voltage driven nature of an electrostatic machine” means they require less cooling than an electromagnetic motor. They also don’t use much if any power when stalled.

Would you like a refresher on how to make static electricity or a deeper dive on how these motors work?

The average Hackaday user could probably piece together a rough model of a simple DC motor with what they’ve got kicking around the parts bin. We imagine some of you could even get a brushless one up and running without too much trouble. But what about an electrostatic corona motor? If your knowledge of turning high voltage into rotational energy is a bit rusty, let [Jay Bowles] show you the ropes in his latest Plasma Channel video.

Like many of his projects, this corona motor relies on a few sheets of acrylic, a handful of fasteners, and a healthy dose of physics. The actual construction and wiring of the motor is, if you’ll excuse the pun, shockingly simple. Of course part of that is due to the fact that the motor is only half the equation, you still need a high voltage source to get it running.

In this case, [Jay] is revisiting his earlier experiments with atmospheric electricity to provide the necessary jolt. One side of the motor is connected to a metallic mesh electrode that’s carried 100 m into the air by a DJI Mini2 drone, while the other side is hooked up to several large nails driven into the ground.

The potential between the two gets the motor spinning, and makes for an impressive demonstration, but it’s not exactly the most practical way to experiment with your new corona motor. If you’d rather get it running on the workbench, he also shows that a more traditional high voltage source like a Van de Graaff generator will do the job nicely. As an added bonus, it can even power the device wirelessly from a few feet away.

So what can you do with a corona motor? While [Jay] is quick to explain that these sort of devices aren’t exactly known for their torque, he does show that his motor is able to lift a 45 gram weight suspended from a string. That’s frankly more power than we expected, and makes us wonder if there is some quasi-practical application for this contraption. If there is we suspect it’ll be featured in a future Plasma Channel video, so stay tuned.

Finally, someone decided to answer the question that nobody was asking: what if [Benjamin Franklin] had had a drone rather than a kite?

Granted, [Jay Bowles] didn’t fly his electricity-harvesting drone during a thunderstorm, but he did manage to reach some of the same conclusions that [Dr. Franklin] did about the nature of atmospheric electricity. His experimental setup was pretty simple: a DJI Mini2 drone with enough payload capacity to haul a length of fine-gauge magnet wire up to around 100 meters above ground level. A collecting electrode made of metal mesh was connected to the wire and suspended below the drone. Some big nails were driven into the soil to complete the circuit between the drone and the ground.

[Jay] went old-school for a detector, using a homemade electroscope to show what kind of static charge was accumulating on the electrode. Version 1 didn’t have enough oomph to do much but deliver a small static shock, but a larger electrode was able to deflect the leaves of an electroscope, power a beer can version of a Franklin bell, and also run a homemade corona motor. [ElectroBOOM] makes a guest appearance in the video below to explain the physics of the setup; curiously, he actually managed to get away without any injuries this time.

Most of the electric motors we see these days are of the electromagnetic variety, and for good reason: they’re powerful. But there’s a type of motor that was invented before the electromagnetic one, and of which there are many variations. Those are motors that run on high voltage, and the attraction and repulsion of charge, commonly known as electrostatic motors.

Ben Franklin — whose electric experiments are most frequently associated with flying a kite in a thunderstorm — built and tested one such high-voltage motor. It wasn’t very powerful, but was good enough for him to envision using it as a rotisserie hack. Food is a powerful motivator.

What follows is a walk through the development of various types of these motors, from the earliest ion propelled ones to the induction motors which most have never heard of before, even an HV hacker such as yours truly.

Franklin’s Electric Wheel

In 1748, Benjamin Franklin invented what he called the electric wheel. This is usually cited as the first electrostatic motor and sometimes as the first electric motor, depending on your definition. The rotor consisted of a wooden hub with 30 glass bars attached to the circumference. These bars were strips cut from window glass. Brass thimbles were attached to the end of each bar. To turn easily, the rotor had a central shaft with an iron point on the bottom and a strong wire at the top, the wire going through a hole to keep it all upright.

