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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 rotation or a brief search of google, forums etc.

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    \$\begingroup\$ Well some people use A,B, and C, just convention I assume \$\endgroup\$
    – michaelyoyo
    Commented Oct 14, 2015 at 1:22
  • \$\begingroup\$ Then of course R,Y,B is used for power phases in some cases. \$\endgroup\$
    – Tom Carpenter
    Commented Oct 14, 2015 at 2:29
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    \$\begingroup\$ It is just a convention, typically a world wide convention. For things like drives, the convention is RST for supply, UVW for output to motor. Motor leads may be labeled XYZ as well. There are many such conventions, In the US, the past 'standard' was L1/L2/L3 for incoming, T1/T2/T3 for outgoing and motors. \$\endgroup\$
    – R Drast
    Commented Oct 14, 2015 at 10:33
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    \$\begingroup\$ In conclusion: L1 L2 L3 or R S T to input electrical energy. U V W for entry into the electric machines, either transformers or electric motors. Finally X Y Z to the end of the windings of electric motors. \$\endgroup\$
    – user107998
    Commented Apr 25, 2016 at 9:01
  • \$\begingroup\$ You may even see some equipment (especially Siemens) designate U1 V1 W1 for input, and U2 V2 W2 for output. For 6-wire delta-loads, X,Y,Z is often added like the under-appreciated answer below. \$\endgroup\$
    – rdtsc
    Commented Jul 25 at 18:55

2 Answers 2

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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

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  • \$\begingroup\$ Complete speculation for why choose U-V-W over, say, A-B-C or X-Y-Z: U-V-W all sort-of-in-a-vague-way look like loops/coils of wire... \$\endgroup\$
    – TripeHound
    Commented Mar 29, 2018 at 14:26
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In normative for manufacture electrical motors there exist two standards: DIN convention (Europe) and NEMA (American countries).

In normative DIN use for terminal leads the letters U, V, W signify the coil head, and letters X, Y, Z signify coil end.

For example, a two pole three phase motor has 6 nests, 3 coils per nest, and 6 leads.

The first head nest start letter U --- (end nest) (one pole north) join end --- X terminal (one pole south) phase 1 north pole and south pole 180° elec grade. Phase 2 is similar to phase 1, V head start ----- Y end head. Likewise, for Phase 3, W ----- Z.

Then we have input voltages of 208, 230 V

      DIN normative (50Hz)          NEMA (60 Hz)          
   1Ø(R)> (U)------ (X) terminal    L1> T1---T4
   1Ø(S)> (v)-------(Y) terminal    L2> T2---T5                             
   1Ø(T)> (w)-------(z) terminal    L3> T3---T6

Now, use for start or wey connection single voltage

For start connection join (X, Y, Z) and connect U (phase 1), V (phase2), W (phase3) and delta join (U, Z) phase 1, join (V,X) phase 2, join (W,Y) phase 3.

Normative DIN (German) uses the first letter U mean (or) V, W (electrical machinery) and (R) (S) (T) for electrical terminal phases of 120°.

Note that R is not Run, S is not Start, and T is not Time

If a motor has 12 lead terminals:

    (U1 ----- X1) (V1----Y1) (W1----Z1)
    (U2 ----- X2) (V2----Y2) (W2----Z2)
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    \$\begingroup\$ why is this downvoted? \$\endgroup\$
    – gbt
    Commented Apr 27, 2021 at 0:00

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