# How is the 90-degree phase shift achieved in a Var transducer?

• kanishke007
In summary: The disk rotated at a set rate and the pulses were counted by a computer that then displayed the wattage being produced.
kanishke007
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 ?

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"

Last edited by a moderator:
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 ?

thanks

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.

"www.electricalmanuals.net/files/RELAYS/ALSTOM/MWTU/R8127B.pdf"[/URL]

Here they use a phase sensitive bridge.

Last edited by a moderator:
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.

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.

## 1. What is the difference between a watt transducer and a var transducer?

A watt transducer measures the real power (watts) in an electrical circuit, while a var transducer measures the reactive power (vars) in the circuit.

## 2. How do watt and var transducers work?

Watt and var transducers work by using a current transformer to measure the current flowing through a circuit and a voltage transformer to measure the voltage across the circuit. They then use these measurements to calculate the power (watts or vars) in the circuit using the formula P = VI for watts and Q = VI for vars.

## 3. What are some common applications for watt and var transducers?

Watt and var transducers are commonly used in power systems to monitor and control energy usage. They can also be used in renewable energy systems to track the amount of power being generated and consumed.

## 4. Can watt and var transducers be used for both AC and DC circuits?

Yes, watt and var transducers can be used for both AC and DC circuits as long as they are appropriately calibrated for the type of circuit being measured.

## 5. How accurate are watt and var transducers?

The accuracy of watt and var transducers can vary depending on the specific model and manufacturer. However, most transducers have an accuracy of around 1% to 2%.

• Electrical Engineering
Replies
8
Views
1K
• Electrical Engineering
Replies
9
Views
42K
• Electrical Engineering
Replies
2
Views
7K
• Electrical Engineering
Replies
8
Views
9K
• Electrical Engineering
Replies
1
Views
6K
• Electrical Engineering
Replies
15
Views
8K
• Electrical Engineering
Replies
3
Views
3K
• Electrical Engineering
Replies
5
Views
2K
• Materials and Chemical Engineering
Replies
1
Views
2K
• Electrical Engineering
Replies
2
Views
3K