Unit 7
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
