Explanation:

When the rotor circuit of a 3-phase induction motor is open, it means that the rotor coils are disconnected from each other and from the external circuit. In this condition, the rotor cannot develop any torque and it behaves like a squirrel cage rotor. However, the stator winding is still energized and it produces a rotating magnetic field that cuts across the rotor bars. This induces currents in the rotor bars and produces a magnetic field that interacts with the stator field. As a result, the rotor experiences a torque that tends to rotate it in the same direction as the stator field.

Therefore, the correct answer is option A, the 3-phase induction motor with rotor circuit open will run normally. However, there are some important points to consider:

- The motor will not develop any useful torque because the rotor is not connected to any load. Therefore, the motor will run at a very high speed (almost synchronous speed) and it may not be able to start from rest because it has no starting torque.
- The motor will draw a very small current from the supply because the rotor impedance is very high due to the open circuit. Therefore, the motor will consume a very small amount of power and it will not get overheated.
- The motor may produce some noise because the rotor bars are not tightly coupled to the rotor core and they may vibrate or resonate at some frequencies. However, the noise level will be very low compared to a normal running motor.

Conclusion:

In summary, the 3-phase induction motor with rotor circuit open will run normally but it will not develop any useful torque, it will consume a very small amount of power, and it may produce some noise. This condition is not desirable for practical applications, but it may occur accidentally due to a fault in the rotor circuit or during some maintenance operations.