synchronous machine Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchrono ...

s, the V curve (also spelled as V-curve) is the graph showing the relation of armature current as a function of

field current A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a electric motor, motor or electrical generator, generator. It consists of a coil of wire through ...

synchronous motor A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Sync ...

s keeping the load constant. The name comes from an observation made by W. M. Mordey in 1893 that the curve resembles a letter V. The lowest point of the curve corresponds to the

unity power factor In electrical engineering, the power factor of an AC power system is defined as the ratio of the ''real power'' absorbed by the load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneous product of vol ...

. For a motor, points on the left of the minimum correspond to underexcitation (and therefore the armature current would " lag" the voltage), on the right - to overexcitation (and "

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

"). Typically multiple V curves are plotted based on the experiments, each corresponding to its own load value. The minimum at unity power factor (

\cos \phi

) is due to the general formula for the power of a synchronous motor,

P = \sqrt 3 V_A I_A \cos \phi

. In order to keep the power constant, with the line voltage at the terminals of the armature

V_A

also constant, any decrease in power factor has to be accommodated by a corresponding increase in the armature current

I_A

. At the low values of the

, the power factor is low, so the armature current is high (and lagging). As the field current increases, the power factor increases too, until the unity power factor is reached (the armature current decreases to its minimum when the motor reaches this ''normal excitation''). If the field current is increased beyond this point, the armature current becomes leading, power factor decreases, and

I_A

grows again. The data from the V curves can be used to set up the synchronous motor to correct the power factor of the overall system, as the power factor of the motor can be changed by simply adjusting the field current. While performing the correction, the motor can either provide the mechanical power also, or run in the idle mode ("float"), working as a

synchronous condenser In electrical engineering, a synchronous condenser (sometimes called a syncon, synchronous capacitor or synchronous compensator) is a DC-excited synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to c ...

Inverted V curve

The inverted V curve is a graph showing the relation of

power factor In electrical engineering, the power factor of an AC power system is defined as the ratio of the ''real power'' absorbed by the electrical load, load to the ''apparent power'' flowing in the circuit. Real power is the average of the instantaneou ...

as a function of field current. The shape of this curve resembles a dome.

References

Sources

* * * * Saadat, Hadi. 2004. ''Power Systems Analysis''. 2nd Ed. McGraw Hill. International Edition. . Electrical generators Synchronous machines Electromechanical engineering {{electric-power-stub

Inverted V curve

See also

References

Sources