When following the proper procedure to read voltage using a PMMC Voltmeter, which type of voltage can be measured accurately?
Direct current (DC) voltage only
To determine the appropriate type of voltage that a Permanent Magnet Moving Coil (PMMC) Voltmeter can measure accurately, it is essential to understand the working principles of the PMMC instrument:
- Principle of Operation: The PMMC instrument operates on the principle of the motor effect, where a coil carrying current in the presence of a magnetic field experiences a torque. This type of voltmeter is used primarily for measuring direct current (DC) voltages.
- AC vs DC Voltage:
- DC Voltage: The PMMC voltmeter is inherently designed to measure DC voltage. The deflection of the needle is proportional to the average value of the current flowing through the coil, which works perfectly for steady direct current.
- AC Voltage: A PMMC meter cannot measure AC voltage accurately without modifications, because it would only respond to the average value of the current, which is zero for a symmetrical AC waveform.
- Conclusion: Given the intrinsic properties of the PMMC voltmeter, it is only capable of accurately measuring direct current (DC) voltage. Therefore, the statement "Direct current (DC) voltage only" is the correct answer.
Other options can be eliminated based on the device's limitations:
- Alternating current (AC) voltage only: Incorrect, as PMMC is not designed for AC measurement without rectification.
- Both AC and DC voltage accurately: Incorrect, because AC requires additional conversion techniques for measurement.
- High-frequency AC voltage only: Incorrect, as the standard PMMC does not measure AC, especially high-frequency, without rectification.
Therefore, the precise functioning and design of the PMMC voltmeter confirm it is suitable only for measuring direct current (DC) voltage.
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