Types of circuit connections for wattmeter circuits for different kinds of phases.
2 – Phase, 4 – Wire circuit (not interconnected) may be treated as equivalent to two single-phase circuits. Two wattmeters are connected; total power is arithmetical sum of the two instrument readings.
2 – Phase, 3 – Wire circuit requires two wattmeters connected; total power is algebraic sum of the two readings. This connection is correct for any condition of load and power factor. One wattmeter may be used, if there is no load across the outer conductors and the phases are balanced as to load and power factor; readings are summed for the two switch positions.
2 – Phase, 4 – Wire interconnected circuit requires three wattmeters; total power is the algebraic sum of three readings. This connection is correct under all conditions of load and power factor. It will be noted that the voltage impressed on first meter is 1.414 times the voltage on the other two meters. Two wattmeters, one in each phase, will give the power only when the load is balanced in all four legs.
3 – Phase, 3 – Wire circuit requires two wattmeters connected; total power is the algebraic sum of the two readings under all conditions of load and power factor. If the load is balanced, at unity power factor each instrument will read half the load; at 50% power factor one instrument reads all the load and the other reading is zero; at less than 50% power factor one reading will be negative. When the load is balanced, power may be measured by one wattmeter, using a Y box. This arrangement, which creates an artificial neutral, has two branches which have the same impedance and power factor as the wattmeter’s voltage circuit, which is the third branch of the Y. Total power is three times the reading of the wattmeter.
3 – Phase, 4 – Wire circuits require three wattmeters; the total power is the algebraic sum of the three readings under all conditions of load and power factor. A 3-phase Y system with a grounded neutral is the equivalent of a 4-wire system and requires the use of three wattmeters. If the load is balanced, one wattmeter can be used with its current coil in series with one conductor and the voltage circuit connected between the conductor and the neutral. Total power is three times the wattmeter reading in this instance.
Reactive power (reactive voltamperes, or vars) is measured by a wattmeter with its current coils in series with the circuit and the current in its voltage element in quadrature with the circuit voltage.
Corrections for instrument transformers are of two kinds. Ratio errors, resulting from deviations of the actual ratio from its nominal, may be obtained from a calibration curve showing true ratio at the instrument burden imposed on the transformer and for the current or voltage of the measurement. The effect of phase-angle changes introduced by instrument transformers is to modify the angle between the current in the field coils and that in the moving coil of the wattmeter; the resulting error depends on the power factor of the circuit and may be positive or negative depending on phase relations.
Wattmeter calibration can be best checked on direct current by using normal potentiometer techniques to measure current supplied to the field coils and voltage supplied to the voltage circuit from independent sources, but with an electrostatic tie (a high resistance) between one current terminal and the terminal at the moving-coil end of the voltage circuit to avoid errors from electrostatic forces between fixed and moving coils.
2 – Phase, 4 – Wire circuit (not interconnected) may be treated as equivalent to two single-phase circuits. Two wattmeters are connected; total power is arithmetical sum of the two instrument readings.
2 – Phase, 3 – Wire circuit requires two wattmeters connected; total power is algebraic sum of the two readings. This connection is correct for any condition of load and power factor. One wattmeter may be used, if there is no load across the outer conductors and the phases are balanced as to load and power factor; readings are summed for the two switch positions.
2 – Phase, 4 – Wire interconnected circuit requires three wattmeters; total power is the algebraic sum of three readings. This connection is correct under all conditions of load and power factor. It will be noted that the voltage impressed on first meter is 1.414 times the voltage on the other two meters. Two wattmeters, one in each phase, will give the power only when the load is balanced in all four legs.
3 – Phase, 3 – Wire circuit requires two wattmeters connected; total power is the algebraic sum of the two readings under all conditions of load and power factor. If the load is balanced, at unity power factor each instrument will read half the load; at 50% power factor one instrument reads all the load and the other reading is zero; at less than 50% power factor one reading will be negative. When the load is balanced, power may be measured by one wattmeter, using a Y box. This arrangement, which creates an artificial neutral, has two branches which have the same impedance and power factor as the wattmeter’s voltage circuit, which is the third branch of the Y. Total power is three times the reading of the wattmeter.
3 – Phase, 4 – Wire circuits require three wattmeters; the total power is the algebraic sum of the three readings under all conditions of load and power factor. A 3-phase Y system with a grounded neutral is the equivalent of a 4-wire system and requires the use of three wattmeters. If the load is balanced, one wattmeter can be used with its current coil in series with one conductor and the voltage circuit connected between the conductor and the neutral. Total power is three times the wattmeter reading in this instance.
Reactive power (reactive voltamperes, or vars) is measured by a wattmeter with its current coils in series with the circuit and the current in its voltage element in quadrature with the circuit voltage.
Corrections for instrument transformers are of two kinds. Ratio errors, resulting from deviations of the actual ratio from its nominal, may be obtained from a calibration curve showing true ratio at the instrument burden imposed on the transformer and for the current or voltage of the measurement. The effect of phase-angle changes introduced by instrument transformers is to modify the angle between the current in the field coils and that in the moving coil of the wattmeter; the resulting error depends on the power factor of the circuit and may be positive or negative depending on phase relations.
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