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Device guide / Circuits
03

Reversing and interlocking

The direction of rotation is reversed with two contactors that swap two phases. The contactors are interlocked so that they cannot energise at the same time.

L1L2L3K1K2M3~UVWK1: L1→U, L2→V, L3→WK2: L1→W, L2→V, L3→ULNS01112S11314K11314K22122K1A1A2S21314K21314K12122K2A1A2
Control circuit: S1 forward (K1), S2 reverse (K2), common S0. In series with each coil is NC contact 21–22 of the other contactor. Power circuit: K2 swaps L1 and L3.

How it works

LNS0S1K1K2K1S2K2K1K21At rest

S0 is closed. The interlock contacts of both coils are closed, but the start buttons are open.

LNS0S1K1K2K1S2K2K1K22Forward

S1 energises K1, which latches. NC contact 21–22 of K1 opens in the circuit of K2.

LNS0S1K1K2K1S2K2K1K23S2 while running

Pressing S2 does nothing, because the NC contact of K1 is open. K2 cannot energise.

LNS0S1K1K2K1S2K2K1K24Stop

S0 drops out K1. Only now can S2 start the motor in reverse.

A three-phase motor changes its direction of rotation when two phases are swapped. In a reversing circuit K1 supplies the motor directly and K2 swaps L1 and L3.

Why an interlock

If K1 and K2 energised at the same time, phases L1 and L3 would be short-circuited through the contactors. That is why the contactors are interlocked against each other.

Interlocking

In series with each coil is an NC auxiliary contact of the other contactor. When K1 is energised, its NC contact is open and K2 cannot energise. Ready-made reversing contactors have a mechanical interlock in addition to the electrical one, which stops both from closing at the same time.

Direct reversing

If the direction must change without pressing the stop button, add a push-button interlock: an NC contact of S2 in the circuit of K1, and an NC contact of S1 in the circuit of K2. S2 then first drops out K1, and the interlock between the contactors still stays in place.

Common mistakes

  • The contactor's own NC contact is used as the interlock. Then the contactor cannot stay energised, because its own NC contact opens as soon as it pulls in.
  • In the power circuit all three phases are shifted by one position. Then the direction does not change. Swap only two phases.

In your region

Showing notes for

United States, Canada

The forward and reverse contactors are often marked F and R. To reverse the motor, two motor leads are swapped, traditionally T1 and T3.

Source: Siemens: Basics of Control Components, part 3 (STEP 2000 course) · Siemens: Basics of Control Components, part 1 (STEP 2000 course)

UK & Ireland, Europe, Australia & New Zealand, Other IEC countries

No regional notes on this page yet. If something is different in your country, please send feedback.

Sources

  1. Schneider Electric: interlock and holding contacts of reversing contactors
  2. ABB AF265: reversing contactor, mechanical and electrical interlock
  3. Sprecher + Schuh: auxiliary contact terminal markings 13–14 and 21–22
  4. Siemens: Basics of Control Components, part 3 (STEP 2000 course)
  5. Siemens: Basics of Control Components, part 1 (STEP 2000 course)