Electrical Engineering Answers

Question 2 20 marks

Describe the concept of harmonics and harmonic distortion. Illustrate your

answer with suitable annotated diagram(s).

Write on ANY TWO practical applications of feedback concepts to real-life

problems. The practical applications can be in any area, e.g. electronics,

control systems, mechanical systems, automobile systems, banking and

finance, business, economics, biological systems, medical sciences, industry,

etc.

Your answer should include a statement of the problem and how feedback

concepts are used for solving it. The answer should be supported with relevant annotated diagrams, and any useful basic equations. Elaborate

equations are not required.

Research any three electrical problems found on any three motor vehicles as well as documentation needed to solve the problems. 

Range: any three electrical problems examples (any lights, wiper motor, hooter cigarette, lighter). Documentation: current flow diagrams, specifications, manufacturer reports.

State the expression for intensity and amplitude of light diffracted through multiple slits. starting from them obtain the conditions for Maxima of diffraction pattern. draw the intensity distribution curve

In a certain eight-pole, 50 Hz three-phase induction motor, the rotor resistance per phase is 0.04 Ω and the maximum torque occurs at a speed of 645 revolutions per minute (rpm). Assuming that the air-gap flux is constant at all loads, determine the torque developed as a percentage of maximum torque:

(a) at starting (8.0 marks) and  

(b) when the slip is 3 per cent (5.0 marks).                                           

Hint: Some derivation is necessary and reasonable approximations/assumptions will have to be made in order to solve this problem.

A 2200-V, 50-Hz, delta-connected three-phase induction motor has a star-connected slip-ring rotor with a phase effective transformation ratio of 3.8. The rotor resistance and stand-still leakage reactance are 0.012-ohm and 0.25-ohm per phase, respectively. Neglecting stator impedance and magnetising current,

(a) determine the rotor current and the corresponding stator and line currents at the instant the motor is started with the slip-rings shorted (4.0 marks).

(b) if the maximum allowable current in the motor supply lines is 600 A, determine the external rotor resistance per phase required to achieve this requirement (6.0 marks).

(c) determine the full-load currents if the full-load slip is 3% (4.0 marks).  

(14.0 marks in total)

A DC shunt motor is driving a constant torque load at the rated speed and rated terminal voltage. The motor has a terminal voltage of 115 V with speed of 312 rpm and the field constant is 3 V.sec. If the terminal voltage of the motor is reduced by 10 %, calculate the motor speed and assuming that the field voltage is also reduced by the same ratio.

A 6600 V, y-connected, 3-phase alternator has a synchronous reactance of

6 Ω

per phase and a negligible resistance. At a certain field current the armature is

known to induce 5800 V per phase. Calculate the maximum power that can be

developed by the alternator at this excitation.