Electric Motor Performance Improvement Using Auxiliary Windings and Capacitance Injection

Abstract

Generally, some electric machines such as induction machines and synchronous reluctance motors require reactive power for operation. While the reactive power required by a synchronous machine can be taken from the power source or supplied by the machine itself by adjustment of the field current, the power factor of an induction machine is always lagging and set by external quantities (i.e., the load and terminal voltage). Poor power factor adversely affects the distribution system and a cost penalty is frequently levied for excessive VAr consumption. Power factor is typically improved by installation of capacitor banks parallel to the motor. If the capacitor bank is fixed (i.e. that it can compensate power factor only for a fixed load), when the load is variable, then the compensation is lost. Some authors (El-Sharkawi et al, 1984, Fuchs and Hanna, 2002) introduced the capacitors using thyristor/triac controllers; by adjusting the firing angle, the capacitance introduced in parallel with the motor becomes variable and thus compensating the power factor for any load. Other works (Suciu et al, 2000.) consider the induction motor as an RL load and power factor is improved by inserting a variable capacitor (through a bridge converter) which is adjusted for unity according with the load. For the above methods, the capacitive injection is directly into the supply. Another method conceived for slip ring induction motor was to inject capacitive reactive power direct into the rotor circuit (Reinert and Parsley, 1995; Suciu, et al. 2002). The injection of reactive power can be done through auxiliary windings magnetically coupled with the main windings (E. Muljadi et al. 1989; Tamrakan and Malik, 1999; Medarametla et al. 1992; Umans, and H. L. Hess, 1983; Jimoh and Nicolae, 2006, 2007). This compensating method has also been applied with good results not only for induction motors but also for a synchronous reluctance motor (Ogunjuyigbe et al. 2010).