V Curves MCQ Quiz in বাংলা - Objective Question with Answer for V Curves - বিনামূল্যে ডাউনলোড করুন [PDF]

Variation in dc excitation of a synchronous motor causes variation in both power factor and armature current. These both can be represented by an inverted v curve.

Explanation:

V-Curve of Synchronous Machine

Definition: The V-curve of a synchronous machine is a graphical representation that illustrates the relationship between the armature current and the field current for a synchronous machine, typically a synchronous motor or generator, operating at a constant load and constant terminal voltage. The curve is named "V-curve" because of its characteristic V-shaped appearance.

Working Principle: In a synchronous machine, the field current is adjusted to vary the excitation level, which in turn affects the armature current. The V-curve demonstrates how the armature current behaves with changes in field current. When the machine operates at under-excitation (lower field current), the armature current increases. Similarly, at over-excitation (higher field current), the armature current also increases. However, at a certain optimal field current (normal excitation), the armature current reaches its minimum value.

Correct Option Analysis:

The correct option for the parameter of the X-axis in the V-curve of a synchronous machine is:

Option 3: Field current.

This option is correct because the V-curve represents the variation of armature current (on the Y-axis) with changes in the field current (on the X-axis). The field current is adjusted to study its effect on the armature current, and this relationship is plotted on the graph. As the field current is increased from under-excitation to over-excitation, the armature current follows a distinct V-shaped pattern.

Additional Information:

To further understand the analysis, let’s evaluate the other options:

Option 1: Armature Voltage.

This option is incorrect. The armature voltage is typically held constant during the plotting of the V-curve, as the curve is meant to study the relationship between field current and armature current under a constant voltage condition. The armature voltage does not serve as the parameter on the X-axis of the V-curve.

Option 2: Armature Current.

This option is incorrect because the armature current is plotted on the Y-axis, not the X-axis, in the V-curve. The graph illustrates how the armature current varies with changes in the field current, making the field current the parameter on the X-axis.

Option 4: Field Voltage.

This option is incorrect. Although the field current and field voltage are related, the field current is the parameter directly adjusted and measured to plot the V-curve. The field voltage is not the primary parameter on the X-axis in this context.

Option 5:

This option is left blank and does not provide any meaningful input for analysis. It does not contribute to the explanation of the correct X-axis parameter for the V-curve.

Conclusion:

The V-curve is an essential tool in understanding the operational characteristics of synchronous machines. It helps in determining the optimal field current for efficient operation, minimizing armature current and associated losses. Correct identification of the X-axis parameter is crucial for interpreting the V-curve. The X-axis represents the field current, while the Y-axis represents the armature current. This graphical representation is widely used in the analysis and troubleshooting of synchronous machines, making it a vital concept in electrical engineering.

Subnormal :

The projection on the x axis of the portion of the tangent to a curve between the x-axis and point of tangency.

Subnormal equation :

\(y\frac{dy}{dx}=k\)                           k : constant

ydy = kdx

y²/2 = kx + C

y² = 2kx + 2C

Above equation resembles with Parabola's equation.

Hence correct option is "3" 

V curve of synchronous machines

V curve is a plot of the stator current versus field current for different constant loads. 

The graph plotted between the armature current (I_a) and field current (I_f) at no load the curve is obtained known as the V curve.

Case 1: V curve for synchronous motor

Case 2: V curve for synchronous generator

Additional Information Inverted V curve of synchronous machines

The characteristic curve plotted between the input power factor (cosϕ) and the field current (If) for a constant mechanical load is known as an inverted 'V' curve.

Case 1: Inverted V curve for synchronous motor

Case 2: Inverted V curve for synchronous generator