I'm going to apologize in advance as I don't have oscilloscope screenshots for the problems I'm about to describe, so words will have to suffice.
I've put together a non-isolated SMPS using the LNK304 as seen below (sorry for LTSpice). Perhaps a better laid out schematic of the power supply can be found on page 4 of the LNK302/304/306 datasheet.
The problem I'm seeing is that when the universal motor (14A) on the right hand-side of the schematic is triggered a noticeable dip repeating at exactly 60 Hz is seen in the LNK302 output, the dip gets progressively larger as the motor is driven at higher and higher currents. At its peak this dip is around 5V. The dip gets less prominent as more capacitance is added to the output but I'm seeing diminishing returns as I add more capacitance (247uF so far).
Additionally, the 5V regulator output sees a dip of the same magnitude on its output.
I did look at the drain input of the LNK302 with an oscilloscope and saw a negative ramp wave, the amplitude of which increased as the motor current draw increased.
I strongly suspect that this issue is related to the neutral line getting yanked around by the motor but I find the repetitive nature of the issue troubling, especially since it continues at steady state. If this is the problem I'm not at all sure how to solve it other than by limiting the current to the motor or designing a fully isolated power supply, unfortunately, neither of these are an option.