how to reduce inrush current of a DC motor

izeman

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i got a little vacuum cleaner. originally powered by 6s 1500mAh NICD batteries.
they got weak after 10 years, and i replaced them by 2s2p 30q samsung 18650 cells.
i also installed a 2s 13A BMS. this was the strongest one i could find on aliexpress. it has a shutdown current of 20A.
and this little motor really draws A LOT OF AMPS on start. my current meter showed 20A plus, and 13A during constant use.

the BMS shuts down when the motor starts to turn. how could i reduce the inrush current automatically? is there a way to do it easily?

thanks!
 
quick and dirty.... add a small bank of capacitors, it will stiffen up the circuit and help keep the voltage sag your are experiencing during start-up from tripping the BMS.

Looks like you might have room for 2 since replacing 6 bats with 4.

If you want a detailed explanation and several different possible solutions
http://www.ti.com/lit/an/slva670a/slva670a.pdf
 
Addy said:
You could also stick an NTC thermistor in series with the motor. These devices start off with a high resistance, until current starts to flow and heat up the thermistor, which lowers it's resistance to stop limiting the current as much.
those thermistors have super tiny legs. how will they be able to carry 10A+ loads?
 
2WheelsMovesTheSoul said:
quick and dirty.... add a small bank of capacitors, it will stiffen up the circuit and help keep the voltage sag your are experiencing during start-up from tripping the BMS.

Looks like you might have room for 2 since replacing 6 bats with 4.

If you want a detailed explanation and several different possible solutions
http://www.ti.com/lit/an/slva670a/slva670a.pdf
after thinking about it addind caps will not solve my problem. caps on the motor side will make the problem worse. so far so clear. caps on the battery side will deplete the battery over time. not so big of a problem, but voltage sag imho is NOT the problem here. it's the BMS that shuts down because of inrush current exceeding the 20A max this BMS can handle.
 
Addy said:
izeman said:
those thermistors have super tiny legs. how will they be able to carry 10A+ loads?

I've seen 15A NTC thermistors. Others may be rated higher than that. Of course, make sure you know what the rating is if you're going to use an NTC thermistor with your motor.
ah ok. didn't know that. i will have a look for those. thanks
btw: i could add 2 or 3 in parallel to allow higher loads, right?
which resistance would i look for? i guess the lower the better?
 
i took what i had at home - in my case 10k thermistors. bundled them in parallel and tried to make the motor work. nothing. there was no current going through it. i just disconnected one motor wire, and put those resistors in line, so the current would have gone through them. correct?
 
I don't think you should use NTC thermistors in parallel. Here's a quote from an application note for NTC thermistors:
"If connected in parallel, the NTC thermistor with the smallest rated resistance consumes almost the entire current in the circuit and the resulting overload could destroy the component."

Also, it sounds like you're using NTC thermistors that are for sensing temperature. Those are way too tiny and have too much resistance to be used for this. The NTC thermistors for inrush current limiting look like this:
B57888S0888M888[1].JPG

You could probably salvage them from old power supplies like a PC ATX supply.
 
Simple: dont use a bms on the motor input. Use it only for charging. The current draw and losses are huge if you use a ntc/ptc and the shitty mosfets on the bms also burn up.

Just put in a low voltage warning buzzer and use the bms only in the charging side
 
flippy said:
Simple: dont use a bms on the motor input. Use it only for charging. The current draw and losses are huge if you use a ntc/ptc and the shitty mosfets on the bms also burn up.

Just put in a low voltage warning buzzer and use the bms only in the charging side

Can you please explain how to do this in more detail? I am asking because I have an inrush issue with my lawnmower.

Thanks
 
simple, you simply connect the device directly to the battery so the current bypasses the BMS.
so you connect the bms as normal but simply instead of using the output of the bms you hook the device directly to the battery - terminal.

but note that you need to keep a close eye on your voltage meter because you can run the cells well below their 2.5v rating.
if you dont have a voltage meter: install one.

charging DOES happen via the bms.
 
The problem is most likely that the battery voltage drops too much because the batteries cannot sustain the voltage at the required current. Add more cells in parallel or use cells with a higher C rating.
 
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