It could be your cheap tester if it is just some lights (most likely), but it could also be that your wiring is screwed up (not very likely, but possible). 277 volts is significant because that is what you would see in a three-phase Y 480/277 volt system where 480 divided by the square root of three gives 277 volts which is the voltage from any of the three high legs to the current carrying neutral. I can't imagine this highly unlikely scenario happening unless somebody got the wiring wrong back at the transformer. I would opt for spending a few more dollars on a better tester.
If you feel like you are flying by the seat of your pants when adding wiring your home, then you ought not be doing it for safety reasons. Also, you didn't need to use 10 gauge wire. It is basically wasted money for no tangible benefit (unless the wiring run was several hundred feet). The NEC doesn't say that you can't use a larger gauge wire, but it might be prudent to also use a receptacle rated for the wiring size just in case the breaker is changed out for a larger 30 Amp breaker sometime later on. Voltage drop is insignificant with 12 gauge wire in a 20 Amp circuit for typical wiring runs in a home although you will read all sorts of nonsense from Internet experts.
Even in the unlikely event that your line voltage were low for whatever reason, it is not an issue as long as the voltage to the VFD is within the range specified by Delta VFD operating manual (not the voltage specified in the Powermatic manual). The AC power to the VFD gets converted to a DC voltage that is then goes through a circuit called an H-bridge that switches this DC voltage at a high frequency to send pulses to the three power leads of the motor. My point in describing this is to say that in a variable frequency drive, the equivalent voltage to the motor is rarely equivalent to the full 240 volt line voltage except when the motor is running at full speed. At slower speeds as the frequency is reduced the voltage is also reduced to prevent magnetic saturation of the motor.
