Voltage Measurement

Voltage is a differential parameter; it is always measured between two points. There are two ways to measure three-phase voltage—between two of the three lines or between a line and neutral or ground. Service and motor voltages are quoted as line to line, unless otherwise noted. Because of the phase difference, line-to-line readings are 1.73 times line-to-neutral readings.

Inside a three-phase motor there are three windings, one for each phase. The easiest three-phase motor connection to visualize is with each of the three windings connected line to neutral. This is called wye because, schematically, it looks like the letter “Y”. A more common connection eliminates the neutral tie and connects the three windings from line to line. This is called delta because, schematically, this looks like a triangle or the Greek letter Delta. The winding experiences 73% higher voltage when connected line to line, so it must be designed for the type of connection it will have. Even if a motor’s windings are internally wye connected, its nameplate voltage rating is the line-to-line value.

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