Synchronous Phase Modifiers
It is well known that asynchronous motor can be made to take either a lagging or leading current form the line by altering its excitation. Idle-running synchronous motors were first employed in connection with power plants to correct for low power factor of the load, and thus reduce the current and power losses in the feeders and generators. The synchronous machines used for power factor improvement are usually referred to as ‘synchronous condensers’ as they are always required to take a leading current.
However, an idle running synchronous motor can be used for voltage regulating purposes by connecting it in parallel with the load at the receiving end of a line. Since the machines used for voltage regulation are required to run part of the time with leading current and part of the time with lagging current depending upon the load conditions, therefore, it is more appropriate to refer to these machines as ‘ synchronous phase modifiers.’
Synchronous phase modifiers differ from the ordinary synchronous motors in as much as they are built for the highest economical speeds and provided with smaller shafts and bearings and special attention is paid for securing a high overall efficiency. Standard machines for this purpose are designed to give their full-load output at leading power factor, and can carry about 50% of their rated capacity at lagging power factor. Machines can, however, be designed to operate at full rating on both leading and lagging power factors but they are larger in size, have poor efficiency and are more expensive than standard machines.