Motor Starting Method (1- DOL Method )

When a motor is switched on , there is a high inrush current from the mains, ( The starting current of an AC motor can vary from 3 to 8 times the nominal current). The high current drawn from the network can cause problems such as voltage drop, high transients and, in

Schneider Electric DOL

some cases, uncontrolled shutdown. High starting current also causes great mechanical stress on the motor’s rotor bars and windings, and can affect the driven equipment and the foundations. Several starting methods exist, all aiming to reduce these stresses To overcome this , There are several starting systems which differ according to the motor and load specifications. The choice is governed by electrical, mechanical and economic factors. The kind of load driven is also important in the choice of starting system.

1- Direct on-line starting 

This is the simplest mode, where the stator is directly connected to the mains supply. The motor starts with its own characteristics. Direct on line starting is suitable for stable supplies and mechanically stiff and well-dimensioned shaft systems. It is the simplest, cheapest and most common starting method. When it is switched on, the motor behaves like a transformer with its secondary, formed by the very low resistance rotor cage, in short circuit. There is a high induced current in the rotor which results in a current peak in the mains supply.

 

Current on starting 

 5 to 8 rated Current.

                                                                                              

The average starting torque is:

T on starting = 0.5 to 1.5 rated T.

 Advantages

 - Simple equipment

- High starting torque

- Fast start

- Low cost

This Method is suitable only when:

- The power of the motor is low compared to that of the mains, which limits interference from inrush current.
- The machine to drive does not need to speed up gradually or has a damping device to limit the shock of starting.
- The starting torque can be high without affecting machine operation or the load that is driven.

 Circuit Diagram: 

The Direct online ( Power and Control ) Circuits consist of :

( Fuse , MCCB , Contactor , Overload relay , Push-buttons  On , Push-buttons Off  )

   

 

 

 

 

 

 

 

 

 

 

 

             

 

 

 

        Control Circuit                                                                   Power Circuit

• Contactor is connecting among Supply Voltage , Relay Coil and Thermal Overload Relay
• L1 of Contactor Connect (NO) to R Phase through MCCB
• L2 of Contactor Connect (NO) to Y Phase through MCCB
• L3 of Contactor Connect (NO) to B Phase through MCCB
• (13-14 or 53-54) is a normally Open NO contact (closes when the relay energizes)
• Contactor Point 53 is connecting to Start Button Point (94) and 54 Point of Contactor is connected to Common wire of Start/Stop Button.
• (95-96) is a normally closed NC contact (opens when the thermal overloads trip if associated with the overload block)
• A1 of Relay Coil is connecting to any one Supply Phase and A2 is connecting to Thermal over Load Relay’s NC Connection (95).
• T1,T2,T3 are connect to Thermal Overload Relay
• Overload Relay is Connecting between Main Contactor and Motor
• NC Connection (95-96) of Thermal Overload Relay is connecting to Stop Button and Common Connection of Start/Stop Button.