Larry Clinton At Frankfort, (Central) Indiana
You are correct - however it doesn't change the basic premise that a couple of volts difference would have any appreciable effect on the measurable horsepower of a motor. It is quite likely that you would see more difference of voltage measured in a circuit between peak and low usage hours than you would due to the 240 and 120 Volt voltage drop. I have hooked up a (laboratory standard) recording volt meter to the circuit in my home many years ago to prove to the local utility that they had an undersized transformer feeding several houses in the area. (They did change it after I proved the voltage dropped below allowable specification at peak hours). I was an Engineer at a manufacturing facility that produced automotive switches and appliance controls and timers and had access to better instruments than the utility owned.