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The CORRECT answer is (d) To avoid sudden LOADING of the prime mover, burning of all switch contacts and avoid mechanical jerking to the shaft

The best I can explain: Brushes are the medium between rotating and non-rotating part of the DC machine. If sudden change in connections are done, whole machine undergoes change in all electrical quantities, which may damage machine. Thus, machines are CONNECTED and DISCONNECTED only at FLOATING condition.

Right ANSWER is (b) 0.22 to 0.40

The EXPLANATION is: Brush friction is influenced by many VARIABLES including brush temperature, spring force, current, atmospheric conditions, mechanical conditions, ring or commutator materials, surface films, speed and other factors. Brush friction is of MEDIUM category when, coefficient of friction LIES in between 0.22 to 0.44.

Right OPTION is (b) 3

To explain I would SAY: The spacing between ADJACENT brushes in terms of the commutator segment is ratio of number of commutator segments with poles for a given DC machine.

C/P= 12/4= 3.

It MAY also be noted that C/P need not necessarily be an integer.

The correct ANSWER is (d) BRUSHES should be of proper grade, size, material, run smoothly in the holders and CONCENTRIC commutator properly undercut

Explanation: Brushes are in contact with commutator. So, for good commutation brushes must be of superior quality so that brushes will give/receive APPROPRIATE CURRENT to and from commutator. Also, the contact between brushes and commutator must be smooth for proper commutation process.

Right option is (C) Using carbon brushes of superior quality

For explanation I would say: Brushes carry current to/from rotating parts from/to STATIONARY part. RIPPLES can be avoided if brushes are MAINTAINED. Else, brushes will have some voltage drop in it and we’ll not get simple repeating part in EMF.

The correct answer is (c) Lie in inter POLAR region

To elaborate: Because of the diamond SHAPE of coils, the BRUSHES which are physically opposite the pole centres are electrically connected to coil-sides lying close to the interpolar region. Thus, electrically the brushes are displaced 90° elect. From the axes of the MAIN poles.

Right CHOICE is (a) Same or more than the number of poles

The BEST explanation: In WAVE windings, the number of parallel paths, A, is always two (2), and there MAY be two or more brush positions. When two adjacent commutator bars make contact with a particular brush, p/2 coils are shorted by the brush in the wave winding.

Right answer is (a) Where LOW voltage and high currents are involved

Easy explanation: DUE to various limitations, copper brushes are used in low voltage applications. For various other voltage ratings, different carbon-graphite proportions are used in manufacturing of BRUSH materials.

Right answer is (d) Severe SPARKING, rough COMMUTATOR surface or imperfect contact

Explanation: BRUSHES are the parts in a DC machine which are always in contact with rotating and stationary parts. Thus, imperfect contact, rough SURFACES, sparking all these may reduce the life of brushes.

Correct choice is (B) False

Easy explanation: Commutator and brush assembly of the DC MACHINE performs the mechanical rectification process so induced AC is converted into DC (Unidirectional). Commutation process provides full wave rectification.

The correct answer is (a) Otherwise commutation will not be possible

Explanation: A DC machine is a heteropolar STRUCTURE with stationary poles and the rotating armature. The armature winding of a DC machine is placed on the rotor to improve commutation i.e. to convert the alternating VOLTAGE produced in the winding into DIRECT voltage at the BRUSHES.

The correct CHOICE is (c) Metal graphite brushes

For explanation I would SAY: Metal graphite brushes are ideal for a VARIETY of applications because of their low resistivity. Thus, DROP will be less in metal graphite brushes. Metal graphite brushes are USED on commutators of plating generators where low voltage and high brush current densities are encountered.

The correct option is (a) Number of ARMATURE COILS

The best explanation: Armature current is induced in a DC machine, which is fed up to the external circuit. Thus, it NEEDS to be unidirectional. So, for converting bidirectional current to unidirectional commutators which are equal in number of armature coils NEED to be used.

The correct choice is (d) Amount of current to be collected

Easiest EXPLANATION: Brushes are in contact with rotating part and STATIONARY part. Thus, if more amount of current is to be carried, it requires more NUMBER of brushes. Hence BRUSH number depends directly on the amount of current that NEEDS to be collected and fed up in or out.

Right OPTION is (c) 24

The explanation: The spacing between adjacent brushes in terms of the commutator segment which is also equal to armature slots is ratio of number of commutator segments with POLES for a given DC machine.

C= P*Spacing= 4*6= 24.

The correct option is (a) Commutator

Best explanation: In any rotary MACHINE current is induced in SINE wave format, according to Faraday’s LAW. For DC machine commutator provides mechanical rectification so that OUTPUT is in the unidirectional format.

The correct choice is (d) COPPER lugs

Easy explanation: COMMUTATOR in CONNECTED to the armature using lugs. Generally, they are made with copper. They are tightly bolted to the armature in order to prevent the centrifugal FORCES from causing the segments to fly AWAY.

The CORRECT choice is (B) FULL wave rectification

Explanation: In any electromagnetic machine the voltage generated is always alternating one as per Faraday’s law. For a DC machine the output MUST be UNIDIRECTIONAL. This is carried out by a commutator. Hence, commutator provides full wave rectification.