PlantFlow 3D › Guides › Forward–reverse motor control and interlocks
Forward–reverse motor control and interlocks
How swapping two phases reverses a three-phase motor, and the electrical and push-button interlocks that prevent a phase-to-phase short.
Reversing a three-phase motor
A three-phase induction motor turns in the direction of its rotating magnetic field. Swapping any two of the three supply phases reverses that field and so reverses the motor. A forward–reverse starter does this with two contactors: KM1 connects L1-L2-L3 in order, KM2 connects them with two phases exchanged.
Why the two contactors must never close together
If both contactors closed at the same moment, two phases would be connected directly to each other through the main contacts: a short circuit. That is why every reversing starter is interlocked.
The electrical interlock puts a normally-closed auxiliary contact of KM2 in the coil circuit of KM1, and a normally-closed contact of KM1 in the coil circuit of KM2. Whichever contactor is in, the other cannot be energised.
Push-button interlocking
Many circuits also use double push buttons: the FORWARD button has a normally-closed contact in the REVERSE circuit and vice versa. Pressing one direction first breaks the other. Mechanical interlocks between the contactors give a third independent layer.
Reversing a large load directly from full speed puts heavy mechanical and electrical stress on the machine. Many applications require the operator to press STOP and wait before reversing.
Fault finding tips
If the motor hums but does not turn in one direction, check the reversing contactor's main contacts. If one direction will not start at all, check the interlock contact of the opposite contactor and its wiring.
Try it yourself
Load the Forward–Reverse starter, run it in both directions, then try removing an interlock wire in the 2D diagram. The inspector flags the danger before you simulate it.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.