Induction Motor


2.3 When the rotor is prevented from rotating, the induction motor can be considered to be a three-phase transformer. The iron of the stator and rotor acts as the core, carrying a flux linking the stator and rotor windings, which represent the primary and secondary windings, respectively. The steady-state equivalent circuit of one phase of […]

DEVELOPED TORQUE (Induction Motor)

2.4 The steady-state per-phase equivalent circuit in Figure 2.14 allows calculation of the stator current and torque developed in the induction motor under steady-state operating conditions. Balanced voltages and currents in individual phases of the stator winding are assumed, so that from the point of view of total power and torque the equivalent circuit represents […]


2.5 Based on Eqs. (2.3), (2.6), and (2.10) through (2.14), stator current, torque, input and output power, efficiency, and power factor of an induction motor can easily be computed. The input power, Pin, efficiency, t|, and power factor, PF, can be expressed as and and the Pin to 5in ratio is equal to the cosine […]


2.6 It has been said that an induction machine rotating with the speed higher than that of the magnetic field of the stator operates as a generator, feeding electrical power back to the supply system. This property is utilized in, for example, induction generators driven by a wind turbine and connected to the grid. The […]

SUMMARY (Induction Motor)

2.7 Operation of the induction motor is based on the ingenious principle of induction of EMFs and currents in the rotor that is not directly connected to any supply source. Three-phase currents in stator windings produce a revolving magnetic field, whose angular velocity, called a synchronous velocity of the motor, is proportional to the supply […]


In this topic, operation of uncontrolled induction motor drives is examined. We briefly outline methods of assisted starting, braking, and reversing. Speed control by pole changing is explained, and we describe abnormal operating conditions of induction motors.


3.1 In a majority of induction motor drives in industrial and domestic applications, the control functions are limited to the turn-on and turn-off and, in certain cases, to assisted starting, braking, and reversing. When driving a load, an induction motor is supplied directly from a power line and operates with fixed values of stator voltage […]


3.2 As exemplified in Figure 2.18, the stator current at zero slip, that is, the starting current, is typically much higher than the rated current. Using the approximate equivalent circuit in Figure 2.16, the starting current, 7S st, can be estimated as In the example motor, the starting current, at about 250 A/ph, is 6.3 […]

POLE CHANGING (Induction Motor)

3.4 A formula for speed, nM, of the induction motor as a function of the supply frequency, /, number of pole pairs, pp, of the magnetic field, and slip, s, of the motor can be obtained from Eqs. (2.4) and (2.6) as On the other hand, with a fixed output power, the speed is inversely […]


3.3 In drives requiring rapid deceleration, the motor needs to develop a negative torque for braking, especially in systems with low load torque and/ or high inertia. Because the torque depends on slip, a proper change in the slip must be effected. Apart from frequency control or changing the number of poles of stator winding, […]