Q2. A non-linear element is connected in series with a resistor across 240 V d.c. supply.
The non-linear element is governed by the la I = kv2. When the resistor is set to 10W the supply current is 12 A.
(a) Calculate EACH of the following:
(i) The resistance value required to reduce the current to 8 A:
(ii) The resistance of the nonlinear element when the current is 8A.
(b) The supply voltage is reduced to 159 V. calculate the power dissipated in the non-linear element if the series resistor is reset to 10W.
Q1. A 120 mF capacitor is charged through a resistance from a 12 V d.c. supply and the instantaneous charging current at switch on is 2.55 mA.
(i) The value of the charging resistance:
(ii) The time constant;
(iii) The voltage across the capacitor after 2 s of charging;
(iv) The energy stored after 2 s.
(b) After 2.5 s of charging the supply is switched off and the capacitor is discharged through a 1.2 kW resistor. Calculate EACH of the following:
(i) The new time constant;
(ii) The voltage across the capacitor after 124ms;
(iii) The discharge current at 124 ms.
Q1. A series circuit comprising a pure resistor R, a coil having resistance and inductance and a capacitor is connected in series across a 60 V variable frequency supply. When the supply frequency is 400 HZ the current reaches its maximum value of 0.8 A and the voltage across the pure resistor 'R' and the capacity are 40 V and 200 V respectively.
Calculate EACH of the following:
(a) The value of the pure resistor 'R'
(b) The value of the capacitor:
(c) The resistance and inductance of the coil.
(d) The p.d. across the coil.
(e) The coil power factor.
Q5. (a) State two disadvantages of operating electrical circuits at a low power factor.
(b) a 3-ph, 440V, 80 kVA transformers supply a unity power factor load of 15 kW and an inductive load of 55 kw and power factor of 0.67. Determine the minimum kVAR rating of a load capacitor bank to ensure that the supply transformer is not overloaded.
Calculate the current supplied by the transformer in EACH pf the following;
(i) Prior to the power factor correction being applied;
(ii) After the power factor correction is applied.
Q1. (a) describe the principle of operation of a 1-ph, a.c. power transformer.
(b) A 500 kVA power transformer has a full load copper loss of 4 kW and iron loss of 2.5 kW.
calculate EACH of the following;
(i) the kVA load at maximum efficiency;
(ii) the maximum efficiency for a load power factor of 0.75 lagging at this kVA rating in Q6(b)(i).
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