Across-the-line starting, also called full-voltage or direct-on-line (DOL) starting, is the simplest way to start a three-phase motor: a single contactor closes and connects the motor straight to the full line voltage with nothing in between to soften the start. Because the not-yet-turning rotor offers little opposition to current, the motor immediately pulls its locked rotor current, commonly six to eight times its rated full-load amps, until it accelerates and the current falls to running level. This is the baseline against which every reduced-voltage method is measured; those methods exist specifically to avoid the full inrush that a DOL start applies. This page explains where across-the-line starting is perfectly acceptable, where the inrush forces a gentler method, and what the surge looks like from a monitoring standpoint.
Across-the-Line Starting in one line: Across-the-line (direct-on-line or full-voltage) starting connects a motor directly to full line voltage through a single contactor, with no voltage reduction during startup. The motor draws its full locked rotor current, typically six to eight times rated, for the second or two it takes to accelerate, then settles to running current. It is the cheapest and simplest starting method and is used wherever the supply and the driven load can tolerate the full inrush.
An across-the-line starter is little more than a contactor and an overload relay. When the start command comes, the contactor's coil energizes and its main contacts close, applying the full three-phase line voltage to all three motor terminals at once. There is no intermediate step, no ramp, and no reconfiguration of the windings; the motor sees rated voltage from the first instant. This is why the method is so cheap and so reliable: there are no power electronics to fail and no timers or extra contactors to coordinate, just one contactor pulling in.
At the moment of energization the rotor is stationary, so it generates no counter-voltage to limit current, and the motor draws its full locked rotor current straight from the line. That current is several times the running value, and it stays high for the brief interval the motor takes to accelerate, usually a second or two on a lightly loaded machine. As the rotor speeds up it builds counter-voltage, the current falls away, and within a couple of seconds the motor settles to the modest full-load amps it draws while running. The whole start is over quickly, which is what makes the large inrush tolerable in most cases.
Because the motor also develops full torque immediately, a DOL start is mechanically abrupt. The shaft jumps to full torque with no easing in, which slams the coupling, belt, or gearbox and can jolt the driven load. On a small pump or fan this is harmless, but on a large or delicate drivetrain the mechanical shock of full-voltage starting is one of the reasons a reduced-voltage or soft-starting method may be chosen even when the electrical inrush itself could be tolerated.
Across-the-line starting is the default and the right choice for the large majority of small and medium motors. Where the supply is stiff enough to absorb the inrush without a disruptive voltage dip, where the driven machine can take the abrupt torque, and where the motor is small relative to the transformer or generator feeding it, there is no reason to spend money on anything more complicated. Most fractional and integral-horsepower motors on a robust utility supply start direct-on-line without a second thought, and adding a reduced-voltage starter would only add cost and failure points.
The inrush becomes a problem as the motor grows large relative to its supply. A big motor starting direct-on-line pulls so much current that it dips the bus voltage, and that dip can dim lights, drop out contactors on other equipment, disturb sensitive electronics, or, on a generator-backed site, sag the frequency and voltage enough to trip other loads. This is acute on weak or islanded supplies, where the source impedance is high and the same motor that starts cleanly on a utility feed would collapse the voltage on a generator. In those cases a reduced-voltage method, a soft starter, or a variable frequency drive is used to cap the starting current.
The driven load also drives the decision. A load that cannot tolerate the sudden full torque of a DOL start, such as a long conveyor that would jerk, a belt drive that would slip, or a pump whose surge would water-hammer a pipeline, may call for a gentler start regardless of the electrical inrush. Utility rules can force the issue too, since some supply authorities limit how large a motor may be started direct-on-line to protect other customers from the voltage dip. The engineering judgment is a balance of motor size, supply strength, load sensitivity, and any imposed limits, with across-the-line starting as the simplest option that is used until one of those factors rules it out.
From a monitoring standpoint, an across-the-line start has an unmistakable signature: current jumps almost instantly to the locked rotor value, holds there for a second or two, and drops sharply to running current as the motor comes up to speed. There is no ramp and no intermediate step, which distinguishes it cleanly from the smooth ramp of a soft start, the two-step profile of a wye-delta start, or the low, controlled current of a drive start. If a monitoring system is sampling fast enough at the moment of energization, that single sharp spike and rapid decay is the fingerprint of a DOL start behaving normally.
The other thing a monitoring system sees at a full-voltage start is the effect on the bus. The inrush pulls the supply voltage down momentarily, and the depth and duration of that dip tell you how the motor and its supply are getting along. A dip that is deeper than usual, or a start where the current stays at locked rotor value longer than the couple of seconds a healthy start should take, points to a problem: a stiffer load, a struggling motor, or a sagging supply. Because these events last only seconds, catching them requires a system that logs current and voltage at start, not just steady-state averages.
For remote and unmanned sites, which is where cloud SCADA earns its keep in oil and gas and similar industries, surfacing the starting current profile turns an invisible event into an observable one. An operator who can see that a pump motor's start current is creeping longer or that the bus dip at each start is deepening has early warning of a mechanical problem or a weakening supply, well before it shows as a failed start or a nuisance trip at a site nobody is standing at. Comparing start signatures across a fleet of similar motors also flags the outlier whose direct-on-line starts have begun to drift from the norm, which is often the first sign that something in the drivetrain is loading the motor down.
They are two names for the same thing. Across-the-line, direct-on-line (DOL), and full-voltage starting all describe connecting a motor straight to full line voltage through a single contactor with no voltage reduction during startup. The motor draws its full locked rotor current until it accelerates. Direct-on-line is the more common term in international practice, while across-the-line is common in North America.
At the instant of starting the rotor is not turning, so it generates no counter-voltage to oppose the applied voltage. The full line voltage drives current through the low impedance of the windings with almost nothing limiting it, so the motor pulls its locked rotor current, commonly six to eight times its rated full-load amps. As the rotor accelerates it builds counter-voltage, the current falls, and within a second or two the motor settles to normal running current.
Avoid full-voltage starting when the motor is large relative to its supply, so the inrush would dip the bus voltage enough to disturb other equipment, which is common on generator-backed or weak supplies. Also avoid it when the driven load cannot tolerate the abrupt full torque, such as a conveyor that would jerk or a pump that would water-hammer, or when the utility limits how large a motor may be started direct-on-line. In those cases a soft starter, reduced-voltage starter, or variable frequency drive caps the starting current.
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