Ingenious Methods to Synchronous Motors Startup

September 18, 2023 Jessica

The synchronous motor, distinguished for its synchronous speed operation, presents an interesting challenge when commencing motion from a standstill position. Unlike asynchronous motors, synchronous motors cannot self-start due to their fixed-speed nature. Therefore, ingenious methods must be employed to initiate their operation. The starting process is crucial as it lays the foundation for the motor’s subsequent synchronization with the supply frequency. Here, we delve into the remarkable techniques that enable the seamless launch of synchronous motors, propelling them from inertia to productive motion.

Synchronous Motors: Methods from Standstill

We know that a synchronous motor cannot start on its own. Hence, some method must be used to create it. The following strategies can obtain the starting of a synchronous motor from its standstill position.

Direct-On-Line (DOL) Starting

This conventional method involves directly connecting the synchronous motor to the power supply. It utilizes the maximum torque capability of the engine during starting, utilizing the strong magnetic field generated by the direct power connection. However, this approach can result in high inrush current, potentially leading to grid instability or damage to the motor windings.

Pony Motor Starting

In this creative approach, a smaller auxiliary motor known as the “pony motor” is employed to bring the synchronous motor to synchronous speed. Once the synchronous motor reaches near-synchronous speed, it’s synchronized with the supply frequency and takes over the load. This method minimizes inrush current and offers controlled starting.

Damper Windings and Starting Induction Motor

This method initiates the synchronous motor’s rotation using damper windings and an induction motor. The induction motor brings the synchronous motor to a fraction of synchronous speed, after which the synchronous motor’s field winding is excited to “pull in” the rotor to synchronism. This approach is beneficial for large synchronous motors.

External Starting Means

An external starting means, like a variable frequency drive (VFD) or a static frequency converter, can be utilized for applications requiring precise speed control. These devices allow gradual acceleration, reducing mechanical and electrical stresses during startup.

Starting Using a Separate Induction Motor

The synchronous motor is made to reach its synchronous speed by using a separate small induction motor mechanically coupled to it. The number of poles in the synchronous motor must be higher than the number of bars in the induction motor. This is to help the induction motor run at the synchronous speed of the synchronous motor. The synchronous motor will have to synchronize with the bus bar in this method. Afterwards, the induction motor can be disconnected. Then, the synchronous motor uniform of its own.

Starting by using a DC Motor

A synchronous motor can be started by using a DC motor. Firstly, the synchronous motor is driven by a DC motor and is brought to synchronous speed. The machine is then synchronized with the bus bars. The machine works as a motor when the synchronous device is connected in parallel with the bus bars. Then, the DC machine coupled to the synchronous machine can be mechanically disconnected from the system or drawn as a generator.

Starting using a Damper winding

A damper winding is made on the pole face slots. Copper aluminum bars are inserted in places on pole shoes, and each side of the poles and end rings short-circuit these bars. A squirrel cage winding is formed by these short-circuited bars. A three-phase supply is given to the stator.

The synchronous motor, provided with a damper winding, starts as an induction motor. This will run at a speed near the synchronous speed. At this stage, DC excitation is given to the field windings. The rotor will be pulled into synchronous speed. This is because the pole of the rotor rotating magnetic slip speed only concerns the stator rotating magnetic field. For higher output motors, the starting current drawn may be many times the full load current. So, the starting current has to be limited to a safe value. For this purpose, an autotransformer may apply reduced voltage. The voltage applied should be about 50 to 80 percent of the full line voltage. The connections for the autotransformer are shown in the figure. To reduce the supply voltage, the switch S₁ is closed, and S₂ is kept open. When the motor picks up, S₂ is fast, and S₁ is kept open to cut out the transformer.

In this method, starting is effected as an induction motor. The starting torque produced is low. Hence, the output motor may not be able to start with full load.

Tests in Induction Motor: No Load and Locked Rotor

Induction motors, the workhorses of the industry, undergo rigorous testing to assess their performance and operational characteristics. Two fundamental tests—No Load Test and Locked Rotor Test—provide crucial insights into the motor’s efficiency, starting behavior, and potential issues.

Evaluating Core Losses and No-Load Current

The No Load Test, conducted at rated voltage, runs the motor without any mechanical load. This test primarily aims to determine core losses, magnetizing current, and no-load current. By measuring input power and input current, it’s possible to calculate core losses. The magnetizing wind reveals the energy required to maintain the magnetic field without load. Efficient magnetization is vital, as excessive current can lead to energy wastage.

Locked Rotor Test

In the Locked Rotor Test, the motor’s rotor is immobilized, simulating a stalled condition. This test examines stator current, locked rotor torque, and impedance. We can assess the motor’s behavior during startup and under stress by measuring the input power, current, and voltage. The locked rotor torque provides insights into the motor’s starting capability and efficiency. High-locked rotor current or torque may indicate potential problems like winding issues or mechanical binding.

Conclusion

Starting a synchronous motor demands a thoughtful approach due to its inherent characteristics. The methods mentioned above reflect the innovative spirit of engineering, addressing the challenge of initiating motion in these specialized motors. The chosen method depends on factors like motor size, application, and desired operational characteristics. As technology advances, new forms may emerge, further showcasing the resourcefulness of engineers in conquering the intricacies of synchronous motor startup.

FAQs

1. Why can’t a synchronous motor start on its own?

Synchronous motors are designed to operate at a fixed speed, known as synchronous speed, which is determined by the supply frequency. Unlike asynchronous motors, they lack the inherent slip necessary for self-starting. As a result, external methods are required to initiate their motion.

2. What are the challenges of starting a synchronous motor?

The main challenge is the absence of inherent slip. During startup, synchronous motors don’t naturally rotate and need external assistance to reach their synchronous speed. The initial high inrush current during certain starting methods can also strain the engine and electrical systems.

3. What is the Direct-On-Line (DOL) starting method?

DOL starting involves directly connecting the synchronous motor to the power supply. While it provides maximum torque for starting, it can lead to high inrush currents, potentially affecting the power grid’s and motor windings’ stability.

4. How does the pony motor starting method work?

The pony motor method initially employs a smaller auxiliary motor (pony motor) to bring the synchronous motor to near-synchronous speed. Once the synchronous motor is in sync with the supply frequency, it takes over the load. This approach reduces inrush current and ensures controlled starting.

5. What is the significance of damper windings and starting induction motors?

This method involves using damper windings and an induction motor to initiate rotation. The induction motor accelerates the synchronous motor to a fraction of the synchronous speed. Subsequently, the synchronous motor’s field winding is excited, pulling the rotor into synchronism. It’s a preferred approach for larger synchronous motors.