A dynamometer wattmeter can be used for - Basic electrical Engineering
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Q. A dynamometer wattmeter can be used for
(A) ac only
(B) dc and ac both
(C) None
(D) dc only
Answer: dc and ac both
Explanation: Dynamometer Watt meter:- If two coils are connected such that, current proportional to the load voltage, flows through one coil and current proportional to the load current, flows through other coil, the meter can be calibrated directly in watts. This is true because the indication depends upon the product of the two magnetic fields. The strength of the magnetic fields depends upon the values of the current flowing through the coils. Advantages: 1) In dynamometer type wattmeter, the scale of the instrument is uniform (because deflecting torque is proportional to the true power in both DC as well as AC and the instrument is spring controlled.). 2) High degree of accuracy can be obtained by careful design; hence these are used for calibration purposes. Disadvantages : 1) The error due to the inductance of the pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect) 2) In dynamometer type wattmeter, stray field may affect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case.
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