Editing Synchronous motor (section)

===Use as synchronous condenser===
{{Main|Synchronous condenser}}
[[File:V curve synchronous motor.svg|thumb|upright=0.9|[[V-curve]] of a synchronous machine]]

By varying the excitation of a synchronous motor, it can be made to operate at lagging, leading and unity [[power factor]]. Excitation at which the power factor is unity is termed ''normal excitation voltage''.<ref name=bhatta>
{{cite book
 | last = Bhattacharya
 | first = S. K.
 | title = Electrical Machines
 | publisher = Tata - McGraw Hill
 | edition = third
 | page = 481
 | oclc = 808866911
|  url = https://books.google.com/books?id=BN9rplPm-wAC&pg=PA481
| isbn = 9780070669215
 | date = 2008-08-27
 }}</ref> The magnitude of current at this excitation is minimum.<ref name=bhatta/> Excitation voltage more than normal excitation is called over excitation voltage, excitation voltage less than normal excitation is called under excitation.<ref name=bhatta/> When the motor is over excited, the back emf will be greater than the motor terminal voltage. This causes a demagnetizing effect due to armature reaction.<ref name=kosow>
{{cite book
 | last = Kosow
 | first = Irving L.
 | title = Electric Machinery And Transformers
 | publisher = Pearson Education
 | edition = second
 | page = 230
 | oclc=222453
 | url = https://books.google.com/books?id=h-965eTcjJEC&pg=PA229
| isbn = 9788131711279
 | date = September 2007
 }}</ref>

The [[V curve]] of a synchronous machine shows armature current as a function of [[field current]]. With increasing field current armature current at first decreases, then reaches a minimum, then increases. The minimum point is also the point at which power factor is unity.<ref>{{cite book
|first1=B L|last1=Theraja |first2=A K |last2=Theraja |title=Electrical technology |page = 1524 |series = II |edition=2010 reprint|publisher=S Chand}}</ref><!-- pls clarify where leading, lagging pf obtain -->

This ability to selectively control power factor can be exploited for [[power factor correction]] of the power system to which the motor is connected. Since most power systems of any significant size have a net lagging power factor, the presence of overexcited synchronous motors moves the system's net power factor closer to unity, improving efficiency. Such power-factor correction is usually a side effect of motors already present in the system to provide mechanical work, although motors can be run without mechanical load simply to provide power-factor correction. In large industrial plants such as factories the interaction between synchronous motors and other, lagging, loads may be an explicit consideration in the plant's electrical design.{{citation needed|date=January 2013}}