Motor control · lesson 7 of 12
Forward / reverse
A reversing starter is two contactors: F feeds the motor straight, R feeds it with two lines swapped. The one rule that matters: they must never both be closed, because together they short one line to another. The interlocks enforce that. The first lab lets you take them out and find out why they are there.
Electrical interlocks
Look at each branch: in front of the F coil is a normally closed R contact, and in front of the R coil is a normally closed F contact. Whichever contactor is in has opened the other's path. To change direction you must press Stop first.
Adelante / reversa con enclavamientos eléctricos
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Dos contactores: F alimenta el motor con L1-L2-L3 y R lo alimenta con dos líneas intercambiadas. Nunca deben cerrar a la vez.
Pushbutton interlocks: reverse without Stop
Give each direction button a second, normally closed contact block and put it in the other direction's rung. Now pressing Rev opens the forward rung first; F drops; its NC interlock recloses; R pulls in. The electrical interlocks are still there as the backstop.
Adelante / reversa, enclavamientos por botones y eléctricos
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Cada botón de dirección tiene además un contacto NC en la OTRA rama.
The power side
On the power diagram, F's three main contacts connect L1, L2 and L3 to the motor's T1, T2 and T3. R's three contacts connect the same motor terminals with two lines crossed — typically L1 and L3 swap places. Close both and L1 meets L3 directly through the contacts. That is why a factory reversing starter also has a mechanical interlock: a bar that physically stops the second contactor closing, which works even if someone has wired the coils wrong.
En la obra
Two contactors feeding the same motor from opposite directions is the one circuit where a wiring slip is a phase-to-phase short. The interlocks are not optional and there are usually three of them.
Common mistakes
- Relying on one interlock. The trade convention is belt and braces: an electrical interlock (each coil through the other's NC auxiliary), a pushbutton interlock where reversing without Stop is wanted, and the mechanical interlock built into the contactor pair.
- Leaving the mechanical interlock out when replacing one contactor of a pair. A welded contact on one side is exactly the case the mechanical link is there for.
- Landing the auxiliary interlock on the wrong contact. It must be a normally closed auxiliary of the other contactor. An NO there does the opposite: neither can run unless the other is in.
- Swapping the wrong pair on the power side. Reversing swaps two of the three motor leads at the contactors — and it must be the same two on both sides of the pair, or forward and reverse are not opposites.
- Plug-reversing a machine not built for it. Slamming from full speed one way to the other is a mechanical event as much as an electrical one.
On a real panel
- The reversing pair is usually a factory-assembled unit with the mechanical interlock and the power cross-wiring already done. If you build one from two separate contactors, you are supplying all of that yourself.
- Rotation is checked with the load uncoupled where anything can be damaged by a wrong turn — a pump, a compressor, an auger.
- Limit switches on a travelling machine go in series with the direction they limit, not in the common part of the rung.
Prove it before you leave
- With the control energised and the motor disconnect open, press Forward. Then press Reverse without stopping. Whatever the design says should happen must happen — and both contactors must never be in together.
- Push one contactor's armature in by hand and try to energise the other. It must not pull in.
- Check the mechanical interlock physically: with one in by hand, the other must not move.
- Close the disconnect, bump forward, confirm rotation, then bump reverse and confirm it is genuinely the other way.
Preguntas frecuentes
How do you reverse a three-phase motor?
Swap any two of the three supply leads. A reversing starter has two contactors: forward feeds the motor L1-L2-L3 straight through, reverse feeds it with two lines crossed.
What is an electrical interlock?
A normally closed contact of each contactor wired in series with the OTHER contactor’s coil. While forward is in, its NC contact in the reverse rung is open, so reverse cannot energize — and the other way round.
What is a mechanical interlock?
A lever or bar between the two contactors that physically stops both closing at once, even if both coils were energized. Reversing starters normally have both kinds, because both closing together is a line-to-line short.
What is a pushbutton interlock?
Each direction button also has a normally closed contact in the other direction’s rung. Pressing Reverse then drops Forward first, so you can change direction without pressing Stop.
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