cameroon gce A level 2024 automatic control of electrical machines 2
cameroon gce A level 2024 automatic control of electrical machines 2
1. Direct current machine
For a given direct current machines, we have the following:
Field resistance: 200£}.
Armature resistance: 0.8Q.
Nominal values are indicated by the manufacturer: supply voltage: U ~ 120V; armature current
rotation 2400tr/min for an excitation of 120V.
The machine been separately excited, we supply separately the field Vvith a 120V.
a) Calculate the field excited current.
b) Determine the value of the starting rheostat needed to limit the starting current at 1
c) Calculate under nominal conditions:
i) The back emf
ii) The total mechanical power developed and the value ot the nominal torque.
la – 10A; speed of
(2 marks)
.5 In. (2 marks)
(2 marks)
(4 marks)
2. Three — Phase Asynchronous Motor > , ^
A three phase Asynchronous cage motor of 220/380V, is supply from a 127/220V, 50Hz network. The resistance
measure between two stator phases is 3.5£2. On No–load test: the motor rotates at a frequency Ns practically equal to
3000rev/min and the two wattmeter method gave the following indications: P|=460W, ?2= –260W. The line current is
3.32A.
( 1 mark)
( 1 mark)
a) How are the motor winding coupled?
b) What is the number of poles in the motor stator? .
c) Calculate:
i) The power absorbed. . .
ii) The no–load power factor
iii) The stator copper losses.
d What is the value of the rotor copper losses under this condition? Explain,
e) Calculate the magnetic losses Pm, knowing that the mechanical loss amount to 20W.
( 1 mark)
( 1 mark)
(2 marks)
(2 marks)
(2 marks)
3. Single– Phase Transformer
and X1=5.2Cl , X2=0.015£2 respectively Calculate for the transformer:
a) Equivalent resistance referred to the primary.
b) Equivalent resistance referred to the secondary.
c) Equivalent reactance referred to both primary and secondary.
d) Equivalent impedance referred to both primary and secondary.
e) Total copper losses using individual winding resistances
f) Total copper losses using referred resistances.
(1.5 mark)
( 1 mark)
(2 marks)
(1.5 marks)
(2.5 marks)
(1.5 marks)