Errors in dynamometer type wattmeters

1. Pressure coil inductance
      In an ideal dynamo-meter type watt meter the current in pressure coil in phase with the applied voltage . But in practically the pressure coil of watt meter has an inductance and current in it will lag behind the applied voltage . If there is no inductance the current in pressure coil will be in phase with the applied voltage . In the absence of inductance in pressure coil of wattmeter , it will read correctly in all power factors and frequency .
     The wattmeter will read high when the load power factor is lagging ,as in that case the effect of pressure coil inductance is to reduce the phase angle between load current and pressure coil current . Hence the wattmeter will read high . This is very serious error.
      The wattmeter will read low when the load power factor is leading as in that case the effect of pressure coil inductance is to increase the phase angle between load current and pressure coil current .Hence the wattmeter will read low.
Compensation for inductance of pressure coil.
       Inductance of pressure coil can be reduced by means of capacitor connected in parallel with a portion of multiplier (series resistance).

2.Pressure coil capacitance.
      The pressure coil circuit may have capacitance in addition with inductance.This capacitance mainly due to the inter turn capacitance of the series resistance.The effect of capacitance is opposite to that due to inductance.Therefore the wattmeter will read high when the load power factor is leading.
       The inductance in pressure coil circuit will always more than inductance,hence the error caused by capacitance will be nullified by that due to inductance.

3.Error due to mutual inductance.
       Errors may occurred due to the mutual inductance between the current and pressure coils of the watt meter.These errors are quite low at power frequencies.But they increased with increase in frequencies.
     The effect of mutual inductance can be avoid by arranging the coil system in such a way that they have no mutual inductance.So we can eliminate the errors due to mutual inductance.The Drysdale Torsion head wattmeter is an example for such type.

4.Eddy Current errors.
      Eddy currents are induced in the solid metal parts and within the thick conductors by the alternating magnetic field produced by the current coil.This eddy current produce their own magnetic field and it will alter that produced by the main current in the current coil and thus error occurred.
  This error can be minimized by avoiding solid metal parts as much as possible and by using stranded conductors for high current applications.

5.Stray Magnetic field Errors.
     The electrodynamometer type wattmeters has a weak operating field and therefore it effected by stray magnetic fields it will resulting in serious errors. Hence these instruments should be shielded against stray magnetic field.

6.Errors caused by vibration of moving system.
     The torque on the moving system varies with frequency which is twice that of voltage.If the parts of the moving system has a natural frequency which is resonance with the frequency of torque pulsation,the moving system would vibrate with a considerable amplitude.These vibrations will cause errors.This error can be reduced by design.

7.Temperature Error.
     The change in room temperature may affect the indication of wattmeter. This is because of change in temperature will change in resistance of pressure coil and stiffness of springs which provide controlling torque.This effect are opposite in nature and cancel each other.The use of material of having negligible temperature coefficient of resistance will reduce change in resistance the pressure coil with change in temperature.