Unit 7

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The reason for the difference between line and phase current in a delta connection is that the current flows through different windings at different times in a three phase circuit.

True

How do you correct the pf and induce VARs?

Add a capacitor bank to circuit

Power is going to be one and half of the single phase.

True

The line current if a delta connection is higher than the phase current by a factor of the square root of 3.

True

If the phase values of voltage and current in a three phase circuit are known, the apparent power can be computed using the formula Va =

3 times phase voltage and phase current

What connection is the line and phase voltage the same in?

Delta

A wye connected three phase alternator supplies power to a delta connected resistive load. The alternator has a line voltage of 480 V. Each resistor of the delta load has 8 ohm resistance. The load is connected directly to the alternator. Therefore the line voltage supplied by the alternator is 160 V.

False

In a wye-connected system, the line voltage is lower than the phase voltage by a factor of the square root of 3.

False

Motor power factor can be computed using the formula pf = Va/P

False

Phase current can be determined by Ip (load) = Z/Ep(load)

False

The horsepower rating of three phase motors and the kilovolt-amp rating of three phase transformers are about 75% greater than for single phase motors or transformers with s similar frame size.

False

The inductive VARs in a three phase circuit can be computed using the formula Va^2 - P^2

False

Formula used to compute voltage in a wye system is

Phase voltage = line voltage/square root of 3

A delta connected alternator is connected to a wye connected resistive load. The alternator produces a line voltage of 240V. And the resistors have a value of 6 ohms. If El(load) is 240V, the. Ep(load) is 138.57 V. Also, the Ip (load) can be computed using ohms law and the value is 23.1.

True

Correcting the power factor of a three phase circuit is similar to the procedure used to correct power factor of a single phase circuit.

True

What connection is made by connecting one end of each of the three phase windings together?

Wye

In a wye connected system the phase and line current are the same.

True

One reason why three phase power is superior to single phase power is that in a balanced three phase system, the conductors need to be only about 75% of the size of conductors for a single phase two wire system of the same kVA rating.

True


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