force that generates a torque to start the motor. An alternator rotor is made up of a wire coil enveloped around an iron core. The magnetic component of the rotor is made from steel laminations to aid stamping conductor slots to specific shapes and sizes. As currents travel through the wire coil a magnetic field is created around the core, which is referred to as field current. The field current strength controls the power level of the magnetic field. Direct current (DC) drives the field current in one direction, and is delivered to the wire coil by a set of brushes and slip rings. Like any magnet, the magnetic field produced has a north and a south pole. The normal clockwise direction of the motor that the rotor is powering can be manipulated by using the magnets and magnetic fields installed in the design of the rotor, allowing the motor to run in reverse or counterclockwise. Characteristics of rotors Squirrel-cage rotor This rotor rotates at a speed less than the stator rotating magnetic field or synchronous speed. Rotor slip provides necessary induction of rotor currents for motor torque, which is in proportion to slip. When rotor speed increases, the slip decreases. Increasing the slip increases induced motor current, which in turn increases rotor current, resulting in a higher torque for increase load demands. Wound rotor This rotor operates at constant speed and has lower starting current External resistance added to rotor circuit, increases starting torque Motor running efficiency improves as external resistance is reduced when motor speed up. Higher torque and speed control Salient pole rotor This rotor operates at a speed below 1500 rpm (revolutions per minute) and 40% of its rated torque without excitation It has a large diameter and short axial length Air gap is non uniform Rotor has low mechanical strength Cylindrical rotor The rotor operates at speed between 1500-3000 rpm It has strong mechanical strength Air gap is uniform Its diameter is small and has a large axial length and requires a higher torque than salient pole rotor Rotor equations Rotor bar voltage The rotating magnetic field induces a voltage in the rotor bars as it passes over them. This equation applies to induced voltage in the rotor bars. where: = induced voltage = magnetic field =conductor length =synchronous speed = conductor speed Torque in rotor A torque is produced by the force produced through