Synchronous motors: methods from standstill
We know that a synchronous motor cannot start itself. Therefore, a method must be used to start it. The following strategies can achieve starting a synchronous motor from standstill.
Direct Start (DOL)
In this traditional method, the synchronous motor is directly connected to the power supply. This utilizes the motor's maximum torque when starting using the strong magnetic field generated by the direct power connection. However, this approach can result in high starting current, potentially causing network instability or damage to the motor windings.
Starting the pony engine
This creative approach uses a smaller auxiliary motor, called a “pony motor,” to bring the synchronous motor up to synchronous speed. As soon as the synchronous motor reaches a near-synchronous speed, it is synchronized with the grid frequency and takes over the load. This method minimizes starting current and allows controlled starting.
Damper windings and starting induction motor
In this method, the rotation of the synchronous motor is initiated using damper windings and an induction motor. The induction motor drives the synchronous motor to a fraction of the synchronous speed, after which the synchronous motor's field winding is excited to “pull” the rotor into synchronism. This approach is advantageous for large synchronous motors.
External starting device
For applications that require precise speed control, an external starting means such as a variable frequency drive (VFD) or a static frequency converter can be used. These devices allow gradual acceleration, reducing mechanical and electrical stress during starting.
Starting with separate induction motor
The synchronous motor is driven to its synchronous speed by a separate small induction motor that is mechanically coupled to it. The number of poles of the synchronous motor must be greater than the number of rods of the induction motor. This helps the induction motor to run at the synchronous speed of the synchronous motor. This method requires the synchronous motor to be synchronized with the bus. The induction motor can then be disconnected. Then the synchronous motor balances its own uniformity.Start with a DC motor
A synchronous motor can be started using a DC motor. First, the synchronous motor is driven by a direct current motor and brought to synchronous speed. The machine is then synchronized with the buses. The machine works as a motor when the synchronous device is connected in parallel to the buses. The direct current machine coupled to the synchronous machine can then be mechanically separated from the grid or used as a generator.Start with damper winding
A damper winding is attached to the slots on the pole faces. Copper-aluminum rods are inserted in a few places into the pole pieces, and each side of the poles and end rings short-circuits these rods. A cage winding is formed by these short-circuited rods. The stator is supplied with three-phase voltage.
The synchronous motor, equipped with a damper winding, starts like an induction motor. This works at a speed close to synchronous speed. In this phase, the field windings are energized with direct current. The rotor is pulled to synchronous speed. This is because the rotating pole of the rotor's magnetic sliding speed only affects the rotating magnetic field of the stator. For motors with higher power, the starting current consumed can be many times greater than the full load current. Therefore, the starting current must be limited to a safe value. For this purpose, an autotransformer can apply a reduced voltage. The applied voltage should be approximately 50 to 80 percent of the total line voltage. The autotransformer connections are shown in the figure. To reduce the supply voltage, switch S 1 is closed and S 2 is kept open. When the engine starts, p 2 is fast and S 1 is held open to turn off the transformer.
This method starts as an induction motor. The starting torque generated is low. Therefore, the output motor may not be able to start at full load.
Induction motor tests: idle and locked rotor
Induction motors, the workhorses of the industry, undergo rigorous testing to evaluate their performance and operating characteristics. Two basic tests – the idle test and the stall test – provide important information about the engine's efficiency, its starting behavior and possible problems.
Assessment of central losses and no-load current
The no-load test is performed at rated voltage and allows the engine to operate without mechanical stress. This test's main objective is to determine the core losses, magnetizing current and no-load current. By measuring the input power and input current, core losses can be calculated. The magnetizing wind indicates the energy required to maintain the magnetic field without tension. Efficient magnetization is critical because excessive current can lead to wasted energy.
Locked Rotor Test
Locking rotor testing involves locking the engine rotor, simulating a stall condition. This test examines stator current, blocking rotor torque, and impedance. We can evaluate the motor's behavior during starting and under load by measuring the input power, current and voltage. Stall rotor torque provides information about engine starting ability and efficiency. High locked rotor current or high locked rotor torque can indicate potential problems such as winding problems or mechanical binding.
Conclusion
Starting a synchronous motor requires a careful approach due to its inherent characteristics. The methods mentioned above reflect the innovative spirit of the engineers and meet the challenge of making these special engines move. The method chosen depends on factors such as engine size, application and desired operating characteristics. As technology advances, new formats may emerge, further demonstrating the ingenuity of engineers in mastering the complexities of synchronous motor starting.
Common questions
1. Why can't a synchronous motor start itself?
Synchronous motors are designed to operate at a fixed speed, called synchronous speed, which is determined by the grid frequency. Unlike asynchronous motors, they do not have the slip necessary for automatic starting. Therefore, external methods are necessary to initiate your movement.
2. What are the challenges of starting a synchronous motor?
The biggest challenge is ensuring that no landslides occur. When starting, synchronous motors do not rotate on their own and require external support to reach their synchronous speed. The high initial starting current associated with certain starting methods can also cause stress on the engine and electrical systems.
3. What is the Direct-On-Line (DOL) boot method?
During DOL starting, the synchronous motor is directly connected to the power supply. Although this provides maximum starting torque, it can result in high starting currents, potentially affecting the stability of the power system and motor windings.
4. How does pony engine starting work?
With the pony motor method, a smaller auxiliary motor (pony motor) is used first to bring the synchronous motor to a near-synchronous speed. Once the synchronous motor is synchronized with the grid frequency, it takes over the load. This approach reduces starting current and ensures controlled starting.
5. What is the importance of damper windings and starting asynchronous motors?
This method uses damper windings and an induction motor to initiate rotation. The induction motor accelerates the synchronous motor at a fraction of the synchronous speed. The synchronous motor's field winding is then energized, bringing the rotor into synchronism. This is a preferred approach for larger synchronous motors.