The power source consisted of two oppositely charged Leyden jars, the original name given to a cylindrical capacitor made using a jar of some sort. The jar, and therefore the dielectric, was glass in Franklin’s day. A wire extended up from the inner capacitor plate and this wire was placed near where the thimbles would pass by. As a thimble passed, a spark would occur between the thimble and the wire, charging the thimble with the same polarity as the inner plate. Being like-charged, the thimble would be repelled from that wire, turning the wheel. As the thimble reached the wire of the opposite Leyden jar, being of opposite charge, it would be attracted to the jar’s wire. As it passed, a spark would occur between the two, charging the thimble to the same polarity as that wire and causing it to be repelled, turning the wheel some more.

The initial direction of the wheel was set by giving it a push by hand. In a letter (page 29), Franklin wrote that it rotated at 12 to 15 RPM, bearing the weight of one hundred Spanish dollars. He also wrote that:

if a large fowl were spotted on the upright shaft, it would be carried round before a fire with a motion fit for roasting.

Electric Whirl

This motor is a bit older than Franklin’s and dates back to 1745. It was made by Andrew Gordon, a Scottish Benedictine monk. It goes by many names but the original name was the electric whirl. The basic idea is that an electric field at a sharp point will be stronger than at a smooth point. This strong electric field ionizes the air near the point, and depending on the polarity, may even emit electrons to the air. This ionized air is called a corona. In either case, the ionized air around the point has the same polarity as the material making up the sharp point. Since like-charges repel each other, the ionized air repels the sharp point. You’ve of course heard of Newton’s third law which basically states that for every action, there’s an equal and opposite reaction. So the air is repelled in one direction, and the sharp point reacts by being repelled in the other direction. Arrange the point on an arm which is free to move around an axis, and the arm will rotate around that axis. The moving ionized air is often referred to as ion wind and for that reason this is also often called an ion wind rotor, or ion wind spinner or pin wheel.

Usually there are two points, each on their own arms, pointing in opposite directions to make for a more stable system. There can of course be more. The one pictured here is sitting on top of a Van de Graaff generator’s dome, which is at high voltage with respect to ground. The electric whirl is in electrical contact with the dome through the supporting shaft. The other end of the electric field is the surrounding air, the bottom of the Van de Graaff generator and the room.

Poggendorff/Corona Motor

The 1860s was a time when many were experimenting with influence machines, machines that used electrostatic induction, called influence at the time, to produce high voltages. One such experimenter was Wilhelm Holtz. In 1867, in a stroke of brilliance,  he connected the output of a similar machine to one of his and found it would run as a motor. And so in 1869, J.C. Poggendorff had a simplified version of Holtz’s machine built that had only what it needed to run as a motor.

Poggendorff’s motor consisted of a glass disk with combs facing it on either side. The combs had sharp points on them arranged so that the points would be near the disk but not touching. The combs could be turned so that they were strictly radial with respect to the disk. In this case the disk was not self-starting and, once given a spin by hand, would continue to rotate in that direction. However, the combs could also be turned to an off-radial angle. In this case the disk would self-start and be unidirectional.

One difference between Franklin’s wheel and Poggendorff’s motor is that Franklin’s places charge onto the thimbles using a spark, whereas Poggendorff’s sprays charge onto the disk through ionized air, or a corona. This corona is the same we mentioned when talking about the electric whirl, and is formed in the same way — the sharp points result in a strong electric field which ionizes the air.

Similar to Franklin’s the area of the disk, approaching a comb is charged with the opposite polarity with respect to the comb, and so is attracted to the comb. And the area moving away from the comb is charged with the same polarity, and so is repelled.

