cameroon gce advanced level 2025 electrical automatic control of electrical machines 2
cameroon gce advanced level 2025 electrical automatic control of electrical machines 2
Here is the data extracted from the image, organized by section and question number.
SECTION A: ELECTRICAL MACHINES
1. Three phase alternator
A 26-pole star connected three phase alternator delivers a line voltage of 5000V at a frequency of 50Hz. The internal characteristics is assumed to have the equation Ea=10.5If. Where Ea is the line emf and If the field current. The resistance of one stator winding is 5.4mΩ. A short circuit test gave the following results: I=3.34A, If=2000A.
Calculate:
a. The rotor speed (4 marks)
b. The synchronous reactance per phase (3 marks)
c. Neglecting stator resistance, find the value of the field current If for a load current I=300A at 0.9 power factor lagging. (4 marks)
d. The stator resistance being 13.6mΩ and the total mechanical losses 400W calculate the efficiency for the same load as in question 3. (2 marks)
2. Single phase transformer
The following tests were performed on a single-phase transformer:
No load test: V10=220V, V20=44V, I10=1.4A, P10=80W
Short circuit test: V1′=40V, Psc=250W, Isc=20A
Full load test: I2FL=100A, cosϕ=0.9 (lagging).
The transformer is supplied by a single-phase supply of 220V-50Hz.
Determine:
a) The transformer ratio. (2 marks)
b) The number of secondary turns knowing that the primary has 520 turns. (1 mark)
c) The equivalent parameters Re1 and Xe1 referred to the secondary. (2 marks)
d) The secondary voltage drop by means of KAPP approximation. (1 mark)
e) The load voltage at the secondary. (1 mark)
f) The power absorbed by the load. (1 mark)
g) The transformer efficiency knowing that the joules loss at no load is negligible. (2 marks)
3. Direct current machine
A perfect compensated bipolar dc shunt motor has the following rated characteristics:
- Supply voltage of the armature:
- Armature current:
- Field current
- Speed
- Armature resistance: Ra=0.05Ω
The motor is operating under a constant voltage of 120V. The constant losses are Pc=785W.
a) Calculate the back e.m.f of the motor. (2 marks)
b) Compute:
i) The useful power of the motor. (2 marks)
ii) The useful torque of the motor (2 marks)
iii) The efficiency of the motor (2 marks)
c) Find the value of the starting rheostat in order to limit the starting current to 2Ia? (I_a being the rated current). (2 marks)
SECTION B: CONTROL SYSTEMS
4. FUN FAIR MACHINE
1- DESCRIPTION
A Fun fair machine of figure 1 consists of a fixed internal column and mobile swinging external column which swings about this fixed column. This mobile part carries two wings on which baskets are hung and they rotate about these columns.
- MOTOR M is three phase wound rotor asynchronous motor, three steps of starting, braking by injection of direct current obtained from three phase rectifier bridge with current limiting resistance and controlled by contactor FR.
- Cylinder U controls the upward and downward movement of the mobile part with the help of a solid rope and intermediary pulleys. This up position are limited by limit switches h and b respectively.
