This question was attempted by few candidates and their performance was fair. Majority of the candidates avoided this question.
In part (a), quite a number of candidates made a fair attempt to the questions on d.c. machines.
In part (b), majority of the candidates were able to give the types of d.c. generators however, some candidates could not state why the starting current of a d.c. motor is dangerous to motors.
In part (c), quite a number of candidates made a fair attempt on the question.
The expected answers include:
- (a) (i) - field winding
- armature winding
- inter pole
- commutator
- brushes
- pole shoe
- shaft
- eye bolt
- brush holder
- main frame
(ii) Armature
(iii) A d.c. generator is used as a booster because of its increasing terminal voltage characteristics with increase in load current from no load to full load.
OR
The output voltage is dependent on the load applied/ voltage increases
with the load
b. (i) - Series (wound)
- Shunt (wound)
- Compound/cumulative compounded/differential compounded
- Self-excited
- Separately excited
(ii) This is the e.m.f. generated in the armature conductors as they cut the lines
of flux of the main field.
(iii) By varying the armature voltage /armature control.
By varying the strength of magnetic field /field control.
(iv) D. C. motor starting current is dangerous because if not limited/restricted, it
gets too large to start damaging/destroying the motor .
(v) By connecting rheostat/variable resistor in series with the armature
OR
By using a face-plate starter
c (i) Squirrel cage
Wound rotor
(ii) - Direct-on-line
- Star-delta
- Rotor resistance
- Auto transformer
- Primary resistance
(iii) By interchanging any of the two lines
d (i) Totally enclosed
Reason: Protect the rotating parts of the motor against dust/particles
(ii) - It prevents transmission of vibration to the shaft
- It absorb impact of loads on the shaft
- It makes motor alignment easy
(iii) (Earth) insulation resistance test. |
