Lab 16: Forward/Reverse with Anti-Plugging Delay

Application Scenario

At a quarry, a heavy reversible shuttle conveyor sends crushed rock either to the left stockpile (FORWARD) or to the right stockpile (REVERSE). The belt and its load are very heavy: after STOP, the belt keeps coasting for several seconds.

In Lab 06, the operator had to press STOP before reversing, but could press the opposite START button immediately after STOP. On this conveyor, that would reverse the motor while the belt is still moving forward. Reversing a spinning motor is called plugging. It draws a current even higher than a normal start, heats the motor, and gives the gearbox, couplings, and belt splices a violent shock.

The fix is an anti-plugging delay (also called a coast-down lockout): after the motor stops, no start is allowed until the motor has been stopped for a set time. The same delay also blocks starts for a few seconds after power is restored, in case the belt was still coasting when the power came back.

In this lab (5 seconds represents the coast-down time):

Learning Outcomes

After this lab you will be able to:

• Detect "motor stopped" by putting N.C. contacts of both direction contactors in series.
• Use an on-delay timer as a start permissive that requires a condition to be true for a set time.
• Place a permissive before the seal-in junction, and an interlock where it can also stop a running motor.
• Make one relay contact do two jobs by choosing where it sits in the rung.

Background

"Stopped for 5 seconds." The motor is stopped when CR1 is OFF and CR2 is OFF. That is two N.C. contacts in series (AND logic). Feeding an on-delay timer through them gives a contact that closes only when the motor has been stopped for the whole set time. If either direction starts, the timer resets at once.

The timer contact is a start permissive. TR1A is placed in series with the FORWARD START button, and TR1B in series with the REVERSE START button. As soon as a direction starts, TR1 resets and its contacts open, so the seal-in must bypass them (remember Lab 04's design question and Lab 07's start permissive).

One contact, two jobs. CR1 has only two contact sets: CR1A is the FORWARD seal-in, so CR1B (N.C.) must provide both the FORWARD→REVERSE interlock and half of the "motor stopped" signal. The trick is to put CR1B at the very beginning of the REVERSE rung, just after N2. The node after it (N6) means "STOP not pressed and FORWARD not running." Both the REVERSE rung and the TR1 timer are fed from N6.

The lamps G1 and R1 are wired in parallel with the CR1 and CR2 coils, as in Lab 06, because CR1 has no spare contact.

Timing Diagram

• Power-up: TR1 starts timing. No start is possible for the first 5 s.
• FWD: CR1 seals in. TR1 resets. The first REV press is ignored (interlock and permissive).
• STOP: CR1 drops. TR1 starts timing the coast-down. A REV press during the lockout is ignored.
• ready → REV: after 5 s, TR1's contacts close, and the next REV press starts CR2.

Functional Requirements

1. PB1 starts FORWARD (CR1, G1); PB3 starts REVERSE (CR2, R1). Each seals in.
2. FORWARD and REVERSE are interlocked: they can never be ON together.
3. PB2 stops either direction.
4. After any stop, and after power-up, neither START button works until the motor has been stopped for 5 s.
5. After a power outage, nothing restarts by itself.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. Use pencil. This design uses 7 grid lines.

Step 1 — Rung 1: FORWARD

1. From L1: PB2 (N.C.) → N2 → TR1A N.O.T.C. (1–3) → N3 → PB1 (N.O.) → N4 → CR2B N.C. (10–2) → N5 → CR1 coil (2–7).

Step 2 — Rung 2: FORWARD seal-in

1. Draw CR1A N.O. (1–3) from N2 to N4, bypassing both TR1A and PB1.

Step 3 — Rung 3: FORWARD lamp

1. From N5, draw G1 to L2 (in parallel with the CR1 coil).

Step 4 — Rung 4: REVERSE

1. From N2: CR1B N.C. (8–5) → N6 → TR1B N.O.T.C. (8–6) → N7 → PB3 (N.O.) → N8 → CR2 coil (14–13).

Step 5 — Rung 5: REVERSE seal-in

1. Draw CR2A N.O. (9–5) from N6 to N8, bypassing TR1B and PB3 (but not CR1B).

Step 6 — Rung 6: REVERSE lamp

1. From N8, draw R1 to L2.

Step 7 — Rung 7: stopped-time timer

1. From N6: CR2D N.C. (12–4) → N9 → TR1 coil (2–7).

Step 8 — Trace your drawing with a pencil

1. Power has just been turned ON (t = 1 s). Is the TR1 coil energized? Can PB1 start FORWARD? Why not?
2. FORWARD is running. Trace why PB3 cannot start REVERSE. How many different contacts block it?
3. FORWARD is running. What keeps CR1 energized, given that TR1A is now open?
4. STOP was pressed 2 s ago. Which contact blocks PB1? Which blocks PB3?
5. What would happen if the FORWARD seal-in were drawn from N3 to N4 (bypassing only PB1)?

Step 9 — 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. The N6 row is done for you.

Get instructor approval before wiring.

Key Wiring Points

1. "Stopped" = CR1 N.C. AND CR2 N.C., in series, feeding the on-delay timer.
2. The timer contacts are start permissives: in series with the START buttons, bypassed by the seal-ins.
3. Interlocks must also act on a running motor: CR2B is after the FORWARD seal-in junction; CR1B is ahead of the whole REVERSE rung.
4. CR1B does two jobs because of where it sits. A contact's position in the rung decides what it controls.
5. The lockout also works at power-up, protecting a motor that may still be coasting after a short outage.

Wiring Procedure

1. Power OFF. Set TR1 to 5 s.
2. Wire the circuit from your approved wiring table, one node at a time.
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 TR1:1 should read near 0 Ω, and high while PB2 is pressed (TR1:1 is on N2).
   • TR1:1 to CR1:5 should read near 0 Ω (N2 → CR1B N.C. → N6).
   • CR2:12 to TR1:2 should read near 0 Ω (CR2D N.C. closed).
   • TR1:1 to PB1:1 should read high (TR1A open with the timer OFF).
   • CR2:10 to CR1:2 should read near 0 Ω (CR2B N.C. closed).
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. What is "plugging," and why is it harmful to a heavy conveyor drive?
2. Why is the anti-plugging timer an on-delay timer fed by "motor stopped," rather than an off-delay timer?
3. Explain why the FORWARD seal-in must go from N2 to N4 and not from N3 to N4.
4. Explain how CR1B acts as both an interlock and part of the "stopped" signal. What would change if CR1B were placed just before the CR2 coil instead?
5. A real system might use a zero-speed switch on the conveyor pulley instead of a fixed time. What advantage does that have?

Applying This Circuit Pattern

The "condition true for a time" permissive pattern: feed an on-delay timer through the condition, and put its contact in series with the START button(s), bypassed by the seal-in.

Design question for the future: Some processes allow restarting in the same direction immediately (the motor is already turning the right way) but still require the delay before reversing. How could you change this lab so that only the opposite direction is locked out?

Extension

Add a READY indicator that lights when a start is allowed. TR1 has no free contacts. Can you build READY with lamp G2 anyway? Hint: look for a node in your diagram that is live only when a start is allowed. Connect G2:2 to L2.