Elementary Induction Machines

In the discussion that followed Equation, the third possiblemethod of producing constant torque was to cause the mmf axes of stator and rotor to rotate at such speeds relative to their windings that they remain stationary with respect to each other. If the stator and rotor windings are polyphase and carry polyphase alternating current, then both the stator mmf and the rotor mmf axes may be caused to rotate relative to their windings. Such a machine will have polyphase stator ac excitation at ωs , polyphase rotor ac excitation at ωr , and the rotor speed ωm satisfying

ω_m = ω_s - ω_r

Let us consider the rotor speed given by Equation and the same phase sequence of sources. A rotating magnetic field of constant amplitude, rotating at ωs rad/s relative to the stator, is produced because of polyphase stator excitation. A rotating magnetic field of constant amplitude, rotating at ωs rad/s relative to the rotor, is also produced because of polyphase rotor excitation. The speed of rotation of the rotor magnetic field relative to the stator is ωm + ωr ,or ωs , if the rotor is rotating with a positive speed of rotation ω_m in the direction of the rotating fields. If so, the condition for energy conversion at constant torque is satisfied. Such a situation is illustrated diagrammatically in Figure. Under these conditions the machine is operating as a double-fed polyphase machine. Normally, in an induction machine with polyphase stator and rotor windings, only a source to excite the stator is employed, and the rotor excitation at the appropriate frequency is induced from the stator winding. The device is thus known as an induction machine.