🔍 Key Concepts

• Series field circuit of a synchronous motor rotor (all 10 coils are in series between the two collector rings) • Effect of an open circuit in a DC series circuit on current and voltage drops • How a voltmeter reads: it shows the difference in potential between its two leads, not the absolute voltage of a point

💭 Think About

• First, imagine ALL 10 coils are good. With 240 VDC across the brushes and equal coils, how would the 240 V be shared along the series path? What must exist for any voltage drop to appear across a coil? • Now, imagine one coil goes open. With no complete path, what happens to the current in the entire series circuit? If the current is zero, what is the voltage drop across every intact coil on each side of the open? • Fix one voltmeter lead to the collector ring that is tied directly to one side of the DC source. As you move the other lead from coil-to-coil around the rotor, what will the meter show before you reach the open, and what will it show after you pass the open?

✅ Before You Answer

• Verify that with an open in a series circuit, current in ALL series elements is zero. • Check that with zero current, the voltage drop across any intact coil (R) is I×R = 0 V, so all points on that side of the open sit at the same potential as the source lead they’re connected to. • Be sure which collector ring the voltmeter’s fixed lead is on, and whether you are talking about points electrically before vs. after the open with respect to that ring, not just their physical position on the drawing.