Class 2 (2nd Engineer) MET 📅 Sep 2023

Exam Question

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

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

(b) A twelve pole three-phase, delta-connected alternator runs at 500 rev/min and supplies a balanced star connected load. Each phase of the load is a coil of resistance 35 ohm and inductive reactance 25 ohm. The line terminal voltage of the alternator is 440V. Determine (10)

(i) Frequency of supply.

(ii) Current in each coil,

(iii) Current in each phase of the alternator, whota power supplied to me load.

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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