Why does a PMDC motor refuse to rotate on half wave rectified AC?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. OP's rectified mains input to transformer.


  But, I can get a half wave rectified output if I feed the same to it. [From your comment to Andy aka.]


No you can't. You might get the same waveform (probably with distortion) but since the output can't have DC then the average voltage must be zero.



Figure 2. Top waveform is DC feed into transformer. Bottom waveform is transformer output.

Since there is no DC component in the output your DC motor can not run.





Figure 3. Note that even if you feed in a sinewave with a DC bias that the output would be a sinewave (provided you hadn't saturated the transformer).