PlantFlow 3D › Guides › How a DOL motor starter works
How a DOL motor starter works
The direct-on-line starter explained step by step: power circuit, control circuit, seal-in contact, stop button and overload protection.
What "direct-on-line" means
A direct-on-line (DOL) starter connects a three-phase motor straight to the full supply voltage. It is the simplest and most common way to start small and medium motors: one contactor closes, all three phases reach the motor at once and the motor accelerates to full speed in a few seconds.
Because the motor gets full voltage while it is still stationary, it draws a large starting current, typically several times its full-load current, until it comes up to speed. That is why DOL is used where the supply can take that inrush and the driven machine can take a sudden start.
The power circuit
The power circuit carries the motor current. It normally has three parts in series: a circuit breaker or fused switch (short-circuit protection and isolation), the main contactor KM1 (switching), and a thermal overload relay (protection against too much current for too long).
In PlantFlow 3D these are QF1, KM1 and F1. Follow L1, L2 and L3 from the supply through QF1, through the main contacts 1-2, 3-4 and 5-6 of KM1, through the overload heaters and on to the motor terminals U1, V1 and W1.
The control circuit and the seal-in contact
The control circuit decides when the contactor coil A1-A2 is energised. Pressing START closes a normally-open push button, current flows through the normally-closed STOP button and the overload contact 95-96 to the coil, and KM1 pulls in.
When you release START the motor must keep running, so an auxiliary normally-open contact of KM1 (13-14) is wired in parallel with the START button. This "seal-in" or "hold-in" contact keeps the coil fed. Pressing STOP, or the overload contact opening, breaks the circuit; the coil drops out and the seal-in contact opens too, so the motor will not restart on its own.
Why the overload relay matters
A short circuit is cleared by the breaker in a fraction of a second, but a motor that is slightly overloaded or has lost one phase draws only a moderately higher current. That will not trip the breaker, yet it overheats the windings over minutes. The thermal overload relay models that heating and opens its 95-96 contact before the insulation is damaged.
The overload is set to the motor's rated (nameplate) current. In the simulator you can calculate a full-load current from power, voltage, power factor and efficiency, but the nameplate value always takes priority for real settings.
Try it yourself
Load the DOL starter, press START and watch the seal-in contact hold the coil. Then open the Check tab, remove the 13-14 wire in the 2D diagram and see what changes.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.