Explore critical concepts of analog measurement tools with this quiz focused on PMMC instruments, Moving Iron meters, and Dynamometer wattmeters. Assess your understanding of construction, working principles, and distinctions among these core analog instrumentation types.
Which fundamental principle does a Permanent Magnet Moving Coil (PMMC) instrument operate on when measuring direct current values?
Explanation: A PMMC instrument operates by the interaction between a permanent magnetic field and a current-carrying coil, producing a deflecting torque. Electromagnetic induction is characteristic of induction-type instruments, not PMMC. Electrostatic attraction applies to electrostatic instruments, and the piezoelectric effect is irrelevant to this context. The unique design of the PMMC ensures high accuracy for DC measurements.
A Moving Iron (MI) instrument is best suited for which type of current measurement scenario?
Explanation: Moving Iron instruments can measure both AC and DC, making them suitable for use in domestic and industrial energy meters. They are less precise than PMMC instruments for DC and are not optimal for sensitive voltage detection or frequency measurement. Digital circuits require instruments with much higher input impedance, while frequency measurements are not a core function of MI meters.
Which key component is responsible for producing the deflecting torque in a dynamometer-type wattmeter during power measurement?
Explanation: Dynamometer wattmeters use a pair of coils—one fixed and one moving—both carrying current to produce deflecting torque proportional to power. Induction coils are not used in dynamometer wattmeters; that concept applies to induction-type instruments. A moving iron vane is used in MI meters, and permanent magnet rotating drums are not part of wattmeter construction.
Why are PMMC instruments generally unsuitable for measuring alternating current (AC)?
Explanation: PMMC instruments measure the average value of the current, which is zero over one cycle for pure AC, making them ineffective for AC measurement. They do not use moving iron, so heating up of iron is not an issue. Sensitivity of the coil is not the limiting factor for AC. Electrostatic fields are also not primarily disruptive in PMMC operation.
If a moving iron instrument deflects due to the attraction of a soft iron piece towards a coil when current flows, which specific type is being described?
Explanation: The described mechanism, where the iron piece moves towards the coil via magnetic attraction, is characteristic of the attraction-type moving iron instrument. Repulsion type uses two iron pieces repelling each other. Dynamometer type uses coils, not moving iron, and thermocouple instruments employ different temperature-based mechanisms.