Sequence Control

Force motors to start in the right order. A conveyor line that starts out of order jams — sequence control makes the order mandatory.

One thing before another

Sequence control (or interlocked starting) forces motors to start in a required order, and often to stop in order too. The classic example is a conveyor line: you must start the last conveyor first and work backward, so material never piles onto a belt that is not yet running. Start them out of order and you get a jam.

The core trick

To make motor 2 depend on motor 1, you put a NO contact controlled by motor 1’s coil in series with motor 2’s start circuit. Now motor 2 physically cannot start until motor 1 is running, because the contact feeding motor 2 is held open until coil 1 energizes and closes it.

• Start permissive: M1’s NO contact in M2’s rung means "M2 may only start if M1 is running."
• Chain it: M2’s NO contact in M3’s rung extends the sequence — 1, then 2, then 3.
• Stop in order: for shutdown sequencing, you can arrange the stop logic so stopping M1 also drops the ones that depend on it.

Real uses

Conveyor lines, lube-oil pumps that must run before a big machine starts, exhaust fans that must prove airflow before a burner lights, hydraulic pumps before press operation. Anywhere starting out of order causes damage or danger, sequence control enforces the rules.

What to take away

Sequence control makes one motor a condition for the next by placing the upstream coil’s NO contact in the downstream start circuit. It is start/stop logic plus a permissive contact — simple to wire, and it prevents expensive mistakes.