Definition : It is a feedback control system in which for a preset value of the reference input, the output is kept constant at its desired value.
In such systems reference input remains constant for long periods. Most of the times the reference input or the desired output is either constant or slowly varying with time. In a regulator, the desired value of the controlled outputs is more or more fixed. Similarly the reference input is also fixed and called set point. Thus the regulator maintains a constant output for a fixed reference input. The problems due to disturbances are mainly rectified by the regulator. A simple example of such regulator system is servostabilizer. We have seen earlier that in voltage stabilizer position of tap on secondary is adjusted by using relay controls. But instead of fixed tap, the entire secondary can be smoothly tapped using a servomotor drive. The servomotor drives the shaft and controls the position of tap on secondary as per the controller signal. Due to the fluctuations in the main input if the load voltage changes, such effects are rectified by the regulator to keep load voltage constant.
The actual scheme is shown in the Fig.1, while its block diagram representation is shown in the Fig.2.
Few other examples of regulating system are,
1) Temperature regulators.
2) Frequency controllers.
3) Speed governors.