With parallel operation using opposite polarity, there is a build up of e.m.f.s. across the two coils a1a2 and a3a4, leading to a high circulating current within the primary coils that blows off the fuse. With d.c. supply, the coils have zero inductance reactance, hence they have very less impedance. With an input of 110 , the current in primary coils is high enough to blow off the fuse. When connected as an autotransformer, we can obtain several different values of voltage across different combination of coil terminals. However, ensure that the current flowing through the coil, or the voltage then do not exceed the rated values(as mentioned in the nameplate) under any condition.