What is the difference between the unipolar and bipolar PWM inverter?

Here I would like to add few basic detail about this.Inverter converts fix DC voltage in to equivalent sinusoidal waveform noticeably by performance.Main idea behind this conversion is fact that sine wave has higher magnitude at its center (90 degree) and its reducing as go far from center both side (toward 0 degree and 180 degree). Thus supplying dc voltage of varying width such that it has high width around center and smaller at far from center resembles sine shape. This DC voltage can be switched from either from 0 to Vdc or from Vdc to –Vdc to produce same effect which call unipolar and bipolar technique respectively. Name unipolar is given as switched DC voltage applied to the load remain positive during positive cycle and negative during negative cycle. While in bilopar method this voltage switches in both the direction throughout the cycle. Thus bipolar method produces twice the voltage stress on load than maximum AC voltage that can be produce by the inverter. Thus unipoar method is more preferable.

Note that its about shape of output voltage and not the pulse. Bipolar apply negative voltage during positive half cycle too but not result in negative current. Applying negative voltage during freewheeling period fasten the rate of current reduction compared to applying zero voltage. Its depend upon load that which shape is more closed to sine wave. Thus having information or estimation about load inductance is important to get idea about harmonics and THD.