Arkansas HVACR NewsMagazine November 2019

S tate, National, Chapter News Tech News

shows a contactor coil circuit with no issues and a 0.5 amp current at 48 ohms

There are a few other factors that make the trouble with voltage drop worse. Let’s say you use an undersized wire to feed a lightbulb, an undersized wire means that the conductor has a lower ampacity (amp capacity) than it should have. Once the circuit is energized the wire will begin to heat up, as it heats up the molecules in the wire begin moving faster which increases the resistance of the wire. The greater the resistance of the wire the greater the voltage drop across the wire resulting in a hot, dangerous wire, increased voltage drop at the bulb, less light from the bulb and decreased circuit amperage ( less total work being accomplished ) . In the case of many loads including inductive (magnetic) loads like a compressor contactor, the resistance in the coil isn’t just resistance you can measure with the contactor de- energized. This resistance that is created within an electromagnet once it is energized is called “inductive reactance” and it is measured in ohms of impedance. In order for the contactor coil to properly engage it requires the correct applied voltage and without the properly applied voltage, the resistance of the coil remains low. The crudely drawn diagram below (I’m no artist)

When you add in a 200 ohm “bad connection” or any other type of resistance, not only does it create huge voltage drop, it also drops the impedance of the contactor coil itself with the result being a very low applied voltage (3.13V) on the contactor coil with it connected and under load. Under these conditions, the contactor won’t try to pull in at all. Under less extreme conditions it may chatter or become noisy.