Not sure if this is the right label but this is an impressive lightweight powerful motor (200grams and 100 watts)
http://revolution-green.com/high-power-electrostatic-motor-95-efficency/ (http://revolution-green.com/high-power-electrostatic-motor-95-efficency/)
I've made some remarks about this motor before. I must say, I'm puzzled.
I have no doubts as to the efficiency of electrostatic motors in general, but the "vacuum" aspect has me puzzled. In order for a "vacuum" to be non-conductive enough to be able to stand off the high voltages needed for efficient electrostatic motor operation, the vacuum must be pretty "hard". That is, it must have a pressure that is below the "Paschen limit" , to the left of the leftmost curve in this diagram:
http://en.wikipedia.org/wiki/File:Paschen_Curves.PNG (http://en.wikipedia.org/wiki/File:Paschen_Curves.PNG)
This kind of hard laboratory vacuum is usually obtained and maintained by using several stages of pumps: a dual-stage vane pump for "roughing" and a turbo or oil-diffusion pump for pumping down to the final vacuum, low enough to be nonconductive at the voltages and spacings in the electrostatic motor.
Surely, the energetic "cost" of running the vacuum pumps must be included in an overall efficiency calculation.
As I've shown in my own primitive vacuum/high voltage systems, a "vacuum" that can be gotten with a simple 2-stage vane pump -- 30 microns or so -- just isn't low enough to provide the insulation that an electrostatic motor needs.
One would actually be better off using _higher pressure_ than atmospheric. High-power VanDeGraaff machines used in particle accelerator systems may operate under 10 atmospheres of CO2 or SF6 gas for insulation! And it's a lot easier to maintain a high pressure than a high vacuum.
Quote from: TinselKoala on January 09, 2014, 10:59:42 PM
I've made some remarks about this motor before. I must say, I'm puzzled.
I have no doubts as to the efficiency of electrostatic motors in general, but the "vacuum" aspect has me puzzled. In order for a "vacuum" to be non-conductive enough to be able to stand off the high voltages needed for efficient electrostatic motor operation, the vacuum must be pretty "hard". That is, it must have a pressure that is below the "Paschen limit" , to the left of the leftmost curve in this diagram:
http://en.wikipedia.org/wiki/File:Paschen_Curves.PNG (http://en.wikipedia.org/wiki/File:Paschen_Curves.PNG)
This kind of hard laboratory vacuum is usually obtained and maintained by using several stages of pumps: a dual-stage vane pump for "roughing" and a turbo or oil-diffusion pump for pumping down to the final vacuum, low enough to be nonconductive at the voltages and spacings in the electrostatic motor.
Surely, the energetic "cost" of running the vacuum pumps must be included in an overall efficiency calculation.
As I've shown in my own primitive vacuum/high voltage systems, a "vacuum" that can be gotten with a simple 2-stage vane pump -- 30 microns or so -- just isn't low enough to provide the insulation that an electrostatic motor needs.
One would actually be better off using _higher pressure_ than atmospheric. High-power VanDeGraaff machines used in particle accelerator systems may operate under 10 atmospheres of CO2 or SF6 gas for insulation! And it's a lot easier to maintain a high pressure than a high vacuum.
TK,
Although we usually think of high voltage when we think of electrostatics, electrostatic does not necessarily mean high voltage. I work with the effects of a substantial amount of electrostatic attraction every day with a mere 100-200V applied.
I don't believe this motor's operating voltage was given, but I would not be surprised if it operates on well under 3-5KV.
PW
I doubt if it could stand off that much.... _unless_ the vacuum is, as I said above, hard enough to get below the glow discharge region. With a rotating shaft seal to get the mechanical power out of the box .... this will require constant pumping to maintain. A magnetic coupling through the walls of the vacuum chamber will have its own set of little problems.
