## V Curves and Inverted V Curves of Synchronous Motor

From the previous article,

it is clear that if excitation is varied from very low (under

excitation) to very high (over excitation) value, then current I_{a }decreases,

becomes minimum at unity p.f. and then again increases. But initial

lagging current becomes unity and then becomes leading in nature. This

can be shown as in the Fig. 1.

Fig. 1 |

Excitation can

be increased by increasing the field current passing through the field

winding of synchronous motor. If graph of armature current drawn by the

motor (I

then its shape looks like an english alphabet V. If such graphs are

obtained at various load conditions we get family of curves, all looking

like V. Such curves are called V-curves of synchronous motor. These are

shown in the Fig. 2a).

be increased by increasing the field current passing through the field

winding of synchronous motor. If graph of armature current drawn by the

motor (I

_{a}) against field current (I_{f}) is plotted,then its shape looks like an english alphabet V. If such graphs are

obtained at various load conditions we get family of curves, all looking

like V. Such curves are called V-curves of synchronous motor. These are

shown in the Fig. 2a).

As against this, if the power factor (cos ?) is plotted against field current (I

_{f}), then the shape of the graph looks like an inverted V. Such curves obtained by plotting p.f. against I_{f}, at various load conditions are called Inverted V-curves of synchronous motor. These curves are shown in the Fig. 2(b).Fig. 2 V-curves and Inverted V-curves |

__1.1 Experimental Setup to Obtain V-Curves__

Fig. 3 shows an experimental setup to obtain V-curves and Inverted V-curves of synchronous motor.

Stator is connected top three phase supply through wattmeters and

ammeter. The two wattmeter method is used to measure input power of

motor. The ammeter is reading line current which is same as armature

(stator) current. Voltmeter is reading line voltage.

Fig. 3 Experimental setup for V-curves |

A rheostat in a

potential divider arrangement is used in the field circuit. By

controlling the voltage by rheostat, the field current can be changed.

Hence motor can be subjected to variable excitation condition to note

down the readings.

potential divider arrangement is used in the field circuit. By

controlling the voltage by rheostat, the field current can be changed.

Hence motor can be subjected to variable excitation condition to note

down the readings.

**Observation Table :**

Now I

_{L }= I_{a}, per phase value can be determined, from the stator winding connections. I

_{L }= I_{aph }for stator connection I

_{L}/?3 = I_{aph }for delta connection The power factor can be obtained as

The result table can be prepared as :

The graph can be plotted from this result table.

1) I

_{a}Vs I_{f }**?**V-curve2) cos? Vs I

_{f }**?**Inverted V-curveThe entire procedure can be repeated for various load conditions to obtain family of V-curves and Inverted V-curves.

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