V curve of Synchronous Motor: Definition, Characteristics & Inverted Curve
A quick guide to V curve of synchronous motor. Includes definition, characteristics and inverted v curve.
What is V Curve of Synhcronous Motor?
The V curve of synchronous motor is a graph explaining the relationship between armature current (la) and field current (lf) at constant speed.
As the field current increases, the armature current decreases and reaches its minimum value and then increases. This results in a curve which forms a “V” shape; therefore, it is called “V curve” of synchronous motor.
Assume the motor is running at constant speed. When the field current (lf) gradually increases, the armature current (la) starts to decrease. At this point, the armature current lags behind the voltage, and motor operates at a lagging power factor.
If the field current (lf) increases further, the armature current (la) continues to decrease and reaches its minimum value. At this stage, the armature current and field current are both in phase. Therefore, the motor operates at a unity power factor.
If the field current increases after this point, the armature current also starts to increase. At this point, the armature current leads the voltage, and motor operates at a leading power factor.
Therefore, the V curve of synchronous motor is drawn between armature current and field current to understand the effect of field excitation.
Characteristics of V Curve
- Shows the relationship between the armature current and field current
- Armature current decreases as field current increases.
- Armature current reaches a minimum value at unity power factor
- After unity power factor, armature current increases again.
When the armature current is at the minimum value, this point is known as unity power factor. The curve which connects all the lowest points of various stages at different power factors is known as the unity power factor compounding curve.
The compounding curve for 0.8 lagging power factor and 0.8 leading power factor are highlighted in red dots in the above figure.
The loci of constant power factor points at the V curve are known as the compounding curves. These curves explain how the field current is to be managed to maintain a constant power factor under different load conditions.
Increasing the field current beyond the minimum level of armature current results in a lagging power factor. On the contrary, increasing the field current beyond the maximum value of armature current results in a leading power factor.
Inverted V Curve of Synchronous Motor
Inverted V curve is the graph which represents the relationship between power factor and field current at constant load. Unlike the V curve, the inverted V curve shows the variation in power factor. It helps to understand the effect of field excitation on the motor's power factor.
Key Features
- Explain the relationship between the power factor and field current
- Unity power factor occurs at the peak point of the curve.
- Power factor increases when the motor is in the under-excitation region
- Power factor decreases when the motor is over-excited
Frequently Asked Questions
Q1. What does inverted V curve represent?
Inverted V curve represents the relationship between power factor and field current of the synchronous motor. It shows how the power factor changes from lagging to leading as the current increases gradually.
Q2. What load conditions considered for V-curve plotting?
V curve plotting depends on various load conditions such as no load, light load, and heavy load. For each load, the field current varies and corresponds to the armature current.
Q3. What is the meaning of the unity power factor curve?
Unity power factor curve is the curve formed by joining points at different V curves. It helps to find the field current required by synchronous motor to operate at different loads.
Q4. Is V curve affected by motor design?
Yes, the V curve is affected by the motor design. Parameters such as winding design, rated capacity and synchronous reactance influence the field current and armature current.
Conclusion
The V curve of synchronous motor explains the effect of field excitation on a synchronous motor. It helps to select the right excitation for a power factor and enables reliable operation.
