Reversing circuits: understand interlocking before changing direction

1. Two directions, two supply paths

A reversing circuit controls a motor in two directions, such as a conveyor moving forward/backward or a mechanism extending and returning. With a suitable three-phase motor, swapping two phases reverses rotation.

Call the forward contactor KM1 and reverse contactor KM2. A contactor switches load power through a controlled coil. The power circuit determines phase order; the control circuit determines which coil receives power.

Build the control circuit with 24 VDC and compatible coils. You do not need a motor yet: observing KM1 and KM2 is enough to understand holding and interlocking. Forward and reverse are command names here; actual motion depends on wiring and machine mechanics.

2. Why is the other contactor’s contact in the supply path?

Recall NO, normally open before actuation, and NC, normally closed before actuation. Auxiliary contacts follow a compatible coil with the same device-linking tag.

The forward path passes through Stop, KM2 NC, the forward Start/holding branches, then KM1’s coil. While KM2 is released, its NC contact remains closed and allows forward starting.

Once KM1 picks up, its NC contact in the reverse path opens. Pressing reverse Start still cannot feed KM2. Likewise, KM2 blocks KM1. Using one side’s contact to block the other is electrical interlocking.

Each holding NO contact must bypass only its own Start button. Do not let it bypass Stop or the interlock; that would create an unintended supply path to the coil.

3. How do electrical and mechanical interlocks differ?

Electrical interlocking blocks a coil’s supply. Mechanical interlocking uses compatible mechanical parts to prevent both contactors closing together. They act in different places; NC contacts on a schematic do not prove a mechanical interlock exists.

In real reversing equipment, simultaneous closure can cause a phase-to-phase short circuit. Select contactors, mechanical interlocks, and wiring according to manufacturer documentation and machine design. Screen colors do not verify real assembly.

One easily missed point: NC interlocks alone do not define command priority when both Start buttons are pressed from rest. Do not assume forward always wins or two NC contacts resolve every scenario. The machine needs suitable simultaneous-command handling and interlocking.

4. Build each branch in EleRun, then combine them

If you understand Start–Stop holding, this is a manageable next step. Place the branches side by side to see which blocks which, and keep the whole exercise on one page.

Do not give both coils the same tag. Forward coil and contacts use KM1; reverse uses KM2. If a contact fails to follow its coil, inspect tags and symbol types before changing wires.

1. Place the 24 VDC supply, Stop NC, two Start NO buttons, and KM1/KM2 coils. Each coil needs one holding NO and one NC blocking the opposite branch.
2. Connect Stop on the common path before the branch split. Complete both supply returns.
3. Build forward as in the table: KM2 NC in series before both forward Start and KM1’s holding branch.
4. Build reverse with KM1 and KM2 exchanged. Check that each holding NO bypasses only the correct Start button.
5. Start Run. Press and release forward Start; try reverse Start while KM1 holds. Then press Stop, release the Start buttons, and select reverse.

5. Five actions worth testing

This exercise changes direction by stopping and selecting again. In real equipment, a released coil does not mean a stationary motor: inertia may keep it turning. Machine design must determine when reversal is allowed.

Compare each action with the table. If it fails, stop Run before editing, change one thing, and retest from rest. Record actions the tool cannot directly reproduce as untested; another action is not a substitute.

6. Wrong behavior? Inspect the right branch before moving wires

If one direction works but the other will not start even after stopping, inspect the blocked branch’s NC interlock: correct type, tag, and endpoints? Then inspect Start, the coil, and its return.

If Start release stops it, inspect that side’s holding NO. If Stop fails, find a path bypassing it. If both coils energize, stop testing and trace both interlock branches; do not ignore it just because the motor picture still rotates.

Understanding means explaining a specific fact: “KM2 cannot pick up because KM1’s NC contact is open here.” Point to that contact. This is more useful than simply saying “the circuit is faulty.”