A variable reluctance motor: -> ( the movie is at the bottom)

Background and theory:

            To make a long and complicated story short, a magnetic circuit with a gap in it will produce a force to close the gap.  Its much more involved that all that, but really, that’s all you need to know.  I was sitting in my power systems class one day, and he introduced the idea of the “simplest possible ac motor.”  It consisted of a magnetic circuit with a core to carry the main flux, a coil of wire to produce a magnetic field when a current is applied, and a non-circular core attached to a shaft that would rotate in a gap of the main core.  This simple device would create a torque on the shaft when the rotating core was not aligned with the main core.  If the applied voltage to the coil was sinusoidal, it was possible to create a net torque, and thus a rotation of the shaft.  This device was named a “synchronous variable reluctance motor.”  A drawing below is taken straight from my notes.

 

 

Synchronous: if the frequency of the applied voltage increases, the rotational velocity of the shaft will also increase, likewise, it the frequency of the applied voltage decreases, the velocity of the motor will also decrease.  The motor will stay in locked step with the applied frequency.

 

Variable reluctance:  the motor operates on a principle of variable reluctance.  The ‘reluctance’ of the magnetic circuit (its resistance to magnetic flux) can be varied as the position of the rotating core is changed.  Thus, a force will be produced when the reluctance of the circuit is high.  This force will act to “close the gap” or reduce the reluctance of the circuit.  As in the drawing above, the force will produce a torque.  This torque however will be zero when the rotating core is aligned with the main core.  The magnetic flux at this point needs to be zero so that momentum can carry the rotating core (or rotor) to a point where when the magnetic flux is increased again, the torque will be in the same direction.

 

Ok, seemed simple enough, and I love to actually see these things working, so I decided to take on the challenge.  Is it really the simplest AC motor?  Lets build one and find out!

I began with a square piece of flat iron.  It measured approximately 3 inches on each side and about 1/8^th of an inch thick.  If I were to construct another motor, I would use thicker, laminated steel to allow more flux before saturating the core (and reduce eddie current losses).  I knew that I needed a circular cross section in the main core for the rotor, so I located a point on the plate that was centered vertically, but off center horizontally (see the drawing above).  I center punched the point, then used an 1-3/8 inch hole-saw to cut the circular cross section

 

 

 

 

After the circle had been cut, I needed to remove material from the steel to create the shape of the core.  I used a ¼ inch end mill to remove the material.

 

With the main core completed, I turned my focus to the task of creating a rotor.  The first step was to create a non-circular core to create the variable reluctance.  I settled on using a piece of 1/8^th inch thick 1-inch wide band steel cut using a 1-1/2 inch hole saw.  This created a non-circular core that fit perfectly in the circular cross section of the main core.

 

 

Oooh.  That looks nice.  Now comes the hard part.  We need to create a shaft to hold the rotor.  The shaft needs to be aligned vertically, horizontally and front to back.  It also needs a smooth but firm bearing.  I did not want to buy ball bearings, so I had to come up with a flexible solution.  First thing I did was take some 3/8 inch steel rod and cut off a piece about 2 inches long.  I then turned a length of it down to 0.263 inches (which was 0.001 inches larger than the hole that got drilled in the rotor core).

 

 

The rotor core was then press fit onto the shaft.  The ends of the shaft were tapered with a 20-degree angle to a sharp point.  These points will sit inside shallow cups drilled into the ends of threaded rod.  This will make up the adjustable bearing.  Here is the completed rotor assembly.

 

 

I then created the uprights that would hold the rotor in place.  I drilled a hole in two pieces of 1/8 inch thick band steel for tapping. 

 

 

these pieces were then tapped to accept a 3/8 – 16 thread.  Then two pieces of 3/8 – 16 threaded rod were cut each about 0.8 inches long, and the ends were cupped using a center drill on the lathe.  Below is a mock up of how the bearing is going to work.  The threaded rod allow me to adjust the pressure on the bearing surface and the horizontal position of the rotor so that it can be perfectly aligned inside the main core.

 

 

you may or may not be able to tell, but it is spinning quite nicely on these makeshift bearings.

 

 

Here are all the metal parts, ready for assembly (sans the base).

 

 

The base was made from a dense and sturdy wood (salvaged from furniture).  3 1/8 inch parallel grooves were cut using a 1/8 end mill.

 

 

The pieces were then assembled into the grooves for ultimate flexibility in placement.

 

 

The coil was then wrapped onto the main core (#26 magnet wire, maybe a hundred turns) wrapped in electrical tape, and everything was aligned.

 

 

Ok, so now I had something that looked like the drawing.  But does it work?  First I tried 60 hz AC from my power supply.  Synchronous speed at 60 hz, that’s 3600 RPM (in the case of this simple 2 pole motor) probably unrealistic speed for this little motor.  Besides, this type of motor is not self starting (at least not a single phase motor of this type) so getting it to 3600 RPM with exactly the right timing to keep it running was looking like a daunting task.  So I devised another approach.  Using a function generator, a strong DC supply and a powerful darlington pair transistor, I created a variable frequency psuedo AC source.  Using this new source, I was able to start the motor at about 9 hertz.  Once running, I could vary the frequency up to about 30 hertz, and down to 9 hertz with the motor keeping in synch.  Below is an early attempt at running the motor.

 

 

 So after about a week of playing with it, I had built a 555 timer circuit (same one used in my variable frequency power inverter) to drive the motor, and had located a 5 volt 1 amp DC wall-wart for bulk power.  below is a video I recorded of the motor operating.  I apologize for the sound, the camera's microphone was a bit too sensitive and it caused some distortion.

  variable reluctance motor.wmv -> ~ 8 mb!  big video file, but worth it if you are interested in seeing it actually run!

Thank you for your interest in my projects!  My email is posted on my home page if you would like to send me an email.