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What Is a Direct on Line Starter?

T.S. Adams
T.S. Adams

A direct on line (DOL) starter is one of the simplest methods for starting an electric motor. It functions by supplying a high level of current of voltage to each terminal in the motor. This method for starting motors is traditionally used in devices such as compressors, fans, water pumps, and conveyor belts. A direct on line starter can be used in conjunction with a second DOL circuit, providing the motor with the ability to run in both forward and backward directions.

This method of starting an engine is a bit like using a defibrillator to jump start a patient who has a stopped heart. By providing a quick surge of maximum power to the engine, the direct on line starter kick starts the motor into life, getting the machinery up and running. This type of starter can only be used if the power supply can handle the massive influx of energy a DOL starter provides. Weak or already overstressed circuits are not the best methods to support DOL starters, as the intense load placed upon them as the engine starts can result in an excessive voltage drop for everything else connected to the line.


Protecting the engine itself against overload is important when using this type of starter. The most common type of protection is a thermal overload relay, which prevents the engine from overheating from the voltage used during starting. There are two general types of thermal protection devices: bi-metallic strips and eutectic alloys. With bi-metallic strips, a dramatic increase in temperature causes a circuit to trip, stopping the power supply to the engine. Eutectic alloys, on the other hand, melt when too much current heats the metal, opening a circuit that stops the motor.

Direct on line starter methods can use multiple circuits, giving the engine the capability to run in both clockwise and counter-clockwise directions. This is a branched switch; only one direction or the other can be selected prior to engaging the direct on line starter in the engine. Depending on the position of mechanical switch on the device which controls the starting direction, either one or the other circuit will be activated when the engine is started. The engine will then begin to spin in the chosen direction.

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Discussion Comments


@Markerrag -- These work well, but you do have to have a good, strong power supply for them to work and not blow out the power in a building. There are some companies that use dedicated circuits for one of these.

That starts to wander into the realm of complexity, doesn't it? But, truth be told, once you get one of these set up and the support to make them work well, you can usually leave them along for years. We're talking about a proven method put into a proven design here.


Why are these so common? Because they work, that's why. Ask a mechanic which is better. Something that will work almost every time or something complicated that can break down quite a bit.

There are more sophisticated ways to start a motor, but the direct on line starter works so well that it is almost foolproof.

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