Poggendorff’s motor also has some advantages over Franklin’s electric wheel. The combs are always spraying onto the disk, continuously producing rotation, whereas with Franklin’s, this happens only when a thimble passes a Leyden jar. Also, there is always the same amount of disk surface area facing the combs, resulting in a constant torque being produced.

Due to the corona being the mechanism for transferring the charge, these types of motors are generally called corona motors. And in many of them the disk is replaced with a cylinder. The combs then become either just the sharp ends of thin wires or long sharp blades as in the one shown here that was run off of atmospheric electricity.

Capacitor Motors

This next type of motor was invented in 1889 by Karl Zipernowsky, but an easier version to illustrate is a 1904 one by N.G. van Huffel^[1]. In it the rotor and stator are both made up of two partial cylinders, or curved plates. The rotor plates have a slightly smaller diameter to fit inside the stator plates.

The stator plates are charged oppositely and the rotor plates receive a charge from their adjacent stator plates via a sliding contact just as they are rotating away from those plates. That way they repel away from the adjacent stator plate and reach a position where they’re then attracted to the next stator plate.

The big difference between all these motors is in the way the charge is moved around, via sparks as in Franklin’s, via corona, or via sliding contacts. However, since sliding contacts are used here, these motors can run at a lower voltage than the others. These can also run on AC.

Induction Motors

The induction motor is the one that operates on perhaps the most interesting principle. The rotor is not electrically connected to any power supply and is made of a dielectric material. The surrounding stator plates are supplied with AC, and so there is a rotating electric field surrounding the rotor. That electric field induces polarization of the molecules on the surface of the rotor. From the stator’s point of view, this appears as a net charge.

The key is that as the stator plates’ fields change polarity, it takes time for the polarization of the nearby rotor to change in response. That creates a lag between the moving field and the polarized molecules. That lag is what causes the rotor to follow the field.

The illustration shown here is of one made by Rocardo Arno in 1892-1893^[1].

Where Have All The Motors Gone?

As we said, electromagnetic motors are far more powerful. But electrostatic motors do find use in MEMS motors of microscopic dimensions. And of course, they haven’t really gone anywhere if you consider electrostatic tinkerers who love resurrecting these old mechanical marvels. I’ve dabbled in electric whirls and corona motors and have thirty thimbles on order from China for something which the attentive reader can guess at. What electrostatic motors have you toyed with in your home laboratory?

Resources:

[1] Electrostatic Motors: Their History, Types and Principles of Operation by Oleg D. Jefimenko

In high voltage applications involving tens of thousands of volts, too often people think about the high voltage needed but don’t consider the current. This is especially so when part of the circuit that the charge travels through is an air gap, and the charge is in the form of ions. That’s a far cry from electrons flowing in copper wire or moving through resistors.

Consider the lifter. The lifter is a fun, lightweight flying machine. It consists of a thin wire and an aluminum foil skirt separated by an air gap. Apply 25kV volts across that air gap and it lifts into the air.

So you’d think that the small handheld Van de Graaff generator pictured below, that’s capable of 80kV, could power the lifter. However, like many high voltage applications, the lifter works by ionizing air, in this case ionizing air surrounding the thin wire resulting in a bluish corona. That sets off a chain of events that produces a downward flowing jet of air, commonly called ion wind, lifting the lifter upward.

But that ionization of air requires current, electrons flowing, in the wires coming from a high voltage power supply. In fact, that ion-filled air gap is the equivalent of a high resistance wire in the circuit, along with some capacitance; it’s a part of the circuit.

A Van de Graaff generator, even a DIY 84kV one, is a low current power source and cannot supply a high enough current to ionize enough air fast enough to produce the necessary strong jet. However, the power supply powering the lifter above converts the energy from a wall socket using a flyback transformer and a Cockcroft-Walton voltage multiplier. That way it produces sufficient voltage and supplies more than enough current.

Another example is the corona motor, a type of electrostatic motor, that also works by producing a high voltage across an air gap, multiple air gaps in fact. The gaps are between sharp metal blades and a neutral plastic cylinder. The blade sprays ions across the gap onto the cylinder.

