Direct On Line Starter

Introduction to Induction Motor Starters

Different starting methods are employed for starting induction motors because Induction Motors draw more starting current during starting. To prevent damage to the windings due to the high starting current flow, we employ different types of starters.

The simplest form of motor starter for the induction motor is the Direct On Line (DOL) starter. The DOL starter consists of an MCCB or Circuit Breaker, Contactor, and an overload relay for protection. The electromagnetic contactor can be opened by the thermal overload relay under fault conditions.

Principle of DOL Starter

To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.

As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.

Parts of DOL Starters

• Contactors & Coil: Magnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits.
• Overload Relay (Overload protection): Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.

Wiring of DOL Starter

The main heart of the DOL starter is the Relay Coil. Normally it gets one phase constant from incoming supply Voltage (A1). When the coil gets the second phase, the relay coil energizes and the magnet of the contactor produces an electromagnetic field, causing the plunger of the contactor to move and the main contactor of the starter to close.

Motor Starting Characteristics on DOL Starter

• Available starting current: 100%
• Peak starting current: 6 to 8 Full Load Current
• Peak starting torque: 100%

Advantages of DOL Starter

• Most Economical and Cheapest Starter
• Simple to establish, operate and maintain
• Simple Control Circuitry
• Easy to understand and troubleshoot
• It provides 100% torque at the time of starting

Disadvantages of DOL Starter

• It does not reduce the starting current of the motor
• High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor)
• Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life
• Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current

Features of DOL Starting

• For low- and medium-power three-phase motors
• Three connection lines (circuit layout: star or delta)
• High starting torque
• Very high mechanical load
• High current peaks

DOL is Suitable for:

A direct on line starter can be used if the high inrush current of the motor does not cause excessive voltage drop in the supply circuit. The maximum size of a motor allowed on a direct on line starter may be limited by the supply utility for this reason.

DOL is not suitable for:

• The peak starting current would result in a serious voltage drop on the supply system
• The equipment being driven cannot tolerate the effects of very high peak torque loadings
• The safety or comfort of those using the equipment may be compromised by sudden starting