Electric Motor Secrets

[Jetijs]

Eric, I am not sure I understood you correctly. I am not very good in electronic circuits as you are, I know only the basics and it is also hard for me to draw a schematic from descriptions. This is how I understood your descriptions:



Is this correct?
Thank you!
Jetijs

[lighty]

@Tecstatic

While everything you wrote is more than true I suspect that most of it is way over head of average experimenter. Also, there are some variables that may occur that could confuse people if they do happen. For example gate series resistor value can be easily calculated but that calculation will be valid only for one value of current. In practice one would have to find the value of that resistor oneself. The easiest way would be to use multiturn non-inductive trimmer, adjust it and then observe input signal on the gate until one achieve as steep rise time as possible.

Also, SMD caps and resistors will help quite a bit with fast transients but I suspect most of people won't know what to do with them and how to solder them and not destroy them. Same goes with PCB- one surely must observe the length, width and geometry of gate tracks. As you very well know there are simple ways to compensate for the tracks inductivity but even the more experienced amateurs have problems grasping that. Also, I agree that ground plane would reduce some of the problems with transients. However, there are no visible transients on the driving side of the MOSFETs so I don't see any point of bothering with it. It simply doesn't look to me as noisy environment worth the effort of all of the precautions one would employ in commercial projects.

Negative voltage on the gate would help getting steeper faling edge but it matters only when MOSFETs are getting hot and when they are passing larger currents. Also, introducing negative gate signal would complicate schematic even further.

Of course I will again sound ominous and arrogant (as some people called me in the past) but I think all of the above is way above knowledge of average amateur. What you could do if you have time is to engineer everything and then simply offer people finished solution that they could copy. If you do that there is a simple solution for most of the driving problems you mentioned- you could use advanced drivers like ST series TD35x- they have Schmitt trigger input, adjustable delay, active Miller clamp (no need for negative drive) and ever desaturation protection (can also be used as overcurrent protection). Of course they can sink "only" 1A so you may add totem pole driver stage and negative voltage drive in order to be able to controll larger MOSFETs conducting larger current. I used TD351 and TD352 in comercial projects and they're amazing at what they can do when switching fast IGBTs (and MOSFETs) that are working near their maximum power ratings.

Also, optical switch rising time can be solved by using integrated reflective optical switches rather than slotted ones. Because one can use reflective tape it's also easier to adjust timing without any additional mechanical contraptions.

[Tecstatic]

Use of a schmitt trigger to make slow flanks fast

To take the last part about my question first.

I have done some reading from the beginning of this thread, and got my question partly answered in post #408 and #421. What I still don't understand is the looping part, I have seen Hector state you can not loop directly back, also this setup does not result in resonant charging of the capacitor as I see it.

But I guess Peter will consider this out of topic here, I think he wants to follow a path to keep it simple and do the learning step by step.

Back to busyness:

Download the data sheet from the link I gave you:
Part: CD40106BC, Manufacturer: Fairchild

So we can refer to the same data sheet. The data sheet is only 5 pages, by reading it you can see the effect of a schmitt-trigger.

The 40000 logic series can be operated up to 15V, so you do not need an additional 5V regulator.

How to connect the IC:
use a 4K7 resistor to connect the output from the fork to pin 1 of the 40106.
connect pin 2 to pins 3, 5, 9, 11, 13.
connect pins 4, 6, 8, 10 and 12 to the input pin (2) of the UCC27321.

Short distance between pin7 (Vss) and pin 4 (AGND)
Use a ceramic 100nF to decouple the 40106 very close to pins 7 and 14.
connect pin 14 to your 12V supply.

The purpose of the 4K7 resistor is to protect the input diodes from input transients. If will not harm the timing.

I have one more advice for fast FET switching: The use of a negative PGND voltage (-3V).
This is the last touch I know for reducing switching losses in the FET.

4. Use of negative gate turn off:
If you want to do that, add a 10K pull down resistor between pin 2 and 4 on the UCC27321.

Disconnect pin4 and 5 from GND and connect them to -3V

Connect the outputs from the schmitt-triggers to pin 2 via a 3.0V zener diode in series with a 1K resistor. Cathode towards the 40106. put a 1nF capacitor parallel to the zener.

Decouple the -3V supply with a 100nF capacitor and parallel the capacitor with a low forward voltage schottky diode eg. BAT60A, cathode towards pin5 (PGND)

I have always used a power supply to deliver the -3V, but as it is normal to bootstrap positive supplies, why not do the same for the negative supply with a charge pump. This way you can generate the -3V with only 6 additional passive components.

If this seems confusing with the descriptions, let me make a suggestion:
Why do "free energy" experimenting using non-free programs. Take a look on the free "Kicad" schematic and PCB program.

It is intuitive to use, and very close to perfect for making diagrams and PCBs. It runs on both Linux, Mac and Windows. An alternative is to offer postscript files but they are bigger than the diagrams, and you can not change it for further work like you can by receiving compact Kicad sheet files.

Eric

[Jetijs]

Jetijs,

Here is my 2 cents on making FET's running cool. I have made many PCB's employing FETS, at first mine ran hot too, now most are only cooled by a small copper area around the transistor, as I solder the FET metal back side directly to a PCB copper area. But it of cause depend of the application and the quality of the FET.

