Short Answer:

Starting a synchronous motor is more complex compared to other types of motors, as it requires the motor to reach synchronous speed before it can begin operating normally. Typically, synchronous motors are started using induction motor methods, such as using a pony motor (a smaller motor to bring the synchronous motor up to speed) or using a damper winding to help the rotor reach synchronous speed. Once the motor reaches the synchronous speed, it locks into synchronization with the power supply.

Other methods include using variable frequency drives (VFDs) or auto-synchronous starting techniques, which control the motor’s speed and phase to achieve synchronization.

Detailed Explanation:

How to Start a Synchronous Motor

A synchronous motor operates at a fixed speed determined by the supply frequency and the number of poles in the motor. Unlike induction motors, which can start by themselves and gradually reach full speed, a synchronous motor must reach synchronous speed before it can begin operating. This requirement arises because synchronous motors rely on the rotor aligning with the rotating magnetic field produced by the stator at synchronous speed. If the rotor is not at this speed, it will not be able to follow the rotating field and will not function as intended.

1. Methods of Starting Synchronous Motors

There are several methods used to start synchronous motors, each of which aims to bring the motor to synchronous speed and then synchronize the rotor with the stator’s magnetic field. These methods are typically used to avoid the problem of the rotor being unable to follow the rotating magnetic field from a standstill position.

2. Starting with an Induction Motor (Pony Motor)

One common method for starting a synchronous motor is to use a smaller induction motor (known as a pony motor) to bring the synchronous motor up to synchronous speed. The pony motor is mechanically coupled to the synchronous motor’s shaft and runs the motor up to the required speed. Once the motor reaches synchronous speed, the rotor locks into place with the rotating magnetic field of the stator, and the pony motor is disconnected. This method is typically used for larger motors and industrial applications where precise speed control is needed.

• Advantages: This method is reliable and widely used in larger industrial applications.
• Disadvantages: The need for an additional motor increases the cost and complexity of the system.

3. Damper Windings

In synchronous motors, especially larger ones, damper windings can be used to facilitate starting. These are windings placed on the rotor, which, when current is applied, create an electromagnetic field. This allows the rotor to experience torque similar to that of an induction motor, enabling the motor to accelerate up to synchronous speed. Once the motor reaches synchronous speed, the damper windings are no longer needed, and the motor begins running synchronously.

• Advantages: This method avoids the need for a pony motor and simplifies the design of the starting system.
• Disadvantages: It is more complex than using a simple induction motor and requires careful handling of the rotor windings.

4. Starting with a Variable Frequency Drive (VFD)

Another modern approach to starting synchronous motors is by using a Variable Frequency Drive (VFD). A VFD can control the speed and frequency of the voltage supplied to the motor, allowing the motor to gradually reach synchronous speed. Once the motor reaches the correct speed, the VFD synchronizes the rotor with the stator’s magnetic field, and the motor begins running at synchronous speed. This method is particularly useful in applications where precise control of motor speed and torque is required.

• Advantages: VFDs offer precise control and smooth starting, reducing mechanical stresses on the motor and extending its lifespan.
• Disadvantages: The initial cost of VFDs can be high, and they require careful integration into the motor control system.

5. Auto-Synchronous Starting

In some cases, auto-synchronous starting is used. This method involves gradually accelerating the motor using variable speed drives or specialized controllers that adjust both the speed and phase of the motor. The controller adjusts the frequency of the power supply to bring the motor to synchronous speed. Once the rotor reaches the required speed, the system locks the motor into synchronization with the rotating magnetic field of the stator.

• Advantages: This method is very efficient and ensures smooth starting.
• Disadvantages: It is best suited for specialized applications, and the equipment required can be expensive.

Conclusion:

Starting a synchronous motor requires methods that bring the rotor up to synchronous speed, after which it can lock into synchronization with the stator’s rotating magnetic field. Common methods include using a pony motor, damper windings, variable frequency drives, and auto-synchronous starting techniques. Each of these methods has its advantages and is selected based on the size, application, and control requirements of the motor. Understanding these methods helps ensure reliable and efficient operation of synchronous motors, especially in large and high-performance systems.