Lab 02: Three-Wire Start/Stop Control with Seal-In
Application Scenario
In a distribution center, a conveyor belt carries boxes from the loading dock to the sorting area. The operator presses START once and the belt keeps running. It stops only when someone presses STOP. Holding a button down all shift, as in Lab 01, is not practical.
This circuit is called three-wire control, because three wires run between the push-button station and the motor starter. It is the most common motor control circuit in industry, and it provides two important features:
- Seal-in (holding): the relay keeps itself energized after START is released.
- Low-voltage protection: after a power outage, the motor does not restart by itself when power returns. The operator must press START again. This protects a worker who may be clearing a jam when power comes back.
As in Lab 01, CR1 represents the motor contactor and G1 represents the conveyor motor running.
Learning Outcomes
After this lab you will be able to:
- Build a seal-in circuit using a relay's own N.O. contact in parallel with the START button.
- Explain why the STOP button is N.C. and placed first on the rung.
- Demonstrate low-voltage protection.
- Read and use node numbers to wire parallel branches.
Required Components
| Device | Type | Role in this lab |
|---|---|---|
| PB1 | Push button, N.O. | START |
| PB2 | Push button, N.C. | STOP |
| CR1 | Control relay | Conveyor motor contactor (simulated) |
| G1 | Green lamp | Conveyor running |
Background
Seal-in. When PB1 is pressed, CR1 energizes and its N.O. contact CR1A (1–3) closes. This contact is wired in parallel with PB1, so it gives current a second path to the coil. When PB1 is released, current still flows through CR1A, and the relay holds itself in.
Why STOP is N.C. and comes first. PB2 sits in series at the head of the rung, so every path to the coil passes through it. Pressing PB2 opens the only path, and the coil drops out. The seal-in contact opens at the same moment, so releasing PB2 does not restart the motor. This arrangement has two safety benefits:
- Stop has priority: if START and STOP are pressed at the same time, the motor stays off.
- Fail-safe: if the wire to the STOP button breaks or comes loose, the circuit acts as if STOP were pressed, and the machine stops. An N.O. stop button with a broken wire would leave a machine that cannot be stopped.
Node numbers. Every point that is electrically the same gets one number. All wires marked with the same node number connect together, even if they land on different terminals. Two devices are in parallel when they connect to the same two nodes. In this lab, PB1 and CR1A both connect between N2 and N3. In industry, every wire is tagged with its node number so a technician can trace the circuit.
Ladder Diagram
- Rung 1 (control rung): Pressing PB1 energizes CR1. Pressing PB2 breaks the path, and CR1 drops out.
- Rung 2 (seal-in branch): CR1A (1–3) closes when CR1 energizes and bridges N2 to N3, so CR1 stays energized after PB1 is released.
- Rung 3 (output rung): CR1B (8–6) closes whenever CR1 is energized and turns G1 ON.
Contact Allocation
| Relay | Part | Pins | Used in |
|---|---|---|---|
| CR1 | Coil | 2–7 | Rung 1 |
| CR1 | CR1A N.O. | 1–3 | Rung 2 (seal-in) |
| CR1 | CR1B N.O. | 8–6 | Rung 3 |
| CR1 | CR1A N.C. / CR1B N.C. | 1–4 / 8–5 | Not used (you will use one in Lab 03) |
Wiring List
| ☐ | Rung# | Wire# | From | To | Purpose |
|---|---|---|---|---|---|
| ☐ | 1 | 1 | L1 | PB2:1 | +24 V to STOP button |
| ☐ | 1 | 2 | PB2:2 | PB1:1 | STOP to START (N2) |
| ☐ | 2 | 3 | PB2:2 | CR1:1 | STOP to seal-in common (N2) |
| ☐ | 1 | 4 | PB1:2 | CR1:2 | START to relay coil (N3) |
| ☐ | 2 | 5 | CR1:3 | CR1:2 | Seal-in jumper on relay base (N3) |
| ☐ | 1 | 6 | CR1:7 | L2 | Coil return to GND |
| ☐ | 3 | 7 | L1 | CR1:8 | +24 V to CR1B common |
| ☐ | 3 | 8 | CR1:6 | G1:1 | CR1B N.O. to lamp (N4) |
| ☐ | 3 | 9 | G1:2 | L2 | Lamp return to GND |
Tip: Wire 3 must start at PB2:2 (node N2), not at L1. If it starts at L1, the seal-in contact bypasses the STOP button and the conveyor cannot be stopped.
