While using wattmeter for power measurement, some errors will happen in the readings particularly when we use in ac systems. They are due to

1. Pressure coil inductance
2. Pressure coil capacitance
3. Mutual inductance between pressure coil and current coils.
4. Power loss in resistance of coils
5. Eddy currents
6. Stray magnetic fields
7. Vibration of moving system.

In this post we will discuss about each of the above causes. Before proceeding further it is recommended to read about wattmeter construction and operation.

#### Pressure coil Inductance

• Due to self inductance, the pressure coil creates phase-shift in the current flowing through it.
• So the true angle between voltage and  current slightly changed, which leads to  an error in the readings.
• A low value capacitor across a part of the multiplier will reduce the effect of pressure coil inductance.

#### Pressure coil capacitance

• The series resistance (swamping resistance and multiplier) in pressure coil circuit has many turns.
• So in addition to inductance seen before, there are inter-turn capacitance also.
• If this capacitance effect exceeds the inductance effect, a phase-shift in pressure coil current will cause an error.
• When pressure coil capacitance roughly equals the inductance, the errors cancel each other.
• In most cases the inductance is more than the capacitance and thus the shunting capacitor mentioned before will serve the purpose.

#### Mutual Inductance

• The mutual inductance between pressure coil and current coil introduces a phase shift in coil currents and hence an error.
• The mutual inductance varies with shifting of position of pressure coil as it deflects.
• The coil systems are so arranged that they are always in zero position for mutual inductance.
• Torsion wattmeter is an example for this kind of design.

#### Power loss in resistance

• The ohmic resistance of both current and pressure coils account for a loss of power and hence an error.
• For small currents, the pressure coil is connected before current coil.
• For large currents the pressure coil is connected after the current coil.
• In the latter case a compensating coil is usually employed to cancel the effect of pressure coil current.

#### Eddy currents

• These are induced in the metal parts adjacent to the coils and also within the thickness of conductors by ac magnetic field.
• The main magnetic fields are slightly altered by these induced eddy currents.
• Metal parts are avoided to the extent possible and when used they are kept far away from current coil.
• When load current in large, the current coils may be wound with stranded conductors.

#### Stray magnetic fields

• Because of air cored coils, any surrounding magnetic field will easily interfere with main fields inside and cause an error.
• A steel cylinder is placed around the coils to shield them.

#### Vibration of the moving system

• In 50Hz ac systems, the power curve crosses zero ( 50 x 2)100 times in a second.
• The moving system as a whole vibrates at this frequency.
• If the natural frequency of oscillation of the moving system is anywhere near 100, the oscillatory effect will be much due to mechanical resonance.
• Hence by proper mechanical design, the natural frequency of the moving system is made far away from 100.