Elect Installation & Maintenance Work 2, WASSCE (SC), 2022

Question 4

 

  1. (a)     The diagram below illustrates a measuring instrument.

                 

                  (i)         Identify the instrument.
    [1 mark]                                                
    (ii)        Name the parts labelles V to Z.
    V:
                              W:
                              X:
                              Y:
                              Z:
    [5 marks]                                                                      
    (iii)       State one use of the device.    [2 marks]        

     

     

     

                                                                             
    (b)        The diagram below illustrates the stator winding of a three-phase induction motor.
    Redraw the diagram to illustrate the connection of the motor in:


    (i)         star;
    [4 marks]
    (ii)        delta.  
    [4 marks]                    
    (iii)       State three methods of starting a three-phase squirrel-cage induction
    motor.
    [3 marks]

    (c)          State two possible causes of the following faults in a d.c. generator:
    (i)         no voltage at the terminals;    
    [2 marks]        
    (ii)        very low voltage at the terminals;
    [2 marks]
    (iii)       sparking at the commutator.
    [2 marks]


Observation

 

Expected response:
(a)             (i)        Identification of the Instrument
                                                Insulation Resistance Tester (mega ohmmeter)                                                         

                                    (ii)        Name of the labelled parts
V:        current limiting resistor
W:       current coil
X:        voltage coil
Y:        pointer
Z:         scale /calibrated metre

(iii)       Use of the Device

                                                - used for measuring insulation resistance    

 

 

 

                       
(b)             (i)         Connection of motor in star

                                                        

(ii)        Connection of motor in delta

                                                        
(iii)       Methods of starting a three –phase squirrel-cage induction motor
- direct - on - line
- star - delta
- auto transformer

(c)               Causes of the following faults in d.c. generator
(i)         no voltage at the terminal
- open circuit on the field
- open circuit in the armature winding 
- open circuit/low contact between the brushes and commutator segment
- short circuit on armature winding
- short circuit on the field winding    
- failure of residual magnetism          

(ii)        very low voltage at the terminals
- dirty commutator
-  poor contact on the brushes
- very low speed
- reduction of magnetic strength
- high voltage drop at the brushes /high brushes resistance

                        (iii)       sparking at the commutator
- worn out brushes/ commutator
- dirty brushes/ commutator
- high mica
- brush spring burnt/loose brush spray

                        - electrical overload