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Automatic phase changer is an electronic changer which changes or shifts the load from one phase (low voltage phase< 200V) to another phase (high voltage phase 200 to 230 V) if voltage of 1st phase goes below 200V.

In three phase application, if we have requirement to run our equipment using any one phase out of three at standard voltage (230 V), but due to some reason if voltage of the phase in use goes down then we will have to change the phase manually. So this problem will be solved by the automatic phase changer.

It will always provide the rated voltage to load voltage to load if at least one phase is of standard level.

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The circuit consists of 3 identical voltage comparing networks. Operational amplifiers are used as a comparator.

The main power supply (R) is step down by a transformer to 12 V. It is then rectified using diode and capacitor to obtain the proper DC working voltage for comparing networks.

The non-inverting input of IC-741C i.e., pin 3 is set at 5.1 V using zener diode and resistor for reference voltage. The inverting input i.e., pin 2 is set slightly greater than reference voltage at normal condition (i.e., supply voltage of 200 V) which is achieved by voltage divider circuit.

When the phase voltage is in the range of 200-300 V, the inverting input is more than reference voltage, then the output pin 6 of the IC gets high. As a result the transistor does not conduct and relay remains de-energized. Hence the load remains connected to phase ‛R’, but as soon as phase voltage gets down, the inverting gets low than reference voltage and output of IC also gets low. As a result the transistor starts conducting and relay gets energized and it switches the load from previous phase to new phase. The other

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two comparing network also works in same manner.

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The circuit is mainly based on voltage comparator. The comparator compares the input voltage with reference voltage and generated output according to the comparison. The circuit consists of following major electronic component:

1.Centre tapped transformer

2.Op-amp(IC 741)

3.Transistors

4.Relays

5.Zener diodes

6.Diodes

7.Capacitors

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TRANSFORMER:

Transformer is a static electrical device which transfers AC power from one circuit to another circuit without any electrical connection between the circuits. The power transfers from one circuit to other due to electromagnetic induction.

TRANSFORMATION RATIO:

V1 N1 I2

V2 N2 I1

Where,

V1 = voltage across primary winding

V2 = voltage across secondary winding

N1 = number of turns in primary winding

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N2 = number of turns in secondary winding

I1 = current in primary winding

I2 = current in secondary winding

K = transformation constant

TYPES OF TRANSFORMER:

1. STEP - UP TRANSFORMER: It increases the magnitude of voltage with corresponding decrease in current.

2. STEP - DOWN TRANSFORMER: It decreases the magnitude of voltage with corresponding increase in current. The centre tapped transformer used here is a step down transformer.

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2. (A). CENTRE TAPPED TRANSFORMER : It is a step – down transformer in which tapping is done at the centre of secondary winding

P = Primary Winding

S = Secondary Winding

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The operational amplifier is a direct coupled high gain amplifier to which negative feedback is added to control its overall characteristic. It is abbreviated as OP-AMP.

V1

VOUT

V2

V1 is a non-inverting input; that means output is in phase with input and V2 is an inverting input which means output is 180° out of phase. A is the open loop gain of OP-AMP. Op-AMPS are very useful integrated electronic device which is capable of comparing two voltages, if it is used in open loop configuration with measuring voltage at one input and reference voltage on other input terminal.

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PROPERTIES OF AN IDEAL OP-AMP:

1.Input resistance , Ri = ∞

2.Output resistance , Ro = 0

3.Voltage gain , Av = ∞

4.Bandwidth = ∞

5.Vo = 0 , when V1 = V2 , independent of magnitude of V1

6.Characteristics do not drift with temperature

7.Slew rate should be infinity

NOTE: Practical OP-AMPS have properties near top ideal OP-AMP.

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DETAILS OF DIFFERENT STAGES:

INPUT STAGE: It is a dual input balanced output differential amplifier. It contributes maximum gain to OP-AMP (approx 60 db).

INTERMEDIATE STAGE: It is a dual input unbalanced output differential amplifier. It is so called because output of this stage is measured with respect to ground.

LEVEL SHIFTING STAGE: It is an emitter follower circuit in order to shift the DC level at the output of the intermediate stage downward to 0 V with respect to ground.

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OUTPUT STAGE: The output stage consists of push-pull complementary amplifier. This stage increases the output voltage swing and current supplying capability of the amplifier.

POWER SUPPLY REQUIREMENT OF

OP-AMP

An op-amp requires bipolar voltage. That is one positive voltage and other equal in magnitude but negative.

The maximum supply voltage for OP-AMP-IC 741 is

±18 V.

COMMON MODE REJECTION RATIO:

It is the ratio of differential gain (Ad) to the common mode gain (Av). It is denoted by ρ (rho).

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Ideal value of CMMR = ∞

Practical value of CMMR = 10000

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PRACTICAL CIRCUIT:

Rin = input resistance (practically its value is 2 MΩ)

Rout = output resistance (practically its value is 150 Ω)

IDEAL CIRCUIT:

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DC CHARACTERISTIC:

The ideal op-amp draws no curent from the source driving it. The circuit response does not vary with temperature.Practical op-amps do not opertae in this way. It draws source current from source and the circuit response vary with temperature.

INPUT OFFSET VOLTAGE:

When the input of an op-amp are grounded, there is always source output voltage,which is called the offset voltge because the input

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transistor have different β values. Ideally this voltage should not appear at the outout. It is ccalled noise voltage. This offset voltage is compensated by applying a voltage (equal in magnitude to offset voltage) at inverting input of op-amp.

The applied voltage should be :

OUTPUT

Fig : OUTPUT OFF

I/P BIAS CURRENT:

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I/p bias current is a current required to bias thre transistor or FET of differentail amplifier of op-amp into its linear region. In transistor this current is provided by external circuit where as in FET this current is leakage current across the reverse biased gate to channel junction. Ideal value of i/p bias current should be zero wheras practical value for IC 741 is 500 nA at ± 18 V.

I/P OFFSET CURRENT:

I/P offset current is the difference between the two current entering the input terminals of a balanced amplifier for o/p voltage zero.

SLEW RATE:

∆ VO

TIME

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VOLTAGE COMPARATOR:

Voltage comparator is an elctronic circuit which compares the voltage with reference voltage and generate the output.

+ VCC

- VCC

Vout = A ( V1 - V2 )

If V1 < V2,

Vout = -ve

If V1 > V2,

Vout = +ve

It shows that op-amps can be used as a voltage comparator.