Q. How many Elements do I need to accurately measure the Power or VARs in a circuit?

A. 1 element less than the number of wires

Examples: 3 Phase, 4 Wires  circuit, a 3 Element transducer
Note: a 2 1/2 Element Transducer may be used to measure
Power in a 3 Phase, 4 Wires circuit but the balance of
The Phase to Neutral Voltages will influence the accuracy
of the measurements.
3 Phase, 3 Wires circuit, a 2 Element Transducer
1 Phase, 2 Wires circuit, a 1 Element Transducer

Q. Why is a Watt Transducer that is rated 120Volts, 5 Amps nominal normally rated at 500 Watts per element instead of 600 Watts per Element?

A. Because 500 Watts is a convenient number to work with
The Calibration Watts of the Transducer indicates a Transfer function
The Watts are not just simply the Current Input x the Voltage Input but are
Also a function of Power Factor and the Phase relationships of the Voltages and Currents.

The Output of the Transducer is proportional to the Watts being measured not to just the amount of Voltage and Current that is applied.

Q. If my 3 Phase, 3 Wire system has 120 Volts, 5 Amps input to both of the Elements and my Load is resistive why doesn't the transducer indicate 1200 Watts?

A. Because in a 3 Phase , 3 Wire system, with sinusoidal Voltages and Currents at Unity Power Factor the Power is not equal to the L-L Voltage x Line Current x 2 but is instead equal to the L-L Voltage x Line current x the square root of 3., therefore 120 x 5 x 1.732 = 1,039.2 Watts not 1,200 Watts.

Q. If I multiply the applied Line to Neutral Voltages x the Line Currents x 3 on a 3 Element Transducer or the applied Line to Line Voltages x the Line Currents x the square root of 3 on a 2 Element Transducer, why is the Output from the Transducer low compared to the calculation(s)?

A. Because the Load may not be at Unity Power Factor.

Example: If each of the three Line to Neutral Voltages equal 120 Volts and each of the Line Currents equals 5 Amps applied to a 3 element transducer a person that multiplies 120 V x 5Amps x 3 would calculate 1800 Watts. This number would be OK for Unity Power Factor but the Power would only be 1,350 Watts at 0.75 PF. (1,800 x .75).


Q. After installing a Watt Transducer in my system why is the Power indicated by the output of the Watt Transducer much less than the Power that is being used by the load.?

A. Quite likely this would be due to wiring errors such as cross connections or one Or more phase(s) being reversed.