An a.c. the generator can be converted to a d.c. generator by replacing the

Analyze the question: We are asked what part of an a.c. generator must be changed to make it produce d.c. output.

Explain the concept:

• An a.c. generator (alternator) has a coil rotating in a magnetic field. The ends of the coil connect to two slip rings, each touching its own carbon brush. The current in the external circuit therefore reverses direction every half rotation, which gives alternating current.
• A d.c. generator (dynamo) has the same basic design, but the coil ends connect to a split-ring commutator. Every half rotation, the commutator swaps the contacts with the brushes. This keeps the current in the external circuit flowing in one direction only, although it pulsates in magnitude.

Step-by-step solution:

1. The coil (armature) and the magnets are the same in both machines, so they do not need replacing.
2. The only difference is how the coil connects to the external circuit: slip rings (a.c.) or a split-ring commutator (d.c.).
3. To convert a.c. to d.c., we remove the slip rings and put in a commutator.

Checking the options:

• A. Commutator replaced with an armature: wrong, an a.c. generator has no commutator.
• B. Armature replaced with a commutator: wrong, the armature is the coil and is needed in both machines.
• C. Commutator replaced with slip rings: this converts d.c. to a.c., the reverse of what we want.
• D. Slip rings replaced with a commutator: correct.

Answer: D

Exam tip: Remember "Slip rings = Standard a.c." and "Commutator = Constant direction (d.c.)". Whenever you see "a.c. to d.c.", look for the option that puts in a commutator.