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http://www.kvc.com.my/EnterpriseChannel/SharedResources/Datasheet/0/?ProductId=1000066755&Filename=SCHNEIDER-LX4-FH024.pdf

I am trying to operate a 24VDC coil with an inrush of approximately 30A, and a holding current of only 250mA. To avoid needing a 750W supply for a holding current of less than an amp, I attempted to use a 4 ohm thermistor to suppress the inrush current down to about 5A and allow me to use a much smaller 250W supply.

Unfortunately I'm thinking this might've been flawed logic. I was thinking the trick was to simply suppress the inrush current, but now I'm thinking maybe that inrush current is critical to closing the contactor. I tried testing the circuit as designed, but the contactor is not closing. The thermistor worked in the sense that my 10A breaker is not tripping, but I can not operate the coil. My assumption is it is not getting enough initial power to close.

My question is, are coil inrush currents something that can be suppressed? If so, I'm assuming there is a more correct way to do so as my idea does not seem to be functional.

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  • \$\begingroup\$ I added this down below but I'll add here as well. Power supply: us.tdk-lambda.com/ftp/Specs/dpp120-240.pdf \$\endgroup\$ – C.J. C Aug 24 '15 at 14:15
  • \$\begingroup\$ Circuit breaker: phoenixcontact.com/online/portal/… It sounds like you all agree I'm not allowing the coil enough power to close. How can I tell how much surge current my supply can output for 50ms? Or am I best to use a capacitor in parallel regardless? \$\endgroup\$ – C.J. C Aug 24 '15 at 14:21
  • \$\begingroup\$ I tried using just the power supply with no luck. The supply appears to be killing my power before the coil gets its needed energy, which is how the supply is supposed to function. I've read some articles about power supplies that have peak current capabilities of over 300% their operational rating, but I'm struggling to find such supplies. Would I be better off finding an industrial motor/contactor specific supply, or using capacitors, as recommended? \$\endgroup\$ – C.J. C Aug 26 '15 at 14:22
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You don't want to suppress the inrush, but rather, find a different way to supply the inrush current — it is required for the contactor to operate properly. It needs about 30 A for up to 50 ms, or about 1.5 C.

If you are willing to allow your supply to sag by, say, 6 V, then you need about 1.5C/6V = 250mF of capacitance to supply the short-term current.

You can use a low-value resistor, thermistor, or inductor between the power supply and the capacitor in order to isolate the power supply from the surge.

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Generally speaking, contactors of this type have two coils - one is used for pulling, the other used for holding.

The large pull-in current is required. If your source can't supply that large current, the contactor won't pull in properly.

You can try adding a large capacitor in parallel with the voltage source. Choose the capacitor value based on a voltage drop of about 30% over 50 ms or so.

The contactor should be sealed in about that amount of time.

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Hold on here. You are fixing the wrong problem.

First, are you telling us a 6W contactor is tripping a 10A circuit breaker? If so, what type of circuit breaker is it, thermal, magnetic, trip curve etc. You might have a circuit breaker with a short curve that is reacting way too fast to your loads inrush current. That or the circuit breaker is defective.

Breakers must be spec'd to hand inrush loads. Inductive loads typically have what are called C or D curve breakers. They allow large inrush currents for a few seconds before tripping. If you have a load behind that breaker in parallel with the inductive load, and it needs faster protection, then give it its own fast breaker/fuse independent of the contactor and other inductive loads.

Power supplies can easily handle the surge though the voltage will momentarily dip. What type of power supply is it? The inrush should only be a few milliseconds and not trip circuit breakers or overload your supply. I'v built a few control cabinets using 24VDC contactors and relays driven from nothing but a 60W 24V power supply. Never had a problem or blew fuses/breakers.

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  • \$\begingroup\$ No, no I never said I was tripping the breaker. We simply used the thermistor to avoid doing so. I never tried activating the contactor without the thermistor. Are you implying I'd be OK by using a 240W supply to supply the 750W surge? I understand it's only a 50ms inrush, but that's still 20A over the rated limit of my supply. I don't see anything in the specs about surge current. Only overcurrent protection, but I believe this is for steady state operation. \$\endgroup\$ – C.J. C Aug 24 '15 at 14:08
  • \$\begingroup\$ Power Supply: us.tdk-lambda.com/ftp/Specs/dpp120-240.pdf Circuit breaker: phoenixcontact.com/online/portal/… \$\endgroup\$ – C.J. C Aug 24 '15 at 14:13
  • \$\begingroup\$ @C.J.Casperson Yes, you will not have a problem with either the circuit breaker or power supply. Inrush currents are never factored in when selecting a power supply as they can handle the inrush thanks to internal filter capacitors on the output. If the power supply is loaded to its limits, then yes an inrush can cause large voltage dips that can cause other devices to misbehave. Always leave headroom when selecting a supply I normally go for 1.5-2x capacity. As for your breaker, check out its trip curve. At 40A it will take around 2 seconds to trip. The inrush is only a few ms. \$\endgroup\$ – Mister Tea Aug 24 '15 at 18:10
  • \$\begingroup\$ Thank you. How do I actually determine what type of surge a power supply can handle? Not that I don't trust your answer but I'd like to understand it better. It's not like I can pull 1000A from the supply, so how do you determine what can be pulled in a surge without harming the supply? Part of my issue is I don't know the term to use. Inrush current refers to the current the device is pulling, so what term refers to the current being supplied by the supply? Surge current, outrush current? Can't find anything. So yes, is there a way I can calculate what it can supply for 50ms? \$\endgroup\$ – C.J. C Aug 24 '15 at 22:12
  • \$\begingroup\$ @C.J.Casperson, I can't answer that as the power supply datasheet should list it. You should contact the manufacturer and see what they say. Also, the proper term to search for would be transient, as in transient state, when describing a sudden change until a steady state is reached. \$\endgroup\$ – Mister Tea Aug 27 '15 at 12:37

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