Student thesis on pulse (keppe) motor

[harctan]

After a lot of delay I got the rotor piece from the university machinery. In the attached picture it is shown with the magnets glued on to it. The piece is made from a material that is only slighty magnetic (I don't know what it is), and the hole in the middle for the shaft is not very symmetric but I'll just have to work with what I have for now. I use 4 neodymium magnets in 2 pairs, each one with dimensions of 25mm diameter and 5mm thickness. Now I'm working on the motor housing which I will make by hand out of wood.

[Peter Kiproff]

Hi Harctan
You magnet arrangment looks similar to mine, however yours is bigger.
I used 3/8" OD plain steel rod, the magnets are 1/4" OD * 1/2" L
They stick well to the center steel rod, My thought was it would act like a single longer magnet, & still be a small package.
I drilled the steel rod to accept 4mm brass rod as axel.
Regards Peter

[harctan]

I cut out the motor housing out of wood as shown in the pictures below. Like everything else in my build it's not perfectly symmetrical. The screws on the housing are brass as to avoid any cogging from magnetic materials. Now I'm thinking about the wiring. I intend on running the motor on 12V for conveniency and safety reasons. I am thinking of using 1000 turns of 0.8mm (AWG 20) wire. That would amount roughly at 200m in length witch will give about 7 Ohms resistance. With 12V and 33% duty cycle the maximum theoretical current should be around 0.5 - 0.6 Amps. Due to the coils and the transistors this should be somewhat lower. Any feedback on my calculations or any other suggestions are very welcome.

[Peter Kiproff]

Just wondering if the axel will stick out from both ends, one for timing the other for load test ? Can you share the sizes for bearings & frame, it almost looks like you could add another magnet in the future if you wanted.
will you use any spacers on your bolts? 1000 turns of wire might tend to squeeze the ends together.

[harctan]

Hello Peter. You are correct, I will use plastic spacers on the brass screws so it won't become a total mess when I wire it. The bearings are standard skateboard bearings meaning 8mm inside diameter, 22mm outside diameter and 7mm thickness. The frame is 70mm diameter. The rotor has 30mm length without the magnets and a total of 50mm with the magnets. I could probably fit an extra magnet on each side but that was not my intention. I just wanted to make sure there would be enough room because I'm building it by hand and can't make the dimensions very precise. Perhaps if this model motor shows promise I can then build another one with properly CNC machined parts so everything will be nice and tight. The timing mechanism and the load will go on the same side so I 'll be able to easily make adjustments and corrections on the timing after everything is wired, glued and screwed in place.

[harctan]

Finally the motor is operational. You can see the first run here: keppe pulse motor first run - YouTube . The values of the components are as follows, L1= 900 turns of 0.75mm (~AWG#21) copper wire of ~9 Ohms resistance, V1=12V, C1=100uF/63V, T1=TIP42C, T2=TIP41C, R1=R2=220 Ohms, d1=d2=d3=1N4007, Q1=H22A1 slotted optical switch. With no load it turned at 1350rpm consuming 65mA. Duty cycle is about 33% and the transistors remain cool. I'll be away this week but when I get back I'll do more testing. Any feedback is welcome.

[Peter Kiproff]

Good to see it running, can you determine how much your saving with recapturing the BEMF ? if it was shunted I wonder how that would effect the 65ma drain ? possibly A/B switch.
Regards Peter K

[Peter Kiproff]

One other question, are you including the currant to operate the opti sw in your 65 ma ? seems like it might be able to reduce that circuit load.

[harctan]

Hello again Peter. The 65mA were measured including the optical switch, which by itself draws 45mA, leaving 20mA for the motor with no load. The gain from the recapturing of the magnetic field collapse is zero for now, which is something that I'm trying to figure out at the moment. In my second post I mentioned that I used the same circuit on another pulse motor consisting of a skateboard wheel with 4 ceramic magnets glued on it, a coil with a welding rods core and a reed switch for triggering. In that motor when using the recovery part of the circuit I got about 30% less current draw with a slight (~5%) increase in rpm. But in this keppe design I'm not getting any change. I didn't have much time last week to investigate this behavior further but I will be doing some tests soon to see if I can get to the bottom of this. If you have any thoughts or suggestions on this please share.

[Peter Kiproff]

The lions share of Pwr goes to the Opti Sw & you say you used a reed sw before, what was the issue with the reed sw as I doubt any arcing would occur from the base of your transistors. On other applications using Opti Sw
I've only used 20 Ma or less, do you need that much drive currant ?
Interesting loss of your BEMF, any chance the transistors are not fully off?
Base resistor to hold off or light bleeding through or around your vane.
I've had problems before that flat black spary paint fixed.

[harctan]

I added a 100K resistor from the base of the npn transistor to ground. If I reduce the current to the optical switch I get lower torque. I can clearly feel it by using my hand to load the motor. I will also try a reed switch to see if anything changes. The scope shots below are in this order: across the drive coil, across collector-emmiter of npn transistor, across emmiter-collector of pnp transistor and across the trasnistor of the optical switch. I'm not sure what they mean exactly so feel free to chime in.

[Peter Kiproff]

Your screan shots are consistant showing a slow turn on / off of the opti sw being carried through to the coil, looks like close to 5 ms rise / fall from opti sw, If you have lost your BEMF it likely has to do with slow turn off, the more abrubt the better.
Might be worth looking @ adding a couple more transistors & a Hal effect device, or useing a small relay coil as a pickup might lower your switching Pwr requirements.

[harctan]

So the problem with the loss of the flyback spike has most definately to do with the optical switch (Motorola H22A1). I tried a reed switch and everything works fine. I strongly prefer to use an optical switch instead of other methods for various reasons. Does anyone know why this problem occurs and if there is a way around it?
P.S. I also tried using an extra transistor between the bases of the drive transistors, which is switched by the optical switch. This helped a little but still didn't solve my problem.

[Peter Kiproff]

The choice of opto slot bears looking into, compare your output currant & turn off speed, to this for example.
http://media.digikey.com/pdf/Data%20...Fs/RPI-128.pdf
I'm not recomending this just comparing as you want to stay with opto slot.
aditional turn off resistors on your transistors might also help.

[harctan]

The choice of opto slot bears looking into, compare your output currant & turn off speed, to this for example.
http://media.digikey.com/pdf/Data%20...Fs/RPI-128.pdf
I'm not recomending this just comparing as you want to stay with opto slot.
aditional turn off resistors on your transistors might also help.

The one I'm using is this: H22A1 - FAIRCHILD SEMICONDUCTOR - OPTO SWITCH, SLOTTED | Farnell United Kingdom. What am I supposed to look for in the manuals? When you say turn off resistors do you mean from the base of the transistors to ground?