Reducing AC inrush current in this setup

[zabala9]

Hello, I have an AC-DC power charger that has a inrush current of 50A at 230V and I need to reduce it to a maximum of 7.82A at 230V to use it with a certain power generator. What NTC thermistor or circuit do you recommend me to put between the charger and the generator, in order to protect the generator?

These are the specs of both devices

charger
MAX. POWER 722.4W
VOLTAGE RANGE 90 ~ 264VAC
POWER FACTOR (Typ.) PF>0.95/230VAC at full load
EFFICIENCY (Typ.) 92%
AC CURRENT (Typ.) 4A/230VAC
INRUSH CURRENT (Typ.) COLD START 50A at 230VAC

generator
Voltage 230V
Max power 2.0** KW
Nominal power 1.8** KW
Current (max) 8.7A
**LPG operation reduces generator power by 10%. THIS IS THE CASE so I calculated a maximum current of 7.82A

Thanks in advance

 

[berkeman]

What happens if you just turn on the charger unit while connected to the generator? Does the output voltage of the generator just drop out momentarily and then recover? Are you sure this will hurt the generator? Is there an output fuse that will blow? Is it already a slow-blow type?

Depending on the generator, it may not care and may recover just fine. The concern usually is for other devices connected to the same AC Mains circuit as the charger, since the momentary dropout can cause the other devices to reset. Are there other devices connected, or just the charger?

 

[Rive]

NTCs are about building circuits => not recommended on line voltage

Google up 'starting current limiter' or 'inrush current limiter'. Available in several different types, from wall-plug to extender-like, or more...

Einschaltstrombegrenzer
Nice

 

[DaveE]

I need to reduce it to a maximum of 7.82A at 230V to use it with a certain power generator

Are you sure? I bet it works fine as is. Generators need to be able to deal with short load transients. That 8.7A current spec. is for steady state, I think.

 

[nsaspook]

I've used these types of current limiters on line voltage devices like air conditioners with compressors that trip battery powered inverters because of high start currents.
https://www.ametherm.com/
https://www.ametherm.com/datasheetspdf/SL3210015.pdf

 

[zabala9]

Hi, as I understand, generators are rated for nominal power and maximum power.. the first for steady values and the latter for maximum peak values such as inrush currents. Any value over the max power hurts the generator.. correct me if I'm wrong.

I've seen NFC thermistors such as this https://amwei.com/icl-ntc-thermistor-inrush-current-limiter-10-ohm-7a-tdk-b57364s0100m000/ but I'm not sure how to calculate the correct NTC. I need to reduce the inrush current from 50A to maximum 7.5A (8.7A is for maximum current at 230V for 2kW, I use LPG on the generator so the max power is 1.8kW or 7.82A at 230V)

Thanks for the last example. The 'Max Steady State Current' of the NTC will be the maximum inrush current of the circuit?? I'm confused with the 'steady state' adjective

 

[berkeman]

Any value over the max power hurts the generator.. correct me if I'm wrong.

Never anthropomorphize generators. They hate it when you do that.

What exactly do you mean by "hurts"? Where have you seen this (links please)? And did you understand my question about an output fuse on the generator? Is the current-limiting behavior of the generator listed in its datasheet? (burst mode, crowbar mode, etc.)

Also, you did not answer my question about other loads being connected to this generator. Is the charger the *only* load connected to this generator?

 

[zabala9]

Well, the whole point is not to exceed the maximum power of the generator. I don't have the generator yet but I don't want to blow its fuse. I only have its specs and they say what I posted first.. AC current (steady) 4A and inrush current 50A. The generator will be loaded with the charger and maybe a 130W notebook (not plugged simultaneously to avoid adding both inrush currents at the same time, but with the notebook I guess there isn't a problem)

anyway, what's the meaning of 'Max Steady State Current' of the NTC?

 

[nsaspook]

Well, the whole point is not to exceed the maximum power of the generator. I don't have the generator yet but I don't want to blow its fuse. I only have its specs and they say what I posted first.. AC current (steady) 4A and inrush current 50A. The generator will be loaded with the charger and maybe a 130W notebook (not plugged simultaneously to avoid adding both inrush currents at the same time, but with the notebook I guess there isn't a problem)

anyway, what's the meaning of 'Max Steady State Current' of the NTC?

Max Steady State Current is the normal circuit current load MAX rating. Much like you would see on a circuit breaker.
Here are some calculators for various devices and parameters: https://www.ametherm.com/inrush-current/calculators

 

[DaveE]

Any value over the max power hurts the generator.. correct me if I'm wrong.

