A) galvanometer
B) wattmeter
C) dc moving coil millivoltmeter
D) none of the above
✅ Answer:
➡️ C) dc moving coil millivoltmeter
🧠Detailed Solution
◆ Step 1: Key Concept
➡️ Rotor current frequency is very low (especially near synchronous speed)
➡️ It is called slip frequency
◆ Step 2: Measurement Principle
➡️ A DC moving coil instrument responds to average value
➡️ When connected across slip rings:
→ Pointer oscillates due to AC rotor voltage
→ Number of oscillations per second = rotor frequency
◆ Step 3: Why Millivoltmeter?
➡️ Rotor voltage is low magnitude
➡️ Hence millivoltmeter is suitable
➡️ It helps observe deflection rate (frequency)
❌ Why Other Options Are Incorrect
◆ A) galvanometer → Not suitable for frequency measurement
◆ B) wattmeter → Measures power, not frequency
◆ D) none → Incorrect (option C is valid)
🔑 Key Points to Remember
◆ Rotor frequency = slip frequency
◆ Very low frequency → needs sensitive instrument
◆ DC moving coil meter shows oscillation of pointer
◆ Frequency determined from deflection rate
🎯 Final Conclusion
➡️ Rotor current frequency is measured using:
dc moving coil millivoltmeter ✔️