Help me out the prop size for a permanent magnet motor

It works like a dream. The software is a modified version from the Microchip software library. I do use 1 Farad capacitors over the current sensing device. However I do recognize the fact that super capacitors will smooth the drain, I will have at high pulsing currents on the battery and I just have to wait for my supplier to open up after the summer Holidays. If his super capacitors aren't what I like to have, I will take 10 x 2000 Farad in series i.e. 200 Farad.
I could do with 10 x 200 Farad in series or 20 Farad, but for that little bit of extra money, I will have a "smoother" ride. The frequency is not that high 9.8 Khz. What do you understand under high capacitance?
Bert

 

Gruss Gott Gerold, I am concerned about interference at high currents, although the motor is mounted in a 3 mm iron box and is submerged under the water surface. I may have some interference coming from the cables, but I hope to smooth it with super capacitors. I may also have to build some thick copper wired ferrite core coils in the plus 24 - 40 Volt line. But that I will address at a later stage, should it be needed. Bert

 

Hey Bert,

I looked at my old textbooks again, to get a feeling for the size of the capacitors. Just to get a general idea: Let's say 10 kHz, so one pulse is 0,1 ms duration. At 50 % power, the "on" time is 0,05 ms. Let' say we don't want the voltage to drop more than 0,1 volt in that time (forgetting the battery, imagining the capacitor is the only current source). So, it's capacity should be C = (100 amps * 0,05 ms) / 0,1 volt = 50 mF = 0,05 F = 50'000 μF.

I haven't done stuff like this in a very long time, so if there's mistakes in there, please tell me. And, in my book, 50'000 μF @ 100 volts (to be on the safe side, to allow for voltage spikes), that's pretty big. And of course, 0,5 Farad would be better, and 20 Farad would be much better.

And I think you're definitely on the right track with the coils. Inductance in series, capacitor across (in parallel), that'll give you a "low-pass" filter.

Hope this helps,

Gerold

 

Gerold, I know that I can do with smaller capacitance, but I like to experiment with super capacitors. 2000 Farad , 10 in series are about 200 dollar. I have only experimented with 1 Farad capacitors (Over my 0.002 Ohm resistor for measuring the current) 0.7RC is 1 x 0.002 = 1.4 millisecond. Good enough for smoothing my input for measuring the avarage current. 10 capacitors inseries on 24 Volt is safe enough. Bert

 

current ripple

RV battery converters, which supply, say, 35 amps of house DC at 12 volts and attempt to manage the battery bank at the same time also potentially have this issue. The output ripple is very small. This is basically the reverse of what you are talking about. The PWM converters feature decent chokes and caps to manage current ripple. So I'm in Sailor's camp for the moment, and am doubting the battery's cell chemistry is aware it is feeding a PWM controller. Do you have any scope traces of this from a shunt? I should also note that I have absolutely no expertise in integrated electronics. At best, I can sometimes muddle my way through basic discrete component circuits on a good day.

At worst, I would think that a pair of 2" chokes with a few turns would ameliorate the situation if it exists at all. The $2 solution? By chokes, I mean iron donuts with several turns of the pos and neg lead wound round it. Usually found staged in pairs.

 

Thanks Phil,

I am having some inductances in my line. But I am curious to see what super capacitors will do for me.
Bert