I see 3 problems related to your hot FETs:

1. Drive of the UCC37321:
The data sheet states:
"The input stage of each driver should be driven by a signal with a short rise or fall time".

The optical forks are usually very slow (it will be with a passive 4K7 pullup), so you need to add a schmitt trigger between the fork and the UCC37321 to obtain clean fast switching.


2. Gate drive:
The gate capacitance is 4nF and you use a gate resistor of 82 ohms.
That gives a time constant of approx. 330ns which results in slow switching and lots of heat, I have seen this myself. As your driver is very fast and able of 9 Amps (good choice), you have to limit the current like you did, but the value must be much less, I would use 1,5 ohm resistor. See if this solves the problem else keep on reading.

I must admit I have not read all posts in this very long thread, but I say this anyway, as I'm not sure of the type and value of your decoupling of the UCC37321. Also the circuit build up is important when we go for very fast switching.

If you don't already employ surface mounted components I can recommend using a double sided PCB with ground plane on one side and signals on the other. Use ceramic multilayer chip capacitors very close to the power pins of the driver 100nF (loop length less than 15mm), the larger 1uf is also a ceramic, keep that within 30mm of the power pins.

Locate the driver close to the FET (less than 30mm) and use two individual traces from the output pins to the non-inductive smd resistor and gate. The design of the PCB layout is important to avoid unwanted spikes in the circuit, a PCB trace is also in most cases an inductor unless striplines are calculated. Keep current loop areas small (close to bifilar) If the PCB is OK, you don't need D1 and D2. The 12V drive is fine.

3. The FET itself:

If the FET still gets hot with correct gate drive, consider using a faster FET with a lower Rds on.

May I recommend the IPW60R045CP from infineon.
Main data:
Vds 650V
Rds on max 0.045 ohms
Id 60A
Rise time 20ns, fall time 10ns
Free datasheets on:
Datasheet archive (search, preview and download electronic components documentation) | doc.ChipFind.ru

I consider converting a step motor for some experiments, I will make the secondary winding turns 5 times the primary winding to follow the advice of Hector. This results in time compression for sure, and has potential to account to other positive effects as well.

Good luck with your continued work !

Eric

Ps. I have a question: Is the recovery optimal when only one battery is used ?

Thanks Eric
You are right about the slow optoswitch rise/fall times. Can you give me an example how to use a schmitt trigger between the optoswitch and MOSFET driver to solve this?

What do you mean with only one battery in your question?
Thanks,
Jetijs

[uusedman]

I am back from my trip and will start hacking away. will post new improved motor results.

[Tecstatic]

Suggestions for running cool FETs

Jetijs,

Here is my 2 cents on making FET's running cool. I have made many PCB's employing FETS, at first mine ran hot too, now most are only cooled by a small copper area around the transistor, as I solder the FET metal back side directly to a PCB copper area. But it of cause depend of the application and the quality of the FET.

I see 3 problems related to your hot FETs:

1. Drive of the UCC37321:
The data sheet states:
"The input stage of each driver should be driven by a signal with a short rise or fall time".

The optical forks are usually very slow (it will be with a passive 4K7 pullup), so you need to add a schmitt trigger between the fork and the UCC37321 to obtain clean fast switching.


2. Gate drive:
The gate capacitance is 4nF and you use a gate resistor of 82 ohms.
That gives a time constant of approx. 330ns which results in slow switching and lots of heat, I have seen this myself. As your driver is very fast and able of 9 Amps (good choice), you have to limit the current like you did, but the value must be much less, I would use 1,5 ohm resistor. See if this solves the problem else keep on reading.

I must admit I have not read all posts in this very long thread, but I say this anyway, as I'm not sure of the type and value of your decoupling of the UCC37321. Also the circuit build up is important when we go for very fast switching.

If you don't already employ surface mounted components I can recommend using a double sided PCB with ground plane on one side and signals on the other. Use ceramic multilayer chip capacitors very close to the power pins of the driver 100nF (loop length less than 15mm), the larger 1uf is also a ceramic, keep that within 30mm of the power pins.

Locate the driver close to the FET (less than 30mm) and use two individual traces from the output pins to the non-inductive smd resistor and gate. The design of the PCB layout is important to avoid unwanted spikes in the circuit, a PCB trace is also in most cases an inductor unless striplines are calculated. Keep current loop areas small (close to bifilar) If the PCB is OK, you don't need D1 and D2. The 12V drive is fine.

3. The FET itself:

If the FET still gets hot with correct gate drive, consider using a faster FET with a lower Rds on.

May I recommend the IPW60R045CP from infineon.
Main data:
Vds 650V
Rds on max 0.045 ohms
Id 60A
Rise time 20ns, fall time 10ns
Free datasheets on:
Datasheet archive (search, preview and download electronic components documentation) | doc.ChipFind.ru

I consider converting a step motor for some experiments, I will make the secondary winding turns 5 times the primary winding to follow the advice of Hector. This results in time compression for sure, and has potential to account to other positive effects as well.

