Electric Motor Secrets

[Jetijs]

Hi,
I am convinced now that the problem is that the transistors do not open fully. I measured the current on the output of the optoswitch and it was just 10mA at 15V. But it should be at least 30mA. So I will buy some more 1K resistors and put them in parallel to those I already soldered to the board, this way I can decrease the resistance twice. This should give an improvement Also I checked the coil series arangement - it is right.
Thanks,
Jetijs

[Jetijs]

Something is not right. I decreased the MJ2501 base resistor values by half and this resulted in just 1-2mA current draw gain in the optotrigger output. Then I decreased the optotrigger LED resistor value by half, this increased the current draw to 18mA at the optotrigger base. The overall current draw was then 2.4A at 15V each phase. Then I decreased the resistance of the 2N3055 base resistors by half. This improved the situation a little, now the overall current draw increased to 3.4A on each phase. The torque is very weak and the RPM's are low, just 50 or so. The transistors get hot quite quickly. I am out of ideas, any suggestions? Maybe I should decrease the resistance some more?

Here, I made a short video:
http://www.youtube.com/watch?v=zvCnPIV7dvs

Just noticed, that you can see in the video how the IR LED lights brightly on the optotriggers. You can not see this without the camera

[Peter Lindemann]

Slow Down

Something is not right. I decreased the MJ2501 base resistor values by half and this resulted in just 1-2mA current draw gain in the optotrigger output. Then I decreased the optotrigger LED resistor value by half, this increased the current draw to 18mA at the optotrigger base. The overall current draw was then 2.4A at 15V each phase. Then I decreased the resistance of the 2N3055 base resistors by half. This improved the situation a little, now the overall current draw increased to 3.4A on each phase. The torque is very weak and the RPM's are low, just 50 or so. The transistors get hot quite quickly. I am out of ideas, any suggestions? Maybe I should decrease the resistance some more?

Here, I made a short video:
YouTube - Lindemann attraction motor V2.0

Just noticed, that you can see in the video how the IR LED lights brightly on the optotriggers. You can not see this without the camera

Jetijs,

Slow down. There are a number of issues here, so let's just take them one at a time. The first is that I recommend you return to the 500 ohm resistor on the IR diodes in the opto units. You'll burn them out if you run them real hot for long.

Second, use 220 ohm, 2-watt resistors in both voltage dividers switching the transistors. That should give you plenty of drive. Also, if you need to, use another NPN transistor to run the first voltage divider and use the output of the opto unit to darlington that NPN.

Third, cut a new commutator wheel with just two openings of 35 or 40 degrees of rotation. In other words, get rid of the multiple slots for now.

Your 2N3055 transistors should be on heat sinks. They are run hot when they are ON, but their duty cycle will keep them cooler during operation. They need the time to wick away the heat produced when they are ON.

Draw up these changes to the schematic and let's see what they look like. This will give you a circuit I have successfully used in the past.

Peter

[Jetijs]

Peter, I think that there are two problems. One is that the transistors are not opening fully and the other is that the rotor "locks" on a certain position in the firing zone if the current is too high. I am already using a commutator wheel with only one big 65 degree gap on each side. The resistor values for the MJ2150 voltage divider are 166 Ohms (three 500Ohms resistors in paralell). The resistance of the 2n3055 transistor voltage divider is now 70 Ohms, this makes the over all current draw about 5.2A at 15v if I hold the rotor in the firing position. But if the current is this high, the rotor wont spin, it just locks down ar some point of the firing position. This might be due to the rotor touching the stator slightly. Switched the optotrigger LED resistor back to 500 Ohms.

[Peter Lindemann]

Thanks for the Update

Peter, I think that there are two problems. One is that the transistors are not opening fully and the other is that the rotor "locks" on a certain position in the firing zone if the current is too high. I am already using a commutator wheel with only one big 65 degree gap on each side. The resistor values for the MJ2150 voltage divider are 166 Ohms (three 500Ohms resistors in paralell). The resistance of the 2n3055 transistor voltage divider is now 70 Ohms, this makes the over all current draw about 5.2A at 15v if I hold the rotor in the firing position. But if the current is this high, the rotor wont spin, it just locks down ar some point of the firing position. This might be due to the rotor touching the stator slightly. Switched the optotrigger LED resistor back to 500 Ohms.

Jetijs,

Yes, you obviously have enough drive. In fact, I would suggest backing off to single 166 ohm resistors for now. Lock-down of the gap can be a problem with these systems if the frame is not up to handling the stress. If the rotor spins freely when the power if OFF, then let's just work with it for now. Secondly, your commutator wheel with a 65 degree opening is probably to large. The field has no time to collapse before the rotor tries to move away. This is part of the problem with low speed. Make a new wheel with about a 40 degree opening. Then play around with the timing wheels position. Advancing the commutator starts to turn the coils ON before the rotor arrives. This may make for harder starts, but definitely makes for higher revs at the top.

It sounds like you are making a rapid set of changes to the machine without carefully considering the consequences. I assume your commutator wheel with the 65 degree opening was made by cutting the slots off of the slotted wheel. Yes, No?

