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QUESTION: 1

Which of the following is not true for a three-phase system compared to 1-φ system?

Solution:

QUESTION: 2

In a balanced three-phase star connected system, the phase voltages

Solution:

Here, line-to-line voltage leads the respective phase voltage by 30°.

QUESTION: 3

Three identical resistances are connected in delta against a balanced three-phase voltage supply. If one of the resistances be removed, the reduction in the power will be

Solution:

When all the resistance are present,

(Since V_{ph}= V_{L} for Δ connection)

When one of the phase resistance is removed,

QUESTION: 4

In a delta connected system, the line current are

Solution:

QUESTION: 5

A balanced Y-connected load is supplied from a balanced 3-phase 400 V system. If the current in each phase is 15 A and lags the phase voltage by 30°, the total power will be

Solution:

QUESTION: 6

In a balanced three-phase circuit V_{AN}= 1500 ∠20° V and V_{CN} = 1500 ∠-100° V.

The value of V_{BN} will be

Solution:

Since V_{CN} lags V_{AN} by 120°, therefore phase sequence will be ACB.

Here, V_{BN} will lead V_{AN} by 120° but, will have the same magnitude.

so,

QUESTION: 7

The value of the rms line voltage V_{L} for the below circuit if the rms line voltage is 100 V will be

Solution:

Converting the Δ-connected load in star equivalent, we have

So, line to neutral voltage for equivalent Y-load

∴ Line current,

∴ Line voltage at source

QUESTION: 8

Two wattmeters, both have reading of 3 kW when connected for the two-wattmeter method with current coils in lines A and B of a 600 V ABC circuit having a balanced A load. The Δ phase impedance will be

Solution:

we have

(Since W_{1} = W_{2} = 3 kW)

or, cosθ= 1

Hence, the load will be purely resistive.

QUESTION: 9

Three capacitances, each of C/3 Farads are connected in delta. Their equivalent star value for each capacitance is

Solution:

We know that,

QUESTION: 10

The instantaneous value of currents in both phases B and C of a 3-phase balanced system are -20 A. For a phase sequence of ABC, the instantaneous value of current in phase A is

Solution:

Since the system is balanced, therefore

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