Because synchronous motors can maintain a steady speed under any load, they are frequently utilized in industrial settings. However, starting synchronous motors presents challenges because they require a method to bring them up to synchronous speed before they can lock in with the grid. This is due to the fact that synchronous motors do not naturally start at synchronous speed on their own, unlike induction motors. Various methods are employed to start these motors safely and efficiently.

This article will explore the four primary methods of starting synchronous motors, explaining how each method works, its advantages, and where it is most commonly used. The methods include:

• Direct On-Line (DOL) Start
• Auto-Transformer Start
• Star-Delta Start
• Wound Rotor (Slip Ring) Start

Now let’s examine each strategy in depth.

Direct On-Line (DOL) Start

How It Works:

The most straightforward and widely used technique for starting a synchronous motor is the Direct On-Line (DOL) method. With this approach, the motor is immediately connected to the supply voltage upon pressing the start button.

The motor’s rotor is initially stationary, and once the current flows through the stator, the rotor experiences a torque that accelerates it toward synchronous speed. For synchronous motors, once the rotor reaches synchronous speed, the motor can lock onto the grid and start functioning at full capacity.

Advantages

• Simplicity: The DOL method is easy to implement, requiring minimal electrical components.
• Cost-effective: Because no additional equipment is needed, this method is cost-effective.
• Quick Start: The motor reaches synchronous speed almost immediately once connected.

Disadvantages

• High Inrush Current: Upon starting, there is a large inrush current, which can cause stress on the electrical grid and the motor.
• Limited Application: DOL starting is best suited for small synchronous motors with low power ratings (typically below 5 HP).

Application

This method is commonly used in small synchronous motors in applications where the motor power requirement is relatively low, such as small pumps and compressors.

Auto-Transformer Start

How It Works

The Auto-Transformer approach lowers the voltage applied to the motor at startup by using an auto-transformer. This lowers the motor’s starting current and gives it regulated acceleration.

The transformer is typically connected in such a way that only a portion of the full supply voltage is applied to the motor at startup. After the motor reaches near synchronous speed, the auto-transformer is disconnected, and the motor is connected directly to the full supply voltage.

Advantages

• Reduced Starting Current: The auto-transformer method reduces the starting current to a fraction of the normal current, which helps prevent damage to electrical systems and other equipment.
• Smooth Acceleration: The motor accelerates smoothly to synchronous speed without the sharp torque surges experienced in DOL starting.
• Higher Power Capacity: This method is suitable for larger motors that require a higher power rating.

Disadvantages

• Cost: The auto-transformer is an additional piece of equipment, which increases the cost.
• Size Limitation: The motor’s starting torque is reduced in this method, which may not be ideal for motors requiring high starting torque.

Application

This method is often used for medium to large synchronous motors, typically in industrial settings like conveyors, mills, and large pumps where power demand is higher, and inrush current needs to be limited.

Star-Delta Start

How It Works

In the Star-Delta method, the motor’s stator winding is first connected in a star configuration to reduce the voltage. The stator windings are changed to a delta design whenever the motor reaches a specific speed. The motor can run at its maximum capacity when the voltage is increased by switching from star to delta.

The motor initially starts in the star configuration, where the phase voltage is reduced by a factor of √3, which in turn reduces the starting current. The motor operates at full voltage and current after the windings are changed to the delta configuration at a specific speed.

Advantages

• Reduced Starting Current: Like the Auto-Transformer method, the Star-Delta method also helps reduce the inrush current by applying a reduced voltage during startup.
• Simplicity: It is easier and cheaper to implement compared to an auto-transformer, as it only requires a changeover switch and additional wiring.
• Widely Used: This method is commonly used in industries for high-power motors.

Disadvantages

• Reduced Starting Torque: The starting torque in the star configuration is lower than in the delta configuration, which might not be ideal for applications requiring high starting torque.
• Switching Complexity: Switching from star to delta at the correct moment requires precise control, and if the switch is done too early or too late, it could result in inefficient operation.

Application

The Star-Delta method is frequently used in large motors, especially in industries like cement plants, water treatment, and other heavy machinery, where the motor’s size can be very large and the reduction in current during startup is crucial.

Wound Rotor (Slip Ring) Start

How It Works

The Wound Rotor method uses a rotor that is wound with a three-phase winding, rather than the conventional squirrel-cage rotor. At startup, the resistance in the rotor circuit is high, which reduces the starting current. The rotor speed increases gradually as the external resistors are gradually shorted out, eventually allowing the motor to reach synchronous speed.

Once the motor reaches synchronous speed, the rotor circuit is shorted out completely, and the motor continues running at full voltage and current. This approach is perfect for applications needing high torque because it offers great control over the beginning current and torque.

Advantages

• Variable Starting Torque: The starting torque may be changed thanks to the external resistors, which is advantageous in applications with high loads.
• Smooth Start: The method ensures a smooth acceleration without high inrush current.
• High Starting Power: This method is capable of starting large synchronous motors with high torque requirements.

Disadvantages

• Complexity and Cost: The use of slip rings and external resistors increases the cost and complexity of the system.
• Maintenance: Proper maintenance of the brushes and slip rings is essential for optimal performance.

Application

This method is typically used for high-power synchronous motors in industries such as steel manufacturing, crushers, and mills where high starting torque is necessary.

Comparison of Methods

Below is a comparative chart summarizing the key aspects of each starting method:

Method	Starting Current	Starting Torque	Cost	Suitability	Maintenance
Direct On-Line	High	High	Low	Small motors (up to 5 HP)	Low
Auto-Transformer	Reduced	Medium	Moderate	Medium to large motors (5 HP to 500 HP)	Low
Star-Delta	Reduced	Low	Low	Large motors in general industry	Moderate
Wound Rotor	Low	Adjustable	High	Heavy-duty motors (500 HP+)	High

Conclusion

The method used for starting a synchronous motor, as a synchronous motor manufacturer would know, largely depends on the motor’s size, power requirement, and the operational needs of the application. Smaller motors with lower power ratings typically use the Direct On-Line (DOL) method, while larger, more power-hungry motors benefit from methods like Auto-Transformer, Star-Delta, or Wound Rotor start. Each method offers unique advantages in controlling the starting current and providing the necessary torque for the motor to reach synchronous speed.

In practice, selecting the appropriate starting method ensures that the motor is protected while optimizing energy use and performance.