MCQs on Electrical Machines

01․ The armature mmf in a dc machine is 1. stationary with respect to field poles 2. stationary with respect to armature 3. rotating with respect to field poles 4. rotating with respect to armature From these, the correct answer is

The armature mmf in a dc machine is stationary with respect to field poles but rotating with respect to armature.

02․ The type of DC generator used for arc welding purpose is a

In case of a deferentially compounded generator with the increase in load current series field flux opposes the main field flux due to which terminal voltage falls rapidly. Because of this character, deferentially compounded generators are not damaged by short circuit. So, this type of generators are preferred for arc welding.

03․ The magnetic circuit of a dc shunt generator with negligible armature resistance and brush contact voltage drop remains unsaturated when it delivers full-load current at rated terminal voltage. The brushes are kept in the interpolar axis and speed is maintained constant. When the load is switched off, the terminal voltage would

As load is switched off, there is negligible current through the armature so replacing of brush to interpolar axis does not affect much. Hence at no load the terminal voltage would remain same.

04․ In a loaded dc generator, If the brushes are given a shift from the interpolar axis in the direction Of rotation, then the commutation will

In a loaded dc generator, If the brushes are given a shift from the interpolar axis in the direction Of rotation, then the commutation will improve but terminal voltage will decrease due to demagnetizing effect .

05․ In a loaded dc motor, if the brushes are given a shift from the interpolar axis in the direction of Rotation, then the commutation will

In case of motor to improve commutation brush should be shifted from interpolar axis in the opposite direction of rotation. If the brushes are given a shift from the interpolar axis in the direction of rotation, then the commutation will deteriorate and the speed falls as magnetizing effect takes place.

06․ In a dc machine, the demagnetizing effect of armature reaction is due to

In a dc machine, the demagnetizing effect of armature reaction is due tomagnetic saturation in half of the field pole.

07․ Which of the following factors improve commutation in a dc machine ? 1. High contact resistance of brushes 2. High inductance of the coils undergoing commutation 3. Shifting of brushes in the direction of rotation for a generator and in the opposite direction For a motor 4. Narrow width for the commutation bars Select the correct answer using the codes given below : Codes :

High contact resistance of brushes will prevent the sparking through brushes i.e. improves commutation. Commutation is also improved by Shifting the brushes in the direction of rotation for a generator and in the opposite direction for a motor.

08․ In a dc generator operating on load with its brushes on the geometrical neutral axis (GNA), the magnetic neutral axis (MNA) is shifted in the direction of rotation. Now, if the brushes are given a lead of 90° (electrical), then the MNA will

For the above condition If the brushes are given a lead of 90° (electrical),then GNA will also shift and coincide with MNA.

09․ The volt-ampere euation for a long-shunt compound motor is given by

The volt-ampere euation for a long-shunt compound motor is V_t = E_a + I_a(r_a + r_s) Where, V_t = terminal voltage E_a = back emf I_a = armature current R_a = armature resistance R_s = series field resistance

10․ Consider the following statements about commutating poles which are fitted on most large dc Shunt motors: 1. The commutating poles are placed in the geometric neutral plane and their number is usually Equal to the number of main poles 2. The winding on the commutating poles is connected in series with the shunt-field winding on the main poles 3. The polarity of the commutating pole must be that of the next pole further ahead 4. The commutating poles neutralize the reactance voltage produced in the coil undergoing Commutation. Of these statements,

The commutating poles are placed in the geometric neutral plane and their numbers are usually equal to the number of main poles and they helps to neutralize the reactance voltage produced in the coil undergoing commutation. The winding of the commutating poles are connected in series with the armature so that its affect can vary with the load.