Monopole BLDC Motor

I originally built  this motor about 6 or 7 years ago from stuff I had laying around, so it was not pretty. Having only a single coil of salvaged wire wound around a 10p nail, and a reed switch controller, it left much to be desired, but it worked, and it satisfied my curiosity about Permanent Magnet Motors (PMM). I then packed the various components and extra parts into a box, and filed it on a shelf for posterity.

OK, posterity arrived in the form of cleaning up my basement. There it was, same as it was when I packed it away. I have learned a lot about PMM’s since then, and I saw some hope of breathing new life into it, so off to the workbench it went, where it waited until the basement chores were done.

Having kept an eye out for anything I could use for the motor, I now stand at my workbench with everything in front of me.  Looks like a pile of junk, but I have salvaged worse.

First off, the coil and reed switch have to go, as I have decided to go with 3 power coils and 1 trigger coil and an electronic control circuit in the new build. I will be using a Bedini SSG circuit to control this motor, so I need to buy a couple things. Namely, a 2n3055 PNP transistor, and a 0-1K ohm trimmer potentiometer,so eBay, here I come. It’s going to be a while before they arrive, but that is OK, I need to wind the coils anyway, and that will take some time.

This is the ssg circuit diagram:

This circuit shows the power and trigger coils wound together on a single core, but I will use a seperate trigger coil so I can change the timing for testing. The drawback to my setup is that I don’t get the battery charging affect that can be gotten using the co-wound coil.

This is my modified circuit.

Here are pics of one of the finished power coils.

There are 10 layers of #24 gauge enameled magnet wire with 50 turns per layer for a total of 500 turns per coil. The multimeter says they have 1.6 ohms each.

This is the trigger coil.

This is wound on a salvaged am radio antenna core after removing the antenna wire. It is made from ferrite and 3/8″ in diameter just over 2″ long. It is mounted in a bracket that is just the right height to align it with the magnets on the rotor.

The rotor is 6″ diameter with 6 equally spaced 1/2″ round by 1/4″ thick NEO magnets around the outer edge, poles facing out. I should explain here that all 6 of the poles are the same, either N or S. I am not sure which polarity my rotor is as I don’t have a polarity checker.

This is the rotor:

I am going to be honest here. This motor did not start life having a reed switch controller. Originally, commutation was controlled with a set of lawn mower ignition points. How?, you may be asking. WELLLL…

Looking at the image of the rotor, the red hub portion that sticks up is the mandrel from a hole saw. I used this because it has a 1/4 hole for the pilot drill and a set screw to secure it in place. The drive shank also is hexagon shaped. This in affect gave me a 6 sided cam to open and close the contact points. This cam also determined that I have 6 magnets on the rotor as well.

It was a simple matter of wiring one lead from the power supply directly to the power coil and the other to the contact points in route to the other end of the power coil.

Timing this motor was pretty easy, as I had set up the mounting plate much like an automobile distributor, so it could pivot about the motor shaft.

Any way this motor used way more power than it should have. The coil got hot to the point where ozone was in the air, and the power supply circuit breaker would kick out.

Well, I got really engrossed in this, and finished quickly. Everything is together and running. Sorry, but like I said I got engrossed in the build and forgot to take pictures, but here are some taken later.

This is the rotor end with the power and trigger coils attatched.

Here are a couple shots of the back.

This is the power supply and motor controller. Because I was not charging batteries, I decided to use a DC power supply instead.

The power supply was taken from the same old boom box that the AM antenna core was salvaged from. It puts out 12 volts DC. Am not sure what its amperage is, but the 120 volt side of the transformer has a 1/2 amp fuse, which limits the output to 60 watts maximum, which would figure out to be 5 amps. Of course, inefficiencies would limit this further to 2, maybe 3 amps @ 12V DC, or 24 to 36 watts available to the power coils.

The power coils are connected together in parallel so they are each fed 12 volts. I tried wiring them in series, but that acts as a voltage divider, leaving each coil with only 4 volts. This was not enough voltage to make the coils repel the rotor magnets with enough force to overcome the magnetic attraction between them.

This motor now runs very smoothly at 1700 RPM and has quite a bit of torque for a 36 watt motor. Nothing gets too hot to touch, and the power supply fuse has withstood everything from rapid starts and stops to a couple days nonstop running.

All in all, I am happy with the results I got from this motor. It has given me lots of ideas for the next one, so keep your eyes peeled.