Motor control circuit simulator

Start-stop with a seal-in contact and forward-reverse are the first two circuits every electrician wires, and the ones you find on machine after machine. Here you can operate them with the push buttons and watch, at the same time, where the voltage goes on the control diagram, which contactors close on the power diagram and which way the motor turns. You can also remove the interlocks or reverse with the motor running, to see what happens when it is done wrong.

How to use it

• Circuit: choose between start-stop with seal-in and forward-reverse.
• Push buttons: a tap is a short press; if you hold the button down, the press lasts as long as you hold it.
• Interlocks (forward-reverse only): you can remove the electrical one, add the push-button one or use both.
• Diagrams: on the power diagram, green shows where current flows; on the control diagram, where there is voltage. Contacts are drawn as they are at that moment, not in their rest position.

Start-stop with a seal-in contact

A push button only passes voltage while it is held down. To keep the motor running when it is released, an auxiliary normally open contact of the contactor itself, KM1 13-14, is wired in parallel with the start button S1. When S1 is pressed, the KM1 coil is energised, the contactor closes and its 13-14 closes too. When S1 is released, the coil is still fed through that contact: the contactor holds itself in. This is called the seal-in, holding or latching contact, and its PLC equivalent is a rung latched by its own output.

To stop, the stop button S0 is wired in series, ahead of everything else. Pressing it removes the voltage from the coil, the contactor drops out, 13-14 opens and, when S0 is released, there is nothing left to feed the coil again. The 95-96 contact of the overload relay F1 is also in series with S0: if the motor overheats, F1 opens the control circuit just as if someone had pressed stop.

The order matters. Because the stop is in series with everything, if start and stop are pressed together stop wins. It is a safety rule: when in doubt, the machine stops. Try holding Stop and pressing Start in the simulator.

Why the stop is a normally closed contact

The stop button and the overload are wired with normally closed (NC) contacts. If a wire comes loose or a contact breaks, the circuit opens and the machine stops or will not start: the fault leads to a safe state. If the stop were an open contact that had to close to stop the machine, a loose wire would leave it without a stop, and nobody would notice until it was needed. That is also why, in PLC programs, the stop arrives as an input that is at 1 while everything is fine.

The seal-in has another safety advantage: if the power fails, the contactor drops out and, when power returns, the machine does not restart by itself, because 13-14 is open. Someone has to press start again. A motor controlled by a two-position switch would restart on its own when the power came back.

The pilot lights

Pilot lights tell the operator what is happening without opening the panel. In the diagram, H1 (green, running) is lit by another normally open auxiliary of KM1, 23-24, and H2 (amber, overload tripped) by the normally open 97-98 contact of the overload relay, which closes when it trips. It is better to light the running lamp from a contact of the contactor than in parallel with its coil: that way it shows the contactor has really closed, not just that its coil has voltage.

Forward-reverse

A three-phase motor changes direction when two of its three phases are swapped. Two contactors are used for this: KM1 connects L1, L2 and L3 to the motor as they are and runs it forward, and KM2 swaps two phases, here L1 and L3, and runs it in reverse. You can see the crossover just below KM2 on the power diagram. Each contactor has its own start button (S1 and S2) and its own seal-in, and both share the stop S0 and the overload F1.

The danger is both closing at once: with KM1 and KM2 closed, L1 and L3 are joined through the contactors and there is a phase-to-phase short circuit. Interlocks are there to prevent it.

The three interlocks

• Electrical: in series with each contactor coil is a normally closed auxiliary contact of the other one (61-62, or 21-22, depending on the contact block). While KM1 is closed, its 61-62 is open and the KM2 coil cannot be energised. This one is essential.
• Push-button: double-contact buttons are used, with the NC contact of S2 in series with the KM1 branch and the NC contact of S1 in series with the KM2 branch. Pressing S2 first opens the KM1 branch and then closes the KM2 branch. It allows reversing without pressing stop.
• Mechanical: a part fitted between the two contactors that physically prevents them closing together. Factory-assembled reversing contactors include it. It is the last barrier if something fails in the control circuit.

The usual choice on a machine is electrical plus mechanical; the push-button interlock is added when reversing on the fly is wanted. What does not work is relying only on the push-button interlock. When S2 is pressed, KM1 loses its supply, but its contacts take a few milliseconds to separate and, as they separate, current keeps flowing through the arc for a few more. If KM2 closes during that time, there is a short circuit. KM1's 61-62 contact only lets KM2 in once KM1 has really dropped out. Remove the electrical interlock in the simulator, keep the push-button one and reverse with the motor running.

Reversing on the fly: plug braking

When the motor is running forward and is connected in reverse without letting it stop, the magnetic field turns one way and the rotor is still turning the other. The motor brakes hard, stops and starts in the opposite direction. During that time it draws more current than a direct-on-line start, and the machine takes a jolt. A few reversals in a row heat the motor enough to trip the overload: try it in the simulator. That is why, unless the machine is designed for it, the normal practice is to force a stop first and, if needed, add a timer that waits for the motor to stop before allowing a reversal.

Common mistakes

• Wiring the stop with a normally open contact: a loose wire leaves the machine without a stop.
• Wiring the seal-in across both stop and start instead of across start only: the stop no longer works.
• Using another contactor's contact for the seal-in: each contactor holds itself in with its own 13-14.
• Leaving out the electrical interlock on a reversing starter, or relying on the buttons alone.
• Swapping phases in both contactors, or in neither: the motor turns the same way with either button.
• Trusting the pilot light: a lit lamp does not guarantee the motor is turning, and a blown lamp does not mean it has stopped.

Teaching a class? There is a printable worksheet for this simulator, with exercises and answers. More material on the teachers page.

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