How to make an over-current fuse/switch to avoid damage to a brushless motor

Hi EK and Porta. Regretful, that was wishful thinking. I did a test with 2 x 4700 uF and a 0.33 Ohm and on my meter, there was no difference. I tried then with a 0.5 Farad super capacitor and also no difference. I think that those electrolytic capacitors need a certain threshold Voltage and milliVolt's is not one of them. Sorry, we have to look for another way to get a more accurate current peak method. Bert

 

OK Bert...it will be interesting to see how my motor/ESC compares to your results. I haven't used my scope in years...hopefully it still works.

Enjoy your time at ENP...I enjoy wildlife and nature photography...the park looks very nice!

EDIT: We posted at the same time...I will measure my system directly using my scope so don't expect a problem.

 

Thank you, I will certainly enjoy this.

Hope you get the dust removed of your scope. Indeed the PWM works probably at 10 Khz, while the ADC at 2 uS + 16uS overhead,+ the loop instructions at 250 nS. I also love to see what you discover.
bert

 

Here some scope pictures of my milli-volt's over the shunt behavior by my Brushless motor. All other 3 where normal motors. Please note the negative peaks and the sharp positive peaks. I do not know whether my probe is picking some radiation up, but I placed a 1 Farad/5 Volt capacitor over the shunt and it appears that the peaks are reduces. Also I have a 22.000 uF capacitor as buffer between battery and plus input of my ESC.
Photo-D1 at about 2 Ampere and Photo-D2 , laboring the motor by hand to 6 Ampere. Bear in mind I have my switch off at 80 Ampere now, while only using 30 - 40 Ampere.
Bert

 

OK, I measured my motor. Same configuration as i would normally run. Battery into watt meter into ESC into motor. I measured across the watt meter's sense resistor.

Peak to peak about 2-3x the average current value. Positive peak about 2x maybe a little more.

Attached is photo...top trace is voltage ripple at the battery terminals...bottom trace is voltage across sense resistor. Both traces are AC coupled.

 

Good morning EK, yes that seems to recollect my memory. However I need to know the time base i.e. was your time-base 5, 10 or 1 mS per sweep.
Also I need to know the current you were measuring and also the sensitivity per block on the screen. 5 mV or 10 mV or 20? I will then be able to superimpose the 20 uS ADC measurements time and see how many times it will see your current pulse. Just to make sure we do not miss any pulse.

Do we stick to 30 Ampere, or have you decided now to move it up to 35 or 40?
Bert

 

The PWM frequency was 8kHz, the vertical block is 10mV per division.

Is your design sensitive to these numbers? I believe my ESC can run at 16kHz also (I haven't tried it...perhaps the motor will run cooler although the ESC will be hotter). Others run at 10 or 12. Does any of that matter?

I am surprised to see the spikes (well not really since you told us about them...). They are worse without the Wattmeter included so it must have some filtering also. I assume it is due to the ESR in the ESC capacitors but don't really know. How are you handling the current ripple? Are you planning to digitally integrate? Otherwise I'm not sure any set point will be stable enough?

 

While the elephants are having a good look at me, I will have a good look at the solution.
Bert

 

I need 1 more parameter to work the solution out. i.e. what was the current (average) what is shown on the scope.
I leave your guys now in piece, I am off to the ADO national Elephant park and will enjoy myself for a week. No problem in handling current ripples or whether it will be stable enough. That is not my worry. I am planning to use again a super capacitor, probably 1 Farad, or 0,5 Farad or 0,1 Farad or a combination. But need to know what the average current was. Will log in end of next week. Bert

 

Well, well well, that was quite a celebration with people having come from overseas to be at my 75th birthday in ADO national elephant park. Now the overseas visitors have gone back, I can give my full attention to our solutions again. While we were waiting at the waterhole for the animals to pass by, I think, I have come up with a few solutions for the unexpected problems, we had with the waveform. Give me some time to get back on my feet again. Bert

 

EK, Porta, I came to the conclusion after having done more tests with my test platform and brushless motor, that the scope, because it is set on 1 mV and 5 mV per screen block, it picks up quite a radiation, negative up 15 mV and positive also up to 15 mV (This over 2 milliOhm resistor) even when the tip of the scope is short circuited, there is quit some activities. After a handful of tests and buggering my spare brushless motor up, I came to the conclusion that readings from the ADC were quite stable and I was able to make good conclusions from it. Don't worry about the motor, I will repair it, In view that the magnets came lose, due to the incredible forces, when I let the motor stall, to measure the ADC readings to the registers. Bert

 

I am so sorry Porta and EK, The spikes are give me a great headache. The problem is, that the spikes are running at a different frequency as the ADC. As soon the start of the ADC catches the spike, and/or synchronized at occasional intervals, the motor stops, although the current is not exceeded. I have tried to use a 1 Farad capacitor, no luck. I need time to do more experiments with other capacitors, and other solutions to see whether I can dampen the starting spike, special the one when an ESC is used at low speed, then the spikes are the greatest.

In my case I use a current between 0 and 30 Ampere at 24 Volt and my cutoff is 80 Ampere. This means that the spikes measured by the ADC are below the 80 Ampere measuring point. But not in your both cases.

I will need lots of time to see what the solution could be. Maybe to find a solution like you EK mentioned, a digital solution after the ADC has been measured. Maybe a delay at the beginning of the ADC, but I have very little hope as the times are not synchronized and thus there will be a moment that the spike is measured at the middle of the ADC timing (10 Khz versus +/- 100 Khz. law of average? it slows the cut off time down. Different capacitors? tantalum, ceramic capacitors? small coil to the ADC input? I am really sorry, I need more time. that is not a problem for you EK, but for you, Porta. My apology. I will get it going, but don't know when.
Bert

 

Bert, happy 75th, give yourself a present and don't hurry or worry yourself about this. I have breakers in place that already work pretty well. More than anything, I am curious if your fuse will have a lower power draw and handle the adjustment to the gradual voltage drop better. Sometimes my breakers won't trip at 20V-21V range with weed balls.

PC

 

I have not read much of this thread, but im not sure what is the problem with this design. What is wrong with the usual approach taken by most of the commercial VSD manufacturers? I do alot of work with air conditioners which run BLDC motors, all of the ones i can think of use a choke coil (large inductor) as part of the solution, not just a capacitor bank... i think you need both

 

Good morning Porta, Thank you, yes I hope to get to the 95, but that does not mean that you have to wait that long. Yesterday, I tried something which was promising for your application, but not for EK. I made an extra loop and if the ADC sees a spike, it goes for 1 or 2 or 3 times more to check and if then all of them are over current, it stops the motor. While if it is below the maximum allowed current, it carry's on immediately with normal validation.

I need to see how long it will take and whether it falls within the maximum time, I like to see the system looping. For EK, it will not work, because the motor is not, like yours, a continuous waveform, but interrupted by the Pulse With Modulation very short pulse or very long pulses. Also he may go to another frequency. There I like to see whether I can synchronize the Motor frequency with the start of the ADC. Just a thought,

What I like to do for your system is to give you an extra Integrated Circuit with slightly higher values in the same parcel, should in case the first IC let your motor stop too soon. You just have to be careful that by replacing the IC, that little dot, or cut is at the same position as the one you took out.
Bert

P.S. I have never seen in my visits to the parks here in the RSA so many Elephants as last week. Too cute, those very young one's, not more than a half meter high. It was 44 degrees Celsius and all the Elephant groups, moved one after the other via the waterhole back into the field.