Class 2 (2nd Engineer) MET 📅 Jan 2025

Exam Question

(a) (i) What is direct-connected alternator? (3)

(ii) How is a direct-connected exciter arranged in an alternator? (3)

(b) Find the synchronous impedance and reactance of an alternator in which a given field current produces an armature current of 200 A on short circuit and a generated e. m. f. of 500V on open-circuit. The armature resistance is 0.4 ohm. To what induced voltage must the alternator be excited if it is to deliver a load of 100 A at a p.f. of 0.8 lagging, with a terminal voltage of 200 V. (10)

Reference Answer

(a) (i) A direct-connected alternator is an alternator that is directly coupled (without any intermediate gearing mechanisms like belts or chains) to its driving source, such as a diesel engine. This integration allows the alternator to be mounted directly on the extension shaft of the engine. Such alternators are typically used in portable engine-driven applications and are generally small in size, with power ratings ranging from 1 to 1.75 kW. These systems are often air-cooled and compact, making them suitable for mobile and low-power setups.
(ii) In a direct-connected alternator, the stator (or armature winding) may be either single-phase or three-phase with distributed winding. The rotor (field winding) is typically a silent pole design, often using permanent magnets for field excitation.
The arrangement ensures stable performance with:
Voltage variations within ±5% from no-load to full-load conditions.

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