The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature, across which the input voltage is supplied.
Rotor losses, an important form of power losses in induction motors, are largely but not entirely proportional to the square of the slip (slip is the difference between the rotational speed of the magnetic field and the actual rpm of the rotor at a given load). Thus, rotor losses are reduced by decreasing the degree of slip for a given load. This is accomplished by increasing the mass of the rotor conductors (conductor bars and end-plates) and/or increasing their conductivity, and to a lesser extent by increasing the total magnetic field across the air gap between rotor and stator.
The electrical efficiency of motors can be improved by replacing the standard aluminum electrical conductor in the motor rotor with copper, which has a much higher electrical conductivity. For more information, see: Copper die-cast rotors.
See also 
- Armature (electrical engineering)
- Copper die-cast rotors
- Balancing machine
- Commutator (electric)
- Electric motor
- Field coil
- High efficiency motors and transformers CD-ROM # A6121". Electrical: Energy Efficiency. Copper Development Association Inc.; http://www.copper.org/publications/pub_list/energy_efficiency.html
- Systematic Design Approach for a New Series of Ultra‐NEMA Premium Copper Rotor Motors, by Fuchsloch, J. and E.F. Brush (2007), in EEMODS 2007 Conference Proceedings, 10‐15 June,Beijing.
- Copper Motor Rotor Project, by the Copper Development Association; http://www.copper.org/applications/electrical/motor-rotor
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