Editing Synchronous motor (section)

==Applications, special properties, and advantages==

===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}}

===Steady-state stability limit===
:<math>\mathbf{T} = \mathbf{T}_\text{max}\sin(\delta)</math>

where,
:<math>\mathbf{T}</math> is the torque
:<math>\delta</math> is the [[torque angle]]
:<math>\mathbf{T}_\text{max}</math> is the maximum torque

here,
:<math>\mathbf{T}_\text{max} = \frac {{\mathbf{3}}{\mathbf{V}}{\mathbf{E}}}{{\mathbf{X_s}}{\omega_s}}</math>

When load is applied, torque angle <math>\delta</math> increases. When <math>\delta</math> = 90° the torque will be maximum. If load is applied further then the motor will lose its synchronism, since motor torque will be less than load torque.<ref name=dubey>{{cite book|first=G K |last=Dubey|title=Fundamentals of electrical drives|publisher=Narosa publishing chennai|page=254}}</ref><ref>{{cite book|first=S K |last=Pillai|title=A First Course On Electrical Drives|publisher=New age international|edition=second|page=25}}</ref> The maximum load torque that can be applied to a motor without losing its synchronism is called steady state stability limit of a synchronous motor.<ref name=dubey/>

===Other===
Synchronous motors are especially useful in applications requiring precise speed or position control:
* Speed is independent of the load over the operating range of the motor.
* Speed and position may be accurately controlled using open loop controls (e.g. [[stepper motors]]).
* Low-power applications include positioning machines, where high precision is required, and [[robot]] actuators.
* They will hold their position when a DC current is applied to both the stator and the rotor windings.
* A clock driven by a synchronous motor is in principle as accurate as the line frequency of its power source. (Although small frequency drifts will occur over any given several hours, grid operators actively adjust line frequency in later periods to compensate, thereby keeping motor-driven clocks accurate; see ''{{section link|Utility frequency|Stability}}''.)
* [[Phonograph#Turntable technology|Record player turntables]]
* Increased efficiency<!--relative to what?--> in low-speed applications (e.g. [[ball mill]]s).