Problems

Author: E.E. Kimberly

	Problem	Answer
1	A group of Y-connected resistors is connected in parallel with a group of Δ-connected resistors to a 230-volt line, as shown in the accompanying diagram. Calculate the line currents.	IL1 = 116.5 A IL2 = 123.6 A IL3 = 106.3 A
2	As indicated in the diagram at the left, an induction motor drawing 50 amp at 220 volts with 50% power factor, lagging, from a three-phase line has connected in parallel with it between two line wires a single-phase electric furnace which draws from the line 25 amp at 100% power factor. Calculate the total active power. Calculate the current in each line.	IL1 = 50 A IL2 = 72 A IL3 = 56 A
3	If, in Problem 2, the motor power only be measured by the two-wattmeter method, what would be the reading of each wattmeter?
4	By use of a given size of wire, a load of 1000 kilowatts can be delivered over a two-wire single-phase line with a loss of 750 watts per wire. How much power could be delivered over a three-phase line through the same distance with the same weight of copper and with the same power loss in the line?	1155 kW
5	A balanced three-phase, Y-connected load of three single-phase impedances each of which is 5+j6 ohms is connected to a 230-volt line. Determine the line current and the power in the load. Draw the vector diagram.
6	Three impedances ZAB = 4+j0.5, ZBC = 5-j6, and ZCA = 10-j0 are connected in delta across a 230-volt three-phase line whose voltage sequence is VAB, VCA, VBC. Calculate the line currents and the total power in the load. Draw the vector diagram of voltages and currents.
7	Calculate the line currents in Problem 6 for the condition of reversed phase rotation.
8	A balanced three-phase load at 60% power factor is being metered by the two-wattmeter method. The total power is 4000 watts. What is the power indicated by each meter?
9	Calculate the reactive volt-amperes in Problem 8.
10	A balanced load of three hundred 75-watt lamps on a three-phase system is measured by the two-wattmeter method. What will be the reading on each wattmeter?
11	An induction motor draws from a three-phase line 5000 watts at 70% power factor. If a single-phase wattmeter is connected with its current coil in line A and with its potential coil across lines B and C, how much will be the reading of the wattmeter and what will it measure?
12	A three-wire line of No. 1 conductors 200 ft long with Type-SN insulation (see Appendix) supplies a 120/240-volt single-phase load with maximum permissible current. What per cent of load increase would be permissible if the system voltage were changed to three-phase 240 volts at the load?	73.2%
13	If three 5-ohm resistors connected in wye draw 10 amp per phase from a three-phase line, how much power will they draw from the same line if they are connected in delta?	4500 W