Vertical magnetic motor runs on 0.0094 volts!

Last update:  5/21/2015                Return to the motor selection page


If you haven't seen the video of the motor running etc you ought to view it now so you will have an idea what this article is about.

Youtube link showing motor running etc


Vertical pulse motor Background:
While there are lots of videos on youtube showing levitated magnetic pulse motors, most builders concentrate on motors that run on a fairly high voltage of 1.5 volts or more. I find it more interesting to go in the other direction to see how low a voltage I can get a motor to run. This article will present the details so you can also build a really low voltage motor.

While investigating what might be done to lower the drive voltage I realized that I would have to do away with the transistor drive that my horizontal motor uses since a transistor base really takes ~0.7 volts to turn fully on. At first I was hoping the motor would run below 0.1 volt.The simplest switching device to use is a magnetically operated reed switch since the motor uses magnets as the drive mechanism anyway.
The next item to consider is that the motor would have very little output torque when running on extremely low voltages. Most magnetic pulse motors have the magnetic poles close to the center of rotation (the axle shaft) and really don't have a lot of inertia at low rpm. Yet they will rotate a long time when turned off after reaching their top speed due to the low frictional losses. In fact I can spin the armature of my horizontal motor with my fingers and it will continue to rotate a little over 3 minutes with no power applied and the coil moved away from the rotor. This is due to the low friction of the armature levitation supporting system. But when trying to run at low voltage the low RPMs can't generate a strong enough triggering pulse for the drive circuitry to function.

I theorized that by using a larger diameter armature the assembly would be able to store more kinetic energy which would allow the motor to run very slowly on the reduced drive power. At the same time a larger diameter would allow the reed switch to have more separation between each of the magnetic poles so it could switch off between each pole after it passed by the switch. The reed switch on my final version of the motor is closed ~43% of the time as each pole magnet passes by it.



Choice of suspension:

Considering that I would be using a large diameter armature to hold the pole magnets, I decided that a vertical magnetic suspension would be more suitable for this motor than a horizontal setup. And since I had already built a horizontal levitated motor it would be something different to work with.  I also wouldn't have to worry about using extremely strong magnets as the vertical suspension magnets since the armature would be hanging from the suspension magnet. In fact if the armature weighed enough so it was just hanging on to the suspension magnet, the mechanical friction of the motor would at be a minimum. My final setup is such that a light finger tap on the top magnet will cause the armature to disconnect from the suspension system and fall to the bottom of the case.

At first the armature used was a peanut jar lid with 4 magnetic poles and an axle from a DVD or CD-ROM player. The actual magnetic suspension and pole magnets utilize Neodymium N42, 1/2" x 1/4" - 1/8" thick ring or disk magnets. I quickly found that when spinning, the armature axle would walk off to one side of a flat surface of the suspension if that surface was not absolutely level.

I had an idea of using a ball bearing between the armature axle and the suspension system (pictures will show this later) so the armature would hang vertical even if the suspension is not vertical. Later I added a much smaller ball bearing to further reduce the rotating friction because the greater curvature of the small bearing against the curved surface of the axle end and the larger bearing. This is so because the smaller bearing reduces the actual contact area.

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Choice of coil form etc:

Now that the suspension was working, I started working with various coils to see which would drive the motor at any voltage and speed. From working on the horizontal motor I knew that the coil had to be an air core form with a rather small ID and mounted very close to the magnetic poles as they passed the coil(s).

I got the motor to run using a large multi-turn coil from a WWII teletype machine. At that point I had the motor running on 0.06 volts. The march to lower voltages really began. I next started experimenting with different coils.

Lidmotor again came to the rescue with a youtube video using a sewing machine bobbin as the coil form. I wound a bobbin full of #28 AWG wire. The drive voltage went down to ~0.03 volts but the motor just didn't have enough power to always rotate the next pole enough to close the reed switch. I tested the motor on a "dead" Ni-Cad battery (starting at ~0.6 volts). After a few days I got tired of watching it run 24/7 (at about 2000 RPM) and went back to experimenting with it.

Using different diameter lids I found the smaller ones caused problems with the reed switch not switching reliably between the poles passing by it and when using really large diameter lids, the motor just didn't have enough rpm at less than 0.03 volts to keep running. I finally settled on a ~3-1/2" diameter peanut butter lid as the most practical size to use. At this point I decided to use the Wendy's large drink cup as the mounting and as a closed case to protect the armature from stray breezes etc.

All my first attempts at a low voltage motor used 4 magnetic poles. I then tried a 5 pole arrangement and the running voltage went down to ~0.02 volts. The final step was to use 2 coils and then 3. With careful placement of the coils and timing the reed switch, the 3 bobbin coil motor runs reliably on 0.0094 volts from a low voltage power supply!

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Input power calculations:
Since the motor runs on 0.0094 volts and the coils have a total of 14 ohms, the peak current is 0.671 ma. when the reed switch is closed. When the motor is running the reed switch is closed ~43% of the time as measured with an oscilloscope. Therefore the average input ma is,  Iavg = 0.671 * .43 = 0.289 ma.

The average input power is,  
Pavg = E * Iavg  = 0.0094 * 0.289 = 0.000,002,7 watts.

That is 2.7 micro-watts!

To visualize how small amount of power this is, it would take 368,471 of these motors to use 1 watt of power! The motor can run on a single 36 mm x 19 mm, 0.5 volt solar cell inside the house on an overcast day with the sun on the other side of the house. It also runs fine using using the solar cell near a circular fluorescent workbench light as the light source. Most solar cells run at greatly reduced power output with that work light but this motor requires so little power it runs fine.

