Join 60,000+ competitive exam aspirants
or with email
Your Cart
SubtotalтВ╣0
TotalтВ╣0
Examoogle тАв User тАв info@examoogle.com тАв EE-2024-8821
Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
Join 60,000+ competitive exam aspirants
Which of the following statements are correct about the errors in PMMC instruments? I. The swamping resistance is included in series with the moving coil to reduce errors. II. The swamping resistance is included in parallel with the moving coil to reduce errors. III. Errors are caused by weakening of the permanent magnet due to aging. IV. Errors are caused by weakening of the spring due to aging and temperature.
Only II, III and IV
Only III and IV
Only I, III and IV
Only I and III
Only I, III and IV
PMMC instruments suffer from temperature errors due to the copper moving coil and potential weakening of magnetic and mechanical components. Swamping resistance is used in series with the moving coil to minimize the temperature-induced variation in the total coil resistance, thereby reducing errors.
IS 1248:2003
PMMC instruments suffer from temperature errors due to the copper moving coil and potential weakening of magnetic and mechanical components. Swamping resistance is used in series with the moving coil to minimize the temperature-induced variation in the total coil resistance, thereby reducing errors.
TdтАЛ=NBIld тАФ Deflecting torque of PMMC
RtotalтАЛ=RcoilтАЛ+RswampтАЛ тАФ Total resistance with swamping
TcтАЛ=k╬╕ тАФ Controlling torque
The torque produced in a PMMC instrument is TdтАЛ=NBIld. Since the resistance of copper changes with temperature (RtтАЛ=R0тАЛ(1+╬▒╬ФT)), a low-temperature coefficient material (manganin) resistor is placed in series to make the total resistance stable against temperature fluctuations. Aging of the permanent magnet reduces the flux density (B), while spring degradation affects the controlling torque (TcтАЛ=k╬╕), both causing measurement inaccuracies.
Swamping resistance is typically made of Manganin, which has a negligible temperature coefficient of resistance.
Permanent magnets lose flux over time due to mechanical vibration, temperature, and external magnetic fields.
Springs undergo fatigue and loss of elasticity due to long-term usage, leading to errors in the controlling torque.
The swamping resistance is added in series to ensure the total resistance remains nearly constant despite temperature changes in the copper coil.
Uniform scale
High torque-to-weight ratio
No hysteresis loss
Restricted to DC measurements
Susceptible to aging of components
Expensive compared to moving iron
DC Ammeters
DC Voltmeters
Galvanometers
Option II is incorrect because placing a resistor in parallel would not stabilize the resistance of the moving coil against thermal variations as effectively as a series swamping resistor.
Aging effects are inherent to physical materials; regular calibration is the standard method for mitigation.
C is correct тАФ Statements I, III, and IV are accurate descriptions of PMMC error sources and mitigation techniques.
Remember that in PMMC instruments, the swamping resistor is always in series to maintain constant circuit resistance; never confuse this with shunt resistors used for range extension.
Get instant step-by-step AI explanations, full shift Previous Year Papers (SSC GD, RRB, SSC JE), and regional rank analysis on the official Examoogle App.
Free Download тАв 100K+ Aspirants