V Curve & Inverted V Curve of Synchronous Motor Why?

What is V Curve and inverted v curve?

The graphical representation of armature current Ia vs field current If is called V-curve since the final view looks like English letter V. At the same time the power factor vs field current is called inverted V-curve of a synchronous motor.

Experimental setup of synchronous motor
Experimental setup of synchronous motor

Just you connect, two three-phase wattmeters and an ammeter to the stator winding of the synchronous motor. Also, connect another ammeter to the field input supply. Continue this experiment, with different field current as well as various load conditions such as load, half load and full load.

Here stator side connected ammeter gives you armature current Ia and field-side connected ammeter gives you field current If,

V curve Ia Vs If:

V curve
V curve

Take If reading at x-axis and Ia reading at Y-axis. Connect all the reading points, we get a V-shaped curve for a synchronous motor is shown in Figure 1.1.

At no-load condition, the real power supplied to the machine is zero since Ia=0, at unity power factor. When you increase the field current above that point, the line current increases with respect to applied field current, (Here the reactive current flows from the motor to source). in such case, the synchronous motor can be used as a synchronous capacitor.

Learn More:   Why Synchronous Motor is Not Self Starting [Simple Explanation]

At load condition, the motor draws a small amount of real current at unity power factor. After plotting the graph, the same V shape is still maintained.

V curve at load
V curve at load

Inverted V-curve:

Now take the reading of the power factor in the y-axis and field current in the x-axis and plot a graph. Look at the shape of the graph, it seems like inverted V.

Inverted V curve
Inverted V curve Power factor vs If

Frequently asked questions:

What is the use of the V curve and Inverted V-curve?

Just understand the characteristics of the synchronous motor both the curves are used.

How to calculate the power factor of the synchronous motor?

In modern days, we can use digital energy meter to calculate power factor, armature current, the power consumed by the motor etc, but to calculate the pf traditionally, we can use two wattmeter method with the help of below mentioned formula

power factor calculation for two wattmeter


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