Various methods are used for starting up induction motors. These motors tend to draw high amounts of current when they start. If this current is not checked, it can cause some form of damage. Specialized starters are used to ensure that the starting process is smooth and efficient.
One type of motor starter is Direct Online (DOL) starter, which is also known as across the line starter. What is it, and are you planning to use it? We will break down everything that you should know about DOL starters.
- 1 What is a Direct Online starter?
- 2 DOL protection role
- 3 Parts of a DOL starter
- 4 Working of a DOL starter
- 5 Advantages of a DOL starter
- 6 Disadvantages
- 7 When should I use a DOL starter motor?
- 8 You should not use DOL starters if:
- 9 Conclusion
What is a Direct Online starter?
A Direct Online starter is a device that is used to start an induction motor. It is designed to supply power directly to the motor, thus bypassing the use of a separate starting switch.
It is used on motors that have low ratings as it protects them from the excessive current. The maximum power of these motors is 5HP.
The directly connected windings characterize the DOL motor starting technique to the main power supply. This explains the reason for the protection against inrush current whenever you start up the motor.
A DOL starter is commonly used to start a motor. It is also used with a starter relay to control the starting current and voltage. It can be used to take high starting current from a battery.
DOL starters are available as either stationary or portable versions. Stationary DOL starters are designed for use in fixed locations such as in the engine compartment of an automobile, or on the dash of an automobile.
DOL starters have been around since 1952 when they were originally invented by John A Hartley and mass-produced by Bendix Corporation (now part of Robert Bosch GmbH). Since then, many other companies have improved through research and development.
DOL protection role
We have seen that DOL starter mainly protects against the excess current from the definition. Overflow of current can happen both at the starting point and during the operation.
The DOL starter has several functions to protect the motor from getting damaged. It has overcurrent protection, starting current protection, and current overload protection. They are all working together to ensure that your motors are safe from being damaged by overcurrent or overload.
The DOL starter has overcurrent protection. It is a function that protects the motor from being damaged by overcurrent, especially during starting. The device limits the current, and if it exceeds the value, it will be cut off automatically.
Starting current protection
The DOL starter has a starting current protection as well. The maximum allowed starting current for induction motors is 0.4 In for 3-phase and 0.8 In for single-phase motors. If you exceed this limit, the motor won’t start up and you will have to wait until the overload is removed before you start up again.
During the operation, the current can also exceed the limit. For this reason, the DOL starter has a function that protects the motor from overload. The current is limited by the device, and if it exceeds the value, it will be cut off automatically.
Overload protection is a function that protects the motor from being damaged by overload. The overload can occur when there is a low voltage or some mechanical damage to the motor.
The DOL starter has a speed control mechanism that allows you to set the speed of the motor. If you want to change the speed, you can simply change the voltage and current.
Parts of a DOL starter
Now that you know the main functions of a DOL starter, it will be good to look at the main components that make up this system and how they work together to achieve a common goal.
The main parts include:
A circuit breaker/fuse
Like in most applications, the main function of a fuse or a circuit breaker is to protect the system from the consequences of short circuit.
The fuse will trip the power supply in a short circuit hence preventing any electrical damage.
These are also integral components of a typical DOL starter. They are used to provide power to the motor. The magnetic field of the contactor will make and break a contact hence controlling the flow of power.
The magnetic field created by the contacts makes it possible for the starter to operate remotely. The field is due to the switched terminals of the starter.
The magnetic contacts of the DOL have different types of contacts. These are the NO (Normally Open) contacts, whose function is to supply power to the NC (Normally Closed) contacts.
The auxiliary contacts (NCs) connect the coil to the voltage source. Since the supply voltage from the source is not the same, the specifications of the magnetic coils are also different.
Here is another important part of the DOL starter that you should know. The component protects the motor from overload. It cuts off the flow of current once it detects an overflow. Even while providing this protection, OLR is designed to tolerate the high starting current but will still ensure that the current does not cause any damage to the motor.
Is current overload dangerous? Yes, it can end up damaging the simulator and even the motor winding.
Overload relays are further divided into three parts. These are a magnetic relay, thermal relay, and electric relay. Each relay has a unique role to play in the working of the DOL starter.
The magnetic relay responds only when there is an overload of electric current. It is rarely affected by high temperatures in the circuit.
Thermal relay, on the other hand, responds in case of a thermal overload n a circuit. The rising temperatures can be due to the current overload.
Electric current comes in handy in case of a sudden current trip. The electric relay responds to this sudden current surge. This is very important in the working of the DOL starter.