Here's what happens when you try to put high voltage.... even a few hundred volts... into a "soft" vacuum. You get a nice, fairly conductive plasma.
https://www.youtube.com/watch?v=niFRhRgY_9M
Quote from: TinselKoala on January 10, 2014, 03:18:13 AM
I doubt if it could stand off that much.... _unless_ the vacuum is, as I said above, hard enough to get below the glow discharge region. With a rotating shaft seal to get the mechanical power out of the box .... this will require constant pumping to maintain. A magnetic coupling through the walls of the vacuum chamber will have its own set of little problems.
Here's what happens when you try to put high voltage.... even a few hundred volts... into a "soft" vacuum. You get a nice, fairly conductive plasma.
https://www.youtube.com/watch?v=niFRhRgY_9M
TK,
"Well under" also includes much lower voltages...
My point is that the narrow clearances and small radii shown in the construction details tend to indicate that this is not a "high voltage" motor.
Regarding the "vacuum issue", I understand what you are saying, and dry air or SF6 would indeed be easier to maintain. Possibly the article misspoke or lost something in translation. Possibly this motor operates at a much lower voltage than one would might think.
Other than the vacuum issue, what are your thoughts regarding this motor?
PW
I agree that it is not a "high voltage" motor. That is my point, in fact: it cannot be, unless it uses a harder vacuum than is practicable considering the shaft seal, etc. Even then as you point out the clearances are tight, putting oppositely charged parts in close proximity.
However for there to be enough electrostatic attraction/repulsion to provide substantial torque I think the field strength must be pretty strong. Certainly small voltage differences acting over very small distances can result in high field strengths and steep gradients. I'd like to see the motor working, I just can't get a feel for how it could be so powerful and I can't imagine why a vacuum would be preferred over pressurization.
From my study of historical Wimshurst literature, being able to jump a spark gap of distance x implies
a more or less linear relation relationship with circuit voltage. This indicates to me that this motor's
input voltage will be in the range of vaccum-tube plate voltage; something like 100VDC -> 1KVDC
or else it's pieces will arc to one another. One way to make some arcing self terminating would be
to operate the motor on a high voltage square wave (AC) input. This may be what is being heard in
the manufacture's web site videos. It may be that the motor operates to some extent on DC but needs
switched AC for full power.
I think the comment as to; "a vacuum operation" is most likely in reference to high-altitude and space
based operation in an unmodified open environment - which would need to be seen on the manufacture's
specification sheet BTW. Hopefully this motor will come with some sort of of plastic or metal enclosure
to keep loose parts or wires from falling into it.
:S:MarkSCoffman
I've had a closer look at the description and diagrams of the motor on Revolution-Green. This motor is, I think, a commutated enhanced Franklin electrostatic motor. It works by exactly the same principle as my enhanced Franklin motor shown here:
http://www.youtube.com/watch?v=vqf3bUL4YqE (http://www.youtube.com/watch?v=vqf3bUL4YqE)
..... except it has optimised electrode geometry that doesn't waste most of the field like my simple "tub-motor" does. My motor is commutated by very small spark gaps between the balls and the primary field plates, whereas this motor apparently uses actual commutator and brush system at the armature just like an ordinary DC motor with stator and armature coils. As I suspected the motor doesn't produce a lot of torque and operates at very high RPM.
The motor design is also interesting in that it is very similar to the electrostatic generator I used in the video above, the Moore's Dirod, which will also operate as a motor if supplied with HV at its field plates. The Dirod's drum/rod assembly is doing just what the rotor does in the motor, and the Dirod's stator plates are doing what the stator rods are doing, and the Dirod is commutated by carbon fiber brushes, essentially wired in the same way as the motor's brushes.
So in response to mscoffman, the motor appears to be supplied with DC and does its own "chopping" by the commutation system. I imagine that it could also be controlled by supply PWM just like an ordinary DC brush-type motor.
Maybe it needs to operate in a vacuum because of the aerodynamic drag of all those rods on the rotor, spinning inside that cage of rods on the stator!!