GCE Advanced Level Past Questions Power Electronics

 GCE Advanced Level Past Questions 

Technical and Vocational Education Examinations

Electrical Power Systems EPS
Power Electronics

Specialty Name (Specialty Code)	Electrical Power Systems EPS (F3)
Subject Title	Power Electronics
Paper Nº	2
Subject Code Nº	7235

 1. Control of a D.C motor         

The circuit shown in figure 1, include the control and the power circuit of a DC motor to make the up and down movements of a system. All the OP-AMP and switches are ideal

a) Study of the control circuit (OA1; R’s and (AO2).

i)      Give the operating mode of the operational amplifiers (OA1; R’s) and (AO2).                  (6 marks)

ii)    prove that the expression of the voltage at the OA1 inverting input terminal in function of V₁ and V₀₁ is 

V- = (V₁ + V₀₁) / 2        (6 marks)

iii) Prove that the expression of the voltage at the OA1non- inverting input terminal in function of V₂ 

V+ = (V2) / 2

iv) Express the voltage at the OA1 output terminal (V₀₁) in function of V₁ and V₂          (6 marks)

   v) What is the expression or the value of V₀₂ when V₀₁ > U, and when V₀₁ < U                    (6 marks)

 b)      Study of the power circuit or motor circuit

i) When V₀₂ > 0; what is the state [conduct (1) or block (0)] of the following elements D₁, D₂, T₁, T₂ and the motor?                                                                                                                                                (5 marks)

ii) When V₀₂ < 0; what is the state [conduct (1) or block (0)] of the following elements D₁, D₂, T₁, T₂ and the motor?                                                                                                                                                (5 marks)

Section B: Optional

2.  Uncontrolled half wave three-phase rectification     

The circuit diagram of figure 2 represents a circuit of a three phase rectification supplying a purely resistive load R. The diodes are supposed to be perfect.

The expressions of the three phase supply voltages are as follow: , , , where ; ; ; T is the period. Take .

Intervals	D1	D2	D3	VL	VD1
0 ≤ θ ≤ π/6
π/6 ≤ θ ≤ 5π/6
5π/6  ≤ θ ≤ 9π/6
9π/6  ≤ θ ≤ 12π/6

Table 1

a) Analyses the circuit of figure 2 above by drawing and completing the table 1                            (10 marks)

b) Draw in bold on the same reference, the wave forms of the voltages V₁, V₂, V₃, V_L across the load R and V_D1 across the diode D₁ using appendix 1                                                                              (8marks) 

c) Determine the maximum reverse voltage across D₁.                                                                      (3 marks)

d) Determine the duration of conduction for each diodes in terms of T.                                                (3 marks) 

e) Prove that the average value of the voltage across the load is                           (6 marks)

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3.               AC-AC converter        

The circuit of figure 3 below is that of an AC-AC converter. The supply voltage V(t) = 220√2sin100πt; its period is T and R is the resistance of the oven which have the value of 100Ω. T₁ is fired at t₀ and T₂ at T/2 + t₀.

 

t	T1	T2
0 < t < t0
t0 < t < T/2
T/2 < t < T/2 + t0
T/2 + t0 < t < T

Table 2

a)     Explain the functioning of the circuit over the period of the supply voltage by filling the table 2. 

N.B if the switch is conducting is 1and when is blocked is 0.                              (4 marks)

b)     Draw the wave form of the load voltage                                              (3 marks)

c)      Find the reading of the Ammeter when t₀ = T/3                                 (4 marks)

d)     Find the power delivered to the oven             n                                   (4 marks)

e)     Express the effective value of the load current in function of t₀                 (4 marks)

f)      Calculate the value of the effective load current when t₀ = T/3                     (4 marks)

g)      For which values of t₀ the power delivered will be maximum, minimum?            (4 marks)

h)     Calculate the maximum and the minimum power.                                         (3 marks)

 4.                   Battery charger 

It is envisaged to design a battery charger for a mobile telephone; the characteristics of the telephone and the battery are respectively 70mW and 300mAh; 3.6V; 0.11Ω. The device chosen for the conception of the charger is shown in figure 4 below.

Assuming that all the components are ideal; V₁(t) = 220√ 2sin wt; f = 50Hz; the transformation ratio of the transformer is 0.0205.

a)     Give the significance of the following indications: 300mAh; 3.6V; 0.11Ω on the battery and 70mW on the telephone.                               (3 marks)

b)     Draw the wave forms of the following voltages and current V₁(t); V₂(t); V_L(t) and I_L(t)    (4 marks)

c)     Compute the value of the resistance R to be connected in series with battery to reduce the maximum current in the load to 18.5mA.       (4 marks)

d)     Determine the turn-on and turn-off time of a diode. Deduce the conduction period      (4 marks)

e)     Calculate the average value of the current in the load.                 (4 marks)

f)      Compute the charging time of the battery; express the answer in days, hours, minutes and seconds.                                                                  (4 marks)

g)     Calculate the effective load current (I_L) and the joule losses                 (4 marks)

I_L = I_Lmas √ [ (t_closing – t_opening)/T ]

h)     Compute the useful power and efficiency of the set up                         (3 marks)

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 5.   Mixed bridge rectification      

Consider the half-controlled single phase bridge rectifier of figure 5. Thyristors and diodes are perfects

The inductance L of internal resistance 0.1Ω is large enough to cause constant current I = 25A flowing through the motor. The bridge delivers an average voltage U = 169V under a firing angle of 45°. The motor speed is 1800 rev/min.

a)      Sketch the waveform of Vs(t) and U(t), indicate the interval of conduction of each component if Vs(t) is a sine waveform of 50Hz frequency                                  (5 marks)                                                    

b)     Calculate the effective value of  Vs                                          (5 marks)                                                      

c)     Calculate the motor’s emf if the armature resistance is 0.4Ω.                (4 marks)                                

d)     What is the mechanical power developed in the armature?                   (4 marks)                   

e)     Calculate the armature input power                                (4 marks)                                                               

f)      What is the new value of emf if the mechanical torque is doubled?                      (4 marks)    

g)     Deduce the new motor’ speed and comment         (4 marks)