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

Your motors do not have a reduction system and don't appear to be as efficient, and what is the rated load? Mine are servo motors about 5 cm diameter by 8 cm length plus an added reduction system. Free running is about 200 ma including the drag of reduction. Torque is 16 in lbs with the reduction system. Normal load is about 1.7 A from charged battery dropping down to 1.1A at stopping. If I quickly change the prop to a more aggressive version at 21 V, I can keep the same thrust and increase ampere levels close to where I started with a fresh battery.

PC

 

Excellent, that explains it all. I have made the program already. I am now able to add all the info into it. Yes, in a 1024 steps, I can select any voltage in 4.8828 steps up or down. I am just battling with getting good quality pc boards made.
Thank you for the info, Porta.
Bert

 

Good morning EK. Porta,
Well, after 3 days trying to make some decent pc boards, wasting a lot of bare copper boards, I had to give up and will have to do it the simple way. Nothing wrong, technical it is the same, just the looks is slightly different. Just 3 days of delay.
4 weeks ago I accidental dropped my special CANON printer and bought a new HP printer. Unfortunately, this printer does not print well on glossy paper and overhead slide paper. Thus I had to print on rice paper. The differentiation between black and see through is not that good. Over and above my can of photosensitive layer was expired. This is not a real problem because; I can adjust this with the exposure time. Over and above I had a different UV lights now, 2 x 125 Watt instead of one 250 Watt and this needs also a learning curve. All of this resulted in wasting a lot bare boards and I will only go to Cape Town in the beginning of October.

EK, be good to me and don’t crap all over me that it takes so long. You will get it. I hope you have a good sense of humor.
Bert

 

Hi Electric Kayak,
Here your prototype for 30+ Ampere. It was really plumbing works than electronics. Now the testing starts and any possible modifications. Thereafter we have to chat on how the final product should look like.
Bert

 

Sorry I have been away and catching up now.

Perhaps but it was good thinking. I wonder if working on the first derivative of the current rather than the actual current would simplify things. I suspect it might...but just idle thoughts.

That sounds good except you mentioned earlier 5 milliOhm was the minimum for reliability (Post #49). Has something changed?

If less than 5 milliOhm is indeed OK then, in addition to the 31 and 62 ampere options, one could solder an extra parallel resistor into each group and trip at 39 amps, or two extra resistors and trigger at 46 amps. So it seems to give a range of good values (31, 39, 46, 62) without modifying the programming.

I'm in no rush at all...and it is a hobby for us all...so please take your time and enjoy the design and development of a useful project!

It is greatly appreciated Bert, thanks.

 

Hi Bert, just wondering if you are concerned at all with voltage spikes when it trips at large currents? Spikes that may damage my ESC or inline RC Wattmeter etc?

 

no EK, A brush less motor has such low inductance, even at high speed in comparison with a brushed motor, that I have not really measured high spikes. Although the PWM switched fast on and off, because the low inductance it should not spike at all. What blows a ESC and motor is a) the very high torque at stalling, also exceeding any of the parameters by a few percent. i.e. 55 Volt is normally 55 Volt and not 58 Volt.
Trust that put you at ease. Bert

 

Yes , I thought already that you all in the Northern hemisphere with this beautiful summer weather, would be out and not sit at home, like we are doing here in the cold winter (and rainy).

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Nothing has changed, I stick to the 5 milliOhm, which represent 5 x 31 Ampere = 155 milliVolt. By 2,5 milliOhm ( one row of 4 x 10 milliOhm) it is no more than 77,5 miliVolt. Only after we heave done some test, we can maybe reduce it from 5 to 2,5. We need 4 in a row to cope with the power dissipation at 20 - 31 Ampere. By using 3 in a row, it will be 3,33 milliOhm, but the power dissipation is then only 2.1 watt, while with 4 at 2.8 watt.

Yes,that is an option. however I am running low on 10 milliOhm resistors and will only be in Cape Town in October. I still need some for Porta, to finish his project.

You have a good sense of humor and I do appreciate that. Bert

 

Are you sure? The inductance is low, but the di/dt is high.

How long does it take your circuit to switch? For example assume 60 uH inductance then switching off 50 amps in 1 uS will produce a 3000 volt spike!
Of course actual spike will depend on what else is in the system.

