A Tesla Turbine is a device invented (or discovered as he called it) by Nikola Tesla in the 1900's. The device works on the boundry layer effect, where by air molecules passing over a flat surface have the effect of "clinging" to the surface and dragging it with the flow of air. The air is exhausted through holes in the center of the device. The holes also serve to add to the air drag by forcing air to move through them and transferring more of their motion to the rotating disks.
My replication of this device uses an old server hard disk (from around 1982!). The hard drive was a seagate 10Mb (oooh!) IDE drive with 4 platters and 4 read/write heads. Each platter is made from highly polished aluminium and weighs 23g.
I have added a further 4 platters (from other old hard disks, all HDD platters are the same size). The spacing between the disks is quite crucial to provide efficient conversion of energy between the air and platters. I choose not to use the orignal HDD spacers (aluminium 4mm) and decided to make my own from 1mm cardboard, giving a total disk spacing of 1mm. Somewhere I have read that there is an optimal spacing between the disks in a tesla turbine that is based on lots of complicated maths! 1mm is good enough for me for this test!
The hard disk motor has been left in place, which is unusually tall with good, solid bearings, which lends itself quite well to this application. The output from the motor is a Delta Star configuration, whereby there are 3 coils arranged in a triangle construction with taps at each of the 3 corners. The output from this is 3 phase. The rotor has 3 neodymium magnets that revolve around the coils.
Leaving the motor in place is useful because it provides high speed bearing (note; probably only rated for 7000rpm), frequency output that can be converted to the current speed (divide by 3 because you will see 3 pulses on each coil per revolution and times by 60 to get rpm) and a usable ac voltage (small, but proves that it works).
An AutoCAD file that can be printed out at 1:1 scale to be overlayed on each of the platters can be found here.
- Large exhaust hole sizes are 10mm
- Small exhaust hole sizes are 6mm
HDD case mods to construct the tubine housing:
- A 3mm perspex top to enclose the turbine and contain the air stream. A hole was drilled in the perspex to allow the spindle to revolve freely and allow air to be exhausted from the turbine enclosure.
- A semi-circular perspex enclosure is fitted around the platters to contain and direct the air flow around the enclosure. It also avoids causing air disturbance from the excess holes and metal around the hard disk case. Two holes are drilled at an angle to allow the air flow stubs to be inserted.
- 2 holes are drilled in the hard disk case to allow 2 air flow stubs to inserted.
- Two 3mm bore aluminium tubes are inserted into the hard disk casing at a 90 degree angle to platters to allow air to be blown into the center of the platters.
- 2 small 6mm holes were drilled into the underside of the hard drive casing to allow air to be exhausted.
The platters together with the spacers must be compressed and held onto the motor spindle by using the original platter top plate. A rubber bung is placed on the unused air stub to prevent any air flow disturbances.
Or download below
Tesla Turbine Demo with power generation
7mins 51Mb DiVX
Efficiency and Final Thoughts:
- Air consumed by the device: 7CFM at 50PSI
- Compressor Power required to maintain the air input: 1300W single phase
- Max output power from HDD motor (3 phase rectified): 5W (9v @ 550mA)
- Input power to output: 1295W loss
- 0.38% Efficiency
- View Forum page on this topic
Now, the thing to remember here is that I am using the orignal hard drive motor run in reverse as a generator and is not going to be as good as a seperate motor. Also, once the turbine is up to speed, the air supply can be reduced to maintain the required rpm.
Tesla originally thought that the design could possibly replace the combustion engines of the time, given that his turbine could produce usable power (elecical or mechanical) from multiple souces, including steam, compressed air and even liquid. A 200Hp version of this turbine was demonstrated for Nikola's 50th birthday. But saddly, this idea has never been taken up.
The device operation is fairly simple and solid state, leaving not a lot to wear out or go wrong, but judging by the effeciency I have observed, I don't believe this turbine could ever be very efficient or usable beyond a demonstration tool.
Saying that, I have noticed that a company called Gyroscope.com has a model available to buy that claims to produce 60W output at 70psi. The input pressure is greater than what I am using and they are also using an external large brushless motor. But even with this, air compressor supply must be about 1600W, so efficiency here would be 3.84%, still very low.
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