Lets assume you are asking about a 480 volt three phase motor. The voltage should read between 460 to 480 volts phase to phase ( if not the length of the conductors is too long for the voltage drop and you need larger size condutors) and about 277 volts phase to ground as there is not a netural in most three phase systems. (there are lighting systems that use 277 volts with a center taped transformer in a "Y" but not in a "Delta" but can still use a ground to get the 277 volts but not used this way)....

Lets assume you are asking about a 480 volt three phase motor. The voltage should read between 460 to 480 volts phase to phase ( if not the length of the conductors is too long for the voltage drop and you need larger size condutors) and about 277 volts phase to ground as there is not a netural in most three phase systems. (there are lighting systems that use 277 volts with a center taped transformer in a "Y" but not in a "Delta" but can still use a ground to get the 277 volts but not used this way).

The reason for the different voltage readings is the potential voltage when reading different ways which will change when reading phase to phase And phase to ground and/or neutral.