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

Hi Porta,
The weather is clearing up and I trust that I can make the pc boards over the week-end. Yours is for 4 resistors in serial (OvercrntPorta.jpg) , the 20 Ampere + is for up to 4 resistors parallel. Bert

 

Hi Porta and ElectricKayak, I may have to postpone the offer to you both, in view that I did some tests with a brushed motor and the surge currents are much higher on the oscilloscope than I thought. In view that we embed the level in the software, I would not like you to throw it into the dust bin, should it not work on the parameters given by you. Most likely you are unable to change the software and thus I like to do some more experiments to come up with a mutual agreed level. I tried to upload a video in 3gp format, but boatdesign.net does not allow me to upload it in 3gp format. Can anybody tell me what the best video format is for me to do a conversion?
Bert

 

Good morning Porta, Electrickayak, The attached photo shows the type of motors I can do tests on.
24 Volt 14 Ampere brushed motor
12 Volt 2.25 Ampere brushed motor
48 Volt brushless motor 3,5 kw max 60 Volt, 60 Ampere
72 Volt Brushless motors 6 Kw max 72 Volt , 90 Ampere
48 Volt brushed motor 4 Kw, 98 Ampere.
One should understand Brushless motors, the windings are very short, low impedance and although a ESC controller maybe able to handle 50% higher currents then the motor is allowed, one should only look at the maximum current the motor can handle. Even when the Pulse Width Modulator is on for a very short time. It is a dead short circuit in principle and therefore when we measure the current in nano seconds or micro seconds, the microprocessor regard this as an over current. Although on the meter it will be the average current. For that reason I have a 0.5 Farad capacitor over the 1 Milli-Ohm resistors to smooth it out a little and slow it down. i.e. 500 uSecond.
I need to know now from you both, what is the maximum time you think you like to have. If you can find out what the delay is for your mechanical overload protection, we can then make a compromise and set a time between the speed of measuring the over current in the microprocessor and surge currents of your brushed motor (Porta) and your maximum current of your motor (ElectricKayak) NOT the maximum current of the ESC. We can then experiment with the motors I have (photo attached) what should work for you. I love to put a video on the thread, but don't know what format is the best. I have only a 3gp format camera. Bert
Bert

 

Your video sounds educational.

My (brushless) motor is rated at 70A with winding resistance of 21 milliOhms.

I currently use a regular fast blo blade fuse. The time it takes to blow depends on the amount of over-current. Around rated current it would be a few seconds, at 3x rated it is probably around 100ms. Those are guesses, I've never measured it. I would replace it with this new circuit so most anything would be a worthwhile improvement. At what point does it become difficult? I believe my motor is quite tolerant, however I would also like to limit the energy impulse into the driveline due to an abrupt stall.

Does your program treat startup the same as when running steady state? I'm wondering if startup requires a separate loop?

Also, given it won't be easy to change the trip current via programming, does your board have space for additional parallel resistors? I may want to use 25A rather than 20A in the future.

I am not in any hurry. I will be away over the next week in any case. Thanks for all your work on this!

 

Comments below. Thanks Bert

PC
QUOTE=BertKu Good morning Porta, Electrickayak, The attached photo shows the type of motors I can do tests on.
24 Volt 14 Ampere brushed motor
12 Volt 2.25 Ampere brushed motor

PC: The 1st 2 motors above come closest for my projects assuming the amps you give are continuous load. Mine are permanent magnet field with good bearings and good efficiency. The 24v would need around a 30 amp rating cut off and my smaller 24v would need about a 2.25 amp cut off.

500 uSecond.
I need to know now from you both, what is the maximum time you think you like to have. If you can find out what the delay is for your mechanical overload protection, we can then make a compromise and set a time between the speed of measuring the over current in the microprocessor and surge currents of your brushed motor (Porta)


PC: The longest possible would probably be best because of prop striking bottom in shallow water causing momentary peaks. If I use a instantaneous fast blow conventional fuse (10X max rating for short circuit protection) in series with a slow blow circuit breaker (presently 10 second trip at stall and 10 second reset), that takes care of 2 things (short + stall) - but I have not tested to point of tripping. The 3rd thing while underway which can damage a motor is running for a long time at overload (but not stall) from a small-medium amount of weeds on the prop. In practice I have an amp meter built in to watch for continuous current overload from the rated or can listen (if it is quiet) for a labored tone to the very slight hum of the motor- to tell about overloads. In addition I face the prop and can see the prop at all times to monitor weed accumulation. First timers and guests wouldn't be alert to any of these things, so that is why I was looking at your system.

