What type of motors are connected to an external source of electricity?

The choice of induction motors as the correct answer stems from their operational characteristics. Induction motors are designed to operate on the principle of electromagnetic induction, which involves creating a rotating magnetic field that induces current in the rotor. The external source of electricity is essential for producing this magnetic field, which is critical for the motor's functioning.

Induction motors are widely used in various applications due to their robustness and simplicity. They do not require any additional components, like brushes or separate exciters, which makes them reliable and low-maintenance compared to other types of motors.

The other types of motors have different operational principles that may not necessarily require a straightforward external electricity source in the same sense. For instance, synchronous motors need specific controls to maintain synchronism with the supply frequency, and servo motors typically operate within a closed-loop system for precise control. Asynchronous motors, while connected to an external source, operate differently from induction motors, as the term often refers broadly to various motor types based on their rotor design rather than their induction principle.