Lab 06: Forward/Reverse Control with Electrical Interlock

Application Scenario

A warehouse loading dock has a motor-driven roll-up door. The same motor opens the door when it turns forward and closes the door when it turns in reverse. On a three-phase motor, reversing is done with two contactors: the FORWARD contactor connects the motor normally, and the REVERSE contactor swaps two of the three supply phases.

If both contactors ever close at the same time, two supply phases are connected directly together. The result is a phase-to-phase short circuit: a loud flash, tripped breakers, and possibly welded contacts or a fire. Every reversing starter must therefore make it impossible for both contactors to be ON together. This protection is called an interlock.

In this lab:

• PB1: OPEN (forward)
• PB3: CLOSE (reverse)
• PB2: STOP
• CR1: FORWARD contactor (simulated) — G1 ON while the door is opening
• CR2: REVERSE contactor (simulated) — R1 ON while the door is closing

Learning Outcomes

After this lab you will be able to:

• Explain why two outputs, such as forward and reverse, must never be ON at the same time.
• Build an electrical interlock by placing each relay's N.C. contact in the other relay's rung.
• Build two independent seal-in circuits that share one STOP button.
• Drive an indicator lamp in parallel with a relay coil when the relay has no spare contacts.

Required Components

Background

How the electrical interlock works. Each relay's N.C. contact is placed in series with the other relay's coil:

• A CR2 N.C. contact sits in the CR1 (forward) rung. When CR2 is ON, this contact opens, and CR1 cannot energize.
• A CR1 N.C. contact sits in the CR2 (reverse) rung. When CR1 is ON, this contact opens, and CR2 cannot energize.

Whichever relay turns on first locks the other one out. To change direction, the operator must press STOP first. This "stop before reversing" rule also protects the motor and the door mechanism from the shock of an instant reversal.

Mechanical interlock. Real reversing starters also have a mechanical interlock: a lever between the two contactors that physically prevents both from closing. Industry uses both interlocks together, because each one covers failures that the other cannot.

Running out of contacts. CR1 has only two contact sets. In this lab, CR1A is the seal-in and CR1B is the interlock, so no CR1 contact is left for G1. The solution, which you tried in the Lab 02 Extension, is to connect G1 in parallel with the CR1 coil: whenever the coil has voltage, the lamp has voltage too. R1 is connected the same way across the CR2 coil, so both lamps work alike.

CR2 pins used in this lab:

Functional Requirements

1. Pressing PB1 starts FORWARD (CR1). It keeps running after PB1 is released.
2. Pressing PB3 starts REVERSE (CR2). It keeps running after PB3 is released.
3. CR1 and CR2 must never be ON at the same time. While one direction is running, the other START button must have no effect.
4. Pressing PB2 stops either direction. While PB2 is held, neither direction can start.
5. G1 is ON only while CR1 is ON. R1 is ON only while CR2 is ON. Both lamps dark means the door is stopped.
6. After a power outage, neither direction 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: three for FORWARD and three for REVERSE.

Step 1 — Rung 1 (grid line 1): FORWARD relay

1. Starting from L1, draw PB2 (N.C.), the STOP button. Label pins 1 and 2.
2. To the right of PB2, draw PB1 (N.O.), the OPEN button. Label pins 1 and 2.
3. To the right of PB1, draw a CR2B N.C. contact (with a slash). Label pin 10 on the left and pin 2 on the right. This is the interlock from the REVERSE relay.
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 N2; between PB1 and CR2B is N3; between CR2B and the CR1 coil is N4.

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

1. On grid line 2, under PB1, draw a CR1A N.O. contact. Label pins 1 and 3.
2. Connect pin 1 up to N2 and pin 3 up to N3.

Step 3 — Rung 3 (grid line 3): OPENING lamp

1. From node N4, draw a vertical line down to grid line 3.
2. Draw G1 on grid line 3 and connect its right side to L2. Label pins 1 and 2.

