Vmax - Try measuring the voltages at the speed control pot relative to the AC line neutral. While the control is indeed using low DC control voltages, the entire drive electronics, including those DC voltages is floating at the AC line voltage. You can not connect those control voltages anywhere without isolation and indeed you can be electrocuted working on the "low voltage" control signals if you are not careful.
Remember, everything is relative, and even though the leads on the speed pot only show low DC voltages relative to each other, this does not mean that they do not have a different voltage realtive to something else like ground.
Pete C.