Class 2 (2nd Engineer) MET 📅 Sep 2022

Exam Question

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

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

(b) A 440V shunt motor takes an armature current of 30A at 700 rev/min. The armature resistance is 0.7ohm. If the flux is suddenly reduced 20 percent, to what value will the armature current rise momentarily? Assuming unchanged resisting torque to motion, what will be the new steady values of speed and armature current? Sketch graphs showing armature current and speed as functions of time during the transion from intal to final, Steady-state conditions. (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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