Just to be clear, on the 737 the DC connector is 90° turned in relation to the AC outlet, the DC outlet is a NEMA 6-15. It looks similar at first glance to a normal US 5-15 in this configuration because the pins are all oriented vertically, but they are different sockets nonetheless.
On a different note I've never seen the cleaners use a transformer (which as was pointed out already would not do anything to the frequency), the ones I have seen all plugged in directly to the galley outlets.