A Synchronous Machine designed to supply reactive power is known as the synchronous condenser. The synchronous condenser or a synchronous capacitor as it is sometimes called is a motor which runs in an overexcited condition. The synchronous condenser is not connected to any mechanical load. 
The synchronous Condenser differs from other synchronous machine by its large synchronous reactance and large field windings. The field current fed into the machine will be high as the the machine usually runs overexcited.
Synchronous capacitors support the network voltage and additional short circuit power capacity.
Synchronous capacitors enable very fine reactive power control. Ordinary capacitors can only provide stepped control. Synchronous condensers also do not generate switching transients or harmonics.
The Synchronous Condenser is usually constructed with two or four poles. Synchronous Condensers can be cooled with air, water or with hydrogen.