Suppressing contactor coil inrush, is this possible?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.