Wiring 20 — Complete DOL Motor Starter

Complete DOL Motor Starter

Build both halves of a direct-on-line starter: the three-phase power path through isolator, breaker, contactor and thermal overload to the motor, plus the 24 V three-wire Start/Stop control circuit with overload trip contact and seal-in.

The device in the real world

A direct-on-line starter is the simplest way to run a three-phase motor: the contactor connects the motor straight to the supply. The power path runs through the isolator, a protective breaker, the contactor poles and the overload relay before reaching the motor terminals. A separate 24 V control circuit decides when the contactor coil may close. Conveyors, pumps and fans in plants around the world use this pattern.

Why it is wired this way

The overload power poles measure motor current, while auxiliary NC contact 95–96 opens the coil circuit on trip. The contactor auxiliary NO 13–14 bypasses only the momentary Start button, never the Stop or overload contacts.

Worked example

Follow one press of START. 24 V leaves the supply, passes the normally closed STOP button, passes the overload contact 95 to 96, reaches the open START contact and waits. When you press START, 24 V reaches coil A1, the coil pulls in and the contactor poles close, so the motor runs. At the same moment the auxiliary contact 13 to 14 closes in parallel with START, so releasing the button no longer matters: the coil is held on through the seal-in. Press STOP, or let the overload trip, and the chain opens, the coil drops and the motor stops. Wiring this lab means building those two paths separately: three heavy phases, then one light control chain.

Common mistakes

• Putting the seal-in contact in parallel with STOP as well as START, which makes the motor impossible to stop.
• Keeping the overload power poles but forgetting the 95 to 96 contact in the control chain, so a tripped overload does not stop the coil.
• Mixing up the order of phases between the breaker, the contactor and the overload relay.
• Forgetting to bond the motor frame to the ground bar.

What you can now do

You can build both halves of a three-wire starter, power and control, and explain how the seal-in keeps the motor running without anyone holding the button.

Commissioning standard

With the circuit isolated, test the control chain with the power poles disconnected, and only then close the isolator to try the motor.

Training material. Follow your site procedures, local electrical code and the manufacturer's instructions. Lockout/tagout and a qualified person are required for real equipment.