Direct on line starter

From Wikipedia, the free encyclopedia

A direct on line starter is often abbreviated DOL starter and 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.

1 Definition
2 Starter Circuits
3 DOL Reversing Starter
4 Example of Motor Starters
5 See Also
6 External Links

[edit] Definition

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.

[edit] Starter Circuits

The motor starter is composed of push button switches, relays, isolators, circuit breakers, transformers and interlock switches. Its circuit is split into two circuits; control and power. The power circuit is the higher-voltage circuit to which the motor terminals connect with the power supply. The control circuit is the lower-voltage circuit and it is electrically isolated from the power circuit. The starter is energized by the control circuit using electro-mechanical relays and push button switches. Modern starters employ solid state relays (electronic relays).

[edit] 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 speed of a motor automatically. Logically, the circuit is composed of two DOL circuits; one for clockwise operation and the other for anti-clockwise operation.

[edit] Example of Motor Starters

A very well-known motor starter is the DOL Starter of 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. An animation of the circuits of this starter is shown below. The speed reversing starter of 3-Phase Squirrel-Cage Motor is shown below. The circuit uses two contactors. The method of reversing the motor is simple which is switching any two phases.