A dual-rotor permanent magnet induction motor (DRPMIM) is a motor having two rotors within the same motor housing. This rotor arrangement can increase power volume density, efficiency, and reduce cogging torque. Some dual-rotor permanent magnet induction motors are a type of induction motors, while others are not actually induction motors.
Stator on the outside
In one arrangement, the motor has an ordinary induction motor stator. A squirrel-cage rotor connected to the output shaft rotates within the stator at slightly less than the rotating field from the stator. Within the squirrel-cage rotor is a freely rotating permanent magnet rotor, which is locked in with rotating field from the stator. The effect of the inner rotor is to reenforce the field from the stator. Because the stator slips behind the rotating magnetic field inducing a current in the rotor, this type of motor meets the definition of an induction motor. 
Stator between rotors
In another arrangement, one rotor is inside the stator with a second rotor on the outside of the stator. The photo labelled FIG. 8 is from a patent application. It shows two rotors assembled into a single unit, with six permanent magnets attached to the outer surface of the inner rotor, and six to the inner surface of the outer rotor.
- Diao, Tongshan (2015). "Study on Dual-Rotor Permanent Magnet Induction Motor and Performance". The Open Electrical & Electronic Engineering Journal. 9 (1): 584–590. doi:10.2174/1874129001509010584. ISSN 1874-1290.
- Hwang, Myeong-Hwan; Lee, Hae-Sol; Yang, Se-Hyeon; Lee, Gye-Seong; Han, Jong-Ho; Kim, Dong-Hyun; Kim, Hyeon-Woo; Cha, Hyun-Rok (2019). "Cogging Torque Reduction and Offset of Dual-Rotor Interior Permanent Magnet Motor in Electric Vehicle Traction Platforms". Energies. 12 (9): 1761. doi:10.3390/en12091761. ISSN 1996-1073.
- "IEC 60050 Section 411-31: Rotation Machinery - General". International Electrotechnical Commission. October 1990. Retrieved March 22, 2021.
- induction motor. HarperCollins. 2021. Retrieved April 9, 2021.
- accessdate=March 14, 2021 US 6924574B2, "Dual-rotor, radial-flux, toroidally-wound, permanent-magnet machine", assigned to Wisconsin Alumni Research Foundation