NDA (II) 2023 General Ability Test · Question 84 of 146

Correct — A, a split-ring type commutator. The coil of any rotating generator has an alternating emf induced in it, because the flux linked with the coil reverses every half turn. What decides whether the machine delivers AC or DC to the outside world is therefore not the coil but the sliding contact that connects it to the load. An AC generator uses two continuous slip rings, so each brush stays joined to the same end of the coil and the reversal passes straight to the load. A DC generator replaces them with a split ring — the commutator — whose two half-cylinders change brushes at the very instant the current in the coil reverses, so the current in the external circuit always flows the same way. The commutator is, in effect, a mechanical rectifier.

• (b)Slip rings and brushes — This is what an AC generator already has, so adding it changes nothing. With continuous slip rings no switching happens at all and the output stays alternating.
• (c)A stronger magnetic field — A stronger field increases the size of the induced emf but does nothing to its direction. A larger alternating output is still an alternating output.
• (d)A rectangular wire loop — The shape of the loop has no bearing on whether the machine gives AC or DC. A simple AC generator already uses a rectangular loop.

A generator works on electromagnetic induction — a coil rotating in a magnetic field has an emf induced across it whose direction reverses each half rotation. Both the AC and the DC machine share that coil, that magnet and that induced alternating emf. The only structural difference between them is the sliding contact. Continuous slip rings preserve the alternation; a split ring reverses the external connection in step with the coil and so straightens the output.

Option (b) is the strongest trap because slip rings and brushes sound like a piece of machinery you would have to add, and candidates who half-remember that generators involve brushes reach for it. The one-line discriminator to memorise is that a continuous ring means no switching while a split ring means switching. It is also worth being clear about what the commutator does not do — it does not make the coil produce direct current. The coil still produces alternating current; the commutator only changes how that current is picked off. One practical note for candidates preparing today, the electric-motor and electric-generator sections were removed from the Class 10 science chapter on magnetic effects of electric current in the 2023 syllabus revision, so this topic has to be picked up from an older edition or from a reference book even though NDA keeps asking about it.

• In a generator the commutator reverses the direction of the current with each half turn, acting as a mechanical rectifier that converts the alternating current in the windings into unidirectional current in the external circuit.
• In an alternator the rotating contacts are continuous rings, called slip rings, and no switching happens, so the output remains alternating.
• The emf induced in the rotating coil is alternating in both machines; only the take-off differs.
• A stronger magnet, more turns or faster rotation change the size of the emf, not whether it is AC or DC.

• Believing the coil itself produces direct current in a DC generator; it does not, the commutator does the converting.
• Picking slip rings, which are the arrangement that keeps the output alternating.

As a which-component item like this one, as the principle a generator works on, or through the function of the commutator in a motor.

• practice — not a real PYQ

  In an electric motor, the function of the split-ring commutator is to

  • (a)increase the strength of the magnetic field
  • (b)reverse the direction of current in the coil after every half rotation
  • (c)reduce friction at the axle
  • (d)convert a DC supply into an AC supply

  Answer(b) reverse the direction of current in the coil after every half rotation — which is what keeps the coil turning the same way.

• practice — not a real PYQ

  A generator whose rotating coil is connected to the external circuit through two continuous slip rings delivers

  • (a)direct current
  • (b)alternating current
  • (c)no current at all
  • (d)a constant voltage with no current

  Answer(b) alternating current — with continuous rings there is no switching, so the reversal in the coil passes to the load.