DOL starter or Direct online starter is a common method used for starting of induction motors. As we know that an induction motor needs a large amount of current during the staring process. There is a risk of windings damage due to high starting current flow. To avoid the risk, we use different types of Motor starter methods. Direct On Line Starter is one of the most common methods used for this purpose.
DOL Starter circuit components
Direct online starter is one of the simplest type of motor starters for the induction motor. The device consists of protective devices such as a circuit breaker or Molded case circuit breaker, main contactor & an overload relay. Which is used for motor starting operations. The electromagnetic contactor is operated using a thermal overload relay normally closed contact. The Contactor is coil operated and is controlled by start and stop push button.
DOL Starter Circuit Diagram and Working Principle
The circuit diagram for Direct Online Starter is given above. Let’s describe the working principle of the circuit diagram. When we press the START button S1, the contactor C energizes from two-line conductors L1 and L2.
The auxiliary contact and three main contacts M closes. The terminals “a” and “b” become short-circuits and the motor connects to the supply mains. under the spring action as the pressure releases the button S1 moves back. As a result, coil C remains energized through ab. Thus, the main contact M remains closed during this operation. While the supply to the motor continues.
During this whole process, The Stop button “S2” is held by the spring. When we press the Stop button. The contact disconnects. Coil C de-energize and the auxiliary contact A and the main contacts M are opened. The Voltage supply to the motor disconnects, and as a result, the motor is stopped.
DOL Starter circuit Protection
- Overcurrent: A large amount of short circuit or ground fault is called overcurrent. It can damage devices. DOL circuit has protective devices such as circuit breakers, relays, and fuses. And it limits the chances of overcurrent flow.
- Overload: If the load connected to the motor is more than the rated load of the motor. It causes motor overload. During the overload period, the load draws more current through the motor. When the safe flow current limit is reached. Wire and Winding become damage. DOL Starter consists of relays. The overload relay trips during the condition disconnect power supply. And the system is protected from overheating.
Direct online starter advantages
The advantages of this motor starting method are:
- Simple in design and no complicated wiring needed.
- The most economical starting method.
- The motor starts with almost its full starting torque.
- Easy troubleshooting process.
- Easy to perform periodic maintenance and electrical inspection.
Disadvantages
- The motor starts with high starting current.
- Causes voltage dip which affect other motors on the same power source.
- DOL Starter reduces the motor expected life time.
Applications of DOL starter
The DOL starting method is mainly applicable for small sizing motors because its starting current is not very high and not effecting the power supply voltage.
It is very common in applications like small water pumps, fans, air conditioners and compressors.
DOL starter circuit maintenance
As you can see above that the direct online circuit is very simple and has only one contactor, one overload, and some basic components like bush buttons and fuses. This simple construction makes it simple to perform periodic maintenance to keep it working perfectly and to prevent unplanned shutdown. Take the following points into account while preparing the maintenance program for DOL circuit.
- Measuring voltage monthly for both power and control circuits.
- Measure motor current.
- Record voltage and current values in a recording sheet.
- Ensure that the motor and its control panel are well grounded.
- Measure the value of grounding resistance and record it quarterly (R should be less than 5 Ohms).
- Check DOL panel for any abnormal noise, dirt, dust and unused openings.
- Thermal imaging the motor body helps in overheating detection.
- Megger the motor annually and check the results with the motor data sheet.