However, the gap width is very short, requiring a lower voltage. And more importantly, the cylinder doesn’t have to be ionized much for the cylinder to start turning meaning that not as many ions are needed. i.e. the required current is lower than with the lifter. In this case perhaps the lowest current high voltage power source I’ve ever worked with, rubbing a PVC pipe with a cotton cloth, is sufficient. That utilizes the triboelectric effect.

And then there are applications where all that’s needed is to accumulate charge in a capacitor until there’s enough, taking as much time as necessary. An obvious example is to simply produce a big spark.

An example of that is a TEA laser. A TEA laser works by accumulating charge across a small spark gap and in two flat plate capacitors. When the voltage has built up sufficiently to breakdown the air in the spark gap, it fires, causing a subsequent sparking across the lasing channel, resulting in a laser beam. In the photos below you can see the laser being powered by a low current Wimshurst machine and by the same powerful Cockcroft-Walton power supply mentioned above for flying the lifter. Both produce an identical voltage, accumulate the same amount of charge and fire an identical laser beam. However, the Wimshurst machine requires around 12 seconds of hand-cranking to do so, resulting in the laser firing only every 12 seconds. The Cockcroft-Walton power supply fires the laser around every 1 second.

How do you choose a high voltage source with sufficient current to match your application? This is largely done by experience. The lifter needs to move a large mass of air, and to do so a large quantity of ions are required, and hence a high current to create those abundant ions. On the other hand, a corona motor works using the Coulomb force. Blades of one polarity repel areas of cylinders that are charged with the same polarity. With just a fan as the load on the cylinder, not much repulsion force is required. Naturally for a bigger load, more force would be required and hence a faster charging rate and so more current. Similarly with the TEA laser, more frequent laser beams require higher the current.

I’ve already taken you through a range of high voltage sources available to you. With the needed high voltage already available from any of those sources, we now need to look at the current available from them.

The flyback transformer and Cockcroft-Walton voltage multiplier power supply gets its power from the wall socket. Taking into account losses in the various resistors, transistors, capacitors, diodes and the flyback transformer itself, there’s still a relatively large amount of current available, even if it is in the single- or double-digit milliamps. For high voltage that’s considered quite a bit (remember that power is the product of voltage and current).

On the other end of the scale, the triboelectric effect works by a transfer of electrons when making contact between two specific materials, and the retention of those electrons when contact is broken; basically, it works by rubbing the materials together. In that case, very little charge is transferred compared to the current coming from a wall socket.

The Van de Graaff generator actually starts with the triboelectric effect. It’s the making and breaking of contact between the rollers and the belt that is the rubbing together of two materials. However, unlike rubbing a cotton cloth against a PVC pipe by hand, in a Van de Graaff generator the rollers can be rotating at hundreds of RPM, generating charge more rapidly. But the amount is somewhere in the low microamps for a tabletop Van de Graff, small compared to wall socket current (that’s assuming you’re taking charge directly from the Van de Graaff’s dome and not waiting for sparks.)

And a Wimshurst machine generates its charge by induction when sectors on the opposing disks pass a neutralizer bar. Surprisingly, it’s possible to dimly light a small 20mA LED, where the LED is placed in series with the neutralizer bar. However, by the time the charge is inefficiently removed at the collectors, it’s reduced substantially. The current from the output of a Wimshurst machine is usually in the single digit microamps (again, we’re not waiting for sparks.)

Those are some of the examples I can recall where people, myself included, have forgotten that just because high voltage is involved, that doesn’t mean that basic electronics no longer applies. I’m curious what examples you’ve encountered, either where you or others have forgotten about the current or maybe even some other electrical property. Let us know in the comments below.

