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6 Pole ElectroMagnet Motor

(aka Simple Brushless motor)

Well this is a new venture. It's taken me a little while to work out how to control this effectively. Then it hit me, use a decade counter to count up on an applied pulse and turn on the appropriate coil. (duh, I here you say!). It counts up to 6 and then resets to 1 (6 is included on). It's basically a modified version of my knight rider light circuit (originally counted to 10).

A 555 timer suplies the pulses to a 4017 decade counter with count number 7 (pin 5) connected to reset. The only trick to getting this motor to work is that you have to apply big currents to the electromagnets to get the bar magnet to turn fast enough at high frequencies and not get left behind. To do this I used a bank of IRF740 mosfet's to drive the coils from the output of the 4017.

The Circuit Diagram:

Click Picture to Enlarge

As for the building of the motor. I used 6 coils with two coils on each pole. Each coil is made from 6.2 meters of wire (doesn't look it! but it is) the 2nd wound over the top of the first. The core is an M4 steel bolt (about 50mm long) with two nuts on either side of a piece of 80mm pvc drain pipe cut down to about 3.5cm high. Each pole is spaced apart by 60 degrees.

Each output count control 2 coils on oposites sides of the motor. one goes north while the other side goes south. To do this I powered up each coil in turn to wire up each output correctly according to which way the magnet spun. Each transistor poweres two coils. Also each coil is made from 28AWG (30swg) enameled wire and at 6.2 meters long the resistance is 2ohms. I ran the coils at 5 volts, so they draw about 2.5 Amps each. But as each count control 2 coils, the coils are in parrallel and so the current draw is a little higher than 2.5 in operation, more like 3 amps.

The mosfets control the grounds to each coil sets and the other ends of the coils are connected permanently to +ve.

Coil operation:

I have labelled my motor with numbers 1-6 going clockwise around the outside.

Position (count)	Coil Set to be on	North direction	North coil Energised	South Coil Energised
1	1+4		1	4
2	2+5		2	5
3	3+6		3	6
4	4+1		4	1
5	5+2		5	2
6	6+3		6	3

As for the stator (permanent magnet), I have used a GeoMag (destroyed one of my set!). A hole was drilled through the center and the magnet was pinned to a plastic base with a small nut and bolt. The plastic base fits snuggly inside the motor housing and the geomag sits horizontally to the coils.

The setup:

In operation the frequency achievable is fairly low about 600Hz as the magnet loses the coil pull after that. After a minutes operation the mosfet's got quite warm, so I will have to make a quick heatsink to avoid buring them out before I go any futher on this project.

Although at 600Hz, with 6 positions in 360 degree. Which works out to 100Hz per revolution. 100 times per second, times 60 makes that 6000 rpm. I don't think my motor quite runs at 6000rpm though! So the magnet probably lags the coils for well below 600Hz at the moment. Hopefully increasing the voltage/current to the coils will help with that.