What type of motor is the simplest of all AC motors, characterized by rotors consisting of a series of bars placed in slots?

The squirrel-cage motor is indeed recognized as the simplest type of AC motor. Its design features a rotor that consists of conductive bars shorted at both ends by end rings, resembling a "squirrel cage," which is where it gets its name. This construction facilitates a straightforward and efficient operation since the rotor does not require any additional components like brushes or external power supplies.

When an alternating current passes through the stator winding, it produces a rotating magnetic field that induces a current in the rotor bars. This induced current interacts with the magnetic field to produce torque, causing the rotor to spin. The mechanical simplicity, low maintenance needs, and reliability are key reasons why squirrel-cage motors are widely used in various applications, from industrial machinery to household appliances.

In contrast, the other motor types listed do not exhibit this same structure or operational simplicity, making the squirrel-cage motor uniquely straightforward in its design and function.