Sort of, but not really. Transients and surges can have very high power for short time periods. Normally these don't count. That's why transient and surge suppressors usually also have an energy rating. If your surge deposits (dissipate and/or stores) less energy than these ratings, the associated power or current isn't specified. Ratings need to be sorted between steady state vs. transient conditions. Often there aren't good ratings for transients; you'll know if you see them*, they will often have some element of duration associated with them and are often complex.

I'm not sure how to calculate the correct NTC

In my prior experience, this can be a PIA for anything that isn't low power. It's hard work in interpreting data sheets and characterizing your system to get a guaranteed fit. Often you aren't really 100% sure on paper in the end. I was also never really comfortable with other issues like reset time, ambient temperature effects, or lifetime. My expectation is that while these normally work, and are common in commercial stuff, they usually don't pass a rigorous worst case analysis. This is why I don't believe I ever used them in a final design; but a lot of other people did. I believe the normal approach includes some trial and error. Good luck. Read and understand every specification for these suckers.

I'm confused with the 'steady state' adjective

Steady state is a relatively constant situation; the opposite of transient. The boundary is fuzzy, unless you have a really great data sheet. It means it can/might do it forever. Forever normally means until thermal equilibrium, which of course depends on several things.

But, I'll ask again (for all of us?), how do you know you'll have a problem? Are you just worried about matching ratings? Do you have to build 2000 of these? If it breaks does someone get hurt? Do you need to explain the reliability of your system to a design review committee?

Or, can you just try it and see?

* Here's an example of a device that has a good data sheet re. transients. It's not what you want, it's a shunt device to control voltage, not series current like an NTC thermistor.

 

[berkeman]

Well, the whole point is not to exceed the maximum power of the generator. I don't have the generator yet but I don't want to blow its fuse.

Well, a key part of any power supply/source design is how it handles overcurrents. Current-limit behavior depends on the device, but in all of the power supplies I've designed, they handle overcurrent loads just fine (that is a design goal).

In linear power supplies, crowbar current limiting is often used, which means that an initial current limit fault needs to be cleared before the system can operate again. There is no physical harm by virtue of the crowbar current limit behavior, but the excessive load needs to be cleared before the power supply will power up its output.

However, with switching power supply circuits (which your "charger" probably uses), the output current limit behavior will likely be "burst" in character, which means that it will try multiple times to bring up the load, even if it is overcurrenting the output circuit. With a high-inrush load, that usually means that the "bursts" from the power supply gradually charge up the input capacitors of the load, and the whole system comes up just fine (on purpose). So other than the dropout in output voltage from the power supply during this time (usually less than a second), there is no other "harm" to the system.

For power supplies with built-in current-limit protection and behavior like this, you would not usually include an output fuse, so again, nothing to break or "get hurt".

Can you post a link to the datasheet of this power supply?

 

[berkeman]

BTW, I have used NTC resistance devices (as shown in the picture by @nsaspook ) on a specialized load device to limit its inrush (because it used relays to connect to the AC Mains and the relays could not withstand the inrush current repeatedly without fusing their contacts). It is very important to consider the power dissipated by the NTC devices during steady-state operation, since there is still a non-zero resistance through the device, and the dissipated power depends on the steady-state current. We had trouble getting UL approval for one such device, due to the internal temperature rise from the NTC protection devices -- that was a royal pain to fix.

 

[zabala9]

Thanks for the info ;) Sure, this is the datasheet of the battery charger
https://files.gwl.eu/inc/_doc/attach/StoItem/8257/Datasheet_MW-NPB-750-12.pdf

 

[berkeman]

Thanks for the info ;) Sure, this is the datasheet of the battery charger
https://files.gwl.eu/inc/_doc/attach/StoItem/8257/Datasheet_MW-NPB-750-12.pdf

Oops, sorry, my bad. I meant the datasheet for the power supply that drives the charger. I want to look at its output current limiting specs.

 

[DaveE]

Thanks for the info ;) Sure, this is the datasheet of the battery charger
https://files.gwl.eu/inc/_doc/attach/StoItem/8257/Datasheet_MW-NPB-750-12.pdf

This charger has a PFC controlled rectifier and "active inrush current limiting" at it's input, so it's pretty much guaranteed to have a soft start profile without a lot of uncontrolled energy storage. This means the turn on transient is just charging the input EMC filter capacitors. It should be a low energy, short duration transient that mostly comes from the generator output filters. You'll be OK, just try it.

 

[zabala9]

The steady state current that the NTC will have, will be a maximum of 5A AC (the charger and maybe a couple of notebooks now I'm thinking). The generator is this https://koenner-soehnen.com/en/inverter-generator-ks-2100ig-s
I don't have a datasheet, just the technical parameters of the website