> 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 - History of the Cockshutt Tractor - by Danny Bowes (Dsl). The son of a very successful Toronto and Brantford, Ontario merchant, and himself quite an entreprenuer, James G. Cockshutt opened a business called the Brantford Plow Works in 1877. In 1882, the business was incorporated to become the Cockshutt Plow Company. Along with quality built equipment, expedious demand and expansion made Cockshutt Plow Works the leader in the tillage tools sector of the farm equipment industry by the 1920's.
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