Watt and Var Transducers

  • Thread starter kanishke007
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  • #1
A Watt transducer has line to line voltages and line currents as inputs. It calculates the Watts and outputs DC mA. The one i am working on is rated 1mA DC for 500W AC. The relationship is linear.

A Var transducer does exactly the same, but it shifts the input AC by 90degrees to get Vars.

My question is: How do we mechanically/electrically achieve the 90deg ph. shft. in the construction of the Var transducer ?

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  • #2
Volt-ampere reactive (var) is calculated by multiplying the rms values of the potential (volts), current (amps), and the sine of the phase angle between them.

http://en.wikipedia.org/wiki/Volt-amperes_reactive" [Broken]
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  • #3
You are absolutely right. However, that was not my question.

The Var transducer is the same physical device as the Watt transducer. However, to make the Watt tranducer output Vars, there is a phase shift of 90degs applied to the real power hence making it cos(90-angle)=sin(angle), which gives us Vars. So, my question is, how is this phase shift applied mechanically ? is it something to do with the way the turns are wound on the coil ? or something else perhaps ?

  • #4
Well, the transducer measures the phase angle and makes the calculation.

Check out Figure 3 (Application diagram for phase to phase reverse VAR connection) for their Type MWTU 11 Forward and Reverse Power Relay.


Here they use a phase sensitive bridge.
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  • #5
Long ago, electricity power meters (kWh, kVAh and kVArh) operated using eddy-current motors that spun a metal disk. The disk was connected via gears to a series of mechanical dials which could be visually read. The various windings for the eddy-current motors were wound such that the mechanical arrangement produced torque in the disk proportional to the product of the electric fields from the current-windings and the voltage-windings. The phase relationship was alterable during the construction via the relative physical relationship between the windings.

The days it is all done with microprocessor devices & digital signal processing methods. For the most part, this is an improvement, as the old electromechanical instruments would develop inaccuracies as the jewel bearings became worn. However, the older meters were actually more accurate in measuring power in non-linear loads.
  • #6
Hi MThornton,

I bet they are still used a lot. In the '80s when I was involved with watt/var metering, these mechanical meters were used in tie-line substations (metering between two areas/companies) to account for MegaWattHours being sold and bought. The disk had a pattern of holes that allowed light to be detected, creating pulses; each representing 1 MWH.

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