NTC inrush current limiter experiments

[AntronX]

Found a bunch of NTC current limiter resistors from an old power supply. Turns out these are perfect size for reducing inrush current from 2 HP air compressor. They are 2 ohm 23A 260Vac SG-420 limiters.


Lets see if they help reduce air compressor peak startup current. Here is a measurement without the limiters. Peak current 50.4A (35.5A rms). Compressor takes 0.25 sec. to start.


Next I connect 2 limiters in series with the air compressor. Peak current reduced to 36.8A (25.9A rms) or 27% reduction in surge current. Compressor takes 0.75 seconds to start.


I tried 3 limiters in series but it did not reduce surge current, it only made it take longer to start.


The results with the chop saw did not come out as good. While it does reduce peak surge from 170A to <50A, on subsequent starts the effect goes away because NTC resistors don't get enough time to cool down which keeps their resistance low.

First start 49.6A peak current.


Second start 59.2A peak current.


Third start 76A peak current.


So it looks like these NTC limiters would work OK for inductive load motor starting once in 5 - 10 minutes (hvac compressor also fits this duty cycle profile). For universal motors that start often these don't work so well and thyristor chopper limiter should work better.

 

[Hedges]

I would think OK for universal motor, not likely for induction motor.

Does your connection scheme take NTC thermistor out of circuit once motor starts? Or does it keep current going through it, keep it hot?

If you have a 230V universal motor, starting at 115V should work.
3-phase motors may be started with auto-transformer step-down. Or wired Wye vs. Delta.

 

[AntronX]

Does your connection scheme take NTC thermistor out of circuit once motor starts?

No it stays always in series. That would require complex circuit to sense line current to drop the bypass relay once the load goes away. May as well use wire wound resistor instead then.

 

[yabert]

Inrush current limiter also work for me with our well pump: https://diysolarforum.com/threads/reduce-well-pump-starting-current.49166/#post-625666
Current when pump start dropped a lot. Sadly I don't have precise measuring tool to know exactly how much the current drop.
Still, a really affordable solution.

 

[AntronX]

Still, a really affordable solution.

Yes, hard to beat at only $6.

 

[matttelz]

I did something similar using ICR's. diysolarforum.com/threads/5000btu-ac-unit-on-1200w-giandel-inverter.86445/#post-1148521

Worked quite well with the switch. I'm sure there's some type of automatic switching circuit you could design to bypass the ICR after the load has started... the ICR's get SUPER hot and I don't like the idea of running that continuously.

 

[AntronX]

If you go through the trouble of making bypass circuit, may as well use a resistor instead of NTC limiter. This way you don't have to wait for NTC to cool down before next start. NTC solution is attractive because of simplicity. They are designed for high temp operation so I would not worry about bypassing them.

 

[yabert]

the ICR's get SUPER hot and I don't like the idea of running that continuously.

Yes, that can be tricky.
So, perfect for intermittent load like well pump or air compressor. Not so for continuous load.