Question

To control voltage of a long shunt dc compound generator, a diverter resistance is connected across

• A. \( \text{shunt and series field windings} \)
• B. \( \text{series field winding} \)
• C. \( \text{shunt field winding} \)
• D. \( \text{series field and armature winding} \)

Hint:

A diverter resistance is always used to bypass current from a series winding because it has a very low ohmic value. In contrast, a field rheostat (high resistance) is connected in series with a shunt field winding to control its current.

Answer 1

The Correct Option is B

Concept: A compound DC generator contains both a shunt field winding (high resistance, many turns) and a series field winding (low resistance, few turns). In a long-shunt compound generator, the shunt field winding is connected in parallel across the combined series combination of the armature and the series field winding. To regulate or control the terminal voltage characteristics of a compound generator (e.g., to adjust it from over-compounded to flat-compounded), the magnetomotive force (MMF) produced by the series field must be adjustable. This adjustment is achieved by controlling the fraction of line current that flows through the series field winding. A low-value variable resistor, known as a diverter, is connected in parallel (across) the series field winding to bypass or divert a portion of the main current away from it.

Step 1: Understand the role of the series field winding.
The series field winding carries the heavy armature/load current and adds crucial compounding flux ($\phi_{se}$) to the machine. The total flux is: $$\phi_{\text{total}} = \phi_{sh} \pm \phi_{se}$$ If the series field creates too much flux, the generator becomes over-compounded, and the terminal voltage rises significantly with load. To control and stabilize this voltage, we must reduce $\phi_{se}$.

Step 2: Analyze the function of a diverter resistance.
A diverter is a low-resistance adjustable shunt path. When connected in parallel with a winding, it divides the incoming current according to the parallel current divider rule: $$I_{\text{series field}} = I \cdot \left(\frac{R_{\text{diverter}}}{R_{\text{series field}} + R_{\text{diverter}}}\right)$$ By adjusting $R_{\text{diverter}}$, we can precisely control how much current enters the series field winding, thereby directly tuning the generated EMF and terminal voltage.

Step 3: Match with the connection options.
To specifically divert current away from the series field without affecting the high-resistance shunt field path or the armature's main path directly, the diverter must be placed exclusively across the series field winding. Hence, option (2) is correct.