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.
Best Answer
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.