past paper remake 2024

Question 1

When rotating machine appear stationary or move under fluorescent lighting. this is called ?

Answer 1

The stroboscopic effect.

This is the effect which the flickering of a discharge lamp has on machinery

Question 2

Two basic losses when a transformer is use are ?

Answer 2

Iron loss
Copper loss

Question 3

The test that can be used to determine the losses are

Answer 3

1)Iron loss -Open circuit test
2)Copper loss - Short circuit test

Question 4

Two ways losses in a transformer can be reduced are

Answer 4

Using silicone steel material

Increase conductor cross-sectional area

Question 5

Data : Resistance = 25ohms
XL =20ohms
XC = 40ohms
Volts = 240v
F = 50Hz

Answer 5

DETERMINE

Question 5

Motor components

Answer 5

Overload
Push button on/off
Emergency switches
Timers

Question 6

Supply Current

Answer 6

I = V/Z
240/32 =7.5A

Question 7

Voltage across R

Answer 7

v = I xR
v= 7.5 x 25
v= 187.5

Question 8

Voltage across XL

Answer 8

v= I x XL
v=7.5 x 20
v= 150v

Question 9

Voltage across XC

Answer 9

v= 7.5 x 40
v= 300v

Question 10

Power factor

Answer 10

R/Z = 25/32 = 0.78 lagging

Question 11

Define Power Factor

Answer 11

Power Factor is the cosine of angle between voltage and current in an a.c circuit

Question 12

Components used to improve power factor

Answer 12

1) Capacitor banks
2) Synchronus condensure
3 Phase advancers

Question 13

Data : Resistance = 10ohms
F = 5.0 H
C = 720 x 10-6

       Calculate resonance frequency

Answer 13

1/22/7x square root Lc

Question 14

Impedance of the circuit at this frequency

Answer 14

The impedance of this circuit is equal
to the resistance of the circuit

Question 15

Data:50A
p.f = 0.6 lagging
volts =240v
F = 50Hz

Answer 15

DETERMINE

Question 16

True power

Answer 16

V x I x p.f

240 x 50A =12 000

12000 X 0.6 = 7200 = 7.2 kva

Question 17

Apparent power

Answer 17

s = V x I = S= 240 x 50A = 12,000

Question 18

What is impedance

Answer 18

Impedance is the name given to the combined effect of resistance (R) and reactance (xl or xc)

Question 19

Impedance Formula

Answer 19

Z² = R² + X²

Question 20

When a circuit has resistance, capacitance and inductance

Answer 20

X ² = xc -xl

Question 21

Inductive reactance XL = 2 pie FL

Answer 21

XL = 2 pie FL

Question 22

D.C = Resistance

Answer 22

V = I x R

Question 23

A.C = Impedance

Answer 23

V = I x Z

Question 24

Current tsken by uf capacitor

Answer 24

I= V/1/2 pie fc 2 pie f c v

Question 25

Illuminance = Ep

Answer 25

I/d²

Question 26

Lumen per m²

Answer 26

I/d² lux

Question 27

Illuminance EQ

Answer 27

I/ h² cos o lux

Question 28

Also

Answer 28

H² = d² + x² And Cos ø = d/h

Question 29

Epb = I/Epa × cos

Answer 29

PA x + x=

Question 30

P.f = cos (ø)

Answer 30

(Ø) = 30

Question 31

d²

Answer 31

Luminance /lux = d²

Question 32

Lumious intensity

Answer 32

Luminous output /power

Question 33

Safety practices when installing lighting system in high risk areas

Answer 33

Use explosive proof light fixtures Ensure proper grounding Use non corrosive material

Question 34

Purpose of a rectifier

Answer 34

Rectifiers convert alternating current to direct current

Question 35

Differences between full wave and half wave rectification

Answer 35

In full wave both halves of he waveform are rectified where as in half only half is allowed to pass through the rectifier

Question 36

Types of Rectifier

Answer 36

Mercury arc recitifier Silicon Copper oxide Diode valve

Question 37

SCR

Answer 37

Silicon controlled rectifier

Question 38

Lumious intensity

Answer 38

Light source

Question 39

Lumious flux

Answer 39

Light emitted from a source

Question 40

Lumination

Answer 40

Density of luminous flux pulling on a working plane