Good luck with your continued work !

Eric

Ps. I have a question: Is the recovery optimal when only one battery is used ?

[Jetijs]

Lighty, I am avare of that. You have to chop the input pulses so that each pulse ir just as long as the current rise time. This can be done just by pulsing the optoswitch LED. Then you get the waveform like this:


That is the next thing I will do, but for now I was just surprised to see this odd waweform:


Because if the ON time is too long, the waveform should look like this:

[lighty]

@Jetijs

With lower RPM you would have to compensate for the prolonged impulses, in fact that's exactly what I suggested over two years ago. Imagine this- as the motor slows down your impulse length would get wider and you would get worse input/recovery ratio. As the RPM goes up your ratio gets better because impulses themselves get shorter due to higher speed. The only way I can see that it can be manipulated is either by centrifugal regulator or by microcontroller.

[Jetijs]

Elias, I doubt that the problem is in RPM's because even at low RPM's the current waveform should look different and at 10k PRM's the pulse should be short enough. Already at 10K RPM the bearings start to heat up and this thing is so loud that I need to wear ear protection, because without that there is ringing in my ears a long time after I have done fiddling around with the motor. So increased speed is not really an option.
Thanks.

[elias]

Hi Jetijs,

It seems that in order to get better recovery you may need more RPM, and to get more RPM you need more voltage. have you run your motor, with 24 volts or more? It may go upto 20000 RPM and increase the recovery of your coil. As you have low resistance and low inductance on your coils it might be a good idea to increase the voltage to decrease the current draw and increase the RPM and thus increase the recovery and efficiency of your design.

Using a gearbox to convert the high RPM to Low RPM with be a great option.

Only some shared thoughts.

Keep up your nice work, it is an inspiration.

Elias

[UncleFester]

unclefester,
I tried to make an attraction motor with magnetite and it completely failed, but i didnt mix it very thick like you suggest Thats a good idea, i didnt think it would make that big of a difference, ill have to try it. Easy construction and improved performance would be great!

It has to be very dense to match or come close to iron or steel. It permeates very well though and has characteristics similar to Metglas.

Tad

[UncleFester]

How about putting the filled mold in a vacuum chamber to cure? Should suck all excess air bubbles out and make a stringer mixture for easier machining. You guys go ahead and make these cores, for now I will stick to what I have and learn a bit more. Please keep me updated about your success
Thank you!

Or vibrate the air bubbles out. I assumed you had already pretty much finished testing on the unit that followed Peter's specs and were ready to see if you could make some improvements. If you haven't finished testing the basic setup I would stick to that till you are done.

I plan on following the original information to the last letter before I go for improvements, but there are couple that I can think of to try later on.

But I will be using a small processor to control timing on the original setup. I may borrow your driver section though if you don't mind. I usually use TC4420 drivers, but I want to try the ones you are using this time. Trigger to the processor will be hall effect and will have a variable timing and variable duty cycle, and be ready for multiple pulse per firing later on once the basic tests are done. Really interest to see if this motor will outperform the Gray motor running at 1200VDC @ 20uF. Should be fun to see the difference.

Tad

[Jetijs]

How about putting the filled mold in a vacuum chamber to cure? Should suck all excess air bubbles out and make a stringer mixture for easier machining. You guys go ahead and make these cores, for now I will stick to what I have and learn a bit more. Please keep me updated about your success
Thank you!

[cody]

unclefester,
I tried to make an attraction motor with magnetite and it completely failed, but i didnt mix it very thick like you suggest Thats a good idea, i didnt think it would make that big of a difference, ill have to try it. Easy construction and improved performance would be great!

[UncleFester]

Hi UncleFester
Thanks for the explanation. I have some transil diodes across the source and drain of each MOSFET for overvoltage protection so everything higher than 400V (the MOSFET max voltage) is absorbed by them unless there is another path for the spike to go, a charging battery for example or the input capacitor.
Also it is a good suggestion about the magnetite core because I can easily cnc a good mold for the core and the result would be a solid core. It is a pain in the lower back to clean all those hundreds of plates, scrape the burnups off and glue them together so that they don't twist and so on. Also the machining would be much easier because I would not have to worry about accidentally dislodged plates. The magnetite core would be a lot easier for me to make. But then again the magnetite cores do not make as good electromagnets as silicon steel does, at least that is what my experiments show and we want here the maximum torque possible. Or am I wrong here?

A diode by itself wont snub the entire transient, but that's ok, as long as it doesn't exceed 200 volts or so. An RC snubber would do better but would take a small amount of power so that is not wanted.

I found the molded cores from pure fe304 to be better on the Bedini machine than the steel welding rod. But I had to make the core very concentrated and almost a putty. It had to be mixed very thick and then compressed into the mold. If I simply made a thick but still liquid mix, it was not as good as the steel.

One of the best systems I built used an fe304 core of 1.5" diam, and a silicon steel rod of 1/2" inserted into the center of the larger core. This core was awesome! and still much more simple to build than an entire core of steel by itself.

If I were you I would test both to make sure of torque differences. I will be joining you shortly. My cnc mill will be running pcb's for the next few days but I will be building my motor after that.

Tad