To reverse the output polarity of a DC generator you reverse the field leads or Armature leads but not both.

Question

To reverse the output polarity of a DC generator you reverse the field leads or Armature leads   but not both.

BlackTom AI · Accepted Answer

Fleming’s right-hand rule gives the polarity of the generated emf: the sign of the induced voltage depends on the direction of the magnetic field (Φ) and the direction/orientation of the armature conductors or their external connections. Key concept: the generated emf E is proportional to flux and speed (E = k·Φ·ω) and its sign changes if either Φ or the effective armature connection direction is reversed.

1) Relevant knowledge points and concepts
- Induced emf magnitude: E = k·Φ·ω (k, ω constant here).  
- Polarity depends on the direction of Φ (field direction) and the effective direction of current from the armature windings (armature leads).  
- Reversing either the field leads or the armature leads introduces one sign reversal; reversing both introduces two sign reversals (which cancel).

2) Step-by-step reasoning
- Start with Φ = +Φ0 and armature connection giving +E = +k·Φ0·ω (original polarity).  
- If you reverse the field leads, Φ becomes −Φ0, so E = k·(−Φ0)·ω = −k·Φ0·ω. The output polarity is reversed.  
- If instead you reverse only the armature leads (swap the external connections of the armature), this changes the sign of the terminal voltage relative to the original, effectively multiplying the measured output by −1, so polarity is reversed.  
- If you reverse both field and armature leads simultaneously: field reversal gives −E, armature reversal multiplies by −1 again, so E_final = −(−k·Φ0·ω) = +k·Φ0·ω, i.e., original polarity is restored.

3) Intermediate algebraic steps
- Original: E0 = +k·Φ0·ω.  
- Reverse field only: E1 = k·(−Φ0)·ω = −E0.  
- Reverse armature only: E2 = −E0.  
- Reverse both: E3 = −(−E0) = E0.

4) Why the correct answer is correct
Either reversing the field leads or reversing the armature leads will flip the sign of the output voltage, producing reversed polarity. Doing both flips the sign twice and returns to the original polarity.

5) Why other choices are incorrect
The incorrect option would be “both” (i.e., reverse both field and armature leads) because the two reversals cancel each other and do not change the net polarity. Any suggestion that neither reversal would change polarity contradicts the dependence of emf sign on field and connection directions.

6) Conclusion
Therefore, reversing the field leads or reversing the armature leads — but not both — will reverse the output polarity. This confirms the provided answer: field leads; Armature leads.

To reverse the output polarity of a DC generator you reverse the field leads or Armature leads but not both.多重下拉选择题

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