Like a computer, the machine is just doing what it is told to do by the circuitry and the physical geometry. Trying to get the machine to QUICKLY do what you want it to do is an emotional reaction. It is important to STUDY what the machine IS doing at each stage. It will tell you the conditions it is operating in. But you have to have the patience to LISTEN, and not just JUMP to the next guess at what might work better.

Slow down. Be patient. You have 99% of the work done. We are just tweeking the circuit now. Everything is just fine. Look at what the machine is doing, and deliberately think it through.

Peter

[Jetijs]

Jetijs,
It sounds like you are making a rapid set of changes to the machine without carefully considering the consequences. I assume your commutator wheel with the 65 degree opening was made by cutting the slots off of the slotted wheel. Yes, No?
Peter

Yes, Peter, I just cut off the slots
I will make a new commutator wheel tomorrow and see what it does. The plexiglass end plates are bolted together rather hard, I do not dare to tighten the bolts harder because I am afraid that the glass could crack. The last thing what I did today was removing the circuit from the motor and then just hit the coil terminals on a battery. I noticed, that if I did this with one phase, the rotor would spin violently for several revolutions. But if I did this with other phase, it spins for only one revolution and the spark at the battery terminals is greater. Might it be that there is a much smaller air gap on the second phase and this makes the rotor to lock on the power pulse? Because you can't really see what happens, because it happens so fast.
Ok, now for the commutator wheel and then we will see

Thank you,
Jetijs

[Peter Lindemann]

Check the Air-gaps

Yes, Peter, I just cut off the slots
I will make a new commutator wheel tomorrow and see what it does. The plexiglass end plates are bolted together rather hard, I do not dare to tighten the bolts harder because I am afraid that the glass could crack. The last thing what I did today was removing the circuit from the motor and then just hit the coil terminals on a battery. I noticed, that if I did this with one phase, the rotor would spin violently for several revolutions. But if I did this with other phase, it spins for only one revolution and the spark at the battery terminals is greater. Might it be that there is a much smaller air gap on the second phase and this makes the rotor to lock on the power pulse? Because you can't really see what happens, because it happens so fast.
Ok, now for the commutator wheel and then we will see

Thank you,
Jetijs

Jetijs,

See if you can slide a thin piece of paper in between the rotor and stator. Check each stator with both sides of the rotor. Its possible that everything is fine, EXCEPT that the bearings are just not quite centered. This could account for one gap behaving differently. You might have to shim the bearing positions a tiny bit.

I agree with you. Don't try to tighten the frame anymore than the plastic can handle.

There also might be an electrical problem with the second coils. Check the resistance of the coils and make sure they are isolated and measure the same resistance as the other set.

Just go through everything methodically. You'll find the problems and fix them.

Peter

[Jetijs]

Just so that you know I will post how I assemble this thing. First I take the stator and insert the rotor into it. Then I use thin metal sheets used as thickness gauges. I take four of them and slide them between stator and rotor, two sheets on each side. 0,08mm thick sheets are rather hard to push between stator and rotor, but it can be done. This ensures that the air gap is even from both sides. Then I press the stator between the two plastic end plates so that it fits into the 2mm deepening. The stator is a tinny little bit smaller than the deepening of the end plates, this can make the stator move a little between the endplates at very powerful motor pulses. This is what I mean:



The only thing that holds the stator in place is the pressure that forms when the end plates are bolted together, but if this pressure is not high enough, the stator can move up and down between the plates for about 0.5mm.
So, I bolt the plastic end plates together and then I fix the bearing block, bolting it to the plates. In theory this should be precise enough and all I have to do now is simply to pull out the metal sheets between the stator and rotor and the rotor should remain in perfect position. But in reality it is not as simple. When I pull those sheets out, the rotor wont rotate freely, it will touch the stator at some point. Then I do the following, I turn the rotor back and forth to get the feel when it touches the stator, then I tighten one of the four bolts that holds the end plates together a little bit. If I feel an improvement, I continue tightening the bolt till I feel that it starts to go worse. Then I do the same thing with other three bolts and bit by bit tighten them to the best position possible. When this is done, I simply tighten the remaining "inner" nuts so that the position is fixed steady. Unfortunately I can not get the gap just right, there will always be that quiet sound, that indicates that the rotor is touching the stator so slightly that you can not even feel. I can just adjust everything so that this sound is as short and small as possible. I think that the only way to solve this would be increasing the gap slightly, but lets leave that for later.

I hope you understood what I mean
Thanks,
Jetijs

[nali2001]

Hi there Jetijs,
First thing I would like to compliment you on your great work and persistence. It is really rare to see someone go to such effort in this field. Sorry to 'hear' about your recent problems. I very much doubt that the rotor is touching the stator, if that were to happen you would certainly hear it clearly and there would be much vibrations and violence going on. One method of knowing is to paint the poses of the stators with some recognizable color, run the motor, and disassemble it again, if the paint is scraped you have found your problem. I see that it indeed does not run that fast and to me it does seem like some kind of switching problem. Personally I am not too much of a fan the way you are driving the thing.
Here is my circuit. http://www.krystyna.nl/Machine/Circuit001.gif It uses a fet driver and mosfets. It really is not all that complicated. Just a few cheap parts.

