Types and methods of starting DoL / Star Delta motors

Star Delta

Starting a DoL / Star Delta motor begins with the operation of a water pump. It requires electrical power to make the motor work. As for the types and methods of starting the motor, there are 3 main types of starting the motor: Direct on-line (DOL), Star-Delta, and Soft Start. This is to reduce the occurrence of various problems that follow, such as Water Hammer. What causes Water Hammer in the pipe system and what methods can help prevent it? Let's take a look.

1. Direct On-line Start-up (DOL): This method connects the 3-phase power supply directly to the motor. This initial start-up method causes the motor to consume up to six times its normal electrical current, leading to high electricity bills. Another major problem with this start-up method is freewheeling motor start-up, or stopping the motor without controlling its speed or torque. This results in water surges and subsequent water hammer, which can damage various equipment, including the water pump.

2. Star-Delta Start-up: This second starting method involves starting the motor in star-phase and then switching to delta-phase. This method reduces the electrical current consumption by three times compared to DOL, thus reducing electricity bills somewhat. However, star-delta start-up presents problems during start-up, as it reduces the system voltage by approximately 42%, reducing both motor torque and load torque. Consequently, the motor cannot start.

3. Soft Starter: This startup method can prevent all problems, including water hammer and insufficient torque for star-delta starting. It relies on the Voltage Ramp Up principle, which starts and stops the motor by gradually increasing/decreasing speed in line with the motor torque. This reduces electrical current, prevents pressure surges in the pipe system, and also reduces problems caused by insufficient torque.

When selecting the size of a soft starter for water pumps, it should be the same size as the motor, as the load has a low moment of inertia. However, if there are more than 10 starts and stops per hour, the soft starter should be one size larger.

The basic electrical wiring method for water pumps works like this. For Leo water pumps, the Star-Delta connection method is typically used for relatively large pumps, such as those with 7.5 HP or higher, and 3-phase power. Furthermore, the cost of installing the electrical cabinet and water pump is significantly higher than with conventional connections. This is to avoid damage caused by water hammer and other potential hazards.

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