AC generators can be utilized in high precision control systems. The AC generator rotor is rotated by the rotating shaft (i.e. the shaft from the motor being controlled). The output AC frequency is proportional to shaft speed. Zero crossings per time interval can be counted to determine speed for a simple control system. If high precision speed and position control are needed, the controller can generate a reference AC with frequency proportional to the desired speed. The control system can monitor the phase shift between the reference AC and the sensor’s output AC (in this case the output from the AC generator functioning as a speed sensor). If the phase shift comparator detects the actual system is lagging the reference, the actuator is driven harder until it catches up with the reference AC hence the name, phase locked loop speed control system.
The figure below demonstrates an AC generator speed sensor and a phase locked loop control system
There is also a DC generator sensor (with a commutator), also referred to as a tachometer. It generates DC proportional to speed and can be used as a speed sensor.
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