What you'll learn
The **three-wire motor starter** is the most common motor control circuit in industrial automation. You will find it on conveyors, pumps, fans, and compressors everywhere. Master this circuit and you have the template for hundreds of real machines.
Lab time: ~15 minutes.
Lesson briefing
Three-Wire Motor Starter (Direct On Line)
The three-wire motor starter is the most common motor control circuit in industrial automation. You will find it on conveyors, pumps, fans, and compressors everywhere. Master this circuit and you have the template for hundreds of real machines.
The goal
A momentary Start pushbutton starts the motor. A momentary Stop pushbutton stops it. The motor stays running after you release Start — it does not need you to hold the button. Pressing Stop, or the overload relay tripping, kills the motor cleanly.
Three-wire control
The name refers to the three wires running to the pushbutton station:
- Wire 1: +24 V supply to the NC Stop button input.
- Wire 2: from the Stop button NC-OUT through the OL relay NC contact to the Start button NO-IN.
- Wire 3: from the Start button NO-OUT to the contactor coil A1.
The contactor's auxiliary NO contact (13/14) is wired in parallel with the Start button. When you press Start and the coil energises, the aux contact closes and holds the coil on after you release Start. This is the seal-in — the defining feature of a three-wire circuit.
Overload relay contact
The overload relay's NC contact (95/96) sits in series in the coil path. In this lab a second NC pushbutton component labelled OL represents it. If the motor overheats, the overload trips — its contact opens — and the coil de-energises, stopping the motor.
Run lamp
The contactor's aux NO contact 14 also drives a run indicator lamp: the lamp lights only when the contactor is ON (motor running).
What to wire
- Seed the +24 V and 0 V distribution buses from the PSU.
- Wire the power circuit: contactor T1/T2/T3 → motor T1/T2/T3.
- Control circuit coil path: tb-24v.t1 → NC Stop IN → NC OL IN → OL OUT (= start button junction) → Start NO-IN; Start NO-OUT → contactor A1; contactor A2 → 0V bus.
- Seal-in: contactor aux terminal 13 → start button NO-IN junction; contactor aux terminal 14 → contactor A1 (seal-in holds coil on).
- Run lamp: contactor aux 14 → lamp+; lamp− → 0V bus.
- PE bonds: PSU PE → ground bar; PSU 0V → ground bar; motor PE → ground bar.
Hints
Start with power distribution: psu-1.+V → tb-24v.t0 and psu-1.0V → tb-0v.t0. These seed the two distribution buses that every device draws from.
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This lesson uses 11 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.
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