 

You confirm for yourself now, that at 12 Volt the stalling current could be as much as 12/0.021 = 570 Ampere. Probably the battery will not supplies this, but if the ESC is not doing its job, that is what you could face. At 24 Volt you are exceeding the 1000 Ampere mark. The 70 Ampere is the maximum current for continuous operation with good cooling. That will probably one of the parameters to measure.

Let see whether we together can come to a solution. A few seconds at very high currents could be fatal. Like I have mentioned in the previous thread, every time the PWM opens up, how short it maybe, 200 uS or whatever, the currents are very high. Only because of the impedance it will be limited, but not when the motor has stalled.

This is different for a brushed motor, special the one Porta is using, the impedance is much higher and also he start up with a resistor in serial. Therefore as soon I have made the pcb's we can do some tests and discuss what the next move would be.

No, I feel not for a brushless motor, but I feel for a brushed motor yes. The reason is a brushless motor does not really have startup multiple folded startup currents, while a brushed motor has. i.e. Porta has a motor running at 2 Ampere, the startup current maybe as high as 10 Ampere (without resistor)

If you go for 25 Ampere, you need less resistors. If you go for lower current you need a higher resistance and more resistors. The reason is by higher currents you will get a higher milli-Volt potential and is easier to measure.

We will indeed need lots of time to work together something nice out. Bert

 

Yes they are.

Good information. I will be able to simulate that.

That is also good information. I can work on those parameters now. Bear in mind we can manipulate the result by
a) more or fewer 10 mOhm resistors.
b) higher or lower capacitor over the resistors 0.5 Farad, 1 Farad or whatever.
c) loop for start up currents
d) loop for continuous overload.
e) loop for maximum continuous currents.
Bert

 

Hi Porta and ElectricKayak, Sadly I had a hiccup in the process of making pc boards, the UV bulb blew. Cannot get the same new one in this area. Needless to say that that is a problem. However a farmer makes a plan, is the saying, thus I have printed the layout out on normal paper and glued this on a blank pc board, drilled the holes, cut with a grinder careful some tracks and now will have to place cello-tape onto the area's which may not be etched away and dump it in the acid bath. This type of process in making a board, will take a little while longer, please be patient with me. Attached the photo's from the front and back of the first pc board . Bert

 

Hi Porta and ElectricKayak, Here is the first module. Now the testing phase starts and the we will have to see what should be changed. In the meantime I have been able to get 2 x 125 watt Philips UV lamp, but they had only 1 choke. Need at least 2 weeks to convert the present Photo layer developer to modify. Thus we carry on with the first module and the second.

You both need to encapsulate it in epoxy or put it in a little waterproof box. However the sound of the buzzer may be dimmed too much.

Question for Porta,
a) Does it make sense to use it for your Brushed motor, as the inductance/impedance/ohm resistance is quite high, probably in the order of 0,5 to 1 Ohm or even higher. Thus the stalling current will not be much more than equal the full load current.

b) If you do want to use it for your 2 Ampere 24 Volt motor, the start up loop is therefore to make sure that if the motor is stalled and you reset the system by switching the 24 Volt off and then 2 seconds later on again, that it verifies that the over-current situation is gone.
Please correct me if the time is too short and that the start up loop of 200 milliseconds is too long or too short and what would you like to have it.

c) I will have the IC in a IC socket, you can then remove it easy, get a friend or somebody from Microchip to read the IC and then to modify the program for whatever reason. I assume you agree.

d) The overload current is pegged at 5 bit, i.e. 5 x 19.53 mV = 97.65 mV = 2,44 Ampere. (8 bit processor)

e) your resistance is 40 milliOhm, i.e. by 2 Ampere you lose 160 MilliWatt and in the 8 milliOhm MOSfet , 320 milliWatt. Total 1/2 a watt.
Acceptable??

f) I cannot make one for both motors. Only one. The reason is short of components and secondly it measures the voltage over the low resistance. For your 2 Ampere motor it is 40 milliOhm (4 x 10 milliOhm in serial) and for your 14 Ampere motor it will be 6.6 milliOhm (3 parallel x 2 sets in serial as per photo, although one could keep the IC the same. Maybe you can piggyback a few 10 millOhm resistors over the planned resistors of 4 in serial.

Questions for you ElectricKayak.

Your brushless motor.

a) If you straight away switch the motor on at the speed you are kayaking, without slowly moving the PWM from zero to the speed you like to have, don't use the unit and tell me not to make one for you. It will not work.

