Motor control · lesson 9 of 12
Sequence control
When two conveyors feed each other, the downstream one has to be running before the upstream one starts, or material piles up. A contact from the first starter placed in the second starter's rung — a permissive — enforces the order.
Secuencia: el transportador 2 no arranca hasta que el 1 funcione
Active JavaScript para presionar los botones y hacer funcionar este circuito.
Haga esto
El transportador 2 descarga sobre el transportador 1. Si el 1 no se mueve, el 2 no debe funcionar.
Where the permissive sits
In the lab the M1 contact is in series with conveyor 2's coil after its Start/seal-in fork. That placement matters: because it is also in the seal-in path, losing M1 for any reason — a Stop, an overload trip, a power blip — drops M2 as well. Put it only in series with Start and conveyor 2 would stay running after conveyor 1 stopped.
Permissives are everywhere once you look: an oil-pressure switch before a compressor may start, a guard-door switch before a machine may run, a lube pump before a gearbox. Each is just a contact in series.
En la obra
A permissive is one contact, and where you put it decides whether the sequence also unwinds correctly when something stops. That is the part people get wrong.
Common mistakes
- The permissive in series with Start only. Conveyor 2 then starts in the right order — and keeps running after conveyor 1 stops, which is the pile-up you were preventing. It belongs after the fork, so it holds the seal-in as well.
- Using a contact that proves the wrong thing. A starter auxiliary proves the coil pulled in, not that the motor is turning. Where turning actually matters, the permissive should come from something that senses it.
- Chaining permissives so one stop takes the whole plant down when only part of it needed to stop. Draw what should fall over before you wire it.
- Leaving no way to run one machine alone for maintenance. A bypass with a key or a documented selector is better than the jumper somebody will fit at 2am.
On a real panel
- Permissives are everywhere once you look: oil pressure before a compressor, a lube pump before a gearbox, a guard door before anything moves.
- Stopping in reverse order is usually as important as starting in order — the last machine in the chain should be the last to stop, so the line clears.
- A permissive from another panel means a wire between panels, with its own terminals, numbers and, often, its own supply. Trace where its power comes from.
Prove it before you leave
- Try to start the downstream machine first. It must refuse.
- Start in order, then stop the upstream one. Confirm the downstream drops out too — or does not, if that is the design.
- Open each permissive by hand and confirm exactly what it takes down.
- Write the sequence on the panel door in plain words. Whoever meets it at 2am will be glad.
Preguntas frecuentes
What is sequence control?
Motors that must start in an order — and usually stop in the reverse order. A contact from the first motor’s starter is wired into the second motor’s control rung so the second cannot run without the first.
What is a permissive?
A contact that has to be closed before something is allowed to run: a starter contact, a pressure switch proving oil pressure, a door switch. It is in series in the rung, so without it nothing completes the path.
¿Ve algo incorrecto en esta página? Repórtelo
Gracias — su reporte fue enviado.
No se pudo enviar. Vuelva a intentarlo más tarde.
Avisos de herramientas que le sirven — por correo, opcional
Deje su correo y marque la casilla para recibir un aviso breve, como máximo cada dos semanas, sobre herramientas, ejercicios y diagramas que encajan con lo que ha estado estudiando — aquí y en bentright.com y fieldcalc.net. Es la misma lista para los tres, se sale con un clic y no se envía nada hasta que confirme desde su bandeja de entrada.