I have a motor that can pull 70+ amps on the inrush and 12A steady state. Voltage on the system is vbat so 14.8v max and around 14v most the time.
How do I actually spec out what I need? The ambient is pretty high. Somewhere around 50c.
Any help would be great!
Thanks
Long enough wires specified for 12A will give enough resistance that you don't need the inrush limiter.
On low voltage systems, inrush current isn't usually a problem. If it is, and it's a motor controlled by an Arduino with a reasonable motor driver, just give it a slow* ramp from zero to 255 PWM.
*slow = 50 milliseconds.
MorganS:
just give it a slow* ramp from zero to 255 PWM.*slow = 50 milliseconds.
That's gonna be an interesting programming challenge considering the typical 490 Hz PWM frequency, giving you only 10 pulses in that time span.
50 ms for ramp up doesn't make much sense to me anyway as it's not likely the motor gets to speed in that short a time, and the current depends on the motor speed. After 50 ms the motor is probably still near stationery, and you still get this 70A current.
You would be surprised how little acceleration time you need. I have done a lot of this type of work with 12V linear actuators. They do not have a lot of inertia so 50ms will get them close enough to the top speed to significantly reduce the starting current.
For one motor I did a lot of tests on, 35A peak current was cut to 12A this way.
Indeed, depends on how much inertia there is. Well, that's for the OP to test.
wvmarle:
That's gonna be an interesting programming challenge considering the typical 490 Hz PWM frequency, giving you only 10 pulses in that time span.
Powerful motors are PWM'd at 4, 8 or 16kHz typically, slower for larger motors where the inductance is bigger.
490Hz is likely too slow.
50 ms for ramp up doesn't make much sense to me anyway as it's not likely the motor gets to speed in that short a time, and the current depends on the motor speed. After 50 ms the motor is probably still near stationery, and you still get this 70A current.
Powerful motors spin up very violently without ramping, but yes 50ms is too quick for anything but a low inertia servomotor I think. 300ms is a more reasonable target.
Yea i was hoping to have something kill the inrush using an NTC transistor. I just am unsure of how to actually spec it out.
How many ohm, how many amps etc.. any hints would be great.
Slythy:
Yea i was hoping to have something kill the inrush using an NTC transistor. I just am unsure of how to actually spec it out.How many ohm, how many amps etc.. any hints would be great.
I believe you need to research inrush current on motors and learn why it happens and why without it your motor will never perform to design specifications.
Paul
I understand why it does it but I am limited by wiring diameter and other hardware to make it happen. I am trying to not trip my power supply.
I have found this 2ohm 18A ICL that I think will work.
https://www.digikey.com/product-detail/en/ametherm/SG110/570-1146-ND/4862336
How do you control that motor.
Motor drivers for that sort of current could already have buildin startup current limiting.
Like this one.
Leo..
it allows for an 80A peak which is my problem.
Motor stall is 70Amp, driver max is 80Amp.
I don't see a problem if your power supply can deliver that 70Amp.
I doubt that 70Amp will ever flow, unless your motor and battery wiring is very thick and very short, and the battery is capable of delivering that current without breaking a sweat.
Post pictures. And the specs of everything carrying that current.
Leo..
Here is an option for you. There are SSRs with built-in time delay. When the SSR turns on, it shorts out a resister in series with your motor.
This last summer I built a big step-up transformer for my brother. 120 volts to 240 volts. When the 120 volts was plugged in the circuit breaker popped. A smaller transformer worked ok. The problem was the inrush current to the much bigger transformer.
I found a 1 ohm 100 watt resistor and also a SSR for 120 volts with a time delay. The SSR was used and no specs could be found. The time delay used an external 1/4 watt resistor. Could not determine the delay, even by changing the resistor. But it worked. Circuit breaker happy and 120 volts in gave 240 volts out. Brother never did use it.
So, there are options that don't even require an Arduino.
Paul
What is the actual power supply? Car batteries can supply current in the range of 800A for starting a car. They don't need protection.
Lithium and gel batteries are usually smaller but even small batteries can support 80A peaks.