The Direct-On-Line (DOL) starting method is the simplest and most widely used method for starting three-phase induction motors. In this method, the motor is connected directly to the full supply voltage without using any voltage-reducing device.
Because the motor receives the full rated voltage at start, it produces high starting current and high starting torque.
Table of Contents
Principle of DOL Starting
When the induction motor is directly connected to the supply:
- Full line voltage is applied to the stator windings.
- A large starting current flows through the motor.
- A strong magnetic field is produced.
- This field interacts with the rotor to produce starting torque, causing the rotor to accelerate.
Starting Current in DOL Starter
At the moment of starting, the rotor is stationary, so the slip
\[
s = 1
\]
The motor behaves like a transformer with its secondary short-circuited. As a result, the starting current becomes very high.
Typically,
\[
I_{start} = 5 \text{ to } 7 \times I_{rated}
\]
Thus, the motor may draw 5 to 7 times the rated current during starting.
Starting Torque
The starting torque of an induction motor under DOL starting is relatively high because torque is proportional to the
square of applied voltage.
\[
T \propto V^2
\]
Since full voltage is applied, the motor develops maximum possible starting torque.
Working of DOL Starter
- When the start button is pressed, the contactor coil is energized.
- The main contacts close and connect the motor directly to the supply.
- The motor starts and accelerates to its rated speed.
- If an overload occurs, the overload relay trips and disconnects the motor.
Advantages of DOL Starting
- Simple and inexpensive
- Easy to operate
- Provides high starting torque
- Suitable for small motors
Disadvantages
- Very high starting current
- Causes voltage dip in the power system
- Not suitable for large motors
Comparison with Other Starting Methods
| Starting Method | Starting Current | Starting Torque |
|---|---|---|
| DOL Starter | Very high | High |
| Star-Delta Starter | Reduced | Moderate |
| Auto Transformer Starter | Reduced | Adjustable |
| Soft Starter | Controlled | Smooth |