Lab 07: Conveyor Sequence Interlock

Application Scenario

In a recycling plant, a short feed conveyor (Conveyor 1) drops material onto a long discharge conveyor (Conveyor 2), which carries it to a sorting station. The two conveyors must follow three rules:

1. Start downstream first. Conveyor 1 may start only when Conveyor 2 is already running. Otherwise material piles up where the two conveyors meet.
2. Stop upstream first. The operator stops Conveyor 1 and lets Conveyor 2 carry away the remaining material.
3. Cascade stop. If Conveyor 2 stops for any reason (its STOP button, an overload trip, a pull-cord), Conveyor 1 must stop automatically.

This is called sequence interlocking. It is used on almost every multi-conveyor system: mines, quarries, airports, grain elevators, and distribution centers.

Each conveyor has its own push-button station:

Learning Outcomes

After this lab you will be able to:

• Use an N.O. contact of one motor's relay as a permissive for another motor.
• Explain the difference between a start permissive and a running interlock, and place each one correctly.
• Build a cascade stop so that a downstream failure automatically stops upstream equipment.
• Wire two independent start/stop stations, one for each motor.

Required Components

Background

A relay contact as a permission. In Labs 04–06, the contacts that controlled a coil came from buttons or from the relay itself. In this lab, a contact from another motor's relay controls the coil. An N.O. contact of CR2 placed in the CR1 rung means: "Conveyor 1 is allowed to run only while Conveyor 2 is running."

Where the permissive contact goes makes all the difference. A seal-in contact bypasses everything it is connected across. So:

• If the CR2 contact is placed before the seal-in junction (in series with PB1 only), it is only a start permissive. It is checked when PB1 is pressed, then bypassed by the seal-in. If Conveyor 2 stops later, Conveyor 1 keeps running.
• If the CR2 contact is placed after the seal-in junction (in series with the CR1 coil), it is a running interlock. It is checked at start and all the time while running. If Conveyor 2 stops, Conveyor 1 stops too.

For a conveyor line, the cascade-stop rule requires a running interlock.

CR2 pins used in this lab:

Functional Requirements

1. Pressing PB3 starts Conveyor 2 (CR2). It keeps running after PB3 is released. Pressing PB4 stops it.
2. Pressing PB1 starts Conveyor 1 (CR1) only if Conveyor 2 is running. It keeps running after PB1 is released.
3. Pressing PB2 stops Conveyor 1 only. Conveyor 2 keeps running.
4. If Conveyor 2 stops for any reason, Conveyor 1 stops automatically.
5. When Conveyor 2 is restarted, Conveyor 1 does not restart by itself.
6. G1 is ON while Conveyor 2 runs. R1 is ON while Conveyor 1 runs.
7. After a power outage, neither conveyor restarts by itself.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. L1 (+24 V) is the left rail and L2 (GND) is the right rail. Use pencil. This design uses 6 grid lines. Draw the downstream conveyor first, because it is the one that starts first.

Step 1 — Rung 1 (grid line 1): Conveyor 2 (downstream)

1. Starting from L1, draw PB4 (N.C.), the Conveyor 2 STOP button. Label pins 1 and 2.
2. To the right of PB4, draw PB3 (N.O.), the Conveyor 2 START button. Label pins 1 and 2.
3. At the right end, draw the CR2 coil. Label pins 14 and 13. Connect pin 13 to L2.
4. Mark the nodes: between PB4 and PB3 is N2; between PB3 and the CR2 coil is N3.

Step 2 — Rung 2 (grid line 2): Conveyor 2 seal-in

1. On grid line 2, under PB3, draw a CR2A N.O. contact. Label pins 9 and 5.
2. Connect pin 9 up to N2 and pin 5 up to N3.

Step 3 — Rung 3 (grid line 3): Conveyor 1 (upstream)

1. Starting from L1, draw PB2 (N.C.), the Conveyor 1 STOP button. Label pins 1 and 2.
2. To the right of PB2, draw PB1 (N.O.), the Conveyor 1 START button. Label pins 1 and 2.
3. To the right of PB1, draw a CR2B N.O. contact. Label pin 10 on the left and pin 6 on the right. This is the running interlock: "Conveyor 2 is running."
4. At the right end, draw the CR1 coil. Label pins 2 and 7. Connect pin 7 to L2.
5. Mark the nodes: between PB2 and PB1 is N4; between PB1 and CR2B is N5; between CR2B and the CR1 coil is N6.

Step 4 — Rung 4 (grid line 4): Conveyor 1 seal-in

1. On grid line 4, under PB1, draw a CR1A N.O. contact. Label pins 1 and 3.
2. Connect pin 1 up to N4 and pin 3 up to N5.

