What is Direct Online Starter?
The DOL starter is a kind of motor starter that connects the electric engine straightway to the line voltage. It’s the simplest and cheapest approach to start an induction motor. In the DOL launch, the outstations of the motor are connected to the powerful force through a contactor and overload safeguard device.
Although DOL starter is the most frequent starting approach accessible on the demand, it’s meaningful to reflect that it has some limitations. The starting current of an induction engine is 6 to 8 times higher than the rated current when straightway connected to the power supply voltage. The control of current and torque isn’t achievable. For this reason, large power motors cannot be competent to start with this system.
A wiring diagram of Direct Online Starter :
The DOL starter attaches the engine immediately to the main supply line of the device connected, the motor draws a high amount of current compared to the full loading current of the motor (up to 6-8 times advanced). The valuation of this bulky current decreases as the motor reaches its rated speed.
A DOL starter can exclusively be implemented in situations when the high flow current of the engine doesn’t generate an intolerable voltage drop in the force circuit. However, a star-delta starter should be used rather, if a high voltage drop needs to be avoided. Direct online starters are generally used to start fragile engines, especially 3 phase squirrel cage induction engines.
The protection offered by DOL Starter-
The Motor starters not exclusively give the safe starting current but also give protection to keep the engine all right during operation. It’s crystal clear that the DOL starter provides the full line voltage but it does give the following protection.
The overcurrent condition can generate damage to the engine, power lines and can be a threat for operators. Such a quantum of current is too hazardous for a brief occasion.
In the DOL starter, we apply a circuit breaker or fuses for security against overcurrent. They unclose the circuit and break the current inflow in a moment until the challenge in the system is resolved. The fuse or circuit breaker is precisely named with its degree kept in mind. Because we don’t demand the fuse to break up but to permit the starting current as well as the heavy load current. The overcurrent breaker’s standing is kept a bit advanced than the rated starting current of the motor.
The condition where the load connected to the motor increases beyond its limitation and the engine draws an extreme quantum of current is called overload condition. During load, the current inflow is beyond the safe boundaries which damages the cables as well as the motor windings. It melts the windings and may invoke fire threats. To safeguard the motor from overloading, we apply an overload relay that trips the power supply and protects the system from overheating. The overload relay monitors the current and breaks the current inflow when it exceeds some limit for a period of moment. The tripping mechanism may differ and depends on the operation of the motor.
Different Types of Overload Relays used for motor protection-
1. Thermal Overload Relay- This class of overload relay works on the principle of expansion due to the heat generated by the current inflow. A bimetallic strip is employed with distinguishable thermal expansion to break up or form the circuit grounded on the temperature.
2. Magnetic Overload Relay- Similar relays of this type work on the principle of the magnetic field generated by the current inflow through a coil. An intolerable current drawn by the engine (that is a predetermined quantum) generates enough magnetic field to trips the contact outstations and breaks the current supply.
3. Electronic Overload Relay- An electronic relay is a solid-state device without any portable region or connections. It utilizes current detectors to cover the motor current and has an adjustable setting that allows the tripping at a wide range of current rankings.
Construction of DOL Starter-
A DOL or Direct Online starter has exclusively two push buttons; Green and Red, where the green button is employed for starting and the red is employed for stopping the motor. The green button connects the outstations and closes the circuit while the red button disconnects the outstations and breaks the circuit.
The DOL starter is manufactured of a circuit breaker or MCCB or fuse, a load relay, and a contactor or coil. The circuit breaker is used for security against short circuits while the overload relay protects the engine from overfilling. The contractor is utilized for starting and stopping the engine where the green and red buttons are compounded.
1. Circuit Breaker or Fuse- The circuit breaker or fuse is straightway compounded to the power mains and it’s used for protection against short circuits. It trips the power supply in case of a short circuit to safeguard the system from any possible hazards.
2. Magnetic Contactors- A magnetic contactor is an electromagnetic switch that operates electromagnetically to exchange the power supplied to the engine. It connects and disconnects multiple connections accessibly by supplying remote control over the operation. The magnetic field generated by the coil is harnessed for switching the terminals. The passing current through the coil magnetizes the iron core that’s encircled by the coil. The magnetic force pulls on the armature to close or open the connections.
3. Overload Relay- OLR or load relay is the last part used in the DOL starter and it’s used for protection against overloading of the motor. It breaks the current inflow when it exceeds a certain limit but it also tolerates the high starting current. So the OLR is precisely named in such a way that its tripping current limitation doesn’t tumble below the starting current range.
Principle of DOL Starter-
The Direct Online starter works on full voltage or across-the-line methodology where the engine is directly compounded to the full voltage supply. Since there’s no voltage deduction, the starting current is genuinely high which leads to high starting torque.
When the engine starts, it’ll drag a huge current generally 5 to 6 times that of its rated full speed current. The huge current draw will effectuate a dip in the line voltage. The gradational increment in the speed will drop the current drawn from the lines but not below a certain speed (typically at 75). Once the engine reaches its rated speed, the current drawn and the line voltage will return to normal.
Also, keep in mind, the jumping current may harm the windings of the motor. Therefore, motors having a low power rating are connected through the DOL starter.
Advantages of DOL Starter-
2. Easy to operate and maintain
3. Cheapest and Economical
4. Provides 100% starting torque
5. Troubleshooting and understanding the system is easier.
6. Connects the delta winding of the motor.
Disadvantages of DOL Starter-
1. Starting current is very high.
2. Starting current can damage the motor
3. The high inrush of current causes a voltage drop in the lines that can be connected to other appliances.
4. High starting is torque unnecessary in many cases.
5. High starting torque produces mechanical stress which reduces the life span of the motor.
6. There is no control over the starting current and torque.
1. Provides high starting current.
2. Provides high starting torque.
3. Produces voltage dip in the power mains.
4. Simplest controlling mechanism.
5. Suitable for low power rated motor.
Applications of DOL Starter-
1. The DOL starters are used in motors having low power ratings.
2. Starting current does not cause huge dips in the line voltage.
3. DOLs are used for small water pumps, conveyor belts, compressors, and fans.
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