In order to increase reliability and reduce system cost, this paper studies particularly applicable method for sensorless BLDC motor drive system with 4-switch inverter. Over the years, traditionally, 6-switch 3-phase inverters have been widely utilized for variable speed alternating current motor drives. Recently, some efforts have been made on the application of 4-switch 3-phase inverter for uninterruptible power supply and variable speed drives. This is due to some advantages of the 4-switch 3-phase inverter over the conventional 6-switch 3-phase inverters such as reduced price due to reduction in number of switches, reduced switching losses, number of interface circuits to supply logic signals for the switches, simpler control algorithms to generate logic signals, less chances of destroying the switches due to lesser interaction among switches, and less real-time computational burden. Also rotor position information is extracted by indirectly sensing the back EMF from only one of the 3 terminal voltages for a 3-phase BLDC motor. Depending on the terminal voltage sensing rotor position, active filter is used for position information. This leads to a significant reduction in the component device of the sensorless circuit.