Two three-phase motors, two terminal blocks. One is marked U1 V1 W1 / U2 V2 W2. The other is marked T1 through T9. They are not the same convention, and the difference is not cosmetic.

Direct answer

IEC-based (U V W) NEMA (T1 T2 T3…)
What the letter says Which phase winding —
What the number says Which end of that winding Lead identity within a scheme
Direction of rotation Defined when letter order matches phase sequence Not defined
Where rotation is viewed from Facing the drive end Facing the end opposite the drive end

The second row of that table is the one that catches people. Under NEMA, the numerals used for terminal markings of polyphase induction machines do not define a relationship between phase sequence, the connection end of the coil windings, and the direction of shaft rotation. And explicitly: no direction of rotation is defined when T1, T2, T3 are used, either alone or in addition to U, V, W.

So on a T-marked machine, connecting L1–L2–L3 in order tells you nothing about which way the shaft will turn. On a U/V/W machine it does.

The IEC marking system, in the order you actually read it

Letters — which winding. On a three-phase machine the primary winding ends are marked U, V and W, with N for the neutral point. The secondary winding of a wound-rotor machine uses a different set — K, L, M — which is why rotor leads never look like stator leads. On a single-phase machine the main winding is U and the auxiliary winding is Z.

Suffixes — which end of that winding. Each winding unit’s two ends take a number pair: 1 and 2 for the first unit, 3 and 4 for the second, 5 and 6 for the third, 7 and 8 for the fourth. One rule makes the whole scheme readable:

The end nearer the supply connection takes the lower number.

So U1 and U2 are the two ends of one phase winding, and U1 is the supply end. They are not two separate windings, and a six-lead block is three windings with both ends brought out — not six windings.

Winding taps get their own series: 11, 12, 13 for the first unit, 31, 32, 33 for the second, and so on, with the tap nearer the start of the winding taking the lower number.

Prefixes — which independent winding. Where a machine has several independent winding units with the same function, they carry the same letter and different numeric prefixes: 1U1, 2U1. On a multi-speed motor the prefix order carries information that is easy to invert:

Prefix order runs from the lowest speed upward.

Two letters you will not see. The marking is upper-case Latin letters and Arabic numerals with no spaces, and the letters I and O are not used — they are too easily read as the digits 1 and 0.

One marking is reserved. The protective earth terminal is marked PE, and no other terminal may carry that marking.

Direction of rotation: the same motor, described from opposite ends

This is where the two systems look like they conflict and do not.

  • IEC-based: direction of rotation is the direction seen when facing the drive end of the shaft. A machine whose markings comply with the standard runs clockwise. If the letter order U1, V1, W1 matches the phase sequence of the supply, the rotation is clockwise.
  • NEMA: the standard direction for polyphase induction motors, when only the markings U, V, W are used and connected to L1, L2 and L3 respectively, is counter-clockwise when facing the end opposite the drive end — unless otherwise marked on the machine.

A shaft turning clockwise as seen from the drive end is turning counter-clockwise as seen from the other end. The two statements describe the same rotation. What differs is where the observer is standing, and that is the detail that gets dropped when a rotation requirement is written into a specification.

The practical rule follows: a direction requirement is meaningless without the viewpoint. “Clockwise” in a purchase specification should say clockwise facing which end.

The arrow is not necessarily on the nameplate

Where a machine is suitable for one direction only, an arrow indicates that direction. But the standard is specific about where it has to be: the arrow need not be on the nameplate — it goes on the enclosure.

That has a consequence for the way replacement enquiries usually arrive. A nameplate photograph is not enough to establish rotation on a single-direction machine, which is one of the items that makes a like-for-like fan motor replacement go wrong.

NEMA adds a related caution from the other direction: where the lead location of a polyphase machine is modified in the field — changing mounting position, for instance — it may be necessary to re-tag the leads with proper terminal markings, replace the leads, or otherwise mark the machine with the direction of rotation. A machine that has been re-mounted may no longer be telling the truth about itself.

Why a motor has 3, 6, 9 or 12 leads

The lead count is a statement about what the winding lets you reconfigure, not about size or quality.

  • Three leads — the internal connection is fixed. One voltage, one arrangement.
  • Six leads — both ends of each phase are out, so the three windings can be connected in more than one arrangement. This is what makes star-delta starting or a two-voltage connection possible.
  • Nine or twelve leads — the phase windings are themselves split into halves, so the halves can be paralleled or put in series as well. That is what gives a dual-voltage machine its two connections.

Several leads may legitimately carry the same marking, provided they have exactly the same electrical function — any of them may be connected. Where a winding is split across several conductors, the marking carries a hyphen-separated numeric suffix to identify the parts. And where no confusion can arise, prefixes and suffixes may simply be omitted, which is why a small three-lead motor shows a bare U V W.

What this article deliberately does not give you

A connection diagram.

The connection arrangements are set out in a normative annex of the marking standard, and in the manual for the specific series — and the safe practice is to work from that page rather than from a diagram reproduced somewhere else. Marking schemes differ between systems, lead counts and machine types, and a wrong connection is not a paperwork error.

What this article is for is the layer above the diagram: knowing which system you are reading, so that the diagram you find is the right one.

What to ask for, and what to check

  1. Which marking system is on the terminal block — U/V/W or T-numbers. It decides whether rotation is defined at all.
  2. The connection diagram for that specific series, from the manual or the product standard — not a generic one.
  3. Required direction of rotation, with the viewpoint stated — facing the drive end, or facing the opposite end.
  4. Whether the machine is single-direction, and if so, where the arrow is.
  5. The lead count and what it is for — reconfigurable voltage, a starting method, or multiple speeds.
  6. Whether the machine has been re-mounted or re-leaded since it left the factory.

LEADGO’s IEC-frame series are marked to the Chinese national standard described here, which is an identical adoption of the IEC terminal-marking standard, and the NEMA-standard range is marked to the NEMA convention — so on a mixed site both systems will be present, and the first question on any terminal box is which one you are looking at. The connection diagram for a specific series comes with the manual; ask for it by series and rating rather than working from a generic diagram.

Going deeper