Michelin ECT Full Course Practice Exam

Induction motors are categorized by how their rotor is built, which determines how torque is produced and how the motor starts and runs. The two main rotor designs are the squirrel-cage rotor and the wound rotor (round wound). In a squirrel-cage rotor, conductive bars are embedded in the rotor surface and shorted by end rings, forming a closed "cage." The stator’s rotating magnetic field induces currents in these bars, generating torque without any electrical connection to the rotor. This design is simple, rugged, inexpensive, and requires little maintenance, which is why it’s the most common type. A wound rotor has windings on the rotor connected to external circuitry through slip rings. By inserting resistors or other control devices in that circuit, you can tailor the rotor resistance, giving high starting torque and adjustable speed control—useful in heavy-load starts or precise drive applications, but it is more complex and costly due to the brushes and rings. The other choices don’t fit because they describe power supply (DC vs AC), motor behavior (synchronous vs asynchronous), or physical components (rotor vs stator) rather than the rotor construction that defines the main classes of induction motors.