Working of a DOL starter
Now that you know the parts and components of a DOL starter let’s look at the working principle of this starter.
A DOL starter connects the 3-phase supply voltage to the terminals of an induction motor.
Two major circuits make up the whole working principle of a DOL starter. The control circuit and the power circuit.
This circuit derives its power from 2 phases of power supply. Its main function is to start and stop the power supply into the motor.
A start button, which in most cases is green in color, is connected to the control circuit. Next to it is a red stop button.
Pressing the green button will allow the power to be supplied to the motor. Pressing the red button will halt the supply of power hence stopping the motor.
But, what exactly happens in the control circuit?
Let’s break down key events that take place.
Press the green button
It all starts with the pressing of the green button. This button is connected to the B-phase power supply and the relay coil. It closes the contacts hence allowing current to flow through.
The relay coil will move the contactor until it assumes a closed position. Power will flow directly to the induction coil.
Release the green button
What happens when you release the start button? The voltage supply will remain the same. It won’t stop. In case there is an overload of current, the contacts will open to facilitate the outflow of excess energy from the coil.
Pushing the stop(red) button
What happens when you press the red button? The voltage supply will stop. The contacts will open, and the current flow through the relay coil will cease.
The contacts in the relay coil are now closed. The current can no longer flow through the coil. So, it goes to the B-phase power supply, which is connected to the motor windings.
In this case, there is no load on the motor, but it can’t run as there is no current flowing through its windings. Thus, a dead motor situation occurs.
This is the second part of the DOL starter. Its principal function is to ensure a smooth flow of power to the motor. The control circuit determines the activities and operations of the power circuit.
Advantages of a DOL starter
Here are the top benefits that you will get from using a DOL starter for your motor:
As we have seen, DOL starter motors dont have numerous components. Instead, they sport a simple design that is easy to handle and maintain. You can easily access the start and stop buttons to determine how the starter should operate. In short, the starter does not require a highly skilled technician to operate.
Apart from simplicity, DOL starters are characterized by their compactness. Every component is simplified into a single device, making it easy for storage and installation.
DOL starters are easy to maintain. You will not require extensive knowledge about the motor repair. In this case, you can easily find the parts that you need to fix your machine and make it functional again.
DOL starters have been designed to withstand harsh conditions like high temperatures, vibration, shock, and other dangerous conditions. These staters have been built with heavy components, which enable them to last longer than other motors without any problems at all. Moreover, these staters are reliable because they do not require much maintenance. You only need to check the condition of the power circuit before using them again.
Provide 100% torque
DOL starters provide 100% torque to the machine. This means that they deliver more power to the machine than other motors. The power circuit has been designed in such a manner that it can withstand high temperatures and vibrations without any problem at all.
DOL starters have been designed in a manner that eliminates vibration and noise. This is because of the heavy components used in their manufacturing process. These starters are quiet, which is an advantage for people who want to use them in places where noise is not acceptable.
DOL starters are inexpensive compared to other motors, and they have an affordable price tag which makes them more accessible for everyone. You can easily find these staters at local stores, online stores, or even at your home if you have an electrical installation system installed at your home or office premises.
Of course, the cost will depend on where you buy and the technical specifications. Buying from a reliable source will guarantee you a quality DOL starter.
Despite the advantages, DOL starter also has some demerits, including:
The starting current must be high: The starting current must be high for a DOL starter to function properly. This is because the starter will not have much power if the starting current is low.
-Starter must be used in conjunction with a starter relay: A DOL starter can only work with a relay in order to control it and make sure that it works properly and in time.
-High torque may be unmanageable: As we have stated, one of the benefits of a DOL starter is it delivers high torque. While this is desirable, it may end up causing some problems. High torque can cause mechanical damage to the motor.
When should I use a DOL starter motor?
DOL starter motor is ideal for the following circumstances:
-If high inrush current does not cause excessive voltage drop on the circuit.
-If there is a need to start the motor quickly.
-If there is a need to take a high starting current.
-If there is a need for high torque (for example, in lawnmower, chainsaw, and so on).
You should not use DOL starters if:
-If a high starting current will cause severe damage to the coil and motor in general.
-If peak current from the source will result in a major voltage drop.
I hope that this article has given you valuable insights about DOL starters. You should be able to understand the features, advantages, and disadvantages of a DOL starter.
And in case you want to buy DOL starters in China, we can be of great help. We are a reliable sourcing agent for electrical components in China. Contact ICRFQ now.
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