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Direct Starters

24.02.2025

Direct starter or Direct on-line (DOL) is the simplest form of motor starter. This method starting cage motors, the motor is connected using a starter across the full supply voltage. Depending upon size and design, motor might takes a low-power starting current about 5 to 7 times its full-load current. In normal conditions such currents and short duration don’t damage the rugged squirrel cage motor, but in some particular cases inrush currents may cause objectionable voltage drops in the power supply lines feeding the induction.

DIRECT STARTER, WHAT IS IT AND HOW IT WORKS?

Direct starter or Direct on-line (DOL) is the simplest form of motor starter. This method starting cage motors, the motor is connected using a starter across the full supply voltage. Depending upon size and design, motor might takes a low-power starting current about 5 to 7 times its full-load current. In normal conditions such currents and short duration don’t damage the rugged squirrel cage motor, but in some particular cases inrush currents may cause objectionable voltage drops in the power supply lines feeding the induction.

Large voltage drops cause unwanted dips and swells 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 workshop when a high power chainsaw motor starts.

Direct starter usually consist of the overload relay , protecting device and contactor- these all are used for the motor starting operations in which the stator winding are directly connected to the main supply and due to this fact we need to protect either operator on motor and electric motor itself.

Important Terminologies

  • Contactor: The magnetic contactors act as electromagnetically operated switches. It uses electromagnetic energy to close switches, which consist of a coil placed on the iron core.
  • Coil of contactor: The most crucial component of a contactor, has two parts, a fixed one and a movable part with a spring connecting both parts. provides the driving force in a contactor that closes the contacts.
  • 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. These secondary switching devices are physically connected to the main switching equipment and work at the same time.

How Direct Starters Work?

When the start button is pressed, the motor is energized, and it begins to operate at full voltage. This direct connection allows for an immediate and full torque output, which is crucial for applications requiring high starting torque, such as compressors, conveyors, and pumps.
Steering voltage is connected thru NO Start contact to the normally closed contact of the STOP button. After pressing the START button circuit is closed and the control voltage is applied to the contactor coil. The contactor closes and the START button contact is paralleled by the auxiliary contact of the contactor. After closing the contactor, voltage is applied directly to the motor windings via its main contacts in contactor. After pressing the STOP button, the contactor’s holding circuit is interrupted and it opens without returning to energized itself.

Advantages of Direct Starters

  • Simplicity: Direct starter design is straightforward, making it easy to install and maintain.
  • Cost-Effectiveness: Starters are generally less expensive than more complex starting methods, such as star-delta starters or soft starters.
  • Immediate Full Torque: They provide immediate full torque to the motor, which is beneficial for applications that require quick acceleration.

Disadvantages of Direct Starters

  • High Inrush Current: One of the main drawbacks of starters is the high inrush current that occurs at startup, which can be several times greater than the motor’s full-load current. This can lead to electrical disturbances and potential damage to the motor and electrical supply system.
  • Limited to Smaller Motors: DOL starters are typically used for smaller motors. For larger motors, the high inrush current can cause issues, making alternative starting methods more desirable.

Conclusion

In summary, Direct Starter / Direct-On-Line motor starters are an essential component in many industrial settings for the control of electric motors. Their straightforward design and immediate torque delivery make them an effective choice for a variety of applications, although considerations regarding inrush current and motor size should be taken into account when selecting a starting method. Understanding the principles behind DOL starters helps engineers and technicians ensure reliable and efficient motor operation..

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Last posts on the blog

24.02.2025

Direct Starters

Direct starter or Direct on-line (DOL) is the simplest form of motor starter. This method starting cage motors, the motor is connected using a starter across the full supply voltage. Depending upon size and design, motor might takes a low-power starting current about 5 to 7 times its full-load current. In normal conditions such currents […]
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21.01.2025

Contactor

Different types of loads, such as resistive, inductive, or motor loads, place varying demands on a contactor. Inductive loads, like motors or transformers, require contactors that can handle high inrush currents, while resistive loads, such as heating elements, place a more constant demand on the contactor.
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See other posts
21.01.2025

Contactor

Different types of loads, such as resistive, inductive, or motor loads, place varying demands on a contactor. Inductive loads, like motors or transformers, require contactors that can handle high inrush currents, while resistive loads, such as heating elements, place a more constant demand on the contactor.
Show more

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