Operating Sequence
| Step | Operator action | CR1 coil | CR1A (1–3) | G1 |
|---|---|---|---|---|
| 1 | Power ON, no button pressed | OFF | Open | OFF |
| 2 | Press PB1 (START) | ON | Closed | ON |
| 3 | Release PB1 | ON (held by CR1A) | Closed | ON |
| 4 | Press PB2 (STOP) | OFF | Open | OFF |
| 5 | Release PB2 | OFF | Open | OFF |
| 6 | Power lost while running, then restored | OFF | Open | OFF |
Procedure
- Power OFF the trainer (or unplug the power cord).
- Write the node numbers (L1, N2, N3, N4, L2) on your copy of the ladder diagram. Each node number is one "connection point".
- Wire Rung 1 in this order: Wires 1, 2, 4, 6. This is the basic start circuit.
- Add the seal-in branch (Rung 2): Wires 3 and 5.
- Wire Rung 3: Wires 7, 8, 9.
- Check your work against the wiring list, one wire at a time. Mark each wire as checked.
- Meter check, power still OFF:
- PB2:1 to PB2:2 should read near 0 Ω, and open while PB2 is pressed.
- PB1:1 to PB1:2 should read open, and near 0 Ω while PB1 is pressed.
- CR1:1 to CR1:3 should read open.
- PB2:2 to CR1:1 should read near 0 Ω (both are node N2).
- Ask your lab partner to do a buddy check, then show the instructor.
- Power ON and complete the test table below.
Test & Acceptance
| # | Test | Expected result | ✓ |
|---|---|---|---|
| 1 | Power ON, do nothing | G1 OFF | ☐ |
| 2 | Press and release PB1 | G1 turns ON and stays ON | ☐ |
| 3 | Press PB1 again while running | No change | ☐ |
| 4 | Press and release PB2 | G1 turns OFF and stays OFF | ☐ |
| 5 | Hold PB2, then press PB1 | G1 stays OFF (stop has priority) | ☐ |
| 6 | Start the conveyor, then switch power OFF and back ON | G1 stays OFF (low-voltage protection) | ☐ |
| 7 | Power OFF, remove Wire 1, power ON, press PB1 | Nothing runs (a broken STOP wire fails safe). Power OFF and reinstall Wire 1 afterward. | ☐ |
Troubleshooting
| Symptom | Likely cause | What to check |
|---|---|---|
| G1 is ON only while PB1 is held | Seal-in branch missing or open | Wires 3 and 5; CR1A must use pins 1 and 3 |
| Relay buzzes or chatters at power-up | Seal-in wired to the N.C. pin | Wire 5 must come from CR1:3, not CR1:4 |
| PB2 does not stop the conveyor | Seal-in branch bypasses PB2 | Wire 3 must start at PB2:2, not L1 |
| Pressing PB1 does nothing | Open path through PB2 or the coil | Wires 1, 2, 4, 6; PB2 terminals |
| G1 is ON at power-up | G1 wired to the N.C. pin | Wire 8 must come from CR1:6, not CR1:5 |
| Relay turns ON, but G1 stays OFF | Rung 3 open | Wire 7 at CR1:8; Wire 9 from G1:2 to L2 |
Review Questions
- Why is CR1A connected in parallel with PB1? What would happen if it were connected in series with PB1?
- If Wire 3 were connected to L1 instead of PB2:2, what would happen when STOP is pressed? Why?
- Why does industry use an N.C. STOP button? What happens if the wire to an N.C. STOP button breaks? What if the STOP button were N.O. and its wire broke?
- After a power outage, why is it safer that the conveyor does not restart by itself? Name one device (for example, a sump pump with a float switch) that should restart automatically, and explain the difference.
Extension
Power OFF. Remove Wires 7 and 8, then connect G1:1 directly to CR1:2 (node N3). G1 is now wired in parallel with the CR1 coil. Does G1 still show when the conveyor is running? Which contact set did you free up? (Engineers use this trick when a relay runs out of contacts. You will use it again in later labs.)