Direct on line starter

Direct on line starter

A direct on line starter, often abbreviated DOL starter, is a widely-used starting method of electric motors. The term is used in electrical engineering and associated with electric motors. There are many types of motor starters, the simplest of which is the DOL starter.


A motor starter is an electrical/electronic circuit composed of electro-mechanical and electronic devices which are employed to start and stop an electric motor. Regardless of the motor type (AC or DC), the types of starters differ depending on the method of starting the motor. A DOL starter connects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. This type of starting is suitable for small motors below 5 hp (3.75 kW). Reduced-voltage starters are employed with motors above 5 hp. Although DOL motor starters are available for motors less than 150 kW on 400 V and for motors less than 1 MW on 6.6 kV. Supply reliability and reserve power generation dictates the use of reduced voltage or not

DOL Reversing Starter

Most motors are reversible or, in other words, they can be run clockwise and anti-clockwise. A reversing starter is an electrical or electronic circuit that reverses the direction of a motor automatically. Logically, the circuit is composed of two DOL circuits; one for clockwise operation and the other for anti-clockwise operation.

Example of Motor Starters

A very well-known motor starter is the DOL Starter of a 3-Phase Squirrel-Cage Motor. This starter is sometimes used to start water pumps, compressors, fans and conveyor belts. With a 400V, 50 Hz, 3-phase supply, the power circuit connects the motor to 400V. Consequently, the starting current may reach 3-8 times the normal current. The control circuit is typically run at 24V with the aid of a 400V/24V transformer.

Motor direction reversal

Changing the direction of a 3-Phase Squirrel-Cage Motor requires swapping any two phases. This could be achieved by a contactor KM1 swapping phase L2 and L3 between the supply and the motor.

Safety Devices within the Starting Circuit

These devices are used to protect the motor as well as the user of the motor being operated:
1. Overload CoilThe overload coil, also known as a thermal overload, is designed to open the starting circuit and thus cut the power to the motor in the event of the motor drawing too much current from the supply. The overload coil is a normally closed device which opens due to heat generated by excessive current flowing through the circuit. Thermal overloads have a small heating device that increases in temperature as the motor running current increases. A bi-metallic strip located close to the heater deflects as the heater temperature rises until it mechanically causes the device to trip and open the circuit, cutting power to the motor should it become overloaded. A thermal overload is basically a circuit breaker that will accommodate the brief high starting current of a motor whilst being able to accurately protect it from a running current overload. This is because the heater coil and the action of the bi-metallic strip introduces a time delay that affords the motor time to start and settle into normal running current without the thermal overload tripping. Thermal overloads can be manually or automatically resettable depending on their application and have an adjuster that allows them to be accurately set to the motor run current.

2. KM1 No-Volt CoilThe No-Volt coil serves the purpose of keeping the holding-in contactor closed, this is connected in parallel across the start switch (which is a normally open switch) in the control circuit. The no-volt coil is energized once the start switch is initiated and keeps current flowing through the control circuit. In the event of no power being supplied to the motor, the motor will stop. In order for the motor to be turned on again, the start switch must be initiated and therefore be the catalyst for closing the holding in contactor and hence energize the no-volt coil. Only once the no-volt coil is energized, will current remain flowing in the control circuit.

ee also

*Motor Soft Starter
*Adjustable-speed drive
*Variable-frequency drive
*Thyristor drive

External links

* [ EduMation] contains technical notes, videos, presentations and animations.

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