Why are the letters U, V and W used in AC motors to represent the windings

motorthree phase

I am connecting up an AC motor and was wondering if this labeling system was representative of something? Or just another set of letter like X,Y,Z?

I could not find any information regarding this in IEC 60034-8,
Rotating electrical machines – Part 8: Terminal markings and direction of
or a brief search of google, forums etc.

Best Answer

The letters are "representative of something"

To assist in providing an international standard the International Electrotechnical Commission (IEC) created that IEC 60034-8 standard. "Terminal markings and direction of rotation"

An electrical machine compliant to 60034-8 will ensure that clockwise rotation of the drive shaft will occur for positive electrical phase sequence U-V-W.

Thus if you were to wire such a machine upto a distribution labeled A-B-C and these electrical phases were A leading B, leading C... you would have the expected mechanical rotation

is it "just another set of letters"? you could say they arbitrarily chose that sequence, a sequence that is just as good as any other BUT they assigned a specific meaning to it and a meaning that is internationally recognised.

MotorA from supplier A and MotorB from supplier B being compliant to this reg & meeting the customer requirements would be drop-in replacements that would minimise any unexpected mechanical rotation