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And finally some pictures and the details of the motor:

To see an enlarged view of the pictures, left click on a picture or right click and select "View Image".

Shown here is the top suspension magnet on top of the original bottom of the cup.

top suspension magnet 


This picture shows the various parts of the motor with the details listed below.

  details of motor
  1. Magnet #1, a flat disk magnet, is almost out of sight in this picture and is actually outside the cup.
  2. The next magnet is also a disk magnet centered within the cup.
  3. The next magnet down is a weak 7/16" diameter cylindrical magnet which is used mostly as a spacer to lower the motor.
  4. The large ball bearing is used to allow the motor to hang vertical and to somewhat concentrate the magnetic field.
  5. The smaller bearing is used to reduce the contact friction between the large ball bearing and the armature axle.
  6. Three sewing machine bobbins are wound full of #28 AWG enameled magnet wire and have a total of 14 ohms resistance.
  7. The green armature is a ~3-1/2" diameter peanut butter jar lid with 5 sets of magnets to make a 5 pole motor.
  8. The small PCB is the mount for the reed switch and the red LED. The PCB switch came from an old computer printer.
  9. The CD is used to give the cup a flat surface to sit on.
  10. And last but not least is the scotch tape that holds all the outside stuff onto the Wendy's drink cup.
Notes:
  • The suspension magnets all have the north seeking poles facing downward so they stick together.
  • The magnets that make up the armature poles have the north seeking poles facing outward.
  • The coils are placed on the Wendy's cup so when current is flowing though them the north pole is directed towards the center of the motor. I use a small hand compass to check the north or south seeking poles of the coils and magnets.
  • The coils are wired in series with the end of one coil connected to the beginning of the next coil. The power supply plus lead is connected to the free end of the 1st coil and the free end of the 3rd coil is connected to one lead of the reed switch. The negative power supply lead is connected to the remaining reed switch lead.
  • The LED is connected across the coils with the cathode of the LED connected to the positive voltage lead.
  • If you run the motor above ~0.7 volts or so the LED will flash in time with the reed switch opening due to the back EMF spike being high enough to illuminate the LED. (NO this is NOT a so called "overunity" motor!)
  • A single 0.5 volt solar cell has more than enough power to run the motor.
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Schematic of the motor:
schematic

Not complicated and easy to build.
Holding the pole magnets on the armature:

This is an inside view of the armature showing the pole holding magnets and the inside axle shaft retaining nut.

inside of armature

If you look closely you can see that I have removed the threads that held the lid on the peanut jar. I started milling them off in my lathe and finished the job with coarse sandpaper. This was done so the inside magnets have a smooth surface to seat on. It's best to remove all of the screw thread to keep the lid balanced. The axle must be located at the exact center and perpendicular to the top of the jar lid.

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Placement of magnets on armature:
Rotor top view
The armature pole magnets have to be placed every 72 degrees around the lid for a five pole motor. The coils also need to be placed every (or multiples of) 72 degrees around the jar lid. The motor will run with only one coil (but not at 9.4 mV)

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Positioning the reed switch to "time" the motor:
The position where to place the reed switch involves several variables because a different size or brand of reed switch take a different amount of magnetism to switch on. The reed switch is normally open and not conducting. It will close when one of the pole magnets is close to it. At the same time the pole magnets will be attracted to the reed switch which tends to slow the motor down.

The important thing to remember is that the motor has very little power so you don't want the attraction of a pole magnet passing the switch to slow the armature down any more than necessary. In affect just enough to get the switch to close for each of the armature poles as they pass near the switch. When you turn the motor off you will probably see that one of the armature poles always stops near the reed switch.

In affect you want the switch to first close when any pole magnet is a little offset from the center of the coil(s). By timing the motor this way the pole magnets will be in position to get the maximum amount of repulsion from the magnetic field of the coil(s) to run in the proper direction. You will have to try running the motor CW and CCW to see which way your timing is set.

To get a rough setting for the timing, run the motor on a higher voltage (maybe 0.3 volt) and move the reed switch around until you find the highest speed the motor runs at. You may find that you can move the reed switch up and down to increase the speed. Then lower the voltage in steps and reposition the reed switch until you have it running on the lowest voltage possible.  It may take several minutes for the motor to slow down to the new voltage.

A quick way to slow the motor down is to lean a small sheet of aluminum against the side of the cup. The action of Lenz's law will slow the motor down quite quickly. Using an oscilloscope to monitor the voltage across the reed switch will give you an indication of how fast the motor is rotating. It takes a lot of patience to time the motor.

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How do you start the motor when it's inside a cup?

The motor is not self starting on low voltages but here is are two very simple ways to start the motor.
  1. The way I normally start the motor is to hold a magnet with the North seeking pole near one of the armature magnets.  This will cause the armature to start rotating due to the natural repulsion between the magnet you are holding and the North pole of the closest armature magnet.
  2. Another way is to just bring a piece of magnetic metal slowly towards one of the armature pole magnets from the direction the motor normally runs at. As the armature starts to turn another pole will approach the reed switch and the motor will start. On high voltage (0.3Vdc or more) the motor may start on its own when you turn the power on.


_WARNING_

If you run this motor on voltages above 1 volt  for an extended time, the magnets will probably loose strength and the motor will more than likely not run on as low a voltage as fresh magnets will run at. This is due to the small gap between the magnets and the drive coils demagnetizing the magnets at higher voltages.  If you want to run the motor on higher voltage you should space the coils further away from the pole magnets, Perhaps use a larger diameter cup for the case.

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