DOL Starter

For cases to avoid damage to the motor, various techniques are used to reduce the starting current with the use of a motor starter, these techniques vary on different factors and also protect the motor from overcurrent and overloading. DOL starter is one of the techniques used and in this article, we are going to discuss DOL starters and the starting of induction motors. We will cover what is starting induction motors and their functions, what are DOL starters, their application, terminologies, theory, construction, working, and the difference between DOL and star-delta starters.

What is a DOL Starter?

Direct on-line is the simplest form of motor starter. This method involves directly switching the polyphase stator onto the supply mains. This is also called an “across-the-line starter.” Depending upon size and design, the motor takes a low-power factor starting current of 5 to 7 times its full-load current. Such large currents of short duration don’t harm the rugged squirrel cage motor, but the high currents may cause objectionable voltage drops in the power supply lines feeding the induction.

These large voltage drops cause undesirable dips in the supply line voltage; consequently, the operation of other equipment connected to the same supply line is affected considerably. A common example is the momentary dimming of lamps and tube lights in the home when a refrigerator motor starts. In the direct-on-line method of starting cage motors, the motor is connected using a starter across the full supply voltage.

This consist of the overload relay , protecting device and main contactor- these all are used for the motor starting operations in which the stator winding are directly connected to the main supply.

DOL Starter

Important Terminologies

• Contactor: The magnetic contractors act as electromagnetically operated switches. It uses electromagnetic energy to close switches, which consist of a coil placed on the iron core.
• Overload Relay: Overload relay prevents motor damage. It monitors the current in the motor and breaks the circuit when a fault is detected.
• Start Button: It helps to supply power to the motor to start the motor.
• Stop Button: It helps to cut down the electrical supply to the motor.
• Auxiliary Contacts: Auxiliary contacts are electromechanical devices used in switching equipment such as contactors, circuit breakers, switches, and relays. These secondary switching devices are physically connected to the main switching equipment and work at the same time.

Construction and Working

Here, will have Construction, Working and Connection of DOL Starter given below :

Construction

The three-phase power supply lines (L1, L2, L3) to the DOL starter are connected in which L1 and L2 are connected to the main contacts of the contactor, while L3 is connected to the common point of the contactor coil as seen the diagram of construction of DOL starter. The three-phase leads from the motor to the other side of the contactor’s main contacts which connect the overload relay in series with one of the motor phases. Therefore, the start button in parallel with the contactor coil is connected and the stop button in parallel with the start button is connected for the working of the starter.

It consists of a coil-operated contractor C controlled by start and stop push buttons which may be installed at convenient places remote from the starter. On pressing the START push button S1 (which is normally held open by a spring) the contactor coil is energized from two line conductors L1 and L2. The three main contacts M and the auxiliary contact A close and terminals A and B are short-circuited.

The motor is thus connected to the supply. When the pressure on S1 is released, it moves back under spring action. Even the coil C remains energized through AB. Thus, the main contacts M remain closed and the motor continues to get supply. For this reason, contact A is called hold-on-contact.

Construction of DOL Starter

Working of DOL Starter

On pressing the START push button S1 (which is normally held open by a spring) the contactor coil is energized from two line conductors L1 and L2. The three main contacts M and the auxiliary contact A close and terminals A and B are short-circuited as seen in the above image. The motor is thus connected to the supply. When the pressure on S1 is released, it moves back under spring action.

Even the coil C remains energized through AB. Thus, the main contacts M remain closed and the motor continues to get supply. For this reason, contact A is called hold-on-contact.

When the STOP push button S2 (normally held closed by a spring) is pressed, the supply through the contactor coil C is disconnected. Since coil C is de-energized, the main contacts M and the auxiliary contact are opened. The supply to the motor is disconnected from the supply.

3 Phase Motor Starter with Overload Protection

In order to fulfill the load requirements , the current exceeds its limit that means current draws more and the load requirement also goes higher which is called as overloading. The 3 Phase Motor Starter with Overload Protection is the protection or security performed when the excessive current or load is drawn by the motor which leads to heating also and effects the equipment.

It does the security of the equipment and also provide the safe starting current to the motor, so it provides the following protection to the motor:

• Overload Protection
• Over Current Protection

Overload Protection

It takes place in the condition when the current increases and the load increases as well beyond their limits ,thus the increasing current damages the wire and the motor windings as well which can cause fire hazards.

Overload relay is used for the protection in this case and it trips the power supply which protects the device from being damaged . In this case when the current overflows then it stops the flow of current as it exceeds its limit. Their are some types of overload relay which takes place are as follows:

• Thermal Overload Relay: It limits the amount of drawn current. As the heat generates by the overflow of current so a bimetallic strip is used to limit or break the flow of current so that it does not damages the device or equipment.
• Magnetic Overload Relay: This relay works on the principle of generation of magnetic field so when current flows then the magnetic field is produced around the coil which trips the terminals and breaks the current .
• Electronic Overload Relay: It is the solid relay which is not movable and it uses the sensors to monitor the flow of current and as the flow exceeds then it trips the terminals and breaks the flow of current.

Over Current Protection

The current flows in the large amount because of the ground fault or short circuit , thus the current which flows is called the faulty current and this condition is called overcurrent.

In this the current flows in the large amount which is not safe and hazardous to the operators and supply. In the cases of DOL Starter , fuses are used to control the condition of the overcurrent. The fuse is selected with proper ratings so that it does not get break with the heavy load or over current condition and thus it tolerated the overload as well as the starting current. The fuse or circuit breaker trips the contact terminals and breaks the flow of the current.

DOL Starter Wiring Diagram

The given figure shows the DOL starter wiring diagram and this shows that the DOL Starter has two buttons which are used for the starting and stopping the motor working and thus control the contacts.

DOL Starter Wiring Diagram

This consist of circuit breaker, contactor and overload relay for the protection purpose. So the motor s started by pushing the green button and voltage appears on the motor , since it contains 4 pole type contactor and contains three normally open contacts which connects the motor to the supply and the fourth one is used to push the contactor coil as the start button gets released which is turn leads to the de-energization of the coil in case of occurrence of the fault and hence the starts gets disconnected from the main supply.

Advantages , Disadvantages and Applications

The Advantages , Disadvantages and Applications of DOL Starter are mentioned below:

Advantages

• Simple to design and maintain
• Economical and cheapest starter
• Gives full starting torque at starting
• Troubleshooting and understanding is easier.
• Connects the delta winding of the motor

Disadvantages

• The starter is mechanically tough to operate
• High starting current
• High starting torque
• Reduces lifespan of machine because of mechanical stress
• No control over high starting torque and current
• Voltage dip in power mains

Applications

• Used for the operation of motor which have lower power rating
• DOL starter are used in fans, compressors and small water pumps.
• Used where winding of the motor does not get damaged

Conclusion

DOL Starter are suitable for various applications such as in small motors because of the dip in voltage. It connects the motor to line voltages directly and designed for switching and protection of induction motors , we can also say that it is method to start a 3- phase induction motor. It is cost effective for the smooth operation of the motor and reduces the stress from the electrical equipment. Thus it protects the motor from being damage because of high current or overload applications and so they are operated manually.

FAQs on DOL Starter

Why is R-DOL Starter used?

It is used to change the direction of the motor by changing the phase sequence with the help of power contactors

What is DOL and VFD?

DOL is direct-on-line starter , soft starters and VFD- variable frequency drives.

What is H-DOL starter?

H-DOL starter is heavy duty direct-on-line motor starter, used in the motors where overloading is frequent.

What is starter in PLC?

DOL , magnetic motor starter are the most common type of started used in PLC.