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In electrodynamometer type wattmeters, the inductance of pressure coil produces error. The error is
  • a)
    constant irrespective of the power factor of the load
  • b)
    higher at higher power factor loads
  • c)
    higher at lower power factor loads
  • d)
    highest at unity power factor loads
Correct answer is option 'C'. Can you explain this answer?
Verified Answer
In electrodynamometer type wattmeters, the inductance of pressure coil...
The error caused by pressure coil inductance is VI sin ϕ tan β. W ith low power factor, the value of ϕ is large and therefore, the error is correspondingly large.
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In electrodynamometer type wattmeters, the inductance of pressure coil...
Error in Electrodynamometer Type Wattmeters

Electrodynamometer type wattmeters are used to measure AC power. However, the inductance of the pressure coil produces an error in the measurement of power. The error is higher at lower power factor loads.

Explanation

The pressure coil of an electrodynamometer type wattmeter is an inductive coil. It is connected in series with the load and carries the load current. The voltage coil of the wattmeter is connected across the load and carries the voltage of the load.

When the load current flows through the pressure coil, it produces a magnetic field around it. The magnetic field interacts with the magnetic field produced by the voltage coil, causing the pressure coil to move. The movement of the pressure coil is proportional to the power being measured.

However, the inductance of the pressure coil produces an error in the measurement of power. The inductance of the pressure coil causes the current flowing through it to lag behind the voltage across it. This lagging current produces an additional magnetic field that interacts with the magnetic field of the voltage coil, causing the pressure coil to move more than it should.

At unity power factor loads, the current and voltage are in phase, and there is no lagging current in the pressure coil. Hence, the error is the lowest. At low power factor loads, the lagging current in the pressure coil is highest, causing the error to be the highest.

Conclusion

In electrodynamometer type wattmeters, the inductance of the pressure coil produces an error in the measurement of power. The error is higher at lower power factor loads due to the lagging current in the pressure coil. At unity power factor loads, the error is the lowest. Therefore, the wattmeter should be calibrated for the load's power factor to obtain accurate measurements.
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In electrodynamometer type wattmeters, the inductance of pressure coil produces error. The error isa)constant irrespective of the power factor of the loadb)higher at higher power factor loadsc)higher at lower power factor loadsd)highest at unity power factor loadsCorrect answer is option 'C'. Can you explain this answer?
Question Description
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