Comparison of powerfactor improvement techniques for squirrel cage induction motors




Prabhu, Sandhya Devakikrishna, 1970-

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Many industrial applications require variable speed drive by electrical machines. This requirement can be met either by de or ac machines. Induction motors with squirrel cage rotors are very useful in many of these applications because of their low cost, minimal maintainence and rugged construction. In a squirrel cage induction motor, three phase ac currents in the stator windings produce a rotating magnetic field in the air gap which reacts with the rotor magnetomotive force (mmf) wave to develop the torque. The rotor mmf is created by the stator induction effect. Also in a squirrel cage induction motor, the magnetizing current (lagging nearly 90 degrees behind the applied voltage) is the major portion of the no load current, which is why this type of motor operates at low power factors under no load conditions. Usually the no load power factor of the induction motor is near 0.15. One effect of the low value of the no load power factor is to decrease the full load operating power factor of the induction motor. Also the voltage drops in the stator and rotor leakage reactances increase the power factor angle between applied voltage and stator current. Hence, the effect of leakage reactances is to decrease the operating power factor. Many times induction motors operate at loads which are lower than the rated load, thus reducing efficiency and reducing power factor. Low power factor means higher stator current and therefore greater ²R losses. Also voltage regulation will be poor. The apparent power (kVA) in an ac circuit can be resolved into two components, the in-phase component which supplies the useful power (kW) and the wattless component (kvar) which does no useful work. Most ac machines draw from the supply, apparent power in terms of kVA which is greater than the useful power, measured in kW. The cosine of the phase angle between the kVA and kW represents the power factor of the load. [...] A low power factor means the supply must provide more current than is theoretically required. In a utility, the transmission lines and transformers will have to carry this extra current. One way to improve the power factor is to connect equipment drawing kVAR of approximately the same magnitude as the load kVAR but opposite in phase (leading) across the load. The resultant kVA required will be smaller and the new power factor [...] is increased. Several other schemes have been devised to improve the power factor of squirrel cage induction motors operating at partial load. The prominent methods of power factor control of induction motors are discussed in this thesis. [...] [Two methods] are tested over a range of torques from zero load up to 100% of rating. The effect of the different techniques on performance and operating efficiency are considered. Test results are compared with equivalent circuit model predictions and are then used to identify optimum conditions for each of these methods. Finally, the advantages and scope are also discussed