If you do the worst case test and not measuring spikes on either the battery or motor side then my mind is at ease! As mentioned I have more than the ESC in the circuit and would not want to blow anything when the circuit trips. :!:

 

I agree with of the above, but the switching time is determent by the capacitor (normally 10 nF) between source and gate , the VGS resistor to - volt and the drain to gate resistor. I have no problems with the N type MOSFets IRFP064N, but getting the correct combination for the P MOSfets IRF4710 parallel, I am struggling. With the correct combinations, you can slow the inrush current from 50 ampere to 3 Ampere down and therefore also di/dt. Bear in mind we will be switching DC and not pulsing with a PWM.
Bert

 

EK, I have a different view. They make the brushless motor with 100 pieces of 1/2 meter long copper wire placed parallel and soldered at both end and it is wound around 4 poles, thus 4 winding's. According to my calculations only a few hundred nano Henry.

2) The ESC has normally large input capacitors as buffers and thus switching fast on and off is for me not an issue.

3) If it is a problem we place a 470 uF capacitor after the MOSfet, this will solve your concern.

4) With the N type MOSfet, I use a DC to DC invertor and discharge a 470uF cap into the gate of the IRFP064N MOSfet and that is enough Milli joule to ensure that the drain to source is switched on properly and has a low 8 milliOhm resistance.

5) With the P type , I haven't found a nice way to do the same. But we will get there.

6) I will get some new Hydrogen Peroxide and will do another series of tries to make a decent pc board this week.

7) I have placed a diode in series with the 5 Volt 7805 regulator. I accidental reversed the plus and minus and destroyed the regulator.

8) All those hiccups will help me to get Porta unit quicker made.

Bert

 

I wouldn't assume the SK3-6374 is built the same. It has many more poles just for example. In any case, I calculate the self inductance of a 1/2 meter wire in *air* to be already higher than a few hundred nH. Inductance would be higher in a motor so our calculations are not agreeing.

Also if the inductance is that low then the motor L/R time constant would be in microseconds and the PWM would not perform well.

Maybe you are correct. I've never measured one. I could be talking nonsense. But now I'm curious as to the actual inductance of my motor. LC meters are cheap these days so I've ordered one off ebay (http://www.ebay.com/bhp/lc-meter) and will measure my motor when it arrives!

PS. I was originally thinking spikes generally, not just related to motor inductance. I agree capacitor in the right places can solve the problem if it materializes.

 

Hi EK, you may be right. It was 55 years ago when I did all those calculations and I probably am wrong. I may not have used the correct frequency. By the way , mine motors has also 12 poles , but the windings are grouped in 3 coils spread out over the 12 poles. although most brushless motor has 14 magnets and not 12. photo attached of an open motor.

At what frequency do you measure?

Indeed I agree with you. Special with Mosfets, a few percent over the allowed maximum voltage and you can say goodbye to your MOSfet. (provided the surge is greater than the build in zener diode)

But I have bigger problems. I cannot, although it should be rather simple, to switch the gate in such a way, so the that IRFB4710 archives 7 milli Ohm (2 parallel). I cannot get it lower then 1 Volt at 2 Ampere and that is a disaster.
Maybe I have damaged the MOSfets already as the circuit for a P type Mosfet is very simple. Attached pdf file. Even a donkey like me should be able to get it going. Will try my N types again. I have better experience with them.
Bert

 

You see E.K., With an N type I have no problems. A place 12 Volt on the gate and the current through the MOSfet is 1,95 Ampere with a voltage drop of 14 milliVolt. Thus 7.2 milliOhm at 28 degrees Celcius.
I must have blown the P type MOSfets. I will try another few P types.

If it comes to the push. Would you have objection to have the switch in the minus line? That would solve all my problems. Otherwise I have to resort it back to the 4 MHz internal oscillator instead of 20 MHz to do the Voltage doubler to get 12 Volt on top of the 12 Volt main DC supply line. Or to go for a larger Microchip.
Bert

 

Hi EK, The formula I used is as follow:

D = Diameter of coil in cm (5 cm)
N = number of turns (4)
a = length of the wire used in the 4 coils (50 cm)

......................D x D x N x N.........25 x 16
Henry in uH ---------------------- = ------------ = 0.067 uH = 67 Nano Henry
Divided by..........38,5 x (D + 3a ).......5967,5

This is in air, not with a metal core. With a metal core it is much more, but I can't find my notes after 55 years. Sorry for that.

I am using 10 Khz for my calculations in the calculation of final inductance.
Bert