Electric Motor Secrets

[Jetijs]

here is the video:
YouTube - Current waveform behavior

BTW, at certain high frequencies and using one of the V3 motors with the same coil wiring and voltages I managed to get 79% recovery.

[lighty]

It seems to me that when rotor is in alignment relative inductance of coils rise and what you get is basically a choke. And by their nature choke tend to lengthen current rise time and to produce more linear current curve. Also higher inductance would account for higher inductive collapse voltage spikes and better recovery.

Could you measure inductance of coil when rotor is in alignment and when rotor is not in alignment?

[Jetijs]

Will do that tomorrow
There is another thing that doesn't give my mind a rest. Why is it that when I adjust the dutycycle the current waveform gets elevated? I mean this is how the current waveform should look like:



But when the duty cycle is increased the waveform starts to look more like this:



The same thing happens also if I take a scope shot from a low resistance resistor on the recovery line, like this:



I mean if there is no current flowing, we get the flat, horizontal line. When the current starts to flow, it rises to a certain point till the core is magnetized, like in the first picture and then the power is removed. So everything clear there. But what is that "elevation"? Seems to me that current shoots up almost instantly and then just starts to increase in an angle. But if the angle line is the current rise time, what is that vertical line?

[UncleFester]

Mosfet characterstics

Jeti,

RDSon is gate on resistance, which is critical when trying to minimize power usage and thus heat. The gate on a mosfet (and also IGBT) will want to be as high as possible to minimize switching losses. We want at least 12V on the gate and even 14-15Volts is ok as long as we dont exceed 20Volts. RDSon for that mosefet appears to be .20 Ohm which is good. We need to see what our peak to peak voltage is on the gate first. Mosfets will get hot if we don't get close to or exceed 12VDC on the gate signal. Most engineers would tell you that you want at least 10VDC on the gate or higher.

Also mosfets burn power in the form of heat when they are being exposed to transients as well. It appears from your scope shots the transients created from switching these coils are not massive (I am assuming the scope shots are 10V/Div). A mosfet will happily switch near its maximum amperage, or voltage, but not both at the same time. For instance, if we are switching 100VDC @ 1.5 Ampere we are ok on the IRFP360, but if we take that to 250VDC @ 12.5 ampere or more we are pushing the limits. The mosfet will get hot when it see large transients even if we are only switching a tenth of it's current carrying capacity.

Fortunately it looks like we are well within range on this device for switching those coils at 12VDC @ 3.5 Ampere as long as the transients don't exceed 250 Volts or so. It might be good to measure these transient peaks to see where we are.

It could also be that we have saturation of the core of each coil before the mosfet turns off. It would be good to test this as well. I have been using black sand cores for my Bedini, Gray and Muller projects because of the better saturation characteristics and the fact they cannot be permanently magnetized, and they are non-conductive. They can also be molded into any shape you like, and they are lightweight and cheap to make. You could easily use your CNC router to create a mold for pouring black sand (fe304) and epoxy or polyester resin mixes.

Tell me if you need help with this. I can send lab grade fe304 and send pictures of how I form these cores for my projects.

Keep up the good work!

[Jetijs]

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?

[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

[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!

[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!

[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

[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

[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

[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.

[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]

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:

[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 ?