Kind regards,
Steven

[lighty]

@nali2001

Your schematic is pretty much the same as the basic one I use although I do use some other tricks to improve switch-off time and reduce oscillation of MOSFETs during shut off and I added several means of protecting both MOSFETs and drivers should anything go wrong (overvoltage for example).

Last week I sent Jetijs a few fast power MOSFETs, advanced MOSFET drivers (with enable pins which are to be used for gating switching signal) as well as some optocouplers and hyperfast diodes, protection diodes, various resistors, trimmers etc. He should get it by the end of this week or during next one at the latest.

Improved components and schematics should make experimenting easier and performance better.

[cdsalmons]

MµCord

I have been looking at magnetic shielding and was wondering what kind of difference it would make to use something like MµCord or some other low EMF wire. Also, if this type of wire is used on on a standerd motor, would it act like a no back EMF motor? I am working on peters design and wanted to see if anyone had any experience with this before I tried it. It is much more expensive, so I hope others have tried it. I found a site that has several types HERE I hope this can help someone, Dustin

[Jetijs]

Peter,
I made a new commutator wheel with one 40 degree gap on each side. This did not make any improvement. I noticed, that the motor can not even make one revolution, because it always locks in the same position each time. I turned on the power and listened to the shaft movement, the hissing sound was there on the same spot, when the rotor locks. I took a thickness gauge sheet 0.04mm thin and tried slipped it between the stator and rotor on various places. I found one place where it was much harder to slip through, but it could be done. I am starting to suspect that the shaft may bend on each pulse a little bit and if the pulse is on the spot where the stator and rotor is almost touching, then both surfaces lock together and no movement is possible. Because if the shaft would not bend I should not be able to get the thin metal stripo between the surfaces on the lock zone. I don't think that the stator is moving between the plastic end plates, also the bearings are high quality two row bearings (they cost me about 30$ each). Any suggestions? I guess that I will have to increase the air gap a little bit

[nali2001]

Idea

Hi there Jetijs,
Well I'm not really sure how thick you rotor axle is but I don't believe it will bend unless much more power used. In fact one way of testing is running the motor on much lower power. So that there is less magnetic strength going on. So it is highly doubtful that at low power the axle will 'still' bend. It does seems more like some strange switch/timing problem. Also with my device the timing was at some points very critical I could at one time, time it go get high speeds, but then it would not start at all. Or just start reversed. What you could do is, remove all the electronics and just make some simple contact points or mega simple commutators that will just make and break the coil - to - battery connection. Like:
http://www.gcsescience.com/Split-Ring-Commutator.gif
http://www.bcscience.com/images/stlouis_commutator.jpg

Regards,
Steven

[Peter Lindemann]

Gaps

Peter,
I made a new commutator wheel with one 40 degree gap on each side. This did not make any improvement. I noticed, that the motor can not even make one revolution, because it always locks in the same position each time. I turned on the power and listened to the shaft movement, the hissing sound was there on the same spot, when the rotor locks. I took a thickness gauge sheet 0.04mm thin and tried slipped it between the stator and rotor on various places. I found one place where it was much harder to slip through, but it could be done. I am starting to suspect that the shaft may bend on each pulse a little bit and if the pulse is on the spot where the stator and rotor is almost touching, then both surfaces lock together and no movement is possible. Because if the shaft would not bend I should not be able to get the thin metal stripo between the surfaces on the lock zone. I don't think that the stator is moving between the plastic end plates, also the bearings are high quality two row bearings (they cost me about 30$ each). Any suggestions? I guess that I will have to increase the air gap a little bit

Jetijs,

The motor I have at my shop currently runs on a circuit very similar to this. It is capable of high speed operation with no problems EXCEPT it tends to "suck the gaps" under high power. Unfortunately, my old Flux-Motor frame has movable stator cores and keeping them stable is a mechanical problem that I have not solved yet.

If you are satisfied that the stators don't move, then fine. I agree with Steven that the probability that the shaft is flexing is very low, since the magnetic forces are balanced from side to side in your design. I agree with you and think that opening up the air-gap a little bit will solve the binding issue. Start by taking a total of .001 inches off the diameter of the rotor (.0005 inches per side) and see if that is enough.

Peter

[lighty]

@Jetijs

Is it possible for you to measure possible bending of shaft? It could be that your shaft is imperfect or that you assembled rotor (or stator for that matter) a bit "off" of certain points along the length of motor.

How thick is your shaft and what material did you use? I doubt that your motor can produce so powerful magnetic pull to bend the shaft if you use any standard quality steel with appropriate shaft diameter.

Also, are you sure that your bearing case don't introduce any lateral movement? Maybe the bearing casing is too lose or have too much tolerance?

You really ought to check out everything before widening air gap because once you do it there is no going back.