Step 4 — Rung 4 (grid line 4): REVERSE relay

1. From node N2, draw a vertical line down to grid line 4. This rung takes its power from N2, after the STOP button.
2. On grid line 4, draw PB3 (N.O.), the CLOSE button. Label pins 1 and 2.
3. To the right of PB3, draw a CR1B N.C. contact (with a slash). Label pin 8 on the left and pin 5 on the right. This is the interlock from the FORWARD relay.
4. At the right end, draw the CR2 coil. Label pins 14 and 13. Connect pin 13 to L2.
5. Mark the nodes: between PB3 and CR1B is N5; between CR1B and the CR2 coil is N6.

Step 5 — Rung 5 (grid line 5): REVERSE seal-in

1. On grid line 5, 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 N5.

Step 6 — Rung 6 (grid line 6): CLOSING lamp

1. From node N6, draw a vertical line down to grid line 6.
2. Draw R1 on grid line 6 and connect its right side to L2. Label pins 1 and 2.

Step 7 — Trace your drawing with a pencil

1. CR1 and CR2 are both OFF, and PB1 is pressed. Trace the path to the CR1 coil. Which contacts does it pass through?
2. CR1 is ON (door opening), and PB3 is pressed. Is there any path to the CR2 coil? Which contact blocks it?
3. CR2 is ON (door closing), and PB1 is pressed. Is there any path to the CR1 coil? Which contact blocks it?
4. CR1 is ON and PB2 is pressed. Is there any path left to either coil?
5. Suppose a student drew the CR1B contact as N.O. by mistake. Starting with both relays OFF, what happens when PB3 is pressed?

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 N3 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: STOP to OPEN (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. Cross the interlocks. CR1's N.C. contact goes in CR2's rung, and CR2's N.C. contact goes in CR1's rung. A relay's own N.C. contact in its own rung would make it switch itself off.
2. Interlocks must be N.C. An interlock contact wired to the N.O. pin blocks its relay all the time, and the relay can never start.
3. Each seal-in goes around its START button only. The interlock contact must stay in series with the coil, so it can still block the relay once it is sealed in.
4. One STOP for both directions. Both rungs take their power from N2, after PB2.
5. Watch the pin numbers. CR1:5 (the N.C. side of CR1B) and CR2:5 (the N.O. side of CR2A) share the number 5 but belong to different relays and different nodes.
6. A lamp in parallel with a coil is a quick way to show that a coil is energized when the relay has no spare contacts.

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 PB2 is pressed (CR2:9 is on node N2).
   • CR1:1 to CR2:10 should read high, and near 0 Ω while PB1 is pressed.
   • CR2:10 to CR1:2 should read near 0 Ω (interlock CR2B is N.C.).
   • CR2:9 to CR1:8 should read high, and near 0 Ω while PB3 is pressed.
   • CR1:8 to CR2:14 should read near 0 Ω (interlock CR1B is N.C.).
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. On a real three-phase motor, what would happen if the FORWARD and REVERSE contactors closed at the same time? Why?
2. Why is CR1's N.C. contact placed in the CR2 rung, and not in the CR1 rung?
3. The door is opening and the operator presses CLOSE. What happens, and what must the operator do to reverse the door? Why is this "stop before reversing" behavior good for the motor and the door mechanism?
4. What could happen if PB1 and PB3 were pressed at exactly the same instant? Name two methods that industrial reversing starters use in addition to the electrical interlock.
5. G1 is wired in parallel with the CR1 coil, not through a CR1 contact. What is one disadvantage of this method? (Hint: think about what the lamp actually tells you.)

Applying This Circuit Pattern

The electrical interlock pattern applies to any two (or more) outputs that must never be ON together: place each output's N.C. contact in series with the other output's coil.

Design question for the future: A real roll-up door has an UP limit switch (N.C., opens when the door is fully open) and a DOWN limit switch (N.C., opens when the door is fully closed). Where would you place each limit switch on your ladder diagram so that the motor stops automatically at the end of travel? Why must each limit switch be placed after its seal-in junction (N3 or N5), and not before?

Extension

Use PB4 (N.C.) to simulate the door's UP limit switch. Pressing PB4 means "the door has reached the top". Add PB4 to your ladder diagram so that pressing it stops OPEN, but does not affect CLOSE. Which wire(s) change? Test it: start OPEN, then press PB4. Can you still start CLOSE while PB4 is held?