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Q5. With reference to electronic control systems A. Draw a simple block diagram for temperature control B. Describe each component shown in the diagram in (a).

Q4. A. Describe a brush less alternator with a.c. exciter static A.V.R.;

B. State the output voltage characteristics for this type of machine.

Q2. A. State the necessary conditions required prior to the synchronizing of electrical alternators.

B. Describe the type of cumulative damage that may be caused when alternators are incorrectly synchronized;

C. Explain how the damage referred to in (b) can be avoided / reduced.

D. For two alternators operating in parallel state the consequences of:

(i) Reduced torque from the prime mover of one machine.

(ii)Reduced excitation on one machine.

Q4. Explain the effect of reduced voltage on standard squirrel cage motors with respect to EACH of the following:

(i) Burn out; (ii) Starting current; (iii) Starting torque; (iv) Speed.

Q5. With the aid of a block diagram, briefly describe the effect which negative voltage feedback has on an amplifier and state the advantages resulting from the use of negative feedback. (16)

Q6.a) What is the effect on the field flux of an alternator current in the synchronous motor that leads the terminal voltage. (6)

b) A 1,000-kVA, 11,000-V, 3-ϕ, star-connected synchronous motor has an armature resistance and reactance per phase of 3.5 Ω and 40 Ω respectively. Determine the induced e.m.f. and angular retardation of the rotor when fully loaded at (a) unity p.f. (b) 0.8 p.f. lagging (c) 0.8 p.f. leading. (10)

Q7.a) Explain the process of voltage buildup in a self-excited shunt generator. (6) 

b) A shunt-wound generator has a magnetisation-curve given by the figures below. The total resistance in the field circuit is 20 Ohm and the armature resistance is 0.02 Ohm. With the machine on load, estimate the e.m.f. generated and the armature current when the terminal voltage of the machine is 140V. (10)

Field current

(I)_amperes
1.2
2.8
5.0
7.0
7.7
9.0
11.0
Generated e.m.f. (e)_Volts
46
88
126
149
154
162
168
 

Q8. a) Explain how excitation of the rotor is produced and supplied. (6)

b) A 75-kW, 400-V, 4-pole, 3-phase star connected synchronous motor has a resistance and synchronous reactance per phase of 0.04 Ohm and 0.4 Ohm respectively. Compute for full-load 0.8 p.f. lead the open circuit e.m.f. per phase and mechanical power developed. Assume an efficiency of 92.5%. (10)

Q9.a) Explain the principles of A.C. Motors starting and speed control, including the effect on efficiency. (6)

b) A 3-phase induction motor has a 4-pole, Y-connected stator winding. The motor runs on 50-Hz supply with 200V between lines. The motor resistance and standstill reactance per phase are 0.1 Ω and 0.9 Ω respectively. Calculate (a) the total torque at 4% slip (b) the maximum torque (c) the speed at maximum torque if the ratio of the rotor to stator turns is 0.67. Neglect stator impedance. (10)

Q10.a) Draw the complete phasor diagram of the transformer under no-load conditions. (6)

b) The following results were obtained on a 50 KVA transformer: open circuit test-primary voltage, 3300 V; secondary voltage, 415 V; primary power, 430 W. Short circuit test primary voltage, 124 V; primary current, 15.3 A; primary power, 525 W; secondary current full load value. Calculate: (10)

(i) The efficiencies at full load and at half load for 0.7 power factor

(ii) The Voltage regulations for power factor 0.7 (i) lagging, (ii) leading

(iii) The secondary terminal voltages corresponding to (i) and (ii)