Synchronous Condensor is also known as Synchronous Compensator or Synchronous Phase Modifier. A synchronous condenser or a synchronous compensator is a synchronous motor running without a mechanical load. It can generate or absorb reactive volt-ampere (VAr) by varying the excitation of its field winding. It can be made to take a leading current with over-excitation of its field winding.
In such a case it delivers inductive or absorbs capacitive Volt-ampere reactive. If it is under the excited condition, it draws the lagging current and, therefore, supplies capacitive or absorbs inductive volt-ampere reactive. Thus, a current drawn by a synchronous capacitor or condenser can be varied from lagging to leading smoothly by varying its excitation.
When the motor power factor is unity, the DC excitation is said to be normal. Over-excitation causes the motor to operate at a leading power factor. Under excitation causes it to operate at a lagging power factor. When the motor is operated at no load with over-excitation, it takes a current that leads the voltage by nearly 90 degrees.
Thus, it behaves like a capacitor and under such operating conditions, the synchronous motor is called a synchronous capacitor.
Since a synchronous condenser behaves like a variable inductor or a variable capacitor, it is used in power transmission systems to regulate line voltage.