Abstract:
Three phase induction- motor is supplied . from a single phase public utilities using a. new unique connection of the machine windings and three capacitors to provide balanced currents and voltages in all of the windings for its efficient operation in single phase system. By -change of the capacitor value in response to load change so that the efficiency in single phase operation of three-phase induction motor is close to the efficiency when-motor connected to three-phase supply, for all shaft loads between 20 to 120 percent ' of full load, and single phase line power factor was near unity for wide range of load variation. This method is applicable to both high efficiency and standard three phase motors without motor modification, where a three-phase supply is not available, this method is applicable. This connection is called a smith connection.
The smith connection is essentially a asymmetric winding connection but the suitable choice of the capacitances, balanced current can be injected in to the motor and g windings for a given speed and load. Inspection equations are established and solved using the method of symmetrical components to simulate the motor performance analysis. Using a phase diagram approach the values of capacitances. required for balanced operation can be computed.
In remote - and rural region where only single-phase power supply is available standard three-phase motors can be used in place of single-phase motors. Low cost, short delivery time superior electrical performance and large power rating are the main reasons favoring the use of three phase machines in favor of single phase machine.
Experimental work has done to verify the theoretical result on a 3.7 Kw induction machine, and also describes the multi-mode operation of three-phase induction motor with the smith connection. This operation involves the selection of appropriate phase balancing capacitances that give minimum voltage and current imbalance in the motor as the rotor speed changes. A simple switching strategy is adopted by using stator input current as the control signal that determines the mode selection. A prototype multimode system (namely L-mode, M-mode and H-mode) is implemented by using a controller as a voltage comparator and relay contactor. Experimental performed on a small induction motor confirm the feasibility of the proposed controller.
