Warehouse conveyor systems often need a motor to keep running after an operator releases the Start button, but they must also stop immediately when a Stop button or safety device opens the circuit. A latching, or sealing, circuit solves this by using an auxiliary contact on a relay or contactor to hold the control circuit energized. This idea is common in conveyor lines, sorters, packaging stations, and automated loading systems.
It matters because reliable start and stop logic keeps materials moving while protecting workers and equipment.
In a typical circuit, the Stop pushbutton is normally closed, the Start pushbutton is normally open, and the contactor coil energizes when Start is pressed. Once the coil energizes, a normally open auxiliary contact wired in parallel with the Start button closes and creates a sealing path around the Start button. The motor contactor then stays energized until the Stop button, overload contact, emergency stop, or interlock opens the control circuit.
This simple control pattern is the foundation for many larger warehouse automation circuits using relays, motor starters, PLC inputs, and safety controls.
Key Facts
- A seal-in contact is a normally open auxiliary contact wired in parallel with the Start pushbutton.
- Basic latch logic: Start energizes coil M, then auxiliary contact M closes to maintain current to coil M.
- The Stop pushbutton is normally closed so pressing it opens the circuit and de-energizes the coil.
- Control power follows the path: L1 to Stop to Start or seal-in contact to coil to L2.
- Motor power is switched by the main contactor contacts, while the pushbuttons control only the contactor coil.
- Ohm's law for a DC control coil: I = V/R, where I is coil current, V is control voltage, and R is coil resistance.
Vocabulary
- Latching circuit
- A control circuit that keeps a relay or contactor energized after the momentary Start button is released.
- Sealing contact
- An auxiliary contact that closes when the coil energizes and provides an alternate current path around the Start button.
- Contactor
- An electrically operated switch used to control power to a motor or other high-current load.
- Normally closed contact
- A contact that conducts current in its resting state and opens when it is actuated.
- Overload relay
- A protective device that opens the control circuit when motor current remains too high for too long.
Common Mistakes to Avoid
- Wiring the sealing contact in series with the Start button is wrong because the coil will drop out as soon as the Start button is released.
- Using a normally open Stop button is wrong in most motor control circuits because a broken wire could fail to stop the machine safely.
- Confusing the control circuit with the motor power circuit is wrong because pushbuttons usually switch coil current, not full motor current.
- Forgetting the overload contact in the coil circuit is wrong because the motor may keep running during an overload condition instead of shutting down.
Practice Questions
- 1 A 24 V DC contactor coil has a resistance of 120 ohms. What current flows through the coil when the circuit is latched on?
- 2 A conveyor control circuit uses a 120 V AC coil rated at 12 VA. What is the approximate coil current while energized?
- 3 In a latching conveyor circuit, explain what happens if the auxiliary sealing contact fails to close after the Start button is pressed.