8/16/2019 Single Phase Electrodynamometer Power Factor Meter _ ELECTRONIC INSTRUMENTATION

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Voltmeter 

 Numerical

InstrumentTransformers Basics

Current Transformer equivalent circuit

Construction of Cur rent Transformer 

Circuit connectionfor current and

 power measurementusing C.T

Relationship incurr ent transformer 

Err ors in currenttransformers

Problems on CT

PotentialTransformer Basics

ActualTransformation ratio

Testing of InstrumentTransformers

Power Factor Meter 

Single phaseElectrodynamometer Power Factor Meter 

Weston FrequencyMeter 

Single phase Electrodynamometer Power Factor Meter

Construction of electrodynamometer type power factor meter.

Construction is shown in Fig.1It consists of two coils 1.Fixed coil which acts as current Coil.

2. Moving coil or pressure coil.

Current coil:

1. Split into two parts and carries the current of the circuit under test.

2. The magnetic field produced by this coil is proportional to the main current.

Pressure coil:

1. Two identical coils A & B pivoted on a spindle.

2. Coil a has a non inductive resistance R connected in series with it.

3. Coil B has a highly inductive choke coil L connected in series with it.

4. The two coils are connected across the voltage of the circuit.5. The value of R & L adjusted to carry the same current at normal frequency.

Working Principle:

1. Current in coil is in phase with the circuit voltage.

2. Current through coil B lags the voltage by an angle 90°().

3. The angle between the planes of the coils is made equal to .

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8/16/2019 Single Phase Electrodynamometer Power Factor Meter _ ELECTRONIC INSTRUMENTATION

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4. There is no controlling torque.

5. Minimum control effect using silver or gold ligaments for connecting moving

coils.

Assumption made:

Current through coil B lags voltage by exactly 90°.

Angles between the planes of the coils is exactly 90°.

 Now, there will be two deflecting torques:

1. Torque acting on coil A.

2. Torque acting on coil B.

The coil windings are arranged in such a way that the torques due to two coils are

opposite in direction. Therefore the pointer will take up a position where these two

torques are equal.

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