The reason is logical. If the motor is not yet started to turn and the PWM is let say; halfway, the speed millisecond gap is such that the motor sees this as a short circuit, because it hasn't build up any inductance. The currents will be a few hundred Ampere large and as the microprocessor is sensing in microseconds, the unit will all the time regard this as an over-current and cut out.

b) because of the above, the motor is made for 70 Ampere and this is for me the level that the motor can handle without any damage to the motor. Thus I select an over-current of 70 Ampere minimum. NO START UP LOOP, just one verification point of 64 uS of 70 x 6.6 millOhm = 462 milliVolt = 24 bit.
acceptable?

c) your losses are 6,6 mOhm x 20 Ampere = 2,4 watt >>> acceptable?
Or do you want only 2,5 mOhm ( 4 parallel x 10 milliOhm resistors) i.e measuring point of 175 milliVolt at 70 Ampere and a loss of only 0,8 watt?
2,5 mOhm or 6,6 mOhm ? >>> which one.

d) the unit consumes just under the 10 MilliAmpere at 12 Volt.
Give me some time to do the testing. Attached photos of the finished board. Because at 12 Volt 20 Ampere, the MOSfets consumes 2.4 watt and therefore I made a small heatsink and painted it black. Bert

 

Hi Porta,

have you ever considered to use a PTC thermistor (Positive temperature coefficient resistor NOT a NTC, negative). It is simple, effective, fast. provided as long you can find the right values for high current and low current. I don't know what your consumption loss would be. You have to calculate that. Bert

 

Hi Porta, It was a nice suggestion, but I checked, the lowest resistance I have been able to find, was 2,3 Ohm i.e. you will lose at 2 Ampere 4,6 Volt and 9,2 watt, what for your purpose would be too much.

The first test with the over current module was a disaster. Only after a few minutes I remembered that The Microchip Analogue to Digital converter (ADC and probably all others also) cannot measure from zero, but only from approx 0,3 Volt due to the threshold levels from the transistors/base used in their ADC. Then I remembered that I solved that problem for my own controller I build with the PIC18F2431 in the following way. I lifted the voltage level up.
I placed 2 resistors 2400 Ohm and 240 Ohm between the plus 5 Volt and the shunt resistor. If there is a current, the voltage divider between the 2 resistors is thereafter lifted by the voltage created due to the current flow from your motor through the shunt resistor. 20 Ampere with 6.6 milliOhm shunt means that the 2400 Ohm/240 Ohm divider, gets lifted nearly by the same voltage, created due to the motor current.
Bingo, now we have a 91% accurate voltage to work with.The correction to 100% we do that in the software. This voltage we are now measuring, we now fed via a 470 Ohm into the ADC. Attached drawing explains it all. Sorry I have to change the pc board layout and lucky we are still working with an experimental board. I would be able to make one unit which you could use for both motors, but I am not too sure how to solve the high current connection in a simple way. My apology Porta, when one gets older we are sometimes forget what we have done a couple of months ago. Bert.

 

Here is the drawing explanation.jpg Bert

 

Thanks, Bert.

PC

 

Thanks.

PC

 

Hi Porta, . I enjoy making something for somebody when it works properly , provided it is very useful for that person. It may take some more time than we original had in mind, but if it is something that will be used all the time with great joy by that person, I enjoy making it. With that in mind, here are my replies

Excellent information, now we can peg the overload routine maybe at +/- 2 Ampere and the stall loop at 2,5 Ampere. i.e. I will try in the software to differentiate between overload and stalling. Give me time to solve it.

Not a problem, I will peg it at 2 second.

Wil do, when you want to improve or optimise it, you can either post JUST the uP back to me and I upgrade the uP or you can find somebody who can do it for you. At least it will not be thrown in the rubbish bin if we have a small problem.

I agree with you. I have found another 24 Volt brushed motor in my workshop, I will do some tests and let you know what my recommendation is.

O.K. in that case at 130 milliAmpere, your no load of the total system will only be 3 watt + 135 MicroWatt + 67 MicroWatt , a total of 3,0002 watt total system. Not bad. Well that is it theoretical, Lets hope it is the same when the unit is finalized and finished.

At present the dimensions are 85mm x 40 mm, with a sticking out heatsink of 40 mm , a total length of 125 mm. The weight is 63 gram, which includes all the copper 2,5 mm2 as seen on the photo's. I had to strengthening the pc board with 2,5 mm2 copper wire as the board as shown is for 20 Ampere and the copper layer would otherwise not be sufficient to carry those currents. In your case at 2 Ampere, the Mosfet does not need an heatsink as the loss is only 2 x 2 x 0.040 Ohm = 160 milliwatt loss. However I will have to add some copper to carry the 2 Ampere.

Thus the length would be only 95 mm. Bert