Q17. A coil of inductance 0.25 H and resistance 100 Ω is connected to a 200 V d.c. supply via a changeover switch. When the switch is operated the coil is disconnected from the supply and connected in parallel with a 75 Ω resistor.
Calculate EACH of the following for the coil in parallel with the resistor.
a) the energy stored in the coil. (4)
b) the induced e.m.f. at the instant of changeover. (2)
c) the time constant. (2)
d) the initial rate of decay of the current. (3)
e) the current after a time equal to one time constant. (2)
f) the time taken for the current to fall to one third of its original value (3)
Q18. A three-phase, 415V, 4 -wire, unbalanced load has currents as shown in Table.
a) Sketch, approximately to scale, a labelled phasor diagram to show the relationship between the phase voltages and the currents given in Table Q3, include angles.
b) Calculate EACH of the following:
(i) the current in the R phase and its phase angle with respect to VRN (4)
(ii) the total load power. (4)
c) Sketch and label the current phasor IR on the diagram for Q3(a), and indicate the phase angle φR
Q19. Fig shows a power flow diagram for a three-phase, 2 pole, 6.6 kV, 60 HZ, Induction motor. At full-load the motor efficiency and power factor are 94% and 0.88 lag respectively, the output power is 200 kW and slip is 1.2%.
a) Calculate EACH of the following:
(i) the power input Pin (2)
(ii) the full-load current Ipfi (2)
(iii) the stator, rotor and mechanical losses. (8)
(iv) the shaft torque. (2)
b) State the two components of stator loss.
Q20. Fig shows a flame failure detector circuit which uses a light dependent resistor LDR, a light emitting diode LED, and a transistor switch. When the transistor is FULLY ON the collector current is 10 mA. The base current is negligible and the collector emitter voltage VCEMIT is 0.15 V.
a) Explain the operation of the circuit.
b) Calculate the minimum LDR resistance at which the transistor turns on.
c) Calculate EACH of the following when the transistor is FULLY ON:
(i) the voltage across the LED.
(ii) the power supplied to the circuit.
d) Describe the change in circuit operation if the positions of LDR and R1 are swapped.
Username or email address *Required
Password *Required
Note: Entering wrong username in the login form will ban your IP address immediately. Entering wrong password multiple times will also ban your IP address temporarily.
Log in
Lost your password? Remember me
No account yet?
