Question

A three phase induction motor draws a starting current which is five times the full load current with direct on line starting, while developing twice the full load torque.
Q2.1. Based on the two facts that starting current is proportional to the applied voltage and that the starting torque is proportional to the square of the applied voltage, determine voltage (as a per cent of rated voltage) required to obtain a starting torque equalling the full load torque. (10 marks)
Q2.2. What is the starting current under this condition? (5 marks)
QUESTION 3 (20 marks)
Q3.1 What are the advantages of shell type transformers over core type?
Q3.2 Why are ac machine cores laminated but dc machine cores are not?
Q3.3 Why starting currents of dc machines are very large compared to their current ratings?
Q3.4 What is transformer inrush current?
Q3.5 Can only two pieces of identical single phase transformers be used to supply a three phase load? Discuss the issue.
Q3.6 In starting a separately excited dc motor, the field circuit is energized before the armature circuit. Explain why this is done.
Q3.7 Why are alternators in an interconnected power system run at a single speed?
Q3.8 What per cent of total losses in a dc motor are due to hysteresis and eddy current? Discuss.
Q3.9 Induction machines are very commonly used in industries. Why not induction generators?
Q3.10 When a 240 volt 50 Hz 4 pole induction motor operates at 1450 rpm, what would be the rotor circuit frequency?
(2 marks each)
QUESTION 4 (5 marks)
A three phase star connected 6.25 kVA 220 volt synchronous generator has a reactance of 8.4 ohms per phase.
Q4.1 Using the generator ratings as base quantities, determine the per unit reactance. (3 marks)
Q4.2 Determine the reactance value as a per unit quantity for a 230 volt 7.5 kVA system.

please provide me a solution.

Answer 1

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