Question Electrical Technology

Q Question 1 SI quantities and units (10 Marks) An electric motor rated at 55 kW and 1460 r/min has a full-load efficiency of 0.845. a. Calculate the rated torque Tr for the motor. (6 marks) b. Calculate the power input Pin when the motor operates at rated load. (4 marks) Electrostatics; capacitance [Covers objectives listed in module 2]

Question 2 (25 Marks) Three parallel-connected capacitors, C1

, C2 , and C3 , with capacitances of 20 μF, 15 μF and

33.0 μF respectively, are connected in series with a 10-μF capacitor C4 and a 120 V DC source E. a. Draw the circuit diagram. Calculate the equivalent capacitance Ce of the parallel connected capacitors. (8 marks)

b. Calculate the total charge QT stored by the parallel-connected capacitors. (8 marks) c. Calculate the steady-state charges Q1, Q2 and Q3 held on C1, C2 and C3 respectively. (9 marks) (Conductors; resistors; heating; electrolysis; batteries [Covers objectives listed in module 3]) Question 3 (15 Marks) A copper wire coil has 2600 turns. The wire has a cross-sectional area of 0.227 mm2

and a mean length per turn of 150 cm. At the normal working temperature of the coil, the resistivity of copper is 20.6 nΩ . m. a. Calculate the resistance of the coil at its working temperature. (7 marks) b. Calculate the power dissipated when the coil is connected across a 120 V DC

source. (8 marks) Question 4 (15 marks) An alkaline cell is discharged at a constant current of 7.5 A for 5 hours, the average terminal voltage being 1.21 V. A charging current of 3.5 A is required to be maintained for 15 hours to bring the cell back to the same initial state of charge, the average

terminal voltage being 1.45 V. a. Calculate the ampere-hour efficiency (percent) of the cell. (7 marks) b. Calculate the watt-hour efficiency (percent) of the cell. (8 marks)Assignment 2: Page 4 of 6. DC principles, circuits, laws, theorems; applications

[Cover objectives listed in module 4] Question 5 (25 marks) A battery E with e.m.f. of 24 V and internal resistance Rint of 0.212 Ω and a resistor

R1 (20 Ω) are connected in series across three parallel-connected load resistors: R2 (25 Ω), R3 (8 Ω) and R4 (12 Ω).

a. Draw the circuit diagram and determine battery terminal voltage VT. (9 marks) b. Calculate the powers P1 and P3 dissipated in R1 and R3 respectively. (6 marks) c. In 40 - 60 words discuss what Pint as a loss represents in the battery, and where

these losses may result from; whether these losses are likely to remain the same throughout the battery life or operation; whether Rint is significant in this circuit; and in what battery array arrangement that Rint becomes highly significant. (10 marks) Question 6 (35 Marks) Given in figure 1: E1 = 200 V;

R1 = 18 Ω; R2 = 30 Ω; R3 = 30 Ω; R4 = 16 Ω and RL = 33 Ω a. Draw the Thevenin

equivalent circuit to determine the value of current IL. (16 marks) b. Check your answer using ∆-to-Y transformation (including

circuit diagram) and

Kirchhoff's laws. (19 marks) Question 7 (15 Marks) For figure 2: E1 = 11.6 V;

E2 = 12.2 V; R1 = 0.32 Ω; R2 = 5.00 Ω; R3 = 15.0 Ω

R4 = 0.125 Ω RL = 30 Ω Use Millman's theorem to develop an equivalent circuit diagram and Ohm's law to

calculate the value of currents IL , I2 and I4 . (15 marks) Figure 1

R1 RL R3

R4 R2 IL V2 V3 I1 I4 I2 I3 E1 + - + - E2 Figure 2 b IL RL R3 R2 R4

R1 a c d E1 +

-Assignment 1: Page 5 of 6. Electromagnetics [Covers objectives listed in module 5]

Question 8 (10 marks) A uniform and straight wire, 350 mm long, is moved at a constant speed of 10 m/s

perpendicular to its length and to a uniform magnetic field. The e.m.f. generated in the wire is 0.7 V. The wire forms part of a closed circuit having a total conductance of 75 S. Calculate the: a. Flux density B of the magnetic field. (5 marks)

b. Force F acting on the wire. (5 marks) Question 9 (10 marks) An iron core of cross-sectional area 250 mm2 and length 420 mm is evenly wound with a coil of 120 turns. When a current of 0.45 A flows in the coil, a magnetic flux density of

0.132 T is set up in the iron core. Calculate the: a. Relative permeability μr of the iron. (5 marks)

b. Reluctance S of the iron core. (5 marks) DC machines

[Covers objectives listed in module 6] Question 10 (40 marks) A shunt DC motor under test gave the following results: Load condition Supply voltage VL No Load

110 V Full Load 110 V

Supply current IL 4.0 A 25 A Field circuit resistance Rf Armature circuit resistance Ra 75 Ω 0.5 Ω

75 Ω 0.5 Ω Rotational speed 1860 r/min …r/min

Assume constant flux per pole. Neglect stray-load and brush losses.

a. Draw a circuit diagram, calculate the percent efficiency η of the motor at full load. (20 marks)

b. Calculate the % speed regulation ε of the shunt motor at full load. (12 marks) c. Calculate the motor output torque To at full load. (8 marks)Assignment 2: Page