a synchronous condenser is over excited synchronous motor under no load

Unlike a static condenser (capacitor bank), the value of reactive power from a synchronous condenser can be continuously adjusted.

A synchronous condenser also has other advantages over static condensers such as:

- not being affecting by harmonics (some harmonics can even be absorbed by a synch condenser)

- producing no switching transients

A synchronous condenser is physically an electric generator/motor without a mechanical power source. Electrically a synchronous condenser changes the timing between rise and fall of current compared to voltage (phase angle). They also use stored energy to smooth small power fluctuations.

If a motor is spinning under no load, it is storing some energy in the rotating mass. This energy can be stored and released in the same way capacitors and inductors store and release energy. This is what a synchronous motor acting like a condenser is doing. The way it appears to the power system is dependent upon the field current applied to it. If used as a motor, this is controlled to keep it spinning at the desired speed under load. If used as a condenser, it is controlled to determine whether it will be releasing power ahead of or behind the power system (absorbing or releasing VARs).

A synchronous condenser (also known as a synchronous capacitor or synchronous compensator) is a DC-excited synchronous computer (large rotating generators) whose shaft is now not connected to any using equipment.