cameroon gce intermediate level 2024 electrical machines 2
cameroon gce intermediate level 2024 electrical machines 2
SECTION A: MAGNETISM AND ELECTROMAGNETISM
1) a) Produce table 1 and fill it indicating the corresponding units of measurement for magnetic quantities
Table 1
| UNIT OF MEASUREMENT | S/N I u |
MAGNETIC QUANTITY Magneto– motive force (cmf) Magnetic flux (<I>) |
| in Magnetic field density (H) IV Flux density (B) |
||
| v | Magnetic reluctance (R) |
(2.5 marks)
b) A current of 3A is flowing through a coil having 2000 turns and a length of 40cm. knowing that the
permeability of free space is An x 10‘7, determine:
i) The magnetic flux O in the coil.
ii) The flux density B of the coil if the cross sectional area is 5cm2
iii) An iron core with a relative permeability pr is inserted in the coil. Find the new value of flux density.
(3 marks)
(3 marks)
(1.5 marks)
2) a) Give four characteristics of lines of magnetic flux
b) What is the difference between a permanent magnet and an artificial magnet?
c) Explain what will happen when a long straight conductor is moved through a uniform magnetic field.
(2 marks)
The diagram of figure 1 shows an electromagnet. Reproduce the diagram and draw the magnetic field in and
around the magnet.
b) A conductor of length 200mm moves at a uniform velocity of lOm/s under a magnetic field having a flux
density‘ of 1 5T. Calculate the cmf induced if the conductor is at:
| i) right angles to the field. ii) 30° to the field. |
(2 marks) |
(2 marks)
(2 marks)
c) A direct current of 2A is passed through a coil of 2000tums and produces a flux of 0.2mWb. Assuming that
the whole flux links with all the turns, calculate the inductance of the coil and the voltage developed across the
coil if current interruption is in 1ms (4 marks)
SECTION B: STATIC MACHINES
4) Tests earned out on a single phase transformer whose primary has 5000 turns gave the following results:
– No load test: Ui = 5000V, U2 = 220V
– Load test: Ui = 5000V, U2 = 210V, I2 = 150A, P2 = 26460W
Efficiency = 0.95
Determine:
a) The transformation ratio and the number of turns of the coil at the secondary.
b) The power factor of the circuit connected to the secondary‘.
c) The current drawn by the primary on load knowing that the primary power factor is 0.9.
5) A single phase transformer has a step down ratio of 4:1 and the high voltage winding is supplied with
440V. If the secondary full load current is 50A at a lagging power factor of 0.6. neglecting the losses, a)
a) Draw the diagram of the transformer described.
b) Calculate:
The rating of the transformer in KVA
The power supplied to the load.
The primary current.
i)
ii)
iii)
6) An ideal transformer lias 150 primary turns and 750 secondary turns. The primary is connected to a 240V,
50Hz source. If the secondary supplies a load of 4A at a lagging power factor of 0.8, determine:
a) The transformation ratio.
b) The primary current.
(2 marks)
(2 marks
