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Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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When load on a synchronous motor running with normal excitation is increased, armature current drawn by it increases because
Back e.m.f. Eb becomes less than applied voltage V
Power factor is decreased
Net resultant voltage ER in the armature is increased
Motor speed is reduced
Net resultant voltage ER in the armature is increased
The correct answer is C. When the load on a synchronous motor increases, the armature current increases because the net resultant voltage ER in the armature is increased, which demands more current to maintain the balance in the system.
The correct answer is C. When the load on a synchronous motor increases, the armature current increases because the net resultant voltage ER in the armature is increased, which demands more current to maintain the balance in the system.
Eb=V−Ia×Ra — where Eb is the back e.m.f., V is the applied voltage, Ia is the armature current, and Ra is the armature resistance.
As the load increases, the synchronous motor attempts to maintain its constant speed. This results in a reduction of the back electromotive force (e.m.f.) Eb, due to the increased load torque, which requires an increase in armature current to provide the necessary torque to continue operation. Consequently, the net voltage available for current flow is affected, leading to an increase in armature current.
In a synchronous motor, the load change directly affects the armature current and the net voltage across the armature.
The operational principle relies on maintaining synchronous speed, which can lead to variations in back e.m.f. and the need for higher armature current.
Synchronous motors have excellent speed regulation.
They can operate at leading or lagging power factors, aiding in reactive power management.
Synchronous motors require an external source for excitation.
They are more complex and costly compared to induction motors.
Used in applications requiring precise speed control such as in mills and rolling machines.
Suitable for industrial applications that require high efficiency and power factor correction.
The standard voltage levels for synchronous motors generally range from 440V to several kilovolts based on the application.
Option A is incorrect as a less back e.m.f. does not cause an increase in the armature current directly, it occurs due to the net voltage change.
C is correct — The increase in armature current is primarily due to the requirement for more current to match the increased load, which changes the net voltage in the armature.
Understand the relationship between load, current, and voltage in synchronous machines as this is a recurring theme in electrical engineering.