Step 5 — Rung 5 (grid line 5): Conveyor 2 RUNNING lamp

1. Starting from L1, draw a CR2C N.O. contact. Label pins 11 and 7.
2. Draw G1 at the right end and connect it to L2. Mark the node between them as N7.

Step 6 — Rung 6 (grid line 6): Conveyor 1 RUNNING lamp

1. Starting from L1, draw a CR1B N.O. contact. Label pins 8 and 6.
2. Draw R1 at the right end and connect it to L2. Mark the node between them as N8.

Step 7 — Trace your drawing with a pencil

1. Both conveyors are OFF, and PB1 is pressed. Is there a path to the CR1 coil? Which contact blocks it?
2. Conveyor 2 is running, and PB1 is pressed and released. Trace the path that keeps CR1 energized.
3. Both conveyors are running, and PB2 is pressed. Which coil loses power? Which one keeps power?
4. Both conveyors are running, and PB4 is pressed. Trace what happens to CR2, then to CR2B, then to CR1.
5. Suppose CR2B were drawn between PB2 and PB1 instead (between N4 and PB1:1). Repeat question 4. What is different?

If any answer is wrong, fix the drawing before you continue.

Step 8 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

Step 1 — List every terminal on each node

Look at each node dot on your drawing and write down every terminal that touches it. The N5 row is done for you.

Step 2 — Chain the terminals of each node together

A node with k terminals needs k − 1 wires, connected in a chain. Keep no more than two wires under any terminal screw. Spread the GND return wires across the panel's L2 terminals.

Step 3 — Fill in the wiring table on the worksheet

Write one row per wire: Rung#, Wire#, From, To, and Purpose. In the Purpose column, include the node number in parentheses, for example: C2 STOP to C2 START (N2).

Step 4 — Get instructor approval

Show your ladder diagram, contact allocation table, node table, and wiring table to the instructor. Do not start wiring until the worksheet is stamped.

Key Wiring Points

1. Downstream first. Draw and wire the downstream conveyor (Conveyor 2) first. The upstream conveyor depends on it.
2. The permissive is an N.O. contact of the downstream relay (CR2B, 10–6), placed in the upstream rung.
3. Running interlock goes after the seal-in junction. CR2B must be between N5 and the CR1 coil. Before the junction, it would be only a start permissive, and the cascade stop would not work.
4. Each conveyor has its own STOP. PB2 stops Conveyor 1 only. PB4 stops Conveyor 2 directly and Conveyor 1 through the interlock.
5. Use spare contacts for lamps. Driving a lamp through an N.O. contact proves the relay actually moved.

Wiring Procedure

1. Power OFF the trainer (or unplug the power cord).
2. Wire the circuit from your approved wiring table, one node at a time. Mark each wire as you install it.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR2:9 should read near 0 Ω, and high while PB4 is pressed.
   • CR2:9 to CR2:14 should read high, and near 0 Ω while PB3 is pressed.
   • L1 to CR1:1 should read near 0 Ω, and high while PB2 is pressed.
   • CR1:1 to CR2:10 should read high, and near 0 Ω while PB1 is pressed.
   • CR2:10 to CR1:2 should read high (CR2B is N.O. and CR2 is OFF). A reading near 0 Ω means CR2B is on the N.C. pin.
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Why must the downstream conveyor start first and stop last? What would happen at the transfer point if the order were reversed?
2. CR2B is placed between N5 and the CR1 coil. What would change if it were placed between PB2 and PB1 instead? Which of the three rules would be broken?
3. Both conveyors are running, and the Conveyor 2 motor overload relay trips. What happens to Conveyor 1? Why is this important for safety and for cleanup?
4. After Conveyor 2 is restarted, Conveyor 1 does not restart by itself. Is this a good or a bad feature? Explain.
5. A plant has four conveyors in a line: C1 → C2 → C3 → C4 (C4 is the last one). Describe where the interlock contacts go. How many relay contacts are needed just for the sequence interlocks?

Applying This Circuit Pattern

The running interlock pattern: a device may run only while another device is running. Place an N.O. contact of the "required" device's relay in series with the dependent device's coil, after its seal-in junction.

Design question for the future: In a real plant, the lube oil pump must run for 30 seconds before the main motor is allowed to start, to build up oil pressure. Which part of this lab's circuit would change? What new device would you need? (You will learn this device in Part C.)

Extension: Start Permissive vs. Running Interlock

Power OFF. Rewire only Rungs 3 and 4 so that CR2B becomes a start permissive: PB2 → CR2B → PB1 → CR1 coil, with the CR1A seal-in connected from N4 (after PB2) directly to the CR1 coil. Remove all existing Rung 3 and Rung 4 wires except L1 → PB2:1 and CR1:7 → L2, then install:

Power ON. Start Conveyor 2, then Conveyor 1. Now press PB4. What does Conveyor 1 do? Explain the result using the ladder diagram. When you are done, power OFF and restore the original wiring.