Lab 17: Controlled Stop vs. Emergency Stop

Application Scenario

A CNC lathe spins a heavy chuck driven by a variable-frequency drive (VFD). The machine has two very different ways to stop:

• STOP (controlled stop): the drive keeps its power for a few seconds and brakes the spindle to a quick, smooth stop. Only after the spindle has stopped is power removed. This is the normal way to stop at the end of a job.
• EMERGENCY STOP: power to everything that can move is removed immediately, and the control system cannot restart anything until the emergency stop has been released and a separate RESET has been pressed. A reset alone must never restart the machine.

The electrical standards for machinery (NFPA 79 in the US, IEC 60204-1 internationally) name these behaviors stop categories:

In this lab you will build both, using the panel's third bus L3 as a switched control bus that the emergency stop can switch OFF.

Learning Outcomes

After this lab you will be able to:

• Build a master control relay (MCR) circuit that feeds a switched bus (L3).
• Explain why an emergency stop removes power from a bus instead of opening one rung.
• Build a Stop Category 1 controlled stop with an off-delay timer.
• Explain why an emergency stop requires a separate reset, and why the reset must not restart motion.

Required Components

Background

The switched bus L3. The panel has a terminal named L3. It is not connected to anything by itself; it becomes live only when you feed it through a contact. In this lab, the MCR contact CR2B connects L1 to L3. Every rung that controls motion is connected to L3, not to L1. When the emergency stop drops the MCR, the whole L3 bus goes dead at once — every coil, every timer, every lamp on it. No single wiring mistake in one rung can keep a motion rung alive.

The MCR circuit. The MCR is an ordinary three-wire circuit on L1: E-STOP (N.C.) in series, RESET (N.O.) as the start button, and a seal-in. After the E-stop is pressed, or after a power failure, the MCR stays OFF until someone presses RESET.

Why the reset does not restart the machine. RESET only restores L3. The RUN relay CR1 was de-energized when L3 went dead, and its seal-in is open. Starting requires a separate, deliberate START. This is the same low-voltage protection you learned in Lab 02, applied to the whole machine.

The controlled stop. TR2 (off-delay) is powered from L3, and its trigger is closed by CR1B while RUN is ON. When STOP is pressed, CR1 drops at once (G1 OFF), but TR2A keeps the drive powered (R1 ON) for 3 s so the drive can brake the spindle. When the E-stop is pressed, TR2 loses its supply (L3) and drops out immediately — remember Lab 10, Review Question 4.

Timing Diagram

• STOP (Category 1): G1 goes OFF at once; R1 (drive power) stays ON for 3 s while the drive brakes, then goes OFF.
• E-STOP (Category 0): L3 goes dead; G1 and R1 go OFF at the same instant.
• Reset: after the E-stop is released, RESET restores L3, but nothing runs until START is pressed again.

Functional Requirements

1. At power-up, L3 is OFF. Pressing PB3 (RESET) energizes the MCR and L3 (PB4 must not be pressed).
2. With L3 ON, PB1 starts RUN: G1 and R1 ON.
3. PB2 (controlled stop): G1 OFF immediately; R1 stays ON for 3 s, then OFF.
4. PB4 (E-stop) at any time, including during the 3 s: L3 OFF; G1 and R1 OFF immediately.
5. After PB4 is released, nothing is ON. PB1 cannot start the machine until RESET is pressed, and RESET alone does not start it.
6. After a power outage, the machine behaves as after an E-stop: RESET, then START.

Ladder Diagram — Step-by-Step Drawing Guide

This design uses 9 grid lines. Draw L1 on the left and L2 on the right as usual. You will also draw a second left rail labeled L3, a little to the right of L1, starting at grid line 3.

Step 1 — Rungs 1–2: master control relay (on L1)

1. Grid line 1: from L1, PB4 (N.C.) → N2 → PB3 (N.O.) → N3 → CR2 coil (14–13).
2. Grid line 2: CR2A N.O. seal-in (9–5) from N2 to N3.

