![]() ![]() The fixed coil is also called the Current coil and the moving coil is named the Pressure coil. ![]() There are two types of coils in this instrument.It states that there is a mechanical force act between two current-carrying conductors It works on the principle of a dynamometer.An electrodynamometer or simply a Dynamometer wattmeter is an instrument which is most commonly used for the measurement of DC and AC power.Increase in temperature decreases the spring stiffness and increases the resistance of coil.The correct answer is option 4):( Uniform scale) The reading of the instrument is affected by temperature variation.Therefore moving iron instruments are not suitable for frequency above 125 Hz. This is because at lower frequency, the eddy current error increases with square of frequency whereas at higher frequency this error is almost constant. Moving Iron Instruments are suitable for low frequency application.If it is used at 50 Hz, calibration must also be done at the same frequency i.e. In fact, its calibration must be carried out at the frequency for which it is used in AC circuit. The calibration of these instruments should be done for both AC and DC.This is the reason accurate readings are not possible at lower range. Its scale is non-uniform and cramped at lower end. The scale of moving iron instrument is not uniform like PMMC instrument.These instruments suffer from error due to hysteresis, frequency change and stray losses.These instruments can withstand large loads and are not damaged even under sever overload conditions.For AC, the accuracy of the instrument may be of the order of 0.2 to 0.3 % at 50 Hz. ![]() A well designed moving iron instruments have a error of less than 2 % or less for DC. These instruments can be designed to provide precision and industrial grade accuracy.
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