Elect Installation & Maintenance Work 2, WASSCE (SC), 2016

Question 4

  1. The diagram below represents a simple Leclanché cell.

 

  1. Name the parts labelled I,II, III and IV.
  2. What is the e.m.f. of this cell?
  3. I.          List the two main defects that are associated with this type of cell.

          II.           Explain briefly how each of the defects listed in (iii) I above could be minimized.
III.         Give three disadvantages of this type of cell.
IV.         Give four applications of this type of cell.
V.          Name the instrument that can be used to measure the specific gravity of this cell.
[ 19 marks]

(b)       Copy and complete the table below.
Faults in d.c. generator


Faults

 Causes

(i)  No voltage at terminals

(1)
(2)

(ii)  Sparking at brushes

(1)
(2)

(iii)  Fails to excite

(1)
(2)

                                                                                                                          [ 6 marks]

 

 

Observation

  This is a test of practical, and a compulsory question. Majority of the candidates responded to it. The performance was fair.
 
In part (a), majority of the candidates labelled the marked parts, and listed four applications of the cell. 
In part (b), majority of the candidates could not state the causes of the faults in d.c. generator.

The expected answers include:

4(a)(i)  -    Electrolyte/Sal ammoniac solution
-     Zinc plate/-ve electrode  
-     Carbon plate/+v electrode
-     Container  

      (ii)      E. M. F of Lechanche′ cell                     1.5V

               Defects of Lechanche′ cell
(iii)    I.    -   Polarization             
-    Local action  

            Methods of minimizing defects of Lechanche′ cell
II.    -    The zinc electrode should be coated with mercury  to minimize local action  
-    The carbon plate should be surrounded  with depolarizing agent 

 

           III    Disadvantages of Lechanche′ cell
-    has low voltage/incapable of supplying large current
-   it cannot be used continuously due to effect of polarization
-   cannot be recharged/irreversible
-   has high internal resistance
-   it is not portable in this form.
-    ageing/short life span/deteriorate faster                                                               

            IV   Application of Lechanche′ cell
-   used in bell circuits
-    calculator
-   railway signal/indicators
-   for controlling switch gear in sub-stations
-   telegraphy/telephone (do not accept mobile phone)
-   radio set.
-   torch
-   microphone
-   remote control
-   watch                                                                                                  
V      Instrument used for measuring specific gravity of Lechanche′ cell
hydrometer                                                                                                                

                   Causes of fault in d.c. generator

                  Faults

       Causes

  1. No voltage at terminals

 

-  failure of residual magnetism
-  speed too low / low speed
-  open circuit on armature/field circuit
-  dirty commutators
-  short circuit on armature/field circuit
                                              

(b)