Some airplanes use 400Hz but home line frequencies are usually 50 to 60 Hertz.
You should try using a small RC filter to filter the line before the meter so you can avoid getting a reading that reflects some of the distortion or noise.
I dont think a pump would break due to distortion or noise, but i guess you never know. Fix the pump or buy a new one, they do go out sometimes and it is not because of the line frequency. If a second one blows out you may have to look deeper with a scope.
I have to also wonder how long did you have this pump running before it blew out.,
