MCQs on Electrical Machines

Page 50 of 114. Go to page
01․ In DC generator, the electricity produced in armature is
armature is not present in DC generator.
none of the above.

In DC generators, varying conductor cuts the fixed magnetic field then emf will be induced that will be in the form of AC but not DC.

02․ Commutators in DC machines have a role of which converts
AC to DC.
Both AC to DC and DC to AC.
high voltage DC to low voltage DC.
none of these.

Commutator will convert AC to DC in DC generators and vice versa in DC motors. Commutator is a cylindrical structure attached to the armature and mounted on the same shaft. In generator the main function of commutator is to facilitate collection of current from the armature conductors and to convert the alternating emf induced (AC) in the armature conductors to unidirectional emf (DC) in the external load circuit. In motor the main function of commutator is to supply current to armature from the external DC circuit and to convert this unidirectional emf (DC) of the external DC circuit to alternating emf in armature.

03․ Eg (generated emf) and Eb (back emf) both are
none of above.

Whenever machine is operating as generator the emf is called as generated emf, when machine is operated as motor the emf is called as back emf.

04․ For high current and low voltage rating which winding is employed to DC machine?
Wave winding.
Lap winding.
Both wave and lap winding.
No wave and lap winding.

Wave winding is a series winding in which the armature conductors are divided into two parallel paths between the positive and negative brushes irrespective of the number of poles of the machine. So wave winding used for low current and high voltage rating DC machine.

05․ For high current and low current rating which winding is employed to DC machine?
Lap winding
Wave winding
Both lap and wave winding
None of above

Lap winding is a parallel winding on which armature conductors are divided into as many parallel paths as the number of poles of the machine and the adjacent ends of coils are connected together or close to each other on the commutator segments by lapping over each other. So, lap winding used for high current and low voltage rating DC machine.

06․ In a DC machine, which of these parameters remain the same whether it runs as a DC motor or a DC generator?
Induced emf.
Mechanical power.
Armature current.
Can not be determined.

In a DC machine induced emf given by, \ It remains the same whether it is a generator or a motor. In case of a generator it is called the generated emf Eg. In case of a motor it is called back emf or counter emf Eb.

07․ An 8 pole DC generator has 500 armature conductors and useful flux per pole of 0.065 Wb. What will be the emf generated if it is lap connected and runs at 1000 rpm. What must be the speed at which it is driven to produce the same emf if it is wave wound?
541.67V, 200 rpm.
2166.67V, 230 rpm.
541.67V, 250 rpm.
2166.67V, 270 rpm.

Eg = 8*.065*1000*500/(60*8) = 541.67V To produce the same emf if it is wave wound then the speed of the generator has to be changed and is given as: Nwave = Nlap * Number of parallel paths in wave wound/ Number of parallel paths in lap wound. So, Nwave = 1000 * 2/8 = 250 rpm

08․ Armature reaction in DC generators will result in
increase in slip.
short circuit.
open circuit.
reduction in generated emf.

The effect of flux produced by the current passing through the armature conductors on the main field flux is called armature reaction. This will result in reduction in flux. As we know that, generated emf, Eg is given by, Hence as flux (φ) reduces generated emf also reduces.

09․ A lap wound DC machine has 400 conductors and 8 poles. The voltage induced per conductor is 2 volts. The machine generates a voltage of
100 V.
200 V.
400 V.
800 V.

Number of conductors Z = 400, number of poles = 8 and voltage induced per conductor = 2 Volts. Number of parallel paths = 8 Number of conductors/parallel path = 400/8 = 50 Generated voltage = 50 * 2 = 100 V.

10․ Which machine is used for battery charging?
DC series generator.
DC shunt generator.
Compounded DC generator.
None of these.

DC shunt generator can be made to provide constant output voltage.