Field controlled D.C. motor is an open loop control system because it does not have any feedback mechanism to compare the output with the desired input. In an open loop control system, the control action is not based on the output of the system. Instead, it relies solely on the input command or setpoint.

To understand why a field-controlled D.C. motor is an open loop control system, let's examine its operation and control mechanism:

Operation of a Field-Controlled D.C. Motor:
- A field-controlled D.C. motor consists of a stator (field winding) and a rotor (armature winding).
- The current through the field winding controls the strength of the magnetic field produced by the stator.
- The armature winding carries the load current and generates the torque required for motor operation.
- The speed of the motor is determined by the armature voltage and the field current.

Control Mechanism of a Field-Controlled D.C. Motor:
- In an open loop control system, the control action is determined solely by the input command or setpoint.
- In the case of a field-controlled D.C. motor, the input command is the armature voltage and the field current.
- The desired speed is set by adjusting the armature voltage and the field current.
- These input commands are manually set and do not depend on the actual speed or position of the motor.
- There is no feedback mechanism to compare the actual speed with the desired speed and make adjustments accordingly.
- The control action is open loop, meaning it does not take into account any disturbances or changes in the motor's operating conditions.

Advantages and Disadvantages of Open Loop Control Systems:
- Advantages: Open loop control systems are simple, cost-effective, and easy to implement. They are suitable for applications where precise control is not required, and the system dynamics are well understood.
- Disadvantages: Open loop control systems are highly sensitive to disturbances and changes in operating conditions. They cannot compensate for variations in the system or external factors, leading to potential inaccuracies in the control action.

In summary, a field-controlled D.C. motor is an open loop control system because it operates based on manually set input commands without any feedback mechanism to compare the output (motor speed) with the desired input (setpoint).