Why would a synchronous machine act like a capactior if it only has inductors??

crutschow

Joined Mar 14, 2008
31,115
I won't go into the math, but if a synchronous motor is operated with an excess of field excitation, then it can generate a leading current (as would a capacitor) into the line.
There are synchronous motor specifically designed to operate with no load and a large excitation voltage to generate leading current to cancel the usual lagging current from induction motors and other inductive devices on the line.
 

Thread Starter

subatomic particle

Joined May 8, 2018
76
I won't go into the math, but if a synchronous motor is operated with an excess of field excitation, then it can generate a leading current (as would a capacitor) into the line.
There are synchronous motor specifically designed to operate with no load and a large excitation voltage to generate leading current to cancel the usual lagging current from induction motors and other inductive devices on the line.
How does it generate a leading current? Can you maybe explain the physics behind it? Do you always get a leading current if you over excite an inductor??
 

nsaspook

Joined Aug 27, 2009
10,679
https://nit-edu.org/wp-content/uploads/2021/09/ch-38-Synchronous-motor.pdf
page 1495

38.8 Motor with Different Excitations A synchronous motor is said to have normal excitation when its Eb = V.

https://electrical-engineering-portal.com/basics-synchronous-machines
The capability to control the reactive power output of a synchronous machine becomes a handy feature also in motor applications. By controlling the excitation of the synchronous motor, it is possible to compensate the variable reactive power needs in an industrial plant, thus minimizing the need of additional compensation devices.
https://en.wikipedia.org/wiki/Synchronous_condenser
An over-excited synchronous motor has a leading power factor. This makes it useful for power-factor correction of industrial loads. Both transformers and induction motors draw lagging (magnetising) currents from the line. On light loads, the power drawn by induction motors has a large reactive component and the power factor has a low value. The added current flowing to supply reactive power creates additional losses in the power system. In an industrial plant, synchronous motors can be used to supply some of the reactive power required by induction motors. This improves the plant power factor and reduces the reactive current required from the grid.

A synchronous condenser provides stepless automatic power-factor correction with the ability to produce up to 150% additional vars. The system produces no switching transients and is not affected by system electrical harmonics (some harmonics can even be absorbed by synchronous condensers). They will not produce excessive voltage levels and are not susceptible to electrical resonances. Because of the rotating inertia of the synchronous condenser, it can provide limited voltage support during very short power drops.

Rotating synchronous condensers were introduced in 1930s[2] and were common in 1950s, but due to high costs were eventually displaced in new installations by the static var compensators (SVCs).[2] They remain an alternative (or a supplement) to capacitors for power-factor correction because of problems that have been experienced with harmonics causing capacitor overheating and catastrophic failures. Synchronous condensers are also useful for supporting voltage levels. The reactive power produced by a capacitor bank is in direct proportion to the square of its terminal voltage, and if the system voltage decreases, the capacitors produce less reactive power, when it is most needed,[2] while if the system voltage increases the capacitors produce more reactive power, which exacerbates the problem. In contrast, with a constant field, a synchronous condenser naturally supplies more reactive power to a low voltage and absorbs more reactive power from a high voltage, plus the field can be controlled. This reactive power improves voltage regulation in situations such as when starting large motors, or where power must travel long distances from where it is generated to where it is used, as is the case with power wheeling, the transmission of electric power from one geographic region to another within a set of interconnected electric power systems.
The basic principle is the control of inductive reactive power by pulling or pushing reactive current into the supply network.
 
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