> We must be thinking of two different things > here. I don't see how opening the contacts > wouldn't interupt current through the field > coils.
The diode is in parallel with the field coil and reverse-biased when the contacts are closed. It is the nature of inductors to resist sudden changes in current (v=Ldi/dt). When the contacts open in the absence of a diode or snubber the current continues to flow (though decaying rapidly). It quickly charges the stray capacitance across the contacts to a high enough voltage to arc over. The diode shorts out this voltage by allowing the current to flow through it. The current then decays at a rate determined by the RL time constant of the inductance and resistance of the coil. A resistor in series with the diode will make it decay faster at the price of higher voltage across the contacts.
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Today's Featured Article - Grain Threshing in the Early 40's - by Jerry D. Coleman. How many of you can sit there and say that you have plowed with a mule? Well I would say not many, but maybe a few. This story is about the day my Grandfather Brown (true name) decided along with my parents to purchase a new Ford tractor. It wasn't really new except to us. The year was about 1967 and my father found a good used Ford 601 tractor to use on the farm instead of "Bob", our old mule. Now my grandfather had had this mule since the mid 40's and he was getting some age on him. S
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