Step 2 — Rung 3: feed L3

1. Grid line 3: from L1, CR2B N.O. (10–6), then connect to the top of the L3 rail. Label the rail L3.

Step 3 — Rungs 4–6: RUN command (on L3)

1. Grid line 4: from L3, PB2 (N.C.) → N5 → PB1 (N.O.) → N6 → CR1 coil (2–7).
2. Grid line 5: CR1A N.O. seal-in (1–3) from N5 to N6.
3. Grid line 6: from N6, G1 to L2 (in parallel with the CR1 coil).

Step 4 — Rungs 7–9: controlled-stop timer (on L3)

1. Grid line 7: from L3, the TR2 coil (2–10) to L2.
2. Grid line 8: the trigger loop: CR1B N.O. (8–6) with CR1:8 → TR2:5 and CR1:6 → TR2:6. Label the two wires N7 and N8. This loop does not touch L1, L2, or L3.
3. Grid line 9: from L3, TR2A N.O.T.O. (1–3) → N9 → R1 to L2.

Step 5 — Trace your drawing with a pencil

1. Power has just been turned ON. Is L3 live? Can PB1 start RUN?
2. The machine is running and PB2 is pressed. Trace what happens to CR1, the TR2 trigger, TR2A, G1, and R1, in order.
3. The machine is running and PB4 is pressed. Trace what happens to CR2, L3, CR1, the TR2 coil, TR2A, G1, and R1. How long does R1 stay ON?
4. PB4 is released. What is ON? What must the operator do to run the machine again, and in what order?
5. Suppose the TR2 coil were connected to L1 instead of L3. What would R1 do after an E-stop? Why is that dangerous?

Step 6 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

List every terminal on each node (with device names), chain them, and fill in the wiring table on the worksheet. Treat L3 as a node like any other: it includes the L3 terminal, CR2:6, and every device that starts from the L3 rail. The L3 row is started for you.

Get instructor approval before wiring.

Key Wiring Points

1. E-stop switches a bus, not a rung. The MCR contact feeds L3, and every motion rung is on L3.
2. The E-stop is N.C. and first in line in the MCR rung (fail-safe: a broken wire acts as an E-stop).
3. Reset restores power only. Every motion circuit on L3 needs its own START after a reset.
4. The controlled-stop timer is powered from L3, so an E-stop overrides the controlled stop.
5. The TR2 trigger is a dry-contact loop (CR1:8 ↔ TR2:5, CR1:6 ↔ TR2:6), not connected to L1, L2, or L3.

Wiring Procedure

1. Power OFF. Set TR2 to 3 s.
2. Wire the MCR section (Rungs 1–3) first. Then wire the L3 section.
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.
   • L1 to L3 should read high (CR2B is open). If it reads near 0 Ω, L3 is tied directly to L1 — the E-stop would not work. Find the error before powering up.
   • L3 to PB2:1, L3 to TR2:2, and L3 to TR2:1 should read near 0 Ω.
   • CR1:8 to TR2:5 and CR1:6 to TR2:6 should read near 0 Ω.
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. Explain the difference between Stop Category 0 and Stop Category 1, and give one example where each is the better choice.
2. Why does the emergency stop switch off the L3 bus instead of being wired in series with each motor rung?
3. Why must RESET be a separate button from START? What could happen if one button did both?
4. In Test 6, the E-stop interrupted a controlled stop. The spindle then coasts instead of braking. Why is this still considered the correct behavior for an emergency stop?
5. Real E-stop buttons latch in the pressed position. Why? What would be the risk of a momentary E-stop button in a real machine?

Applying This Circuit Pattern

The master control relay and switched bus pattern appears in almost every industrial control panel: a safety function switches a bus, and all hazardous functions are fed from that bus. Modern machines use certified safety relays or safety PLCs for the MCR function, with redundant (dual-channel) contacts and self-monitoring.

Design question for the future: The last row says the PLC's inputs stay powered from L1 while its outputs are powered from L3. Why is it useful for the PLC to keep running and reading inputs during an E-stop?

Extension

Add lamp R2 as an E-STOP / RESET REQUIRED indicator that is ON whenever L3 is OFF. CR2C (11-7-3) is free. Which pins do you use, and which rail must R2 be fed from? Connect R2:2 to L2. (Your wiring table will go past 23 rows; use the back of the worksheet.)