CHAPTER 10 QUESTIONS

10.1     QUESTIONS

1.            What do you understand by ‘active power’?  What type of circuit absorbs only active power?  What is the relationship between current and voltage in a circuit passing only active power?

2.            What do you understand by ‘reactive power’?  What is the relationship between current and voltage in a circuit passing only reactive power?  In what unit is reactive power measured?

3.           What is ‘apparent power’?  What is its relation to active and reactive power?

4.            In a 3-phase system with line-to-line voltage V_L and balanced line current I_L at a phase angle j, write down expressions for (a) active power, (b) reactive power and (c) apparent power, giving the correct units.

5.            Describe the meaning of ‘power factor’.  A single-phase circuit, supplied at 250V has a resistance of 40 ohms and an inductive reactance of 30 ohms in series.  What is (a) the power factor and (b) what active power flows?

6.            In a 250V single-phase system carrying a current of 50A at a power factor of 0.75 what is (a) the active power and (b) the reactive power?  Give the units in both cases.

7.            In a 3-phase system the line voltage is 11kV and the line current 200A at a power factor of 0.8.  What is the total active power transmitted?

8.                What is the power factor of a 50Hz single-phase circuit consisting of a 50 ohm resistor and a 0.1H inductor in series?

9.              In the circuit of Q.8 what is (a) the active power P and (b) the reactive power Q transmitted when a current of 25A is flowing?

10.             

 

In the balanced 3-phase system shown in the figure determine:

(a)    the line currents I_L

(b)    the overall power factor

(c)    the total active power P

(d)    the total reactive power Q

11.         What difference would it make if the generator in Q.10 were delta-connected?

12.         A purely reactive (90° lagging) a.c. circuit is switched on at an instant when the voltage is passing through zero.  Sketch the ensuing current wave.

13.         When a balanced 3-phase system is switched on, why do the initial currents in the three phases show different degrees of asymmetry?

14.         To what uses can a diode be put in simple power supply or control circuits?

15.         How is an a.c. current converted into d.c.?  What element is used to do this?  If a single-phase a.c. current of 10A (rms) undergoes half-wave rectification, what d.c. current level is achieved?

16.         With a 3-phase, full-wave rectifier what a.c. voltage (rms) must be applied in order to achieve 24V d.c. output?

17.         How can a rectifier bridge be adapted to give a variable d.c. output?

18.         Why is high voltage used to generate large bulk quantities of electricity?

19.         A platform has a 6.6kV generation and a 440V distribution system.  An oil transfer pump is driven by a motor rated at 1 200kW.  At what voltage would you expect it to operate?  Why?

20.         A system has six motors with nameplate ratings 200 hp, 150 hp, 2 x 100 hp, 80 hp and 50 hp.  What is the total full-load output in kW_m ?

21.         What different types of instrument can be used to measure alternating voltages and currents?  What voltage or current will they indicate?  What do you expect to notice about their scales?

22.         Why cannot a moving-coil instrument be used to indicate a.c. directly?  How can it be made to do so?

23.         What type of instrument would you use to indicate watts on a 3-phase a.c. system?

24.         What type of instrument would you use to indicate vars on a 3-phase a.c. system?

25.         Why are transducers often used in modern instrumentation?