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.
Sequence: conveyor 2 cannot start until conveyor 1 runs
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Conveyor 2 feeds onto conveyor 1. If 1 is not moving, 2 must not run.
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.
Questions people ask
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.
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