Synchronous motors are widely used in industrial applications due to their ability to operate at a constant speed regardless of load, high efficiency, and excellent power factor correction capabilities. However, one of the challenges with synchronous motors is their starting process. Unlike induction motors, synchronous motors cannot start on their own because they require the rotor to be brought up to near synchronous speed before the rotor’s magnetic field can lock in with the stator’s rotating magnetic field.
In this blog, we will explore the various starting methods for synchronous motors commonly used in industrial settings, including their working principles, advantages, and limitations. Understanding these methods is crucial for selecting the right approach to ensure smooth motor startup, reduce mechanical stress, and optimize energy consumption.
1. Using an external prime mover
One of the most straightforward methods to start a synchronous motor is by using an external prime mover, such as a diesel engine, steam turbine, or another motor.
How It Works:
- The external prime mover mechanically brings the synchronous motor’s rotor up to near synchronous speed.
- Once the rotor speed is close to synchronous speed, the DC excitation is applied to the rotor winding.
- The rotor’s magnetic field locks in with the stator’s rotating magnetic field, and the motor runs synchronously.
Advantages:
- Provides precise control over the starting speed.
- Eliminates the need for complex electrical starting circuits.
- Useful in applications where the synchronous motor is coupled with a prime mover, such as in hydroelectric plants.
Common Applications:
Large industrial equipment, such as compressors and pumps, where a prime mover is already available or needed.
Limitations:
- Requires additional equipment, increasing system complexity and cost.
- Not practical for all industrial applications, especially where space or cost constraints exist.
2. Damper Windings (Auxiliary Windings)
Damper windings, also known as amortisseur windings, are embedded in the rotor of synchronous motors to facilitate starting.
How it works:
- The damper winding acts like a squirrel-cage rotor in an induction motor.
- When the motor is started, the damper winding carries induced currents that produce torque, allowing the motor to accelerate.
- As the rotor approaches synchronous speed, the DC excitation is applied to the rotor winding.
- The rotor then locks into synchronous speed, and the damper winding currents diminish.
Advantages:
- Allows the synchronous motor to start as an induction motor without external equipment.
- Smooth transition from asynchronous to synchronous operation.
- It is widely used in industrial synchronous motors due to its simplicity and reliability.
Common applications:
Large synchronous motors are used in industrial machinery, power generation plants, and facilities requiring smooth, reliable startups.
Limitations:
- Starting torque is limited by the damper winding design.
- Not suitable for very large synchronous motors where starting torque requirements are high.
3. Variable frequency drives (VFDs)
Variable Frequency Drives (VFDs) are modern electronic devices that control the frequency and voltage supplied to the motor.
How it works:
- The VFD gradually increases the frequency and voltage from zero to the rated values.
- This allows the synchronous motor to accelerate smoothly from standstill to synchronous speed.
- Once the motor reaches synchronous speed, the rotor excitation is applied, and the motor runs synchronously.
Advantages:
- Provides smooth and controlled starting with minimal mechanical stress.
- Reduces inrush current and energy consumption during startup.
- Allows for speed control during operation, enhancing process flexibility.
Common applications:
Pumps, fans, compressors, and various applications require precise speed control and gradual acceleration.
Limitations:
- The higher initial cost is due to the VFD equipment.
- Requires careful tuning and protection settings to avoid motor damage.
- May introduce harmonic distortion in the power system if not properly.
4. Inrush Current and Gradual Excitation
Starting a synchronous motor involves managing the inrush current and the excitation of the rotor field.
Inrush current:
When the motor is started, a large current (inrush current) flows due to the motor’s low impedance at standstill. High inrush current can cause voltage dips and mechanical stress.
Gradual Excitation:
To mitigate this, the rotor excitation is applied gradually. Initially, the motor is started as an induction motor (using damper windings or other methods).
Once the rotor speed is near synchronous speed, the DC excitation is slowly increased. This gradual excitation reduces sudden torque changes and stabilizes the motor operation.
Advantages:
- Protects the motor and power system from electrical and mechanical shocks.
- Enhances the lifespan of the motor and connected equipment.
Common applications:
Occasionally used in very large motors where other starting methods might not be feasible.
Limitations:
- Requires control systems capable of managing excitation current.
- May increase the complexity of the motor control system.
5. Auto-Transformer or Star-Delta Starter
These are traditional electrical starting methods used to reduce the starting current and torque of synchronous motors.
Auto-Transformer Starter:
An auto-transformer reduces the voltage applied to the motor during startup. The motor starts at a reduced voltage, limiting the inrush current.
After the motor reaches a certain speed, the full voltage is applied, and the motor synchronizes.
Star-delta starter:
The motor is initially connected in a star (Y) configuration, reducing the voltage applied to each winding to approximately 58% of the line voltage.
This reduces the starting current and torque. After the motor accelerates, the connection switches to delta (Δ) for full voltage operation.
Advantages:
- Simple and cost-effective methods for reducing starting current.
- Widely used in industrial applications with standard motors.
Common applications:
Large motors are used in mining, manufacturing plants, and power generation industries.
Limitations:
Not suitable for all synchronous motors, especially those requiring precise starting control. Mechanical switching can cause transient disturbances. It does not inherently bring the rotor to synchronous speed; instead, it is often combined with damper windings.
Why are special starting methods necessary?
Synchronous motors cannot start by themselves because their rotor needs to match the synchronous speed of the rotating stator field to operate effectively. Without special starting techniques, the rotor would simply oscillate without catching up, failing to start the motor. Each of these methods overcomes that limitation by either pre-accelerating the motor or using auxiliary systems to initiate rotation.
Conclusion-starting methods for synchronous motors
Starting synchronous motors in industrial applications requires careful consideration of the motor size, load characteristics, and operational requirements. Each starting method has its own advantages and limitations:
| Starting Method | Key Advantage | Typical Application | Limitations |
| External Prime Mover | Precise speed control | Large motors, hydro plants | High cost, complex setup |
| Damper Windings | Simple, smooth transition | Most industrial synchronous motors | Limited starting torque |
| Variable Frequency Drives | Smooth start, speed control | Modern industrial processes | High initial cost, complexity |
| Gradual Excitation | Reduces inrush current | Motors with excitation control | Requires control systems |
| Auto-Transformer/Star-Delta | Reduces starting current | Standard industrial motors | Mechanical switching, limited control |
Starting methods for synchronous motors table
At ZHENLI Motor, we understand the importance of selecting the right starting method to optimize motor performance and reliability. Whether you are dealing with small synchronous motors or large industrial units, our expertise and product range can help you achieve efficient and reliable motor startup tailored to your specific needs.