A direct in line starter (DOL) is a simple method of starting an electric motor by supplying high voltage current to each terminal. It is commonly used in compressors, fans, water pumps, and conveyor belts. A thermal overload relay is used to protect the motor from overheating. DOL starters can be used in both forward and reverse directions with a branch switch.
A direct in line starter (DOL) is one of the simplest methods of starting an electric motor. It works by supplying a high level of voltage current to each terminal of the motor. This method for starting motors is traditionally used in devices such as compressors, fans, water pumps and conveyor belts. An in-line direct starter can be used in conjunction with a second DOL circuit, providing the motor with the ability to run in both forward and reverse.
This method of starting an engine is a bit like using a defibrillator to start a patient who has a stopped heart. By providing a rapid increase in maximum horsepower to the engine, the direct on-line starter starts the engine, putting the machinery into operation. This type of starter can only be used if the power supply can handle the massive influx of energy provided by a DOL starter. Weak or already overloaded circuits are not the best methods of supporting DOL starters, as the heavy load placed on them as the motor starts can result in excessive voltage drop for everything else connected to the line.
When using this type of starter it is important to protect the motor itself from overload. The most common type of protection is a thermal overload relay, which prevents the motor from overheating due to the voltage used during starting. There are two general types of thermal protection devices: bimetallic tapes and eutectic alloys. With bimetal strips, a dramatic rise in temperature trips a circuit, cutting power to the motor. Eutectic alloys, on the other hand, melt when too much current heats the metal, opening a circuit that stops the motor.
Direct in-line starting methods can use multiple circuits, giving the motor the ability to run in both clockwise and counterclockwise directions. This is a branch switch; only one direction or the other can be selected before inserting the direct inline starter into the motor. Depending on the position of the mechanical switch on the device that controls the cranking direction, one or the other circuit will be activated when the engine is started. The motor will then start spinning in the chosen direction.
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