Having hacked away with high voltage for many years I’ve ended up using a large number of very different high voltage sources. I say sources and not power supplies because I’ve even powered a corona motor by rubbing a PVC pipe with a cotton cloth, making use of the triboelectric effect. But while the voltage from that is high, the current is too low for producing the necessary ion wind to make a lifter fly up off a tabletop. For that I use a flyback transformer and Cockcroft-Walton voltage multiplier power supply that’s plugged into a wall socket.

So yes, I have an unorthodox skillset when it comes to sourcing high voltage. It’s time I sat down and listed most of the power sources I’ve used over the years, including a bit about how they work, what their output is like and what they can be used for, as well as some idea of cost or ease of making. The order is from least powerful to most powerful so keep reading for the ones that really bite.

Triboelectric Effect

You’ve no doubt encountered this effect. It’s how your body is charged when you rub your feet on carpet and then get a shock from touching a door knob. When you rub two specific materials together there’s a transfer of electrons from one to the other. Not just any two materials will work. To find out which materials are good to use, have a look at a triboelectric series table.

Materials that are on the positive end of the table will become positively charged when rubbed against materials on the negative end of the table. Those materials will become negatively charged. The further apart they are in the table, the stronger the charging.

An example of where I’ve used this is to power the corona motor shown here. I vigorously rub a PVC pipe with a cotton cloth, and as the pipe emerges from the cloth, a sharp wire a few millimeters away takes the charge from the pipe. You can see this corona motor being powered by other power sources in the video here.

This would be considered an electrostatic power source because charge is accumulated on surfaces. Being insulating materials, that charge can’t move around.

The amount of charge transferred between the materials per unit of time is small meaning that the current available is small. You won’t be powering any heavy loads with this, but the corona motor powered this way turns at around one revolution every 5 seconds and can be stopped with the light touch of a finger. You already have a feel for the power from getting mild shocks from touching doorknobs. This is of course an easy power source to make.

Wimshurst Machine

A Wimshurst machine is also a high voltage/low current power source. It consists of two counter-rotating disks, usually rotated with a hand crank. The disks have metal sectors on them that are spaced apart. The charging occurs where the neutralizer brushes contact the sectors as they pass by. That charge is then removed at collectors on the left and right edges of the disks and is usually accumulated in Leyden jars (capacitors) and across a spark gap where a spark occurs when the voltage is sufficient to break down the air in the gap.

But if you’re making use of the Wimshurst machine then you’re usually not producing sparks. In the photo you see wires going from the Wimshurst machine to a ball cyclotron, making the balls in it rotate around inside the bowl.

The voltage with this one is limited by your losses in the Leyden jars and spark gap and your load. That’s why efforts are made to have everything be well rounded. The spark gap also limits the voltage and with this one I’ve produced sparks around 3 inches/7 centimeters long.

The current is indirectly determined by the disks’ diameter. That’s because larger selectors will produce more charge than smaller sectors. Also, the faster the disk turns, the more sectors will pass by the collectors per second and so more charge will be available.

A Wimshurst machine can’t provide enough current to make a lifter fly. However, it does provide enough current to power a smoke precipitator.

They aren’t too hard to make. I find that the trickiest parts are to find or make the pulleys needed for transferring the hand crank rotation to the disks. The disks can be acrylic which you can cut with a scroll saw or laser cutter, and often small Wimshurst machines are made using CDs.

Van de Graaff Generator

From the outside, a Van de Graaff generator looks like a big ball, or dome, on top of a vertical tube and more stuff at the base of the tube. While that dome is hollow, inside the tube is a belt on rollers. The stuff at the base of the tube includes a motor to turn the rollers and belt. The outer surface of the belt is charged by a combination of the same triboelectric effect we spoke of above, and some nearby sharp pointed brushes. That charge is transferred to the outer surface of the dome.

The amount of charge that can accumulate on the dome is limited only by its diameter. A smaller diameter dome can be thought of as a sharper object than a larger diameter dome. Sharper objects have stronger electric fields surrounding them, which break down the air more easily, taking charge from the object. The big Van de Graaff pictured is rated at around 400kV and the small one is around 80kV.

