<p>This paper presents a novel 3-phase current calculation and reconstruction method using a current ratio function with a 1-shunt resistor. The 1-shunt sensing method is attractive due to its low cost and compact size. However, it has non-detectable current regions when the PWM (Pulse Width Modulation) duty ratios of each phase are close. For continuous current detection during each PWM period, the actual PWM durations of switching voltage vectors must exceed the required sensing time. Otherwise, limited PWM time leads to current distortion and degraded control performance. In the proposed method, the current ratio between the detectable phase current and the non-detectable medium phase current in a 3-phase BLDC(Brushless DC) motor is used to estimate the latter. The third phase current is then calculated from the detected and estimated currents. This method removes the duty ratio limitation, enabling sinusoidal phase currents with reduced distortion and improved control performance. Furthermore, the method does not rely on complex, parameter-dependent models. Instead, it uses a simple position-based current relation function to reconstruct the undetectable phase current. The proposed method is validated through simulations and experiments on sensorless control of a fan blower BLDC motor for electric vehicles. Results show enhanced current sensing and improved sensorless control performance.</p>