Transformers

Problem 653: The full load copper loss and iron loss of a 75 kVA transformer are equal to 1 kW. During a given day, the transformer is loaded as follows: full load at unity pf for 8 hours, one-half load at unity for 8 hours and no-load the rest of the day. Calculate the all day efficiency.

Problem 654: A 6.9 kV/600 V, multi-tap, single-phase distribution transformer is connected to the far end of a distribution line for which the rear end voltage is maintained at 6.9 kV. The equivalent impedance of the transformer referred to low voltage side is 0.05 + j0.25 \(\Omega\) per phase. The impedance of each line is 1+ j2 \(\Omega\). It is required to maintain a terminal voltage of 600 V when the line current of 300 A at 80% pf lagging is delivered to the load. What approximate percentage tapping is required in the primary side? Neglect changes in impedance due to changes in turns ratio.

Problem 655: A 60 Hz alternator generates a single-phase voltage of 120 V and has a reactance of 0.25 ohm. The alternator feeds a line, which is essentially resistive with an impedance of 0.1 ohm. The line is terminated with a 2:1 step up transformer. What should be the load impedance to be connected to the secondary side to have maximum power transfer to it?

Problem 656: A transformer consists of a primary winding with 500 turns and two secondary windings of 125 turns and 36 turns. The 125-turn secondary winding has 60 ohms connected to its terminals and the 36-turn secondary winding has 3 ohms connected to its terminals. If the primary is connected to a 120-V, 60 Hz source, determine the current in the primary windings.

Problem 657: A 3-phase, 60 Hz transformer has primary, secondary and tertiary windings. The primary winding is delta connected and rated 6,600 V. The secondary winding is delta connected and rated 1000 V. The tertiary winding is wye connected and rated 440 V. If the secondary winding has a 3-phase balanced load of 100 kVA at 80% pf lagging and the tertiary winding has a balanced 3-phase load of 50 kW at unity pf, what is the current in the primary winding and its operating pf?

Problem 658: A 20-kVA, 500-V load is to be supplied by an ideal step-up autotransformer from a 400-V source. Find the current in the common winding.

Problem 659: The two windings of a 10-kVA, 2300/230 volt, distribution transformer is connected in series to form an autotransformer giving a small reduction to potential difference from a 2300-volt line. Determine its kVA rating as an ideal autotransformer, operating in the above conditions.

Problem 660: A 10 kVA, 440/110 volt, single-phase transformer has an efficiency of 96% at a rated load of unity power factor. The full load copper loss is 250 watts. If this transformer is to be converted as a 550/440-volt autotransformer, determine its efficiency when supplying full load at 85 percent power factor.

What should be the kVA rating of each two transformers in an open-delta bank when the three-phase balanced load is $290.4$ kVA?

A V or open delta connected transformer bank, composed of two identical units serves a balanced $3$-phase load of $16$ kVA at $230$ volts. Solve for the minimum size in kVA of the transformers needed.

A V (open delta) connected transformer bank serves a balanced $3$-phase load of $40$ kVA at $230$ V and $0.866$ pf lagging. Solve for the minimum kVA of each transformer needed to accommodate the load without overloading.

Two single-phase distribution transformers are banked in V (open delta). If each transformer is rated $37.5$ kVA, solve for the maximum $3$-phase load that the bank can carry without overloading, considering that the load has a power factor of $0.85$ lagging.

An open delta bank, consisting of two single-phase transformers is operating with a balanced three-phase load of $50$ kVA, $440$ V at $0.8$ pf lagging and a single-phase load of $10$ kW resistive connected across c-a leg. Determine the minimum ratings of the two transformers. Assume a phase sequence of a-b-c.

Two (2) single-phase distribution transformers, connected in open delta will supply power to a $200$ hp, $3$-phase induction motor, operating at $0.70$ power factor and $0.90$ efficiency. Solve for the minimum size in kVA of each transformer needed to supply the power without being overloaded.

The Utility Company supplies two single-phase transformers banked in open delta. If each transformer is rated $75$ kVA capacity, what is the maximum $3$-phase load in kilowatts that the bank can carry without suffering overload considering that the load has a lagging power factor of $0.80$.

Two identical $1$-phase transformers connected in open delta will serve a $3$-phase motor load of $250$ hp, $0.70$ pf lagging, $0.90$ efficiency. Solve for the minimum size in kVA of each transformer required to serve the load without overloading.

An open-delta connected transformer bank serves a balanced $3$-phase load of $40$ kVA at $230$ V and $0.866$ pf lagging. Solve for the real power and pf at which each transformer is operating.

An open delta, three-phase transformer consisting of two single-phase transformers is operating with a balanced three-phase load of $50$ kVA, $440$-V at a lagging pf of $0.8$. Determine the operating pf of each transformer.

Two single-phase transformers are connected in V (open delta) and serving a delta connected impedance load, each impedance is equal to $16 \angle 36.87^\circ$ ohms. If the transformer voltages impressed on the impedances are: $V_{ab} = 240 \angle 0^\circ$, $V_{bc} = 240 \angle 240^\circ$, $V_{ca} = 240 \angle 120^\circ$ volts. Solve for the kVA delivered by each transformer.

Two single-phase transformers in an open-delta bank served a three-phase balanced load of $290.4$ kVA. If a third transformer is added for delta operation, what percent increase in load does this represent?