They’re still a low current source, as the current is produced by the triboelectric effect at the belt and rollers and mechanically transported by the movement of the belt. Wider belt and rollers and a faster rotation gives higher current.

They’re medium hard to make. Since the triboelectric effect is involved, the rollers and belt have to be the right combination of materials. For a small one the dome is often a soda can and for a large one it’s often made using metal salad bowls or a large garden ball.

Flyback Transformer

Moving on to the higher current power sources, an often used type is a circuit using a flyback transformer and one or more transistors. As expected, the flyback transformer has a primary winding which induces a current in the secondary winding. However, there’s also a feedback winding which at the same time shuts down the transistor which stops the current going to the primary. This causes the magnetic field to collapse and a large high voltage spike to appear at the secondary. Since there’s now no more current on the feedback coil, the transistor turns on again and the cycle repeats.

A similar circuit using MOSFETs exists called the ZVS flyback driver but as I haven’t made one I’ll refer you to this one put to use making smores.

Flyback transformers can be bought online but can also be salvaged from old CRT TVs and CRT PC monitors. They’ll most often have high voltage diodes built-in after the output of the secondary winding, which means the output is DC.

I built mine into the small cube shown above. You can see the nice continuous arcs you can get from it. I’ve also powered a Jacob’s ladder. Mine produced around 20kV output with a high current.

Flyback transformer plus Cockcroft-Walton voltage multiplier

If you’re lucky enough to find a flyback transformer with no built in diodes like the one shown here, then at the output you can add a Cockcroft-Walton voltage multiplier circuit. This multiplier consists of capacitors and diodes that take the flyback’s alternating output and smooth it out to flat DC but while multiplying the voltage over some number of stages. The number of stages simply depends on the number of sets of  capacitors and diodes you add on. Each added stage increases the output voltage.

The voltage will have been stepped up, but the current will be lower than without the multiplier. It will still be high though, high enough to provide sufficient ionization to make a lifter fly.

You can make your own multiplier boards or you can buy multipliers. The ones you can buy are usually called triplers since they have three stages. They’ll raise a 20kV input to 30kV, also sufficient for flying a lifter.

An almost out-of-the-box source of these types of power supplies is to use an old CRT PC monitor. Simply remove the high voltage wire going into the cathode ray tube and use that wire. I do find that long sparks will damage these monitors easily so be sure to include around 240 kilohms of at least 2 watt rated resistance in series with the output.

These are the interesting high voltage sources with which I have experience. But I’d love to hear about your own high voltage hacks in the comments below. I’d also enjoy hearing questions or ideas on using or building high voltage supplies so don’t be shy.

[Steven Dufresne] of Rimstar.org is at it again with another very functional science experiment. This week he’s showing us how he made a large electrostatic motor, also known as a Corona Motor.

A Corona motor makes use of a cool
phenomenon called the Corona discharge, which is the ionization of a fluid
(in this case, air) surrounding a conductor that is energized. He’s done other high voltage experiments that take advantage of this, like his Ion Wind propelled Star Trek Enterprise!

The motor works by using an even number of electrodes on the motor, each electrically charged; positive, negative, positive, negative, etc.

Because each electrode is the opposite charge, they want to repel each other — but since the cylinder is electrically insulated, the charges have no where to go — instead the cylinder begins to rotate as the charges attract back and forth — when a positive charge on the insulation meets a negatively charged electrode, the charge is removed by ionization (creating the corona effect), and the cycle continues. The direction of rotation is determined by the angle of the electrodes. The motor can get going pretty fast but doesn’t have that much torque or power.

For a full explanation of the project, check out [Steve’s] explanation in the following video:

And how to make it!

This is actually [Steve’s] second Corona motor, as he already designed a simpler one that is easier to build previously.