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Principle of the Direct On Line Starter (DOL starter)

In this article is explaining Principle of the Direct On Line Starter (DOL)

Start the green push button, the contactor is closed, the motor line is in full winding voltage. The motor will pull a very high inrush current for very little time, the magnetic field in the iron, and then the current rotor of the current motor will be limited to the current. The motor rotor will develop the torque and start moving towards full speed.

Direct On Line Starter, DOL starter


As the motor is fast, the current will start to fall, but by the time the motor does not run fast, it will not fall significantly, usually about 85% of the synchronous speed. The original starting current curve is a work of motor design and terminal voltage, and is completely independent of the motor load.

The motor load will affect the time of the motor to accelerate the full speed and therefore the high starting period, but not the magnitude of the initial current.

Provided the motor developed by the motor crosses the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque provided by the motor is less than the torque of the load at any speed during the start of the cycle, then the motor will stop accelerating. If the initial torque with a Direct On Line Starter (DOL starter) is insufficient for load, the motor should be replaced with the motor which can develop a higher starting torque.



Acceleration torque is a torque developed by the motor minus the load torque, and the speed of the motor will accelerate due to motor speed torque curve and load speed torque curve. The start time is dependent on acceleration torque and load inertia.

At the beginning of the DOL, there is a maximum start current and maximum start torque. This may be the cause of an electrical problem with the supply, or it may be a mechanical problem with powered load. It will be inconvenient for the users of the supply line, always experience the voltage drop while starting the motor. But if this motor is not a high power then it does not affect it much.

Direct On Line (DOL) Motor Starter

Direct Theory on Direct On Line Starter (DOL) To get started, the contactizer is off, to implement full line voltage for motor winding. The motor will pull a very high pressure for very little time, the magnetic field in the iron, and then the current rotor of the current motor will be limited to the current. The motor rotor will develop the torque and start moving towards full speed.

As the motor is fast, the current will start to fall, but by the time the motor does not run fast, it will not fall significantly, usually about 85% of the synchronous speed. The original starting current curve is a work of motor design and terminal voltage, and is completely independent of the motor load.

The motor load will affect the time of the motor to accelerate the full speed and therefore the high starting period, but not the magnitude of the initial current.

Provided the motor developed by the motor crosses the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque provided by the motor is less than the torque of the load at any speed during the start of the cycle, then the motor will stop accelerating. If the initial torque with a Direct On Line Starter (DOL starter) is insufficient for load, the motor should be replaced with the motor which can develop a higher starting torque.



Acceleration torque is a torque developed by the motor minus the load torque, and the speed of the motor will accelerate due to motor speed torque curve and load speed torque curve. The start time is dependent on acceleration torque and load inertia.

At the beginning of the Direct On Line Starter (DOL starter), there is a maximum start current and maximum start torque. This may be the cause of an electrical problem with the supply, or it may be a mechanical problem with powered load. It will be inconvenient for the users of the supply line, always experience the voltage drop while starting the motor. But if this motor is not a high power then it does not affect it much.

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4 Comments

My husband is well versed in such schemes and that is why he is so interested in your blog, and I can not understand anything looking at such drawings.
SecurePak said…
Nice article, which you have shared here . Your article is very informative and useful for enhancing the knowledge. Thanks for sharing this article here. bottle cap torque
This is one of the best answer I've read on principle of DOL starter. I've worked with single phase DOL starter. Thanks for sharing this.
Unknown said…
Good presentation..would have been much better if the parts,material details had been put