Lab 01: Jog (Inching) Control
Application Scenario
An overhead crane operator uses a pendant control to move the hook a few inches at a time so a load can be lined up precisely. The motor must run only while the button is held down and stop the instant it is released. This is called jog or inching control. The same idea is used to position a conveyor belt during maintenance, or to feed a wire-winding machine slowly by hand.
In a real machine, the push button never carries the motor current. The button energizes a relay coil (in industry, a contactor), and the relay's contacts switch the load. In this lab, CR1 represents the motor contactor, and G1 represents the motor running.
Learning Outcomes
After this lab, you will be able to:
- Tell the difference between a relay coil and a relay contact.
- Find the common, N.O., and N.C. pins of a relay contact set by pin number.
- Build and test a two-rung ladder circuit: one control rung and one output rung.
- Explain why the jog control stops when the button is released.
Required Components
| Device | Type | Role in this lab |
|---|---|---|
| PB1 | Push button, N.O. | JOG button |
| CR1 | Control relay | Motor contactor (simulated) |
| G1 | Green lamp | Motor running |
Background
How a relay works. When voltage is applied to the coil, it becomes an electromagnet and pulls every contact to its opposite state. When the coil is de-energized, a spring returns the contacts to their normal (shelf) state. "Normally open" and "normally closed" always describe the contact with the coil OFF.
Contact pins. Every contact set has 3 pins: a common (a), an N.C. pin (b), and an N.O. pin (c).
- a–b is closed when the coil is OFF (N.C.).
- a–c is closed when the coil is ON (N.O.).
For CR1, contact set CR1A uses pin 1 (common), pin 3 (N.O.), and pin 4 (N.C.). The coil is pins 2–7, with pin 7 connected to L2 (GND).
Reading a ladder diagram.
- The left rail is L1 (+24 V) and the right rail is L2 (GND).
- Each horizontal line is a rung. Inputs (buttons, contacts) go on the left, and exactly one output (a coil or a lamp) goes on the right.
- A rung's output turns ON when there is a closed path from the left rail through every contact to the output.
- A relay's coil and its contacts have the same name (CR1) but may appear on different rungs. This is how one rung controls another.
Ladder Diagram
- Rung 1 (control rung): Pressing PB1 closes the path from L1 to the CR1 coil, so CR1 energizes.
- Rung 2 (output rung): When CR1 energizes, its N.O. contact CR1A (1–3) closes, and G1 turns ON. When PB1 is released, the coil drops out, the contact opens, and G1 turns OFF.
Contact Allocation
| Relay | Part | Pins | Used in |
|---|---|---|---|
| CR1 | Coil | 2–7 | Rung 1 |
| CR1 | CR1A N.O. | 1–3 | Rung 2 |
| CR1 | CR1A N.C. | 1–4 | Not used |
| CR1 | CR1B | 8–6 / 8–5 | Not used |
Wiring List
| ☐ | Rung# | Wire# | From | To | Purpose |
|---|---|---|---|---|---|
| ☐ | 1 | 1 | L1 | PB1:1 | +24 V to JOG button |
| ☐ | 1 | 2 | PB1:2 | CR1:2 | Button to relay coil |
| ☐ | 1 | 3 | CR1:7 | L2 | Coil returns to GND |
| ☐ | 2 | 4 | L1 | CR1:1 | +24 V to CR1A common |
| ☐ | 2 | 5 | CR1:3 | G1:1 | CR1A N.O. to lamp |
| ☐ | 2 | 6 | G1:2 | L2 | Lamp returns to GND |
Operating Sequence
| Step | Operator action | CR1 coil | CR1A (1–3) | G1 |
|---|---|---|---|---|
| 1 | Power ON, no button pressed | OFF | Open | OFF |
| 2 | Press and hold PB1 | ON | Closed | ON |
| 3 | Release PB1 | OFF | Open | OFF |
Procedure
- Power OFF the trainer (or unplug the power cord).
- On the relay base, locate CR1 pins 1, 2, 3, 4, and 7. Compare them with the panel layout drawing.
- Wire Rung 1 (Wires 1, 2, 3).
- Wire Rung 2 (Wires 4, 5, 6).
- Check your work against the wiring list, one wire at a time. Mark each wire as checked.
- Meter check, power still OFF: Set a multimeter to continuity or ohms.
- PB1:1 to PB1:2 should read open, and near 0 Ω while PB1 is pressed.
- CR1:1 to CR1:3 should read open. CR1:1 to CR1:4 should read near 0 Ω.
- 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; relay silent | ☐ |
| 2 | Press and hold PB1 | Relay clicks; G1 ON | ☐ |
| 3 | Release PB1 | Relay clicks; G1 OFF immediately | ☐ |
| 4 | Tap PB1 quickly several times | G1 follows every press | ☐ |
| 5 | Power OFF, then ON again | G1 stays OFF | ☐ |
Troubleshooting
| Symptom | Likely cause | What to check |
|---|---|---|
| G1 is ON before PB1 is pressed | G1 is wired to the N.C. pin | Wire 5 must land on CR1:3, not CR1:4 |
| Relay clicks, but G1 never turns ON | No power at the contact common, or the lamp return is open | Wire 4 at CR1:1; Wire 6 from G1:2 to L2 |
| No click when PB1 is pressed | The coil circuit is open | Wires 1, 2, and 3; coil pins are 2 and 7 |
| G1 turns ON but there is no click | G1 is wired through PB1, bypassing the relay | Compare with the wiring list |
| Relay chatters or G1 flickers | Loose terminal | Re-tighten all screws; tug-test each wire |
Review Questions
- If Wire 5 were moved from CR1:3 to CR1:4, what would G1 do in each step of the operating sequence? Explain why.
- Why does G1 turn OFF when PB1 is released? What would you need to add so the motor keeps running after the button is released? (You will build this in Lab 02.)
- On a real crane, why does the push button energize a contactor instead of switching the motor current directly?
Extension
Rewire Rung 2 to use CR1B instead of CR1A. Which pins will you use? Does the circuit behave the same way?