Choosing an NTC for inrush current limiting

[GrantV]

Hi there,

I am struggling to find an NTC for my coil and wondering if I might have miscalculated my requirements.

My inverter is full wave rectified 230V mains and I have 3300uF bus capacitance.

If I use 325.22V as my peak voltage I get 175joules as my energy so an NTC with max energy rating of 250J should be fine.

As my supply is from a 13Amp socket, I could use 10A as my max inrush current and that would give me a R@25degC of 32.522 ohms (nearest NTC is 33ohms so all good)

However, if I look for NTC's with a max steady state current of between 13Amps and 15Amps, the max I can find are 10ohm thermistors.

Am I doing something wrong or is it normal to use multiple NTC's in series?

Do you have any recommended NTC's?

OR ... is there a better way?

OH .... another question:
If I do not bypass the NTC after power up and choose to rather leave it in circuit, I take it that once it has charged the bus capacitors, it is going to start cooling (before I start the coil from my interrupter) which would mean that I would likely discharge my bus caps almost entirely before the NTC kicks back into action? and the same would happen between every run?

I am probably just explaining to myself why not bypassing the NTC is a bad idea LOL .... or is the NTC fast enough for this to not be a consideration?

Is there a way to configure a relay on the inverter as a delayed bypass WITHOUT using low voltage components?

[Anders Mikkelsen]

There's little reason to keep the inrush as low as 10A. Uncontrolled inrush is usually in the thousands of amps, if you limit the inrush to the high tens to low hundreds of amps, you should be fine. If you want to be conservative, a B-characteristic circuit breaker will handle minimum 3 times nominal current for a few seconds, so you could start with 40 amps for example. And since we're talking about seconds time-scale rather than microseconds, you can use the RMS voltage to calculate the current. This gives a minimum of 6 ohms, so 10 would be fine. Many breakers will be C- or D- characteristic, and the time to charge the capacitor will be in the low milliseconds, so you could likely get away with 4.7 or even 3.3 ohms as well, but if you can get a 10 ohm NTC with the right energy and voltage rating you should be good.

[Felix B.]

Hi

In my DRSSTC with a bus capacitance of 2800uF at 565V I just made some high power 10 Ohm resistors out of a heating element and used the turn-on delay of a contactor to bypass the resistors after about 50ms. I never tripped my C16 breaker and got no problems with heating.

Felix

[GrantV]

There's little reason to keep the inrush as low as 10A. Uncontrolled inrush is usually in the thousands of amps, if you limit the inrush to the high tens to low hundreds of amps, you should be fine. If you want to be conservative, a B-characteristic circuit breaker will handle minimum 3 times nominal current for a few seconds, so you could start with 40 amps for example. And since we're talking about seconds time-scale rather than microseconds, you can use the RMS voltage to calculate the current. This gives a minimum of 6 ohms, so 10 would be fine. Many breakers will be C- or D- characteristic, and the time to charge the capacitor will be in the low milliseconds, so you could likely get away with 4.7 or even 3.3 ohms as well, but if you can get a 10 ohm NTC with the right energy and voltage rating you should be good.

Hi Anders, thanks for that!

I had assumed that the breaker would immediately trip at anything over it's rated current. Your explanation makes sense and gives me a far clearer idea of how to handle the inrush current